Iremember writing in one of my early booksentitled Servicing with the Oscilloscope,first published in 1969 by Newnes-Butterworth, that the oscilloscope is a‘measuring and diagnostic instrument of

prodigious versatility’. Some 35 years later Istill regard the instrument in exactly the same light.

The oscilloscope has been my benchcompanion since as far back as I canremember. I think my very first ‘feel’ of theoscilloscope’s potential was after I hadimprovised a very basic hook-up for examiningthe mains supply waveform and its harmonics.That must have been just before the start ofThe Second World War, before I had emergedfrom my apprenticeship (as a typewritermechanic!).

No Financial Excuse!As Rob G3XFD, the Editor, has taken pains toemphasise in the May to September issues ofPW in his excellent on-running Radio Basics(RB) series, the oscilloscope (or ‘scope for short)is most definitely the primary of allinstruments that should always be at hand tothe enthusiastic experimenter and RadioAmateur in his or her workshop or shack.There’s not really any financial excusenowadays for not being equipped bearing inmind the incredibly low price for which ‘scopesof earlier specifications can now be acquiredfrom rallies and car boot sales, etc.

Once you are conversant with anyreasonable ‘scope, regardless of its age, then ifnothing else, it can be put into immediate usefor gauging the magnitude of a voltage andthe repetition frequency of a waveform. Inother words, a couple of the basic applications

of the ‘scope are those of a high-resistancevoltmeter and a frequency indicator. However,before we get too carried away let’s first havea look at how the ‘scope works and itsprinciples of operation.

The heart of the analogue ‘scope, the typeof instrument that we shall be investigating inthis series, is the cathode-ray tube (c.r.t.). The‘scope c.r.t. includes many of the basicprinciples of the television picture tube; butthere are differences.

For Radio Amateur use we would notnormally expect our ‘scope to provide colourdisplays; and a screen the size of that intelevision receivers would be going a bit overthe top too! There are other technicaldifferences, including magnetic deflection, butwe shall pick these up as we go along.

Cathode Ray TubeA good idea now would be to look again atthe skeleton presentation of a ‘scope’s c.r.t.that appeared in RB in the June 2004 issue ofPW (Fig. 1). Right at the narrow end of thetube’s glass envelope you will see theelectrode conglomeration from which theelectron beam is ‘fired’ at high velocity. This isknown as the ‘electron gun’ for obviousreasons.

The beam from the gun then passesthrough the space between two pairs ofplates, known as the X and Y deflection plates,before arriving at the screen at the front ofthe tube where it impinges at a remarkablevelocity upon the luminescent screen

phosphors with which the inside of the screensurface is coated. The glass ‘tube’ in which allthese things are contained is airtight and avirtual vacuum.

The cathode in the gun assemblyproduces the electrons, which eventually formthe electron beam. This is brought up totemperature by the heater, which is the samesort of thing as used in a thermionic valve, andthe electrons thus emitted pass through thegun electrodes that effectively concentratethem into a forward flowing beam.

Almost completely surrounding the

28 Practical Wireless, January 2005

Part 1 - Basic Principles

THE OSCILLOSCOPE PART 1 - BASIC PRINCIPLESTh

eGordon King G4VFVexplains the basic principlesof the oscilloscope, a piece oftest equipment that he saysshould be every RadioAmateur’s benchcompanion.

Oscil

losc

ope

Did You Know?The afterglow effect on a ‘scope screen iscaused by the time taken for the light todecay after the electron beam has movedaway?

, , p p g

cathode is the grid that containsa small aperture at its far endfrom which the electronsemerge. An anode positionedafter the grid prevents unduedivergence of the beam, while asecond anode serves as a sort of‘converging lens’ that allows thebeam focus to be optimised atits impinging point on thescreen. Operating controls onthe front panel of the ‘scopework in relation to theelectrodes to provide adjustmentof brightness and focus - thereare many other controls, ofcourse, as we shall later discover!

Post Deflection AccelerationThe velocity at which theelectrons are initially firedtowards the screen, and hence

the pre-deflection ‘stiffness’ ofthe beam, is related to thevoltage applied to the thirdanode of the electron gun. Thehigher the voltage here thegreater the velocity andtherefore the stiffer the beam.

Now, because the sensitivityof the beam deflection plates(see later) decreases as the beambecomes ‘stiffer’ and the displaybrighter, further beamacceleration might well beprovided after the deflection hastaken place. This is known aspost deflection acceleration(p.d.a.) and neatly bestows thec.r.t. with a higher displaybrightness withoutcompromising deflectionsensitivity. It’s achieved by aneffective forth anode thatconsists of a conductive coatingin the form of a helix inside the

flare of the tube, between thedeflection plates and the screen.One end is held at the voltage ofanode three and the other endat an even higher voltage, the

ratio between the two beingknown as the p.d.a. ratio.

Beware High Voltages!To provide the required beamvelocity and display brightnessthe potentials on anodes threeand four, relative to the tube’scathode, might well be in theorder of thousands of volts. Such

high voltages should not beregarded lightly!

High voltages could causemore than a nasty jolt if touchedby the human body, so bewarned when fiddling aboutinside any ‘scope. Make sure it’sswitched off, any high voltagecapacitors discharged (includingthe feed from the high voltagesupply) and the mains supplydisconnected.

Screen PhosphorsNow, because electrons have adefinite mass, even thoughremarkably diminutive, the highvelocity at which they areaccelerated results in themacquiring kinetic energy (energypossessed by a body in virtue ofits motion) on their travel fromthe electron gun to the

Practical Wireless, January 2005 29

WT2398 Horizontal (X) deflection plates

Vertical (Y) deflection plates

Focused stream of electrons

Electron beam gun andelectric focussing unit

Annular (beam forming) anode

Phosphor coated (inside) screen,

held at an high voltage, glows

when struck by the electron beam

Glass envelope

X1Y1

X2

Y2

NEWSERIES!

Fig. 1: Simplified outline diagram of the type ofcathode-ray tube used in an oscilloscope. Theelectron gun ‘fires’ electrons in the form of afocused beam to the phosphor-coatedfluorescent screen. Horizontal and verticaldeflection respectively results from voltagesapplied to the X and Y deflection plates, asexplained in the text. (Note: this diagram was firstpublished in Radio Basics on page 27 of the June2004 issue of PW).

Did You Know?A photograph taken of anoscilloscope screen is calledan oscillogram?

Fig. 2: Showing above the vertical Y and the horizontal X deflection plates through which the electron beam passes from the electron gun to the fluorescent screen, and(right) the directions of beam deflection resulting from the indicated polarity of the potentials applied to the deflection plates.

(b)

(c)

Y1 (+)

Y2 (-)

Y1 (-)

Y2 (+)

X2X1 X1 X2

Y1

Y2

Y1

Y2

X2(-)

X1(+)

X1(-)

X2(+)

WT2595b

WT2595a

Electron beamfrom 'gun'

Electron beamto screen

Y1

Y2

X1

X2

(a)

, , p p g

30 Practical Wireless, January 2005

fluorescent screen. This energy isreleased in the form of lightradiation immediately theelectrons hit the screen. Thecolour temperature of theradiation and hence the actualcolour of the display, is governedby the chemical characteristics ofthe phosphors.

For optimum effect, differentapplications may requirephosphors of different colourtemperatures. While thephosphors of a television screenare required separately toproduce the three primarycolours of red, green and blue

light, the aim being to achieve‘white light’ with the right mix ofthe three colours, the phosphorcharacteristics chosen for thescreen of the type of oscilloscopec.r.t. we are considering wouldnormally be for a single colouronly, such as green or blue/greenfor a general-purposeinstrument.

It’s worth noting that thewhite light for the televisionscreen is known as ‘illuminant D’,and that its colour temperature isequivalent to the light radiatedfrom a solid when raised to atemperature of 6,500° Kelvin.Phosphors of silver-activated zincsulphide radiate blue light and ofsilver-activated zinc-cadmiumsulphite produce yellow light, theproportions being chosen for therequired colour-temperature.

We shall see later that formaximum definition of thedisplay the electron beam needs

to be sharply focusedat the point of impactwith the fluorescentscreen. However, let’sassume for now thatthis has actually beenachieved and that thebeam is striking thecentre of the screen.This would beindicated by thepresence of a small,round bright spot atthe centre of the

display area. We certainlyshouldn’t let this remain for anylength of time, at highbrightness, anyway, because allthe energy of the beam would beconcentrated in that little (very)bright spot and the light-producing properties of thephosphors around that areamight then suffer as a result, andeventually appear on a full-screen display as a patch ofreduced brightness.

To prevent the screen fromretaining secondary electrons asit is struck by the beam electrons,the phosphors are nowadaysbacked by a thin layer ofaluminium connected to theconductive layer inside the flareof the tube. The aluminium layeralso helps to prevent thephosphors from being undulydamaged by the electron beam,while also reflecting phosphorillumination (active as a metalmirror) forward to the front ofthe screen, thereby enhancingdisplay brightness.

The light produced by thephosphors doesn’t immediatelyvanish when the electron beam issuddenly extinguished, or quicklyshifted from the centre to someother area of the screen. Animportant characteristic of thefluorescent screen is the timetaken for the light to decay afterthe beam has moved away. This isof the persistence phosphor,sometimes called ‘afterglow’.

For general applications amedium-persistence phosphor isgenerally adopted, but when it’srequired to investigate displays ofrelatively short duration aphosphor of longer persistencewould be more suitable. Blur orsmear is reduced when very highspeed displays are investigated bya screen composed of short-persistence phosphors. Phosphorsare classified in terms ofpersistence (in the range fromless than a microsecond togreater than a second),

fluorescent colour anddesignation.

Oscillograms &PhotographyOver the years I have taken manyoff-screen photographs toillustrate my books and magazinearticles, and although it has beensaid that a short-persistence

phosphor of bluish hue is the bestbet, most of my photographs(called oscillograms) have beentaken from ‘scopes with screensof green, medium-persistentphosphor. A camera that I’veused very successfully for suchshots is the Polaroid CR-9 LandCamera For Oscilloscope TraceRecording. However, with theadvent of digital photography,Polaroid films for this camera arenow extinct, which is a shame!

Let’s start to round off thisopening instalment with a lookat how the electron beam isactually deflected. In Fig. 2 areillustrations of (a) the two Ydeflection plates (Y1 and Y2) andthe two X deflection plates (X1and X2) through which theelectron beam passes from thegun to the screen. The reason forthe plates being curvedoutwards at their ends is to avoidthe beam from striking themwhen it is fully deflected.

At this point it is importantto note that in conformity withconventional graphs the Y platesare responsible for the verticaldeflection of the beam and the Xplates for the horizontaldeflection. The diagrams at (b)show the path taken by thebeam through the two pairs ofplates, and the positive andnegative potentials required forthe indicated directions ofdeflection.

As with television, it is ourpersistence of vision that makesit possible for us to discern anoscilloscope display. In one of hisRB articles about the ‘scope, Robillustrates persistence of vision asthe subjective disappearance ofthe spokes in a bicycle wheelwhen the wheel is rotated fast .

Another analogy would bethe reflection of sunlightthrough a small mirror onto awall. The resulting patch ofbright light would be seen tomove up and down the wall asthe mirror is twisted slowly in thehand. By increasing the twistingrate the eye would perceive themoving bright patch of light as acontinuous bright line.

Exactly the same effectresults from the fast movingbright dot on the c.r.t. screen.This is because of the relativelyslow decay time (about 80milliseconds) of the eye/brainresponse when the light stimulusis removed or moved from onepoint to another.

Dual Display OscilloscopesFinally, for this month it shouldbe mentioned that pretty well allbut the most basic of ‘scopesnowadays have provision for atleast two simultaneous displayson the one c.r.t. screen. This canbe extremely useful, for example,to display the primary waveformon one trace and the harmoniccontent of that very same signalon the other trace.I shall be having more to sayabout dual display ‘scopes as thisseries progresses, but for now it’sworth noting that ‘scopes withdual display facilities can beprovided either by a c.r.t. withtwo beams, or by a single beamtube and electronic beamswitching. Such ‘scopes areknown respectively as dual beamand dual trace.

Well, that about ties thingsup for this time. Cheerio for nowand I’m looking forward tocatching up with you again nextmonth. PW

Did You Know?The heart of the analogueoscilloscope is the cathode-ray tube?

Did You Know?The white light for thetelevision screen is known as‘illuminant D’, and that itscolour temperature isequivalent to the lightradiated from a solid whenraised to a temperature of6,500°K?

High Voltages!When fiddling inside an Oscilioscope beware of high voltages,make sure: The ‘scope is switched off Any high voltage capacitors are discharged (including the

feed from the high voltage supply) The mains supply is disconnected

You have been warned!

THE OSCILLOSCOPE PART 1 - BASIC PRINCIPLES feature

Fig. 3. One of the author’s early ‘scopes set up foraction in his Brixham laboratory/shack.

, , p p g

In the previous instalment, dealing withthe basic principles of the ‘scope, we sawthat the stream of electrons yielded bythe gun system arrive at the fluorescentscreen at high velocity. Also, that the

electrons are formed into a focused beam bythe voltages (positive with respect to thecathode) applied to the anodes, and thenfurther accelerated, after having passed thedeflection plates, by a positive potentialapplied to a helical conductive coatinginside the flare of the tube.

Since the beam is composed of negativelycharged electrons, it’s attracted towards apositive-going deflection plate and repelledby a negatively going one (remember thatlike repel and unlike attract), which, ofcourse, is the underlying principle of theelectrostatic beam deflection system.

I also mentioned last month that in sometubes a thin layer of aluminium is depositedover the screen. This is then held at a highpositive potential to counteract the adverseeffects of secondary electrons, which ifretained by the screen could impair thebrightness of the display.

We saw that the aluminium backing alsoimproves the brightness by reflectingphosphor illumination forward. There arenumerous other technical details of theoscilloscope c.r.t., but I think we haveconsidered enough to keep us going for thetime being!

Basic PointsBefore venturing into real practicalapplications of the ‘scope there are still anumber of basic points that we need tobecome acquainted with. First, let’s supposethat a spot of suitable brightness is focusedin the centre of the c.r.t. screen and then asinewave signal is applied to the Y input.

Assuming that the oscilloscope’s Y inputsensitivity (this will be dealt with later) suitsthe amplitude of the sinewave, the spot willbe deflected vertically from screen centre inone direction and then the other on thepositive and negative swings of thewaveform, resulting in a vertical trace.

The repetition rate of the deflection, ofcourse, will be the same as the frequency ofthe sinewave. The display will becomeflicker-free at frequencies greater thanabout 10Hz, while the overall length of thevertical line will correspond to the peak-to-peak amplitude of the sinewave, as shownin Fig. 1(a). At very low frequencies, andhence relatively low deflection rates, theactual movement of the scanning spot willbe discernible as it slowly moves across the‘scope screen.

Although such a basic display wouldprovide a measure of the peak-to-peakamplitude of a sinewave, from which thepeak and the root mean square values couldthen be calculated (such measurements willbe shown later), it would not be of muchmore use. However, when the scanning spotis deflected linearly across the screen at thesame time as it’s being deflected vertically, acompletely new world of application opensup! This is where the instrument’s ‘secondheart’, its timebase, comes in.

Timebase CircuitAll practical ‘scopes embody a timebasecircuit, which is a basically a sawtooth (orramp) generator. The circuit is called atimebase because it provides a time scalealong the X-axis.

As the ramp signal rises linearly inamplitude, so it takes the scanning spot withit from the left to the right-hand side of thescreen. At the end of this forward stroke thewaveform drops rapidly to its base level(giving the waveform its sawtoothcharacteristic), which results in the spotswiftly returning to the left-hand side of the

30 Practical Wireless, February 2005

Part 2 - Basic Applications

GETTING TO KNOW THE BASIC APPLICATIONS OF YOUR ‘SCOPETh

e Oscil

losc

ope In the second part of his

series, Gordon King

G4VFV introduces the

basic applications of the

’scope on your workbench.

You’re about to discover

it’s hidden talents!

p p g

screen (called the return traceor ‘flyback’) ready to startanother scan. The idea isshown in Fig.1(b).

There are many applicationsof the ‘scope that require theelectron beam to be deflectedhorizontally across the screenwhile the waveform underexamination is causing thebeam to be deflected vertically.For example, Fig. 2 revealshow a sinewave signal is tracedout on the screen. Here thesinewave signal applied to theY input for display is shown atthe bottom of the diagram,above this the timebasewaveform with its rising rampvoltage as applied to the Xinput, and at the top of thediagram the display as it wouldbe traced on the instrument’sscreen.

The vertical dotted linesmake it easier to follow howthe sinewave is progressivelytraced on the screen as theramp signal from the timebaserises and deflects the scanningspot from the left across thescreen. Note: Although theretrace is shown on the displaydiagram, this is usually blankedout by the oscilloscope’scircuitry. An interesting aspecthere is that the time taken bythe ramp voltage to rise fromits base level to maximum

corresponds to the time takenby one complete cycle ofsinewave. If this weren’t thecase, the screen would notdisplay just one complete cycleof the sinewave input.

Time & FrequencyNow, we’ve arrived at aninteresting but important pointof application - namely timeand frequency. When we’redealing with screen displays weshould always remember thatfrequency is being related totime. Actually, of course, theyare one of the same thing!

Let’s just consider one cycleof a sinewave for the moment.If this has the frequency of the50Hz mains supply, then fromthe start to the finish of thewaveform the time taken willbe 1/50th of a second (0.02second). Referring back to Fig.2 again, it’s clear, then, that ifthe frequency of the sinewaveat the Y input is 50Hz, thetimebase ramp voltage willneed to rise from its base levelto its maximum value in 0.02second in order for a full cycleof signal to be traced on thescreen.

With the timebase runningat, say, half that ratementioned, so that a fullsweep takes 0.04 seconds,

there would be two full tracesper cycle of signal. The screenwill then display two full signalcycles instead of one.Conversely, at twice thetimebase rate, only half a cyclewould be traced. This, then,brings us neatly to the way inwhich the ‘scope timebase iscalibrated.

Timebase CalibrationWith an analogue test meterthe pointer deflection iscalibrated against a scale (volts,amperes, etc.); with a ‘scopethe spot deflection is calibratedagainst a graticule at the frontof the c.r.t. Such a graticule isshown in Fig. 3. On the

Practical Wireless, February 2005 31

Screen display

TimebaseX input

Sine waveY input

Time X

Am

plit

ud

es Y

Fig. 2: This diagram

reveals how a

sinewave Y input is

scanned across the

screen by the timebase

ramp waveform at the

X input to appear on

the screen as a replica

of the of the Y input

signal. Note: The

retrace shown on the

screen display is gen-

erally blanked out (see

text).

WT2595

Y E

0

+

-

Vertical traceon c.r.t.

XE

0

+

-

Horizontal traceon c.r.t.

Timebase

Retrace

0

Fig. 1: Showing at (a) how a vertical trace results from a signal

waveform applied to the Y input, and at (b) how the ramp wave-

form from the timebase generator causes linear deflection of the

spot from left to right across the screen (see text).

NEWSERIES!

Did You Know?

Since the beam is composedof negatively chargedelectrons, it’s attractedtowards a positive-goingdeflection plate andrepelled by a negativelygoing one.

Did YouKnow?

All practical‘scopes embody atimebase circuit,which is a basicallya sawtooth (orramp) generator.

p p g

32 Practical Wireless, February 2005

graticule shown there are tenequal divisions horizontallyand eight vertically, providing80 equal-sized squares in all.

In practice all ‘scopes areequipped with a controlcalibrated in terms of time perdivision (time/div.). It’s the jobof this control simply to set thetimebase ramp generator todeflect the spot across onehorizontal division of thegraticule in the time indicated.

Say, for example, we wishto examine a 1,000Hz (1kHz)sinewave over the wholewidth of the graticule. Well,the speed taken by a 1kHzsignal is equal to 1/1,000second, which works out to 1millisecond (mS). You justenter 1,000 in your calculatorand then press the 1/x (thereciprocal key) to get theanswer!

But don’t forget that thereare ten horizontal divisions onthe graticule, so we need to

set the timebase control to 0.1ms/div, the ten adding up tothe required 1m total time. Bysetting the timebase control to1 ms/div. we would have acomplete 1kHz sine waveappearing in each of the tenhorizontal divisions.

Most ‘scopes, even those ofearly specifications (such asmight well be on sale at radiorallies), have quite a widerange timebase control.Incidentally, the ‘scoperunning in my ‘den’ while I’mactually writing thisinstalment, ranges from 0.5microseconds to 0.2 secondsper division, with the finalposition on the switch marked‘external’. This switch positionallows the internal timebase tobe disconnected so that anexternal X input signal can beconnected in its place. (Thereare many applications wherean external X input would berequired - more on this later).

The GraticuleLet’s return once again to thegraticule in Fig. 3. Thesinewave displayed on thisalmost fills the entiredeflection area. We shall seelater that the Y inputattenuator control is alsocalibrated, but in this case interms of volts/div.

But for now, let’s say thatthe control is set to 1 volt/div.and the timebase control to 10microseconds/div. So, if that’sthe case - what would be thefrequency and the amplitudeof the sinewave?

Okay, then, to find theanswer let’s take frequencyfirst. We see that the fullwaveform occupies all 10 ofthe horizontal divisions of thegraticule, and since eachdivision is equal to 10microseconds, the total timetaken by the waveform is 100microseconds, or 1x10-4

second.To find the frequency we

merely find the reciprocal ofthe time in seconds (e.g., 1/(1x10-4), which worksout to 10,000. The frequency ofthe sinewave is thus 10,000Hz(10kHz). So it’s really simpleisn’t it!

Looking At AmplitudeNow let’s now take a look atthe amplitude. When this ismeasured between the positiveand negative peaks it’s knownas the peak-to-peak amplitude,which in the example occupiessix of the vertical divisions ofthe graticule.

Since each divisioncorresponds to 1V, the peak-to-peak value must be 6V and thepeak value is 3V. Using a basica.c. voltmeter to measure thesame waveform, assuming thatthe voltmeter is accuratelyresponsive up to 10kHz, areading of 3V would not beobtained. It would besomething less than this. Why

would this be? Well, to answer,the basic a.c. voltmeterprovides its reading in the rootmean square (r.m.s.) value ofthe waveform, while the ‘scopedisplays the peak values.

However, the r.m.s. valuecan easily be found simply bymultiplying the peak value by0.707. This means that ther.m.s. value of the example 3Vpeak waveform would be inthe order of 2.1V, which is thevalue that would be indicatedon an a.c. voltmeter.

I shall be having more to sayabout the parameters ofvoltage later, but to round offthis month’s instalment Ithought it would be a goodidea to look at a real sinewavedisplay. The oscillogram, Fig. 4,was taken sometime ago fromthe screen of the ‘scope thathas long become part of myshack’s furniture!

Timebase & SensitivityThe oscilloscope’s timebase wasset to 1ms/div. and the Xsensitivity to 0.5V/div. Thesesettings mean that the displayhas a frequency around 227Hzand a peak amplitude close to1.25V corresponding to 0.88Vr.m.s.

Although we’ve seen that a‘scope is able to measurefrequency and amplitude, thereadout accuracy is obviouslybelow that achievable withmodern digital frequency andvoltage measuring instruments.Despite this, you should beable to achieve an accuracyaround 5%, even from some ofthe early instruments - so it’swell worth keeping an eyeopen for that rally bargain‘scope! PW

High Voltages!When fiddling inside an Oscilioscope beware of high voltages,make sure: The ‘scope is switched off Any high voltage capacitors are discharged (including the

feed from the high voltage supply) The mains supply is disconnectedYou have been warned!

GETTING TO KNOW THE BASIC APPLICATIONS OF YOUR ‘SCOPE feature

Fig. 3: With the timebase set to 10 microseconds/div. And the Y attenuator to

1V/div., this sinewave would have a frequency of 10kHz and a peak amplitude

of 3V.

Fig. 4: This oscillogram has an amplitude of about 0.88V r.m.s. and a frequency

around 227Hz, as explained in the text.

Did You Know?When amplitude is measuredbetween the positive andnegative peaks it’s known asthe peak-to-peak amplitude.

p p g

Imentioned in the last instalment how the‘scope can easily be arranged to provide ameasurement of frequency and amplitude.While becoming conversant with the operationof the ‘scope though, you’ll soon discover that

a sine wave, albeit, significantly lacking in purity,can be resolved on the screen simply by holding afinger on the Y input and suitably adjusting thevolt/div and the time/div controls.

The display results from the human body actingas an antenna picking up the 50Hz radiation fromthe mains supply wiring and passing it on to the‘scope. Don’t worry, it’s at a low level, and highimpedance, so there’s no problem!

In my radio shack environment I can get adisplay of good amplitude with the Y input set to1V/div. With a graticule of 10 horizontal divisions,the timebase will need to be set to 2mS/div, givinga full-screen sweep time of 20 milliseconds, whichis the time taken by one complete 50Hz cycle (e.g.,1/50=0.02S=20mS).

Incidentally, the calibration of the Y and Xdeflection on some ‘scopes might be given in termscentimetres instead of graticule divisions, such asvolt/cm and time (S, mS etc.)/cm.

This, then, neatly brings us to Fig. 1, whichdepicts the amplitude and time aspects of asinewave that were introduced in the previousinstalment. In the case of a sine wave the r.m.s.value works out to 70.7% of the peak amplitude,

and since the time period of the completewaveform from start to finish is shown as 0.02-second, we now know that its frequency is equalto the reciprocal of 0.02, or 50Hz.

Integral calculus is required to derive the r.m.s.value of a waveform, but in the case of a sine waveit simply resolves to the peak value divided by thesquare-root of two (√2 = 1.414), which you willfind is the same as the 70.7% of the peak value inthe diagram. Most a.c. voltmeters are scaled inr.m.s. values based on a sine wave, although themovement responds to the average value of thewaveform, which in the case of a sine wave is63.7% of its peak value.

The electric power supply is also r.m.s. rated,meaning that our 240V a.c. mains supply has apeak value of just under 340V and a peak-to-peakvalue of twice that value! Looked at another way,the r.m.s. value is equivalent to the d.c. value thatwould dissipate the same power and henceprovide the same heating effect. With that littlebit of maths, it’s time to get back to the ‘scope onyour workbench.

Main ControlsSeeing a ‘scope on offer for a very reasonable priceat a rally (and we must keep in mind that it is theaim of this series of articles to consider ‘scopes ofthat category, as distinct from up-market latter-daydigital instruments!), you might veer away fromthe purchase of a good and useful bargain becauseof the multitude of front (and, perhaps, rear)controls. However, my job with this series ofarticles is to make sure you’re not intimidated. So,let’s see now if we can get to grips with some ofthe main controls.

The two controls associated with Y amplitudeand X sweep time have already been investigatedin some detail. In practice though, you’ll usuallydiscover that each one works in conjunction with acontinuously variable ‘fine’ control, which may ormay not be calibrated. In the latter case it’s thennecessary to set the control to one end of its rangefor the calibrated positions on the main switchedcontrol to hold true.

Brilliance ControlThe Brilliance is a primary display control whichmerely adjusts the intensity of the fluorescent spot

24 Practical Wireless, March 2005

Part 3 - Controls, synchronisation & triggering

Gordon G4VFV helps you control your ‘scope and synchronise!Th

e Oscil

losc

ope In part three of his major

new series on the oscillo-scope, Gordon King G4VFVlooks at the controls, syn-chronisation and triggering.Gordon say there’s no needto be intimidated by thecontrols on your ‘scopebecause you’re in charge!

, , p g

on the screen. It’s equivalent to atelevision receiver’s brightnesscontrol.

In practice the brilliance controlworks by way of a potentiometerarranged to provide an adjustmentto the voltage applied to the gridelectrode of the c.r.t. As the controlis retarded (‘turned down’), so thegrid voltage becomes more negativewith respect to the cathode andfewer electrons strike the screen,thereby causing the brightness ofthe display to diminish.

Conversely, when the gridbecomes less negative and thedisplay brighter as the control isadvanced (‘turned up’). The numberof electrons making up the beam isinversely proportional to the gridvoltage, the beam being cut offcompletely when the grid is madestrongly negative with respect tothe cathode.

So when first trying out a‘bargain ‘scope’ that seemingly lacksa trace, make sure that thebrilliance control is notinadvertently turned down too far.This has happened! The brilliancecontrol provides a voltage swingfrom zero to about -50V relative tothe cathode. The grid can alsoreceives pulses of suitable polarityfor ‘retrace blanking’ and display‘bright-up’.

Focus ControlNow it’s on to the focus control.And, as would be expected, the jobof this control is to focus the c.r.t.beam so that it impinges upon thescreen as a small round dot.

Again, this function most‘scopes is handled by apotentiometer. But this time foradjusting the voltage applied to thetube’s focus electrode.

Overcoming AstigmatismAstigmatism is generally somethingwe might hear about when havingour eyes tested! But it can alsoimpair the sharpness of the ‘scopedisplay owing to elongation of thespot.

When the elongation lies in thevertical plane as shown at (a) in Fig.2a, a square wave display would beaffected after the style of Fig. 2b,where the horizontal parts of thewaveform are seen to be thickened.Elongation, which could also be inthe horizontal plane, results from adegree of electrostatic asymmetry(irregularity) while the beam isbeing accelerated through the gun.

However, astigmatism can becounteracted by relativeadjustment to the c.r.t. electrodepotentials. This is achieved by afunction provided by a controllabelled ‘astig’, which is short for

astigmatism. In practice, the focusand astig controls are adjusted inturn until the beam achieves itsmaximum symmetry and thesharpest display is obtained.

Quite a few of the ‘scopes thatcome up for sale at rallies willincorporate a built-in generatorproviding two 1kHz square waveoutputs, one of around 5V and theother of 50mV, so look out forthese. Although handy foroptimising the astig. setting, theseoutputs (often found convenientlysited on the front control panel ofsome ‘scopes) have particularapplications for checking thecalibration of the volt/div andtime/div controls.

Vertical & HorizontalShiftNow we’ll move on to the verticaland horizontal shift controls. Theseare required because it’s frequentlynecessary to move the display so itcan be related to the graticule lineswhen making amplitude andfrequency measurements (forexample).

All ‘scopes are equipped with

front controls for shifting thedisplay both vertically andhorizontally. These also work frompotentiometers that adjust the d.c.potentials across the Y and Xdeflection plates, so that thedeflected electron beam, and hencethe whole screen display, can beshifted up, down and sideways asrequired.

It’s possible to shift the spot ortrace so much that it goes off-screen. This is an important point toremember if a display fails toappear on the screen, and it seemsas though there may be a moreserious fault in the instrument!Note: To overcome this problem(there surely can’t be many ‘scopeusers who haven’t been caught outin this way) some instruments areprovided with a ‘Beam Finding’control. All the operator has to do(when the display seems to havedisappeared) is to press the button.The missing spot will then re-appear, no doubt to a sigh ofrelieved frustration!

A number of additional controlswill be found on most ‘scopes,depending upon their ages and

Practical Wireless, March 2005 25

Fig. 2a: This example of

astigmatism is caused by

vertical elongation of the

scanning spot, which is

largely correctable by the

front astig control. The

spot is shown statically on

the screen. As can be seen

the vertical size is larger

than the horizontal width.

This state would indicate

some form of astigmatism

control is required (see

text). WT2674

100

70.7

0

Root Mean Squared(r.m.s.) level

Positivepeak

Negativepeak

Leve

l (%

)

Total time = 20ms(each mark = 1ms)

Peak to peak

Fig. 1: Sine wave showing the relationship between the peak and root-mean-

square values. The average value of a sine wave is 63.7% of its peak value.

With a time period of 0.02 second (20mS), the frequency of the wave is 50Hz

(e.g. 1/0.02).

NEWSERIES!

Fig. 2b: With the spot problems exhibited in Fig. 2a, the horizontal areas of signals

become difficult to see in detail due to blurring. Note the effect shown on the tops

and bottoms of the square wave shown here.

, , p g

26 Practical Wireless, March 2005

levels of sophistication.We shall be looking atthese as we wend our waythrough some of the‘scope’s innumerableapplications.

Block DiagramAt this point I think it willbe as well to take stock ofwhat has so far beendiscussed, and then to givethe timebase, sync andtrigger functions a littlemore attention. With thisin mind, I’ve provided ablock diagram, Fig. 3, ofthe basic features of a‘scope, showing just asingle Y channel.

Starting with the Ychannel first, you’ll seethat the test input can be appliedto the Y attenuator either directlyor through a capacitor. It thenpasses through the Y amplifier tothe Y plates of the c.r.t. (Most‘scopes are equipped with ana.c./d.c. switch like this).

In the d.c. position the ‘scopewill respond to inputs right downto d.c. However, d.c. isolation maybe desirable when looking for alow level signal (a.c.) ‘sitting on’ ahigher level of d.c. voltage. Inwhich case the input, which wouldthen be of a.c. form, would bepassed to the Y attenuator by wayof the capacitor.

The deflection sensitivity of a‘scope c.r.t. is pretty low and, as wehave already seen, is dependent onthe final anode potential. Thehigher the potential, the lower thesensitivity (this is because theelectron beam will then have moreenergy and will require moreenergy - a bigger push if you like -to deflect it).

The X-amplification as shown isthus provided between thetimebase sweep and the c.r.t. Xplates. When the associatedinternal/external switch is set to the‘ext’ position (a position usuallylocated on the time/div control) itthen becomes possible to connectan external source in place of thebuilt-in timebase to provide thehorizontal deflection.

The timebase is composed ofthe trigger and sweep circuits

shown in the diagram. The sweep(or timebase scan) can be triggeredeither internally from pulsessynthesised from the test signal inthe Y amplifier, or from an externalsource, as determined by thesetting of the associated switch.Another switch allows the selectionof either positive- or negative-going trigger pulses.

Sweep LinearityEssentialFrom my earlier description of theformation of a ‘scope display, you’llrealised that for the least displaydistortion a high degree of sweeplinearity is essential. This is becausewhen the display consists of severalcycles of a sine wave (for example)a lack of linearity (e.g. curvature) ofthe rising ramp waveform willresult in different time spans beingshown for each of the separatecomplete waveforms displayed. Thiswill in effect give a sort ofcompression over the full X scan.

Some very early valved ‘scopestended to suffer from thecompression problem; but ‘scopesof more recent vintage usingtransistors suffer far less from thisshortfall. However, if your bargainpurchase is found to exhibit non-linearity of this kind, there’s noneed to worry. Instead,time/frequency measurements canoften be made with improvedaccuracy by resolving severalcomplete cycles, and then using the

horizontal shift control to align asingle cycle for measurementagainst the graticule to the mostlinear part of the scan.

Steady DisplayClearly, with a free-runningtimebase it would be very difficult,if not impossible, to retain a steadydisplay, even by carefully adjustingthe timebase frequency. Some sortof relationship must exist betweenthe timebase and the Y inputsignal. This was accomplished inearly ‘scopes by a ‘sample’ of the Ysignal being fed back to thetimebase by way of a controlmarked sync (short forsynchronisation).

When the level was optimisedby the sync control, the retracewould occur at the exactly the righttime. The result was that successivecycles of Y signal were then tracedfrom left to right across the screen,one upon the other, to provide a‘locked’ or synchronised display.

In the trigger mode, which willbe a feature of most rallypurchased ‘scopes, the timebase

remains static until ‘triggered’. Herea pulse, tailored from the Y inputand adjustable by a front control interms of level and polarity, is thenfed to the timebase in such a waythat it initiates, or ‘triggers’, thestart of a scan at the same point onthe Y signal for every sweep.

The speed at which the scanoccurs is, of course, established bythe time/div rotary control switch. Asteady display, with more suitablefor time measurements than waspossible from the early sync-mode‘scopes, results from the use of theprecise triggering action.

Okay, then, that just about takescare of the ‘scope for this month,there’s more than enough for youto read, enjoy and digest! Nexttime we’ll look together some moreaspects of this versatile instrument,including things like sweep delay,bandwidth, rise time and so forth.

There’s one thing for sure -playing about with the ‘scope iscertainly a good way of getting toknow more about the various kindsof signals that are involved in ourhobby! Cheerio for now. PW

High Voltages!When fiddling inside an Oscilioscope beware of high voltages,make sure: The ‘scope is switched off Any high voltage capacitors are discharged (including the

feed from the high voltage supply) The mains supply is disconnectedYou have been warned!

GETTING TO KNOW THE BASIC APPLICATIONS OF YOUR ‘SCOPE feature

Polarity Level

X select

WT2676

Y inputattenuator

Yamplifier

Xamplifier

Sweepcircuit

Y2

Y1

X1 X2

CRT

External

Internal

External

Triggerselect

DC

AC

Y input

Trigger circuit

Fig. 3 Elementary block diagram of

single Y-channel ‘scope as explained in

the text.

, , p g

21 Practical Wireless, April 2005

Last time I concluded the March instalmentwith a peep back at the early way inwhich the timebase was synchronised tothe signal under examination; where async control was used in conjunction with

the timebase frequency control to achieve astabilised display. However, nowadays, evenrally-acquired second-hand ‘scopes are unlikely tofeature just such a basic sync control.

Unless you come up against a pretty ancientmember of the ‘scope species, the instrument willbe of the kind possessing a timebase that in onemode, anyway, remains quiescent until triggeredby the Y signal. When the Y signal is recurring,then, of course, there will be successive sweeps,resulting in a steady screen display - much steadierthan achieved with the early ‘sync’ system.

Trigger LevelIt’s most likely that the front panel of your ‘scopewill feature a control for adjusting the trigger leveland most likely a means of selecting positive ornegative polarity. This will allow the timebase tobe triggered from pulses derived either frompositive or negative-going Y input signals.

To ensure that the leading edge of afast-occurring signal under examination is fullydisplayed, there will probably be a delay line in theY amplifier channel. The idea of this is to delay theY signal fractionally so that the sweep starts a triflebefore the Y signal arrives at the c.r.t.

Some ‘scopes also have an Auto Sweepfunction, which triggers the timebaseautomatically at a relatively low sweep rate, evenwhen there’s no Y input signal. Otherwise, without

the application of a Y input signal, the screenremains blank, which has led inexperienced usersincorrectly to suspect that the ‘scope is at fault.This is a point worth having in mind whenbecoming acquainted with the operation of thatrally acquisition!

When the trigger switch is set to External, thetrigger circuit is disconnected from the internalpulse-forming circuit. It then becomes possible forthe timebase to be triggered from an externalsource, such as from a functions generator.

Second TimebaseSome ‘scopes will be found equipped with a secondtimebase, with its own time/div control, andprobably with an additional control labelled delaysweep. Oscilloscopes of this kind allow parts ofthe trace to be increased in brightness andpositioned against the graticule for the mosteffective measurement.

The main timebase is generally labelled Asweep, and the second timebase B sweep. Eitherone of the timebases, or the two in combination,can be selected. The sweep delay function,activated by a separate switch, works inconjunction with a multi-turn delay time control.

Basically speaking, the B sweep is triggered fromthe A sweep at a precise point on the display (asestablished by the setting of the delay control),which then makes it possible to measure timeintervals and pulses with enhanced accuracy. Whilethis might be an important requirement forprofessional applications, it’s unlikely to be viewedin a similar light by the practical amateur.

Dual ChannelLet’s now take a look at the type of ‘scope thatprovides two simultaneous displays. We’ve alreadylearned that ‘scopes of this kind use either a c.r.t.with two beams and two deflection systems (thetrue dual-beam ‘scope) or an electronic means ofobtaining two beams from a single-beam c.r.t. (thedual-trace scope).

The block diagram, Fig. 1 (on page 22), showsthe basic arrangement of a dual-trace version,which is the kind most likely to represent a rallypurchase. While having some duplicated featuresof its single-channel counterpart (see Part 3), theconfiguration has now expanded into two Y inputchannels, called channel 1 (Ch1) and channel 2(Ch2), whose signals for examination pass to the Ydeflection plates of the c.r.t. through an electronicbeam switching circuit.

TAKING THE MYSTERY OUT OF DOUBLE BEAM AND DUAL TRACE INSTRUMENTS feature

Part 4 - Dual Trace Principles and XY ApplicationsTh

e Oscil

losc

ope This month Gordon King

G4VFV takes the mystery

out of double beam ‘scope

and dual trace instruments

as we learn more about

these wonderfully versatile

items of test equipment.

, , p g

TAKING THE MYSTERY OUT OF DOUBLE BEAM AND DUAL TRACE INSTRUMENTS

There are two dual-trace modesof operation. One, called thealternate mode, sweeps Ch1 andCh2 alternately, and works in thefollowing manner: At theconclusion of, let’s say, a Ch1 sweep,a pulse from the timebase operatesthe beam switch and instigates Ch2sweep, while at the same timecutting off Ch1. At the conclusionof Ch2 sweep, the sweep of Ch1 isinstigated and Ch2 is cut off, and soon. (Note: The alternate sweepsonly become apparent when the‘scope is operating at very lowsweep speeds).

The other way, often referred toas the ‘the chopped mode’, switchesbetween the two channels at amuch higher rate, under the controlof an built-in multivibrator. Thismeans that each trace then consistsof many closely adjacent alternatesegments, corresponding to theswitching rate.

The chopped mode suffers lessfrom flicker at low sweep speeds,and boasts better phase integritybetween the two channels than thealternate mode. But the separatesegments do tend to become moreapparent as the sweep speed isincreased.

The block diagram shows thatthe mode selecting switch also haspositions for independentoperation of Ch1 or Ch2. A fifthposition labelled Add might also beincluded. On this setting the twochannels are added so that thedisplay then becomes acombination of the Ch1 and Ch2

input signals.It’s possible, of course, to make

vertical adjustments to each traceby its own shift control. Thetimebase can usually be triggeredfrom either the Ch1 or Ch2 signal,or from an external source, as theblock diagram shows.

The X & Y InputsAt this juncture let’s change tack abit and look at one application ofthe ‘scope where external signalsare applied to both the X and Yinputs. On some ‘scopes the Xdeflection circuit can bedisconnected from the sweepgenerator by setting the time/divswitch of the main timebase to aposition marked external.

The display will then consisteither of a vertical trace (if a signalis being applied to the Y input) ormerely a bright, luminescent spotsomewhere on the screen, asdictated by the setting of the shiftcontrols. To obtain horizontaldeflection a signal must somehowbe applied to the X plates, which onsome ‘scopes may be by way of anexternal X input.

However, the deflectionsensitivity of the c.r.t. alone is prettydismal! It’s expressed in terms ofmere millimetres of deflection pervolt, and there are very fewapplications where an external Xsignal could be connected directlyto the X plates of the c.r.t. Realistichorizontal deflection from smallsignals, therefore, generally calls fora fair degree of amplification

between the external X source andthe X plates, and it’s useful if thiscan be adjusted by a calibratedcontrol in the same way as theY signal.

Dual-trace ‘scopes often satisfythis requirement by adopting oneof the Y channels for amplifyingand setting the level of the signaldestined to provide the Xdeflection. This, known as X-Yoperation, is a neat way ofachieving well-balanced andcontrolled horizontal and verticaldeflection from external X and Ysignals.

Accuracy of display requires ahigh degree of linearity to beretained through the amplifiersover their full dynamic range. Thebandwidth, too, must be adequatefor the task in hand, a factor thatwill be considered as we continueon our way.

Phase Shifts & AnglesLet’s now switch our attention todiagram (a) in Fig. 2. Here is shownan oscillator feeding a sinewavesignal to the input of a deviceunder test (it could be an amplifier,filter, simple or complicatednetwork, etc.).

You’ll be able to see that thesame signal is simultaneouslyapplied to the Y input of the ‘scope,while the X input is receivingoutput signal from the device undertest. Depending upon the nature ofthe device it may be necessary toconnect a load resistor across itsoutput, as shown. (This requirement

would apply particularly to anaudio amplifier).

The plan initially is to obtainequal deflection vertically andhorizontally by adjusting thecontrols and signal levels. This iseasy if the ‘scope is equipped withan X level control, as just explained;otherwise it will be necessary toemploy some other method ofexternal adjustment, such as theadjust X potentiometer shown inthe diagram.

When the X and Y signals areapplied to the ‘scopesimultaneously any phase shiftbetween the input and output ofthe device under test will berevealed by the display. A forwardsloping diagonal line indicates zerophase (or 180° when sloping in theopposite direction), a perfect circle(assuming correctly balanced X andY signals), as distinct from anellipse, indicates a phase shift of90°, while forward or backwardsloping elliptical displays indicateintermediate phase shifts.

By aligning the display centrallyagainst the graticule lines, as at (b)in the diagram, the sine of thephase angle can be calculated bydividing distance A by distance B.For example, if A corresponds toone graticule division and B to twodivisions, then the A/B ratio is equalto 0.5.

By using a scientific calculator ora table of trigonometric functionsthere’s no trouble in finding theangle, which, of course, is 30°(e.g., the sine of 30° equals 0.5). Theforward sloping ellipse shown onthe ‘scope in diagram (a) indicates a

WT2717

CH1attenuator

Y AmplifierCH1

CH2attenuator

Y AmplifierCH2

Channelswitching

circuit

Xamplifier

Sweepcircuit

Triggercircuit

Y2

Y1

X1 X2

CRT

43 2

1

1 CH12 CH23 Chop4 Alternate

Beam switch

CH1

CH2

External

CH1in

CH2in

Ext. intrigger

Modeselect

Fig. 1: This block diagram of a dual channel scope is

explained in the text.

Fig. 3: This indicates a phase angle of

around 55° (see text).

22 Practical Wireless, April 2005

, , p g

Practical Wireless, April 2005 23

Feature

phase angle of around 35°.There are times when servicing

or optimising an item of electronicsthat an idea of its overall phaseshift could be useful. An audioamplifier, for example, may befound to go unstable and generateits own signals owing to anabnormal shift of phase at theextremes of its pass band.

Under normal conditions awell-designed amplifier wouldprobably indicate a reasonablysteady 180° phase difference whena signal generator is used to swingthe input signal over its designed-for range of frequencies. Thiswould be shown on the display as adiagonal line, though a shift of afew degrees, indicated by thediagonal line display tending tobreak into elliptical formation,could possibly be tolerated!

Application of the ‘scope’sfacilities can also be very usefulwhen designing filters (which willinvariably reveal phase shift),balanced push-pull drivers and soforth. It can also be revealing whenexamining an audio amplifier inthis way to see how much the tonecontrols and filters impair its overallphase integrity. Additionally, in thecase in mind this case, possibly alsoaffect the performance of thenegative feedback, sometimestending to make it more positivethan negative, but that’s anotherstory!

Lissajous FiguresNow it’s time to look at Lissajousfigures. To start, an off-screen shotof a phasing display is given in

Fig. 3, where the phase angleworks out to about 5°, calculatedfrom its A/B ratio, whose value isapproaching 0.82 (e.g., the sine of5° equals 0.819). Displays of thiskind are known as Lissajous figures,a technique named after the Frenchphysicist Jules Lissajous (1822-80).

In practice Lissajous figures cantake on numerous and complicatedconfigurations when signals ofdifferent frequencies are appliedsimultaneously to the X and Yinputs. We’ve already seen thatwhen two signals of the samefrequency (e.g., a frequency ratio of1:1) are applied simultaneously tothe X and Y inputs the display takesthe form of a circle, an ellipse or adiagonal line, depending upon howthey differ in phase.

When sine waves of differentfrequencies are appliedsimultaneously to the X and Y

inputs, displays such as illustrated inFig. 4 are obtained. The frequencyratio between the two signals isindicated on this kind of Lissajousfigure by the number of loopsoccurring along the horizontal andvertical sides.

Looking at Fig. 4, at (a) the twoloops along the horizontal side andthe one loop along the vertical sideindicate a frequency ratio of 2:1.For example, a figure like thiswould result from inputs of 50Hzand 100Hz. Similarly, the figure at(b) indicates a frequency ratio of3:1. The more complicated figure at(c) has three loops horizontally andtwo vertically, indicating a 3:2 ratio.

Displays in the orientationshown are obtained when the Xinput corresponds to the lowerfrequency signal and the Y inputthe higher frequency signal. Whenthe frequency of, say, the X signal is

known it then becomes possible tofind the frequency of the Y signalsimply by dividing the number ofhorizontal loops by the number ofvertical loops, and multiplying theresult by the known frequency.

Distorted displays occur whenthe inputs differ from a sinewave,and for the display to hold steadythe frequency of one wave needs tobe a simple multiple or fraction ofthe other. Drifting frequenciescause the display to change andmove erratically, making loopcounting difficult, especially on themore complicated figures.

Well, once again, this just aboutties up this month’s instalment.There are still plenty of interestingthings to discover about ‘scopes, sodon’t forget to focus on nextmonth’s story. Cheerio for this time.

PW

WT2718

Y XE

Oscillator Device undertest

Load

AdjustX

Earth

A B

sin θ = ( A )B

(b)

(a)

Fig. 2: Showing at (a) how a ‘scope

can be applied to measure the phase

angle of an active or passive device,

and inset at (b) how the phase shift

between the input and output of the

device can be assessed (see text).

Fig. 4: These examples of Lissajous figures illustrate the ratios of two sine waves applied to the X and Y inputs of a ‘scope when adjusted for suitable

X and Y deflections (see text).

(a) (b) (c)

, , p g

30 Practical Wireless, May 2005

We have already discovered thatwhen the Y attenuator is set to,say, 1mV/div and a sinewave of5mV peak-to-peak (p-p) isapplied to the input, the overall

amplitude of the display, between the positive andnegative peaks, would correspond pretty closely tofive vertical divisions on the graticule.However, this calibration will only holdtrue within a given frequency range,which brings us to two importantrelationships of the Y amplifier; namely,bandwidth and rise time.

Because the Y channel (or channels)can be switched to respond right down tod.c., ‘scope bandwidth is essentially thefrequency range to a defined upperfrequency limit, which is commonly thefrequency where the response roll-off is 0.707 timesits mid-band value, known as the -3dB bandwidth. Ifthis upper frequency limit is, say, 10MHz, and the Yinput control is set to 1mV/div as before, then theoverall amplitude of a 10MHz display would be ashade over three and half vertical divisions with a5mV p-p input, instead of five vertical divisionsobtained at lower frequencies.

Rise TimeFor the measurement and examination of pulse andtransient-type signals, a ‘scope must be capable ofdisplaying fast-rising waveforms with the leastdistortion, which means that the rise time of the Yamplifier or amplifiers must be shorter than that ofthe input signal itself. So, what is rise time? Well, it’snothing more involved than the time it takes anamplifier or a network to respond to a fast-risingsignal, such as the leading edge of a pulse orsquarewave. By definition it is the time taken for awaveform to rise between two points on its leadingedge, which correspond respectively to 10 and 90%of its peak amplitude, as shown in Fig. 1.

Because of the relationship between bandwidthand rise time, knowing one makes it possible toobtain a fair approximation of the other. For

example, bandwidth is given by k/rise time, and risetime by k/bandwidth. When the bandwidth is inMegaHertz (MHz) and the rise time in microseconds,the value for k could range between 0.3 and 0.5,depending on how the terminal frequency isdetermined, the characteristics of the response andthe nature of the upper-frequency roll-off.

However, when the terminal frequency refers tothe -3dB bandwidth, which is a common definition,the value for k is generally taken as 0.35. Based onthis value, therefore, a bandwidth of 10MHz relatesto a rise time of 0.035 microsecond (35 nanoseconds).Similarly, a rise time of 0.1 microsecond (100nanoseconds) relates to a bandwidth of 3.5MHz.

The display shown in Fig. 2 represents theleading edge of a rectangular or square wave as it

may appear on the screenof a ‘scope. With thetimebase sweeping at therate of 0.1 microsecondper horizontal division, afair assessment of thetime taken by the signalto rise from 10 to 90% ofits maximum value canbe calculated from the

graticule. This is shown to be around 0.3µs,corresponding to a -3dB bandwidth of about1.16MHz.

Expanding X DeflectionMost of the ‘scopes obtainable from rallies andsimilar places will almost certainly be equipped witha switch for expanding the X deflection, usually by afactor of 10. Times 10 x expansion can be very usefulwhen assessing fast rise times.

For example, if the timebase is set for a sweep of0.5µs/div, which could be the ‘scope’s maximumsetting, switching in the times 10 × expansion willdecrease the sweep time to 0.05µs/div, making itmuch easier to read off rise times against thegraticule, and to examine the detail offast occurringpulses. Indeed,the rise timedisplayed in Fig.2 could well havebeen obtainedwith the timebaseset to 1 µs/div andthe times 10 ×expansion activated.

The expansion, of course, also has the effect of

IMPORTANT OSCILLOSCOPE Y AMPLIFIER RELATIONSHIPS

Part 5 - BBaannddwwiiddtthh,, RRiissee TTiimmee,, SSqquuaarree WWaavveess aanndd AAnncciillllaarriieessTh

e Oscil

losc

ope Gordon King G4VFV continues

with his look at the theory and

practices of the Oscilloscope.

What is Rise Time?

Answer: It’s the time taken for

a waveform to rise between

two points on its leading

edge, which correspond

respectively to 10 and 90% of

its peak amplitude.

X Deflection Expansion

Times 10 x expansion can be very

useful when assessing fast rise

times. Many ‘scopes are

equipped with switch for

expanding the X deflection.

p g

making the trace that much longer.With times 10 expansion (anothercommon value is times 5) the effect istantamount to the trace becomingten times longer than withoutexpansion. This might mean thatwhen the expansion is switched onthe part of the waveform it isrequired to examine suddenlyvanishes outside the range of thescreen. This is no problem, though, asit can easily be brought back into thescreen area by the horizontal shiftcontrol.

SquarewavesThe discussion brings us neatly tosquarewaves! A squarewave iscomposed of a sinewave at thefundamental frequency plus a seriesof harmonically related sinewaves,and it is the addition of essentiallyodd harmonics in specific phaserelationship and amplitude thatendow the waveform with its squareshape. Owing to this widebandstructure, a squarewave represents aparticularly potent test signal whenused in conjunction with a ‘scope.

Consider now the two off-screendisplays at (a) and (b) in Fig. 3. Theoscillogram at (a) was obtained with

a ‘scope connecteddirectly to the outputsocket of a low distortionsinewave/squarewaveoscillator, with the sweepset to 10µs/div.

Since the time takenby a complete cycle (one half-cyclenegative-going andthe other positive-going) is shown tobe approximately54µs (54 x 10-6

seconds), therepetition frequencyworks out to about18.5kHz (1/54 x 10-6 =18.518Hz). The sweep is notsufficiently fast on this display todetermine the rise time, but by theapplication of times 10 × expansion itwas found to be in the order of0.1µs, corresponding to a -3dBbandwidth of about 3.5MHz.

Since the same timebase sweepof 10µs/div was also used forobtaining oscillogram (b), the time ofa complete cycle is shown to be closeto 35µs, indicating a repetition

frequency around 28kHz. However,the rounded corners of thiswaveform point to an impaired high-frequency response, which was in thecircuits through which thesquarewave was passing (and,indeed, testing) - not in the ‘scope’s Y

channel. Therewas no need inthis case for Xexpansion todetermine therise time,which can beseen to bearound 3

microseconds, corresponding to a -3dB bandwidth of around 116kHz.

Ancillary EquipmentThat more or less puts rise time andbandwidth in their places for now.The next article will continue to keepthese themes in perspective; but toconclude this instalment, let’s take alook at how the ‘scope can be linkedto ancillary equipment to providespectral analysis and frequencyresponse displays.

Fairly recently KenwoodElectronics (in conjuction with VannDraper Electronics, suppliers ofKenwood test instruments) launchedan interesting 1GHz SpectrumAnalyser Adaptor Model SAE 1001,which is pictured in Fig. 4 along withthe Kenwood Dual-Trace 20MHzOscilloscope Model CS 4124.

The interesting and usefulKenwood partnership, together withthe Model FCE 1131 hand-heldfrequency counter, was the subject ofa review of mine in the July 1997issue of Practical Wireless. If furtherinsight is required into theapplication potential of ‘scopes withX-Y facilities (see Part 4), and an ideaof the more advanced tests thatbecome possible when a fairly basic‘scope is linked to ancillaryequipment, then this article may beworth another read.

Who knows, one day a bargainspectrum analyser adaptor mightcome up at a radio rally to partner abargain ‘scope! Spectrum analysersare remarkably expensiveinstruments in themselves, sohooking such an adaptor onto a‘scope opens up many other channelsof interest within the budget of ahobby, while certainly aiding the on-going learning philosophy ofAmateur Radio.

This instalment would not reallybe complete without some

Practical Wireless, May 2005 31

feature

Am

plit

ud

e (p

erce

nt)

Time

100

90

10

0Risetime

WT2746

WT2742

Fig. 1: By definition the rise time is the

time taken for a waveform to rise

between two points on its leading

edge, which correspond respectively to

10 and 90% of its peak amplitude, as

this diagram shows.

Fig. 2: Rise time of a step wave can be assessed in

conjunction with the ‘scope’s graticule and possibly X

expansion. The display shown is the leading edge of a

rectangular or square wave, and with a sweep of 0.1

microsecond/div the rise time is seen to be around

0.3µs.

Fig. 3: Oscillogram (a) shows a squarewave taken directly from the output of a low distortion sinewave/squarewave oscillator.

With the sweep control set to 10µs/div, a single cycle takes about 54µs, which means that the repetition frequency is close to

18.5kHz and the rise time about 0.1µs, but needing a faster sweep for a realistic assessment. Using the same sweep setting,

oscillogram (b) takes about 35µs, putting the repetition frequency around 28kHz; but in this case the rise time is significantly

longer at about 3µs.

Squarewaves

A squarewave is composed of

a sinewave at the

fundamental frequency plus a

series of harmonically related

sinewaves.

(a) (b)

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Practical Wireless, May 2005 32

reference, at least, to the way inwhich the ‘scope, again inpartnership with ancillaryequipment, has been adopted overthe years by radio buffs, bothprofessional and amateur, to helpwith the design, alignment andresponse tailoring of the r.f., i.f. andfilter circuits of radio and televisionreceivers and hi-fi tuners. The basicset-up for plotting the overallresponse characteristics of a radioreceiver, for example, is depicted at(a) in Fig. 5.

The signal from the output ofthe swept generator is coupled tothe input of the receiver underexamination, while the d.c. outputfrom the detector is coupled to thed.c. input of the ‘scope’s Y channel.Let us suppose that the sweepfunction is de-activated and that thefrequency of the input signal isadjusted manually over the receiver’spassband. In this case the Ydeflection will be seen first toincrease to a maximum as thefrequency rises up the response curveto the response peak, and then todecrease as the frequency passesdown the other side of the curve.

It would be virtually impossible,of course, to determine the precisenature of the response characteristicmanually. But when the sweepfunction is activated the operationbecomes entirely automatic, andbecause the frequency of the r.f.output is continuously swept overthe passband in direct synchronismwith the ‘scope’s horizontal Xdeflection, it follows that the ‘scopewill display a detailed repetitiveimage of the receiver’s responsecurve. It is essential to ensure thatthe ‘scope’s X and Y levels arecarefully set to avoid overloading,which could flatten the responsecurve, and that the frequency sweepof the generator is set to embracethe full frequency range of theexpected response curve.

It’s often necessary to identifyparts of the curve in terms of actualfrequency, and this is where themarker generator comes in. Whenthe signal from this generator islightly coupled (too tight a couplingcauses distortion) a small ‘pip’appears on the response curve at the

frequency to which the markergenerator is tuned, as shown on the‘scope display in the diagram Fig. 5.

Diagram (b) of Fig. 5 gives moredetail showing how the responsedisplay is produced from the X and Yinputs and the rising ramp voltagefrom the ‘scope’s timebase. It is likelythat many of the ‘scopes for sale atradio rallies will have a ramp outputinterface (possibly located at therear) suitable for driving a sweptgenerator. Although in some casesadditional amplification might benecessary, depending on the typeof ‘scope and swept generator.It’s noteworthy that the sweptgenerator is an instrument that wasvogue in radio workshops and labsnot too long ago boasting the title‘wobbulator’.

When examining the responsecharacteristics of tuned amplifiers,filters, etc. that do not incorporatea detector stage, a simple low-capacitance detector probe allowsthe ‘scope’s Y input to be picked up

from almost any point in a receiveror amplifier, or even at the output ofa filter circuit. Some of the earlyswept generators included a low-capacitance probe as an accessory,while other instruments might haveincluded an inbuilt detector.

That just about ties things up forthis instalment. The plan for the finalpart is to venture more deeply intothe practical applications of the‘scope in the workshop and radio

shack, and to discover just what sortof displays are possible from thedual-trace instrument. Until then,keep an eye open for that possiblebargain - not forgetting theancillaries for spectrum analysis andresponse plotting. PW

WT2718

Y E

Sweptgenerator

Markergenerator

X

Y

Marker pip

Receiver Detectorload

Ramp

(a)

(b)

Ramp for X sweep

Display

Responseof receiver

Y

X

Centrefrequency

Fig. 4: A Kenwood pair. The SAE 1001

Spectrum Analyser Adaptor working in

conjunction with the CS 4125 dual

trace ‘scope.

Fig. 5: Linked to swept and marker generators,

it becomes possible for a ‘scope to trace out

response curves as illustrated at (a). Diagram

(b) shows how such a display is formed from

the X and Y inputs and timebase ramp.

IMPORTANT OSCILLOSCOPE Y AMPLIFIER RELATIONSHIPS

Errors & Updates On page 23 of Part 4 (April issue) of my Oscilloscope series in the first paragraph under the subheading LissajousFigures my 55° was incorrectly printed twice as 5°. You the reader will have undoubtedly already realised themisprint since the 55° is correctly given in Fig. 3 caption. Sorry about this mishap. G4VFV

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If you’ve been following this series you should nowhave a basic idea of the working principles of the‘scope, how time and amplitude measurements aremade and interpreted, rise time and bandwidthimplications, the characteristics of sine and square

waves, elementary applications and so forth, so now is thetime to consider the use of the bargain ‘scope in theworkshop and shack. Actually being able to see thenature of the signals you are dealing with can be ofimmense help to the ‘self training’ philosophy of ourhobby - and let’s face it, that is a primary aspect ofAmateur Radio.

Coupling the Y input of a ‘scope to the test signalthrough an open length of wire is prone to 50Hz rippleand spurious signal pick-up and should generally beavoided. Using screened cable terminated by a couple ofcrocodile clips for ease of connection is satisfactory forrelatively low-frequency signals, and for pulses of not tooshort rise time. However, there are occasions requiring amore specialised interface, such as when application is toa point of high impedance, or when the voltage oramplitude of the test signal is greater than theoscilloscope’s Y input capability.

Compensated ProbeWhen a ‘scope is connected directly to a radio frequency(r.f.) source of high impedance through screened cable,the total shunt capacitance of the cable and the Y inputcapacitance could well approach 100pF, which couldadversely affect the circuit under test. This problem can bereduced by using a probe of suitable characteristics tointerface the source to the ‘scope.

The circuit of one example of a passive probe, knownas a compensated voltage-divide probe, is shown in Fig. 1.When used with a ‘scope whose Y input resistance is 1MΩ,which would apply to many practical ‘scopes of the kindwe are looking at, the resistance at the tip of the probewould rise by a factor of ten to 10MΩ, as established bythe series resistor, but at the expense of a ten-to-onereduction in sensitivity.

Relative to the capacitance of the screenedconnecting cable and the Y input capacitance of the‘scope itself, the trimmer capacitor Cc is adjusted to dividedown the capacitive reactance equally, thereby reducingthe tip capacitance by a factor of ten. All frequencies areequally attenuated when the trimmer is adjusted for theleast rounding or overshoot at the leading corner of a1kHz squarewave applied to the probe tip. (A squarewave

generator is usually built into the ‘scope to cater for thisand other requirements).

When assessing the amplitude of a signal picked upby a probe of this kind, account must be taken of thevoltage-divide ratio in conjunction with the setting of the‘scopes volts/div control. However, there are active probesthat provide a high tip impedance without impairing theinput sensitivity.

Amplitude ModulationA classic example of amplitude modulation (a.m.) isshown in the oscillogram in Fig. 2. Here a modulated r.f.signal, provided by a Marconi signal generator applied tothe Y2 input, is shown by the lower trace, while the audiofrequency (a.f.) sinewave responsible for the modulation,provided by a Radford low distortion oscillator coupled tothe Y1 input, is shown by the upper trace.

The oscillogram (Fig. 2) nicely illustrates how a dual-trace ‘scope makes it possible to display two relatedwaveforms simultaneously. In this case, direct comparisonbetween the modulation envelope of the lower displayand the modulating sinewave of the upper display gives abasic indication of any modulation distortion. Hardly anyin this example, though.

Because the timebase sweep needs to be set to suitthe frequency of the modulating signal, the separatesinewaves of the higher frequency carrier wave, of course,are much too close together to be discernible. Themodulation percentage can be determined from thelower display by dividing the difference between themaximum and minimum amplitudes within the envelopeby their sum, and then multiplying the result by 100.

In the example given, the difference and sumrespectively are about 2.2 and 6, signifying a modulationpercentage close to 36.6% (2.2 divided by 6 times 100).Increasing the modulation to 100%, so that the minimumamplitude within the envelope just drops to zero, resultsin the peaks of the modulation envelope rising to twicethat of the unmodulated carrier wave.

Oscilloscope MonitoringThe ‘scope can also be arranged to monitor the signalradiated by your transmitter by connecting a short lengthof wire, acting as an ‘antenna’, to the Y input through alength of coaxial cable, and setting the Y sensitivity andtimebase controls accordingly. Troublesome 50Hz mainsripple can be reduced, but at the expense of some r.f.amplitude, by connecting a 50Ω resistor between theinner conductor and outer screen of the coaxial (a radio-frequency choke could be a better alternative).

The Y bandwidth will need to embrace the frequencyof the transmission, of course, but most practical ‘scopesshould be capable of responding to the signals of thelower frequency Amateur bands. Working 4W QRP on the7MHz band into my roof-space trap dipole antenna, I geta display of around 0.6V peak-to-peak (p-p) whenmonitoring my transmission in this way.

TAKING THE MYSTERY OUT OF THE OSCILLOSCOPE feature

Part 6 - Workshop andRadio ShackApplications

The Os

cillo

scop

e Gordon King G4VFV now guides

you through the workshop use and

applications of the oscilloscope.

WST2759

tip

Probe

Earth

inY

9M

1M

3 - 30P

CcinC

Oscilloscope

Screenedprobe cable

ProbeFig. 1: This example circuit of a simple

compensated voltage-divider probe

increases the resistance and reduces the

capacitance of the Y input, but at the

expenses of a ten time reduction in

sensitivity, as explained in the text.

17 Practical Wireless, June 2005

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TAKING THE MYSTERY OUT OF THE OSCILLOSCOPE

By setting the timebase to thefastest sweep rate and activating theX10 facility, it should be possible toresolve the separate sinewaves of anunmodulated carrier. Under theseconditions most rally-acquired ‘scopeswill achieve a maximum sweep rate of0.05 microsecond/div. One completesinewave of a 7MHz carrier wouldthen occupy about 2.86 divisions onthe graticule.

By noting the vertical amplitudeof the display when the transmitter issending a low-level c.w. signal, itbecomes possible to determine theoutput at other amplitudes ofdeflection. For example, let’s say thatthe amplitude of the display is 0.6Vwhen the transmitter is sending 4W,then at an amplitude of 6V theoutput would have increased to400W.

So, how do we know this? Well,the arithmetic is simple. We merelydivide 6 by 0.6, square the result andmultiply by 4; e.g., (6/0.6)2x 4 = 400.The deflections are squared becausepower is proportional to the squareof the voltage (see below), whilebeam deflection is directlyproportional to the voltage. I’ve usedthis method of ‘scope monitoring tokeep an eye on the p.e.p. whiletransmitting in s.s.b. mode. It’s oneway to avoid overdriving andinadvertently peaking above the400W p.e.p. limit.

Clearly, care must be taken toavoid interference to other Amateursand users of the band when runninga transmitter while monitoring off-airfor test purposes. Detailed tests andmeasurements using a ‘scope requirethe transmitter to be driven into adummy load with a means ofmeasuring the power, and anarrangement for coupling a smallsample of the signal across the load tothe Y input.

When a ‘scope is used to assesstransmitter performance, a two-tonemodulating signal of suitable level forapplying to the microphone input canbe useful. The two tones should below distortion sinusoidal, equal inamplitude, non-harmonically related

and, of course, within the rig’smodulation bandwidth. (Note: ThePW Two Tone Oscillator by TonyNailer G4CFY, published in theFebruary 2005 issue, would make auseful piece of kit to partner your‘scope. The article also describes howthe oscillator can be used with a‘scope, along with details of testresults. It also tells how an in-linesniffer unit can be constructed to yielda low-level r.f. Y input).

Power, Voltage andResistanceNow let’s look deeper into power,voltage and resistance. The r.f. voltageappearing across a non-reactivedummy load at the output of atransmitter is equal to the square rootof the power in watts multiplied bythe load resistance in Ohms. At 400Wpeak envelope power (p.e.p.),therefore, the amplitude of the r.f.would be quite substantial andbeyond the Y input capability of apractical ‘scope.

This is why it’s necessary toattenuate the voltage by using asuitable probe, coupling circuit orësnifferë. Moreover, when dealingwith strong r.f. voltages extreme careis required to avoid r.f. burns toperson and serious damage to testequipment.

On the audio side, though, it’sgenerally less hazardous to connectthe audio frequency (a.f.) voltageappearing across a resistive load atthe output of an a.f. amplifier eitherdirectly to the Y input or through asimple attenuator. This is because a.f.power is often measured in tensrather than in hundreds of watts,while the load resistance is commonlyin the order of four or eight Ohmsrather than 50Ω. For example, ther.m.s. voltage across a 4Ω loadconnected to the output of an a.f.amplifier delivering a sinewave of,say, 25W average power would be10V, corresponding to 14.14V peakand 28.28V peak-to-peak.

It’s worth keeping in mind thataverage power, sometimeserroneously called r.m.s. power, is

equal to the square of the r.m.s. valueof the voltage (see Fig. 1 in Part 3 PWMarch) divided by the load resistancein ohms, or W = V2 /R (or I2xR; where Iis the r.f. current), and that the p.e.p.is the average power in one r.f cycle atthe highest crest of the modulationenvelope delivered to the antenna (orload).

Key ClicksBy keying a transmitter in A1A mode(c.w.), the keying display will give anidea of whether key clicks are likely tobe troublesome. The basic keyingwaveform is tantamount to asquarewave that, as already told inPart 5, is derived from thefundamental (keying) frequency plusa wide range of essentially odd-orderharmonics.

Harmonics from an untreatedkeying waveform yield multiplesidebands that can extend many kHzeither side of the carrier frequency. Itis the energy carried by thesesidebands that is responsible for theannoying key clicks sometimes heardfar removed from the operatingfrequency.

The trick is to limit the rise (andfall) time of the keying waveform.Most transmitters from commercialsources take care of this quiteadequately, indicated by the

controlled rise and fall times of the akeying waveform.

An impression of the leading partof a keying waveform of an earlytransceiver of mine, based on theoscilloscope’s timebase running at1mS/div, is given in Fig. 3. I think thisis quite a fair result as witnessed overmany years of working c.w. with therig and not having had anycomplaints of key-click QRM.

If the rise time is increased toomuch above about 5mS the keyingwill sound over ‘soft’ and the codemore difficult to read. A rise time of5mS relates to a bandwidth ofbetween 70 and 100Hz (Part 5, PWMay), depending on the nature of theclick suppression treatment.

The power that exists at thepeaks of the modulation enveloperepresents the p.e.p. This, of course, isalso the case with a monitored single-sideband (s.s.b.) audio frequencydisplay, but here the general natureof the display, and the rises and fallsin amplitude, are related to thefrequency, harmonic content and theloudness of the modulation.

With a little practice it becomespossible to glean a rough idea from aspeech-derived s.s.b. display whethera rig is suffering from peak clipping,overload, instability or any othersignificant shortfall. More serioustests, of course, require the use ofadditional instruments, such as anaccurate output power meter, two-tone oscillator, r.f. coupler, etc., asmentioned earlier.

Fig. 2: The modulated carrier wave and

the signal responsible for the

modulation of this dual trace a.m.

oscillogram are shown respectively on

the lower and upper traces.

Modulation percentage can be

calculated from the ratio of the

maximum and minimum amplitudes

within the modulation envelope, as

explained in the text.

Fig. 3: An impression of the leading

edge of a radiated keying waveform

based on the oscilloscope’s timebase

running at 1mS/div.

Fig. 4: Example oscillograms. (a) Squarewave sag resulting from limited low-frequency

response. (b) Squarewave with slight overshoot showing ringing in oscillatory circuit

triggered by pulse. (c) Sinewave with clipped positive-going half cycles resulting from

incorrect amplifier biasing. (d) Noise display heard as ‘hiss’ and sometimes referred to

as ‘white noise’. (e) Electrical interference on 50Hz mains waveform radiated from

fluorescent light fitting. (f) Total harmonic distortion (lower trace) carried by the

sinewave (upper-trace) remaining after removal of the fundamental frequency by a

steep and narrow notch filter.

(a) (b) (c)18 Practical Wireless, June 2005

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Practical Wireless, June 2005 19

Learning CurveAfter first acquiring a ‘scope, whetherancient or modern, new or second-hand, you will have triggered a veryinteresting continuing learning curve.I know, I started on my curve 60 yearsago! Like Rob G3XFD said, his firstintroduction to the ‘scope’s usefulnesswas when it revealed to him a biasfault in a misbehaving tape recorder.

It’s impossible, of course, within afew thousand words to delve deeplyinto the ‘scope’s myriad applications.But I think it would now beappropriate to round off this shortseries by highlighting a few of themore interesting oscillograms relatedto my own diagnostic andperformance testing activities overmany years, yet which still remainperfectly valid.

Looking At OscillogramsThe oscillograms are broughttogether in Fig. 4, where the first oneat (a) depicts a low frequencysquarewave with horizontal sagresulting from its passage through anamplifier or circuit that tends toattenuate (or roll-off) the lowerfrequencies slightly relative to themiddle and higher frequencies. This isthe converse of high-frequency roll-off which, as we saw in Part 5, impairsthe rise time and rounds the leadingcorners of a squarewave.

The squarewave at (b) reveals a

trace of overshoot at the upperleading corner, an indication ofamplifier instability. ‘Ringing’, adiminishing amplitude oscillation, islikely to occur when a fast-rising pulsetriggers an undamped, high Qoscillatory circuit. The clippedsinewave at (c) is a fairly typicaldisplay as might stem from the signalpassing through an incorrectly biaseda.f. amplifier.

Noise signal such as (d) isresponsible for the gentle hiss behinda weak radio signal or from a poorlydesigned high-gain a.f. amplifier,sometimes, though not alwayscorrectly, referred to as ‘white noise’.Interference generated by a poorlysuppressed fluorescent light can beseen on the 50Hz mains waveform at(e), revealing the remarkableversatility of the workshop ‘scope.Finally, the dual trace display at (f)shows a ‘pure’ sinewave on the uppertrace, and the total harmonicdistortion (THD) contained within thesinewave on the lower trace,obtained by ‘notching out’ thefundamental frequency.

Frequency ResponsePlotsThe dual trace display in Fig. 5 showshow I have also used a ‘scope to plota.f. response curves. Here the uppertrace shows the frequency responseof the speaking channel of a stereogramophone pick-up playing afrequency response recording, andthe lower trace the breakthroughsignal in the non-speaking channel.

The graticule is scaled in 5dB/divvertically and Hz-times-tenhorizontally, corresponding to verticaland horizontal ranges of 40dB and20Hz to 20kHz respectively. I have alsoused this technique for checking thefrequency response of stereo radioreceivers and hi-fi tuners.

The idea is similar to thatdescribed for tuned circuit alignment(Part 5), but for these audio tests aswept functions generator was usedin conjunction with a Telequipmentdual beam ‘scope. The Y deflectionwas made logarithmic by using a

home-constructeda.c./d.c. converterwith a logarithmicamplifier.

The Z InputIn addition to theX and Y inputs,most ‘scopes areusually equippedwith a Z input that links to the grid ofthe c.r.t. By coupling positive - ornegative-going pulses to this input,which is often located at the rear ofthe instrument, the trace can beeither intensified or darkened for theperiod of the pulse. In this way timingmarkers can be superimposed on thedisplay by the application of suitablepulses from acalibrated generator,representing analternative methodof reading sweeptime, for example.

Having nowarrived at Z my storytoo has virtuallyended, but notbefore a few wordsof encouragement tonewcomers. Theoscilloscope has beenof immense help to me during my lifeworking with things electronic. Wecame together when I was young andrepairing radios on the home front atthe start of the war, and we becameeven more of a technical duo duringmy war time activities in the RoyalCorps of Signals with SpecialCommunication and ExperimentalUnits in SE Asia.

The reintroduction of televisionand the revival of hi-fi sound in stereonot long after the war finished,rendered the ‘scope an even morepotent aid for procedures related toboth design and servicing (Fig. 6). Italso became a primary instrumentduring the time I was pioneeringcoaxial relay (cable TV) anddeveloping electronic devices such asthe King Telebooster, electronic carrev counter, automatic slide changeunit, etc.

The ‘scope continued to flourishin my domain during the happy yearsI subsequently spent testing andreviewing hi-fi equipment for theaudio magazines, and writing manytechnical books and hundreds ofarticles for the technical press. Now,after more decades than I care toremember, the ‘scope and I find ithard to be parted. Together, you willfind us still in the radio shack one wayor another (Fig. 7). Have funbecoming acquainted with yourbargain ‘scope! PW

5 10 50 100 500 2k1kHz

dB

Fig. 5: Frequency response and stereo

separation curves of gramophone

pickup playing a special test record as

displayed on a ‘scope in conjunction

with a swept function generator, where

the vertical scale of the graticule is

5dB/div logarithmic and the horizontal

scale Hz ×× 10 (also see Fig. 5 in Part 5).

Fig. 6: Gordon G4VFV managing an

electronics division not long after the

end of the Second World War, showing

an early Cossor ‘scope along with a

‘mini-scope’ (on the side table). Despite

its limited features, radio buffs and

service departments in the 1950s often

chose this relatively inexpensive mini-

scope.

Fig. 7: G4VFV on the air in his lab/shack

at Brixham, showing a Telequipment

Oscilloscope, Marconi Signal Generator,

HP Spectrum Analyser and other

associated items used by Gordon for

his design and magazine reviewing

activities.

(c) (d) (e) (f)

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