CRT Display Design

© Display Laboratories Inc.

Session 2Video System

Featured Seminars Introduction to CRT Displays Video and Tube Biasing Deflection and High Voltage Micro-Control and Waveform Generators Special Topics and Miscellaneous Circuits

CRT Display

Video AmplifierAssembly

Intro to the Video System. Block diagrams General description Input signals and timings CRT gun characteristics Video amplifiers Bias and Blanking amplifiers Focus and Horizontal Static Convergence Other Considerations

Block DiagramPower Supply

Cathode Ray Tube

Vertical & HorizontalDeflection Amplifiers

Video Amplifier& Blanking

RGB

H+V Sync.Sepr.

Focus andConvergence

Block Diagram (Video System)

Power Supply

Cathode Ray GunBias & Focus Amplifiers

Video AmplifierRGB

G2 Focus

R G B

Heater

G1Back Porch Clamp and Blanking

Block Diagram (Input Signals)

RGB Analog Video

Separate Sync. Composite Sync. DVI Digital Video

Video AmplifierRGB

Deflection AmplifierH V

Video AmplifierRGB

H+V Sync. Separator

General Description The video system controls the operation of the gun Provides interface of video source to the CRT gun Controls intensity and color balance of the image Performs gain adjustment of drive level Blanking amplifier provides retrace blanking Bias amplifiers establish operating point & spot kill Focus amplifier controls spot shape and size Electrostatic Horizontal Convergence (Static)

Video System Components CRT Gun

Gun Structure Triode Section Lens Section

Cathode Drive Spot Size Cutoff Drive requirements Operating Point Color Balance Black Level Balance

Video Amplifiers Preamplifier Output Amplifier Black level Amplifier Back Porch Clamp

Other Services Retrace Blanking Astigmatism Correction Sync. Separation Input Selection On Screen Display

Future Seminars Deflection and High Voltage Micro-Control and Waveform Generators Special Topics and Miscellaneous Circuits

END Notes:

END Notes:

Video System Block diagram Preamp Input select Termination Contrast Sync Separator

Video System Sync tips Black level Back Porch Clamp Output Amp Cathode voltage swing T-rise/T-fall Cathode capacitance

Video System Beam current CRT Bias Cutoff Brightness Black level Arc suppression

Video System Color Tracking White balance Preset Temp Variable Temp White to Black color tint White uniformity Circuit considerations

Input Signals

Typical sync on green input signal.

Composite Sync. Types Serrations Half line Interlace Doubles Equalizing pulses Missing sync. pulses

Input Connectors BNC – RGB analog video DB-15 – RGB analog video DVI – Digital CRT video Timing and resolutions •

Sync. Types Sync. on Video/Green Separate Sync.

Composite 1 wire Separate 2 wire

END Notes:

END Notes:

Input Signals & Timing

Input Signals and Specs. Display Industry (VESA) Input Signal Specification (VESA) Input Signal Definition (VESA) Video Signals and Sync Types DVI Digital Video Interface GTF General Timing Format P&D Plug and Display DMT Default Monitor Timings

VESA Monitor Timing Specifications Version 1.0 Rev. 0.8

Standards and Guidelines Summary

Input Signal Specification

Input Signal Specification

Input Signal Definition

Input Signal Definition

Input Signals

Sync Signals during Vertical

Double H Sync during Vertical

Vertical “Or” with Horizontal

Vertical “XOR” with Horizontal

Proper Composite Sync.

Separate H and V sync.

D V I Digital Video Interface

D V I Digital Video Interface

D V I Digital Video Interface Shown is a connector by JAE Electronics for the Dual

protocol video interface. Both Digital and Analog may exist simultaneously on

this connector. The Digital is made of up to two 3bit low voltage

differential channels with about 180MPixel/Sec each. They share a common Clock.

DDC Data and Clock, Vertical Sync and +5v are also supplied.

RGB and Horizontal Sync are provided over Coaxial cable.

DVI should open a new range of display improvements.

Video Rates, Dot Clock, Bandwidth Dot Frequency & Bandwidth Dot Frequency & Rise Time Specifying CRT Drivers Rise Time of Cascaded Stages

Dot Frequency Dot Rate is a property of the Signal. Bandwidth is a property of the Amplifier. Dot Time = 1/Dot Clock Frequency. Dot Clock Frequency = (Active H Pixels)x

(Active V Pixels)x( Vertical Refresh Rate)x(t total/t active) If the video signal contains alternating white and black

pixels the resulting square wave will be ½ the Pixel Rate.

The video amp should reproduce this adequately.

Dot Frequency & Bandwidth 90% Rule; The high frequency dot amplitude should

not be less than 90% of the low frequency dot amplitude.

The response of an amplifier is generally down to 90% at ½ the –3db frequency.

Amp 3db BW = ½ to 1 times the Maximum Dot Clock frequency.

The designer may pick within this range depending on the overall performance requirements of the monitor.

Dot Frequency & Rise Time The dot period = 1/Dot Frequency. The rise time of the signal at the cathode should be

less than 1/3 the dot period. Designers should specify CRT drive requirements in

terms of rise and fall time at 50 Vp-p.

Bandwidth Requirements Determine timing to be supported Determine trace time (Active Raster) Compute Dot rate and Pixel period Rise time should be 1/3 of Dot period Determine Large Signal Bandwidth Design or Choose Amp. components

Rise Time of Cascaded Stages It has been said that the whole is the sum of it’s parts. Or

at least the square root of the sum of it’s squares. For a generator, amp, load, and probe t rise and t fall is given

by;T meas = [T gen2 + T amp2 + T scope2]½

For the amplifier the times are;T amp = [T meas2 - T gen2 - T scope2]½

4 ~= [72 - 42 - 42]½

4 ~= [49 - 16 - 16]½

END Notes:

Video Amplifiers

Video Amplifier Sections Transistor Amplifier General Theory Pre-Amplifiers Output Amplifiers

Open ‘Loop’ Amplifiers Closed ‘Loop’ Amplifiers

Black Level On Screen Display

Video Channel Block Diagram

CRT Gun

Black Level Amp

Output AmpPre Amp

Input select and Termination Bias and FocusOn Screen

Display

Pre-Amplifiers

Pre-Amplifier System The pre-amp has become much more than a

simple variable gain wideband amplifier. Modern Chips include;

RGB three channel integration Common Gain ‘CONTRAST’ Separate ‘DRIVE’ Voltage or I2C buss control Black level clamp DC restoration OSD input selection or integrated OSD Blanking input Brightness insertion into gain path

Typical Three Channel Pre-Amp

Single Channel Pre-Amplifier

Variable Gain Test Circuit Using a Spice model of the

proposed gain stage the transfer function can be plotted.

In addition to plotting the gain / control voltage, the effects of R4/R5 can be studied.

R4/R5 will adjust the range of linear control of the V2 input voltage.

R2/R1 determines the max/min variable gain range.

For Drive control a 2 to 1 (6db) range is sufficient for color balancing.

General Contrast range of 10:1 (20db) or more my be needed.

Voltage Controlled Gain Plot

Contrast and Drive Gain Stages This is a simple cascode

Contrast and Drive gain stage. Input video is AC coupled to

an internal bias supply. Then buffered by an emitter

follower driving a cascode differential transistor pair that has maximum gain range.

Then into a second differential pair with a 2:1 voltage range.

Buffered by a Darlington emitter follower with an active current mirror load that is used to reestablish DC bias across R13.

Note: that the maximum AC gain of this stage is;(R10+R11)/R9 = 2.

Low AC gain and wide band!

Black Level Clamp Offset voltage

between pins 18 and 19 produces a + or – current at pin 12.

This current charges or discharges the bias cap that reduces the offset.

The DC level of the pre-amplified signal is sampled for the clamp time and set to the desired level.

Preamp Output Buffer Negative feedback amp

with a single voltage gain inversion stage Q22.

Input is buffered with an emitter follower.

Non-inverting summing junction Q20 with Darlington connection Q21 to inversion stage Q22.

Output is buffered with an emitter follower and an active load.

Gain set at R21/R18 = 16.

O S D Mixer The OSD On Screen Display

signal needs to be inserted into the video during active trace time.

This is generally done over the normal video. That is covering or masking the normal video. A mux or mixer is used to switch from one source to the other.

Care must be used as to not inject unwanted switching noise or blanking artifacts into the image.

O S D Mixer Some times the video must be blanked to provide

the desired LOOK.

Blank Insertion Black level clamping

occurs during blanking. The output of the pre-

amplifier can be switched to a ‘blacker than black’ level during blank time to drive the output amplifier to a ‘Blank’ level.

The Black level clamp should clamp to the preamp and not the input to the output amplifier.

High End Peaking When G2 is moved blacker the slowest channel of

video will disappear first. When G2 is further reduced only the over peaked channel appears.

The most accurate way to set high frequency peaking is to (after color balance) set G2 so that the white block is about 1/4 as bright.

Contrast should be set for 2/3 of maximum. Adjust the high frequency capacitors on the video board for the same shade of color in the high frequency block.

This diagram shows what happens when G2 is set to cover up most of the video.

Cathode Connection

G2Video Output

Amplifier

Black LevelAmplifier

G1R

Single point ground

Carbon Comp. Resistors

END Notes:

Output Amplifiers

Output Amplifiers Open Loop

Class A Class A with Class B Load buffer

Closed Loop Class A with Class B Load buffer Class A with charge pump Class A with Active Load Dual Class B

General Theory Speed Limitations Flash-Over susceptibility ESD protection Negative Feedback

Open Loop Closed Loop

Open “Loop” Amplifiers

Current Feedback Made Easy

General Description Amplifiers that are not corrected with

voltage feedback The Cascode Amplifier Negative “Feedback” Voltage verses Current Feedback Simple Wide Bandwidth Topology

The Cascode Amplifier The Cascode amp is two

amps in one. Input is buffered by an

emitter follower Q1. Output is a grounded

base configuration Q2. This combines high input

impedance and wide bandwidth characteristics.

Passive speed up can be used in the collector.

Emitter “Peeking” will help offset some of the capacitive load effects.

Cascode Amplifier Current Feedback The current in R3 is from

the emitter of Q1. This current is the sum of

the base current and the collector current.

The collector current is almost exactly the same as the output voltage. V=Ic*(R2||Cload).

If Cload is small then Ic and Vout are similar.

And so is V-R3. The base of Q1 and its

emitter act like - & + inputs to a differential amp.

Hence V-R3 is “Negative Feedback”

Cascode Amplifier Current Feedback This type of Class A linear Amp

is amongst the fastest and most stable topology available.

It’s main draw back is the load current required to charge Cload at high slew rates.

The fastest amps SONY has used are in the DDM 20”x20” monitors. Tr, Tf < 1.5nS

These were specially developed by Tektronix for the 1GHz Oscilloscopes in the late 1980’s.

The PCB and collector cathode connection use strip line layout.

The Cascode Amplifier Transistor Q2 is operated in the

grounded base configuration, which does not have a Ccb feedback effect.

The collector base capacitance appears from collector to ground and does not have a multiplication of 1+Vgain.

Transistor Q1 is also operated as an emitter follower that reduces the Ccb effect. Node 9 is a 12 volts supply.

Node 2 is at about 11.3 volts and appears much like a voltage supply.

There is very little A.C. signal at node 2. The input signal is at node 7, 3 and 4. The Ccb effect of an emitter follower

appears as input capacitance Why R5? We just learned that a grounded base amplifier is fast. Why slow down the amplifier with base resistance?

The Cascode Amplifier There are two functions of a base

resistor for Q2. This type of amplifier is being pushed to

have more gain at high frequencies by the peeking capacitor C1 and inductor L1. At the same time the hFE of Q2 is dropping because of fT. The two effects collide and cause instability and possible oscillation. Resistor R5 roles off the amplifier gain at a point higher than its operating range.

The second reason for base resistance in the output stage of a video amplifier transistor is for CRT tube arc protection. Five to ten ohms of base resistance reduces the base current under arc conditions.

Generally try to keep R4 in the range of 1/5 to 1/10 of R3. Once again C1 (peeking capacitor) pushes the gain and phase upward while capacitance and fT causes the gain and phase to drop. If the peeking capacitor is allowed to push the gain up uncontrolled there is likely to be instability and oscillation.

Transistor Q2 will have a longer life with base resistance.Why R4? It is obvious that R2 and Cload are compensated for by R3 and C1. Why limit the effect of C1 with R4?

It is hard to see 300mhz oscillation in a video amplifier using a 100mhz oscilloscope

Cascode with Emitter Followers This circuit buffers the capacitive load from

the collector of Q2. The buffer only improves the speed if Cload is

larger than that added by Q3 + Q4! In a B/W monitor there is a direct connection

from video amplifier to CRT, cathode Cload may be too small to justify the buffer stage.

Color monitors have additional circuitry added between video amp and cathode. The Cload is large and emitter flower buffers are common.

Remember; 1.)      Minimize the load on Q2 collector. 2.)      Ic of Q3 and Q4 are due only to Cload charging current. 3.)      Q3 and Q4 heat sinks are connected to supply and ground and do not add capacitance. 4.)      Cload charging current is not limited by R2. Current from R2 is multiplied by hFE of Q3.5.)      Cload charging current does not pass through Q2 thus minimizing beta limiting.

Closed “Loop” Amplifiers

Voltage Feedback

General Description Amplifiers that are corrected with voltage

feedback Evolution Complementary Class B Voltage verses Current Feedback Gain Bandwidth Frequency Compensation

Cascode with Charge Pump The Large signal

bandwidth is limited by R1’s ability to charge the capacitive load.

By adding Q5 and driving It on through C1 Q5 can increase the slew rate of the rise time at D1.

This increases the performance without adding to DC power consumption

Gain = R1/R4

Feedback with Active Load In an effort to reduce

parts and power dissipation a dual stage complementarily design evolved.

It had fewer parts but was unstable without negative feedback.

It also lacked a natural operating bias point.

R2 is needed to “Close “ the loop.

With R1 and R9 a DC operating point could be established.

Feedback with Active Load Q2 is biased with R4,5,&6

to form a current source. C1 can drive Q2 to act as

an active load when Q1 is driven off.

Vin is essentially a current input node.

Vin is really an Iin summing junction.

Frequency compensation networks are used to control the response of this type of amplifier.

Closed ‘Loop” Amplifiers Now is a good time for a

network analyzer. If you have one it is

easy to determine the response of the basic output amp under load conditions.

Simply hook up input and output and plot frequency response.

Then compute or look up values for the RC’s and get a cup of coffee.

Closed ‘Loop” Amplifiers If your lab is not so well

equipped or you don’t have the time to figure how to use the thing you can use the

“cut and try” method. First input a test signal.

Square waves are best. With only Rin look at the

response at the cathode. If it is under peaked, and it

will be, start by adding a small pF trimmer 3-30 pF at C1. And adjust.

The leading edge of the waveform should rise. Bring it up to level with the body of the waveform.

Frequency Response Now it will fall back for a short time. Next you need a RC at R2,C2. Use the same kind of variable

cap as above but add a small value resistor in series say 10 ohms.

Adjust. This should start to bring the droop up to flat and over peak the leading edge. Back off C1.

If you were very lucky this will do it. For the rest of us we look to find if the new effective peaking

budges or droops. If it holds for a short time and then droops before it returns.

increase the resistance. If not decrease it. In bad cases a second RC may be needed to give further mid-

band peaking. An RC may be placed across the Amp if the response is over

peaked at mid band.

Two types of video peaking are used to get the best video response. High band peaking is used to set the response of the first pixel or a single pixel. The mid-band peaking sets the response of pixels 2 through about 7.  

Between the preamp and the video output amplifier are the video peaking components.

At low frequency CA, CB and RB do not effect the circuit. The drive for low frequency signals, is sent by RA. At a high frequency determined by CB, the amount of drive is

set by RA and RB in parallel. At a very high frequency CB increases the drive even more. We have experience that the value of CB will be the same for

all video power amplifiers with the same date code. Amplifiers made at a different time may require a different value.

Peaking

High End Peaking There are three video preamplifiers

and three power amplifiers each with it's own frequency response at the very high end.

There is no way to get three perfectly matched preamps and three matched power amplifiers.

On the video board there are trimmer capacitors to set the very high end frequency response for each channel.

If the back ground is set for black then the variations is high end frequency response is hard to see. In the black ground is set very black then the differences in frequency response is very noticeable.

High End Peaking The video generator produces a large

white block and a block of every other pixel on next to each other.

The vertical line on the right would show the frequency of the large white block. The vertical line on the right shows the frequency of the every other pixel region.

If the color balance is done correctly then the white block will look white. If the high frequency peaking is not adjusted correctly then there will be a color shift in the high frequency part if the test pattern.

If G2 is adjusted so only the tips of video can bee seen then the differences in video will be easy seen.

High End Peaking When G2 is moved blacker the slowest

channel of video will disappear first. When G2 is further reduced only the over peaked channel appears.

The most accurate way to set high frequency peaking is to (after color balance) set G2 so that the white block is about 1/4 as bright.

Contrast should be set for 2/3 of maximum.

Adjust the high frequency capacitors on the video board for the same shade of color in the high frequency block.

This diagram shows what happens when G2 is set to cover up most of the video.

R L C Impedance Chart

R L C Impedance Chart

END Notes:

The CRT Gun

The Electron Gun The uni-potential gun can be broken down into

two sections, the triode and the lens section. The triode is the electron emitting and shaping

section formed by the electron source (cathode), the control grid (G1) and the accelerator (G2).

The lens of the uni-potential design has three elements G3, G4 and G5.

Cathode Life Flash-Over protection

The Triode A 6.3 or 12-volt filament heats the cathode.

Operating at an elevated temperature causes electrons to "boil off" the surface of the cathode and form a cloud.

The electron-emitting surface of the cathode is often impregnated with tungsten and barium to lower the work function of the surface.

The effective area of the cathode is determined by the aperture size of G1 (first control grid).

The Triode The video signal is typically applied to the

cathode along with video blanking. This permits the G1 and G2 elements to be biased relative to the cathode for uniform operation from CRT to CRT.

The physical relationship of G1, G2 and the Cathode, influence the lower beam angle, center focus voltage, and the spot size for a given gun design.

The Triode Electrons carry a negative charge. They

are attracted to positive elements within the tube like G2 and the faceplate (1st and 2nd anodes) and are repelled by the relative negative charge of G1.

The more negative G1 appears relative to the cathode, the greater the reduction in the flow of electrons.

The Triode Beam current is attracted through a small

hole in G1, called the aperture, by a positive voltage applied to grid two (G2, the first accelerator or anode).

The bias voltage applied to G2 is set in conjunction with G1 and Cathode to establish “cut-off” of electron flow, at black levels in the picture.

The bias voltage between the cathode (+) and G1 (-) has the largest influence on spot size after physical gun design.

Triode Cross Section (G1 ref.)

Cathode

Heater

G1G2

0v 600-900v

130v70v

70v

100v

100 v

30v110v

Determining Operating Point Select spot size (Line/in.) & Intensity Determine Ik from the charts Determine Vk/G1 bias point from chart Determine Drive requirements from chart Determine G2 range for Vk/G1 from chart Using signal ref (Green Default) Determine R/B .vs. G cutoff limits Black level amps cover these values

Spot Size / G1 Cutoff @ Ik

Intensity / Cutoff @ Ik

Cathode Current / Drive Voltage

Drive curves are shown on this chart.

Note that the three lines do not match exactly.

This must be taken into account when setting black level and white balance.

Each gun will have a different black (cutoff voltage) level and video gain setting.

400uA

40 volts drive

Bias Chart Ik .vs. Vk @ G1 This chart is for an

older Trinitron tube. It shows three different

lines for Ekco voltages. The more “remote”

the cutoff (to the right) the more drive voltage is required to attain a Ik current.

Adjustable G2 for Fixed G1 This is for an older

Trinitron Tube. It shows the range of G2

voltages for Ekco voltage. All tubes made of this gun

type should have G2 voltages within this range.

The chassis must be able to generate voltages that more than cover this range.

The Lens Electrostatic lenses are used to focus the

electron bean onto the phosphor screen in the front of the tube.

The combination of G3, G4, and G5 form the lenses. G3 and G5 are connected to the fixed high voltage of the anode (originally called the 2nd anode).

The voltage on G4 is varied to control the focal distance. Static focus voltage and any dynamic voltage, if required, are applied to G4 through the base connector.

The Lens High-resolution displays require dynamic

focus to maintain pixel quality into the corners of the CRT. This is even more critical on flat profile CRTs.

Electron guns can be optimized for dynamic focus or flat focus applications. A flat focus gun will provide a compromise of focus quality from center to edge and is appropriate for many low to medium resolution applications.

Displays requiring higher resolution such as those in desktop publishing, graphics terminals, document processing, and medical imaging, require dynamic focus.

The Lens The function of each of these gun elements

and their interaction is critical to the overall performance of the CRT.

From cathode to the face of the CRT, it is the relationship between each of these individual elements that determines the final appearance of the un-deflected spot in the center of the tube face.

The Electron Gun (Color PIL)

Example of Hitachi Elliptical Aperture, Dynamic focus (A-EADF) Electron Gun

Hitachi Elliptical Aperture, Dynamic focus (A-EADF) Electron Gun At the heart of Hitachi's high performance

monitors is the EADF electron gun which ensures the sharp focus, high definition, distortion free image.

The elliptical aperture lenses produce maximum focusing control while minimizing distortion effects due to centerline offset.

Hitachi Elliptical Aperture, Dynamic focus (A-EADF) Electron Gun Hitachi's dynamic focus capability means

that even FST screens have consistently sharp focus, right into the corners where the beam path length is substantially greater than at the center.

An electro-static Quadra-pole lens which makes constant adjustments to the cross sectional shape of each beam ensures that the landing spot is precisely circular whatever the deflection of the beam or the position on the screen surface.

Focus Voltage and Modulation The type of gun determines static focus

voltage requirements. Uni – potential Bi – potential High Bi – potential Dual – potential

FBT provides the large static DC for gun Focus Amplifier drives AC waveform

Beam Shape & Focus The electron beam leaving an electron gun will

normally be circular in shape. If the gun were at the radius of curvature of the faceplate then the spot at the faceplate would be round.

The problem is that most CRTs are short, the gun is very close to the faceplate.

This causes the electron beam to strike the phosphor at a non right angle.

The corners will have an elliptical shaped spot-landing pattern.

Beam Shape & Focus A Dynamic Quadra-pole Lens found in some

electron gun enables the beam to be made elliptical as it leaves the gun.

Naturally the Quadra-pole lens will distort the beam in a direction that is at a right angle to the normal elliptical effect.

The addition of the two elliptical effects will result in a round but larger spot.

The beam’s spot shape, and therefore picture sharpness, remain the same all over the screen.

Beam Shape & Focus Dynamic Focus voltages may be applied to

the gun of the CRT to optimize the spot size. A complex 3- D waveform is often needed on

very flat and/or large CRT’s. Several hundreds of volts of drive are used

to effect the beam at this point in the gun. The voltage may be different for the four

corners of the tube.

Quadrapole Focus Lens As Gun design evolves new

and more elaborate methods will be devised to correct spot performance.

This is one example in the evolution that shows the effect of electrostatic field shaping on beam shape.

One set of focus electrodes are held at a static voltage while another set adds a dynamic waveform.

Focus Chart Focus control voltages

may not be symmetrical in all directions.

Focus is dependent on the “throw” distance from the gun to the inside of the face plate.

The outside surface we now view may be flat but inside it is still curved.

Check the chart for your tube.

Focus Modulation Chart

Thermionic Emission Thermionic emission is to electrons what

evaporation is to the water molecules in a hot cup of coffee.

It is a process by which some of the electrons inside a piece of metal can 'boil away', leaving the surface of the metal into the surrounding space.

Thermionic Emission Inside a metal the electrons are not

stationary, but are constantly moving, with an average speed that is controlled by the temperature of the metal.

It is important to realize that this is only the average speed of the electrons, that some of them will have speeds that are significantly larger than this.

It is these higher speeds, and therefore higher energy electrons, which have enough energy to escape from the metal.

Thermionic Emission Since the speed of the electrons increases

with temperature, the number of electrons with sufficient energy to escape also increases with temperature, in fact exponentially so.

At room temperature (300 K) the number is very small, but if the wire is heated to 1000K the number of electrons escaping is dramatically increased.

Heater & Cathode Condition Heater voltage affects the

life of the display tube as shown in Fig 1.

DO NOT operate heater beyond the proper voltage.

This voltage should be regulated to a range of 95% to 102% of nominal spec.

The heater bias voltage with respect to the cathode should be +0v to –200v.

Power save state must completely turn off the heater supply.

The Heater and/or G1 must NEVER be biased more positive than the Cathode.

CRT Life Expectancy "What is the life expectancy of a CRT"? There is no ‘Single’ answer to that question. This information will provide some reasonably

accurate guidelines so that you may form your own opinion taking into account assumptions about variables that apply to your application.

Many factors determine the useful life of a CRT. The two major contributing factors are cathode

emission and phosphor aging, and are the only two that will be addressed here.

It should be noted that references to phosphor aging do not include localized burns such as logos, stationary data and statistics.

Cathode Emission The two most common types of

cathodes in use today are the oxide cathode and the dispenser cathode.

This chart shows the approximate decrease in emission under very heavy loading conditions for both types in an accelerated life test.

As you can see, for a given beam current, the oxide cathode will be at 50% of its initial emission value after approximately 3,500 hours of operation.

Under the same loading conditions, a dispenser cathode will have lost only 7% of its initial emission value after 7,500 hours.

Phosphor Emission All phosphors age with use, each

phosphor having its own aging characteristics.

Phosphor aging manifests itself in two ways: a reduction in luminance and a visible discoloration of the screen.

An accepted theorem used to characterize phosphor aging states that, other factors remaining constant, aging of a phosphor is a function of the accumulated charge deposited on it.

This is known as coulomb aging. The chart to the right shows approximately the long-term aging characteristics of P4, P104 and P45 phosphors.

Re-calibration In addition to the effect of phosphor aging there will also be a small

reduction in luminance caused by cathode degradation. The loss of luminance caused by both phosphor aging and cathode

degradation can be recovered by re-calibration of the monitor. Monitors may be designed with sufficient reserves in the drive circuitry

to allow re-calibration several times. This will eventually affect spot size as the cathode current must

increase to overcome the luminance loss due to phosphor aging. Calibration to the original luminance can be repeated until one of the

following limits are reached:

1. The reserves in the drive electronics have been depleted. 2. The spot size has become objectionable. 3. The luminance uniformity has become unacceptable.

END Notes:

Bias and Blanking Amplifiers

Contents G2 Bias Amplifier Vertical Focus Amplifier Blanking Amplifier Horizontal Static Convergence Video Reference Bypassing Gun Supplies

Bias Amplifiers The CRT has two grids that determine the

operating point of the triode section. Once the Cathode / G1 voltage is set, G2

needs to be adjusted to establish ‘Cutoff” G2 is controlled by the micro-controller

through a high voltage amplifier G2 must be properly filtered to maintain

luminance uniformity

G2 Bias Amplifier This figure shows a typical G2

amplifier The grounded base configuration

provides sufficient flash-over protection.

Due to the high supply voltage required and flash-over susceptibility proper resistors and layout are required.

Often series resistors are used in the collector to meet the ‘SOA’ save operating area.

Due to the high impedance involved Collector leakage/temperature must be considered as well as input leakage of the error amplifier.

Focus Modulator Dynamic focus Vertical Horizontal Mixer Flyback transformer

Vertical Focus Amplifier 200 to 400v P-P are used to focus

the vertical. A high voltage bias amp like for G2

may be used. The amp is required to have a

bandwidth that will reproduce the vertical focus waveform.

This is typically a parabolic shape and needs about 3 times the fundamental to produce the tips.

The capacitive loading and slew rate determines the collector resistance needed.

Flash-over and layout must be considered.

DC Bias for V focus amp When Focus Modulation is AC

coupled to the CRT the absolute DC bias point of this amp is not critical.

Simple bias the amp for DC out of 400 – 600 v by setting ratio of R1 and R2.

If Waveform applied to R3 is AC coupled then we can find R2 after we pick R1.

R1 must provide enough current to get the slew rate needed to charge Cload @ the collector of Q1.

VR1 = Ic*R1 = 500VR2 = Ie*R2 = 12 - 0.7Ic Ie R1/R2 = 500/(12 - 0.7)R2 = R1*11.3/500

DC Bias for V focus amp The AC gain is R1/R3 If the CRT requires 300vp-p for

Vfocus and the waveform generator output is 3vp-p then;R1/R3 = 300/3R3 = R1/100

There will be some thermal drift to the amplifier but it will be small enough to be ignored.

A special group of transistors have been designed for this high voltage low current application.

Horizontal modulation is sometimes done with this kind of amp. But the H freq. is limited by Ic and Cload.

Horizontal Component The horizontal parabola is

mixed with the vertical component.

This combination is level shifted in the Flyback focus assembly and applied to the CRT.

Due to the high voltages present attention to materials used and mechanical clearances must be observed.

The horizontal waveform is often 2 times the voltage of the vertical and comes from the horizontal section of the chassis.

Generation and modulation of the horizontal waveform will be covered in the section on Deflection.

Retrace Blanking Retract blanking is used to hide from view the scanning

beam during times when it is traveling from the end of one scan line to the start of the next or from the bottom of the screen to the top.

During these times the Video signal should be inactive. However this is not always true! New proposals are being offered to use this ‘dead’ time

to communicate information between the HOST and the Display.

Some of the proposals include White and Black Reference levels, Border information as well as remote display control commands.

Retrace blanking is needed to mask these signals from view.

Separate Blanking Amplifier Some Video amplifier

configurations require an additional amplifier to completely blank the image.

The inserted signal must be at least as large as the maximum video to insure a blank screen.

The rise and fall times must be close to that of the video rate. Otherwise the edge of the image may look soft or twinkle.

The source of the blanking signal can also cause jitter on the edges of the raster being blanked.

Newer designs blank the video amplifiers.

Output Amp Blanking Blanking may be inserted into the video before the output amp. By clamping the input of the output amp the output is forced as

positive (Black) as possible. Care must be taken to insure that the amp can handle this offset and

recover instantly.

Preamp Blanking Another option is to

switch the preamp to supply a Black or Blank level output during the Blanking interval

This had the added advantage of using this as a ‘pedestal’ to diode clamp the DC Cutoff voltage for each gun.

Black and white color tracking is enhanced by passing both through the Drive and Contrast sections of the preamp.

Horizontal Static Convergence

Horizontal Static Convergence

Horizontal Static Convergence H Stat amplifier to be adjusted by the micro-processor.

H stat, G2 and Vfocus amplifiers have similar topology.

Video Bias Reference The CRT cathode, G1 and G2

control the flow if electrons to the screen.

The voltages between these elements determine the flow.

Variations on any one of these will be seen.

G1 and G2 may be ‘static’ voltages but they must share a common reference with the video amp.

In this example of a simple video amp. The +80 v supply can have bad ripple and noise that will never be seen because of the common mode bypassing of G1 and G2.

Black Level Cathode Connection

G2Video Output

Amplifier

Black LevelAmplifier

G1R

Single point ground

Carbon Comp. Resistors

Black Level Amp The black level amp will

need an output range of Vcutoff +- 20% (~80v to ~120v).

The actual Black level at the cathodes will be this voltage – Blank insertion voltage.

This amp will need to be DC stable but adjustable.

To accommodate the three guns Ekco’s.

An amp much like G2’s but lower voltage.

END Notes:

Flash-Over Protection

DANGER30,000 volts

Do Not Touch!

Flash-Over Protection Flash-Over Current Soft-flash guns and tubes Limiting resistors Aqua-dag return path P C B Layout Spark Gap Tubes Protection Diodes Surge limiting Resistors Spark gap sockets

Flash-Over Current

Soft-Flash Tube

Soft Flash-Over Gun Structures

Most Tube manufacturers today use guns that incorporate ceramic limiting resistors between gun elements and the socket.

This impedance greatly reduces the surge currents in the G2, Focus and HCV Circuits.

Limiting the flashover in gun elements that are operated at on near the Anode voltage is critical.

Clean assembly techniques reduce the contaminates loose in the tube.

It is recommended that the CRT NEVER be shipped, stored, or used with the gun down.

Loose phosphor and other contamination can lodge between gun elements.

P C B Layout One my clients had a monitor

that lost vertical sync after a hard tube arc.

Their repair group replaced U2, the vertical sync buffer on the video board.

Their engineers had added resistors and diodes to the sync lines with minor effect.

Adding MOVs and small capacitors did not help. Nor did adding diodes on both sides of the input resistor.

Here is the schematic, as traced from the printed circuit board.

“A picture is worth a thousand words”.

Output Amplifier Protection

The most common and effective Flash-Over protection devices are Clamp Diodes and a special Spark Gap.

Most low cost high speed switching signal diodes have the speed and can handle the surge current with limiting by R1 of 33 to 100 ohms.

One of the best and lowest cost Spark Gap is made by MMC in Japan It is made like a metal film resistor except that the trimming laser cuts

a precise gap completely around the middle of the ceramic body. The lead and path lengths to supplies and Ark return MUST be SHORT!

Desired arc current flow is from PCB ground below the CRT connector through the shield and then through the braids and/or metal chassis to the CRT DAG and HVPS.

Desired Arc Current Return Paths

Braid

DAG Straps

DAG Coating

Anode

HV

PSChassis

Arc Protection System DesignGoal: Keep High Currents Away From Video Board Electronics

Arc Protection and the

Recommended Application Circuit Good arc protection is required for reliable operation

All products should be tested using a bench top tester similar to the figure below. With the Output Amp installed in the neck board for the testing, Apply the test voltage of 25 kV, it passes if no failures after 25 discharges to each channel.

Check the data sheet for recommended values for each device.

High Voltage

Power Supply

VariableTransformer

1 2

S

ACInput

R

C Output toUnit Under Test

50 Meg Ohms

2000 pFdc

High Voltage Source for Arc Testing

The Application Circuit

Device R1 (Ohms) L (nH) R2 (1/2W, Ohms)LM2409 110 0.82 100LM2407 91 0.56 33LM2405 91 0.22 33LM2415 91 0.39 33LM2403 100 0.12 33LM2413* ? ? ?LM2402 75 0.05 33LM2412* ? ? ?

Inductor L1 reduces the voltage stress on the outputs of the device during the initial High frequency ringing of the arc.

Resistor R1 reduces voltage stress and limits short circuit current that flows from the device while the spark gap is still active after the initial burst of the arc.

Diodes D1 and D2 reduce voltage stress on the outputs by clamping the the voltage to the Vcc supply and ground respectively.

Resistor R2 limits the current into the protection diodes and also limits short circuit current.

Capacitor C3 minimizes the voltage rise at high frequencies at the cathode of clamp diode D1.

Arc Protection and the

Recommended Application Circuit

RLC Network Transient Response

• Case 1- Overdamped (R/2L)2 > 1/LC

• Case 2 - Critically Damped (R/2L)2 = 1/LC

• Case 3 - Underdamped (R/2L)2 < 1/LC

R=100, 200, 300, 400

100

300

400

L=0.22 uHC=10 pf

C=8pf, 10pf, 12pf

R=200 OhmsL=0.22 uH

8pf

12pf

L=0.12, 0.22, 0.32

R=200 Ohms

L=0.22 uH

0.12

0.32

Determining the Output Network

Use a RL network as shown below to dial in the network. For a given L Value Adjust R for best/desired response Start with L’C resonant frequency, f=1/(2SQRT(LC)), at about

the -3dB BW of the Device L’ includes trace inductance on the PCB

After determining a good network, replace the R with a fixed value and retest.

END Notes:

High Voltage Arcs in CRT Monitors

RFI and EMI Shielding Filtering Minimizing

Shielding Shield around strong emitters Thickness and material Holes < 1/20 wavelength 1Ghz =~ 0.5” Bond seams < 0.5” Al or Cu for Electrical fields High Perm for Magnetic fields Signal returns inside coaxial shields Ground both ends to chassis Ferrite cores around cables Perrite cores in connector shells

Filtering Ferrite beads for decoupling and loss Filter caps Perrite* beads for signal leads and surge

suppression Filter G2, it couples to Rk and Gk at tube socket SHORT leads on all bypass caps!

*Green Tree Technology 1 612-473-3700

Impendence 1uF, 0.1uF, 10nF

Minimizing EMI Generate Less!

Use slowest rise times as possible Short Signal path lengths Close return paths

Keep it in the BOX Use shields and ground planes Common mode filters around cables

END Notes:

END Notes:

Color Balance and Tracking

Additive Properties of Light

Color Colors are rays of light, i.e. electromagnetic

waves with wavelengths between 380 nm and 780 nm.

We perceive them with our eyes and our brain translates them into what we call "colors". In other words: colors are products of our brain.

This means that one person may perceive colors slightly differently from another.

Color To display colors, monitors use what is

called "additive color mixing", using red, green and blue light. If we mix red, green and blue light together, we get white light.

When white is required on the screen, three electron guns hit the red, green and blue dots, or different shapes, of phosphor that coat the inside of the screen, which in turn glow together and produce white light.

Color Metrics The idea of tri-receptor

vision was worked out far before the physical mechanism of retinal pigments was understood.

A common diagram for describing human color perception was developed by the International Commission on Illumination (CIE).

The CIE diagram is an attempt to precisely quantify the tri-receptor nature of human vision.

Color Balance Color Temperature is controlled by the relative

level of intensity of Red, Green and Blue light generated.

These in turn are controlled by the relative voltage of the three cathodes verses G1.

Each Cathode has its own unique characteristics.

The specification sheets give data that has been gathered from many tubes and represents the limits for that type of tube.

The Video section must adjust to accommodate the entire range of values.

White Point A popular way of defining a

color is by color temperature. Max Planck established a

scale of color from heated material.

When a black body is heated to a high temperature it begins to emit light.

The higher the temperature the brighter and bluer the color.

The Planchian Locus of Color temperatures are plotted on this CIE chart.

Color Tracking White temperature

Three Drive (gain) curves Black level and color temperature

Three Cutoff voltages Hue Saturation

Ik verses Ek

80 70 60 50 4085Ek Volts Eg1 = 0

IkuA

100

500

200

20

10

50

IkuA

1

5

2

0.2

0.1

0.5

Color Tracking For accurate Color at both

highlight and lowlight points both the cutoff and drive must be set.

If the Color temperature is to be changed all 6 of these values must be changed.

As seen previously all three guns behave differently.

The start and end range for each Cathode will depend on the desired Temperature.

10v 20v 40v 80v

Black Color and Ekco Each gun has a different Ekco. The three guns share a common G1. Ekco must be set on each cathode. The ratio of highest to lowest is given in the data sheet

but generally runs about 20%.

Black Color and Ekco The designer could set a range of as small as 25% and then

adjust the most remote Cathode for cutoff at the highest voltage. This would cause the cathodes tp be at random settings from

tube to tube. As we shown that the spot size depends more on Ekco. I like to set the most visible color (green) to the design Ekco and

then set the other colors. This causes a greater range for the Ek amps of +- 20%Ek.

DC Setting Ekco The simplest DC setter is to

clip the blackest part of the video against a diode to the Ek bias amp.

The AC coupled Video must be returned to the Ek supply to keep the DC up against the Diode.

There is one problem with this approach. If the video black/blank level is not the same, in the case of Sync on Green the black color will shift to Green in that case.

The preamp must insert true blank level.

Ratio of cathode current

END Notes:

Other Considerations

Circuit components they won’t show you in the

‘SCHEMATIC’And Other Tricks

Topics P C B Effects Stray Capacitance Transmission Lines Oscilloscope Probes Handy Tips

P C B E f f e c t s As video speeds get into the RF region

the printed circuit board becomes a major part of the design.

PCB traces add Capacitance, Inductance, Delay and possible Reflections to signals.

For the formulas below we define a PCB as folows:

Trace width W and thickness T and PCB thickness is H.

The dielectric coefficient ( d ) of the insulating material. 2 < d <6.

d ≈ FR4 fiberglass = 4.7d ≈ G-10 fiberglass = 5.0 to 5.3

T ≈ 0.0015” for 1 oz. Cu orT ≈ 0.0030” for 2 oz. Cu

H ≈ 0.062” or 0.031”

P C B E f f e c t s

TWHLn

dCo

8.098.5

14.167.0

TWHLnLo

8.098.5

67.0475.0017.1 dtpd

 

CoClincrease 1

The above example is for a trace without capacitive loading. If the far end of the trace is connected to a transistor with capacitance then the delay is larger.Cl= load cap. Co= cap. of trace

nS/foot

DelayIt is of interest that the propagation delay of the line is dependent only on the dielectric constant and is not a function of the width or spacing.1.3ns/foot

nH/inch

Trace inductanceAt just above the video rate inductors and even transformers are routinely built using PCB traces! Typically 9 to 10nH per inch

pF/inch

Trace CapacitancePrinted circuit trace capacitance runs around 1.67pF per inch. It is very typical to find 1 to 3pF on most nodes. 1.67pF/inch

P C B E f f e c t s

ohms

Strip line ImpedanceImpedance of a strip line. If a 75Ω video needs to move across a PCB with out reflections then a strip line should be used. 75 Ω ≈ 0.062” board, one ounce Cu, 0.100 trace width50 Ω ≈ 0.062” board, one ounce Cu, 0.049 trace width

Delay with LoadingIf a trace having a delay of 1.77ns/ft and a length of 2 inches having a capacitance of 3.5pF, is terminated in a load of 2pF the resulting delay is 2.21ns/ft.

CoCldtpd 167.0475.0017.1

ftnsftnstpd /21.25.3

21/77.1

TWHLn

dZo

8.098.5

41.187

Transmission Lines A transmission line (either coaxial

or as a strip line on a PCB) is a way of moving a signal over a distance with minimum distortion.

The maximum distance that a signal should be moved without a transmission line is the rise time of the signal divided by twice the propagation delay of the line.

A video signal with a rise time of 1.7ns should use a transmission line to travel over 6 inches.

Video boards rarely have any traces 6 inches long. With that logic it looks like we will never need to make a strip line.

tdtrL

2max

Transmission Lines Wrong! The video DAC in the

computer is at least three feet away from the video board in the monitor.

The transmission line should not end at the back of the monitor.

It should not end at the edge of the video board.

It must continue with 75 ohms impedance across the PCB right up to the video pre-amp to the termination resistor.

In high speed strip lines, the shape of the trace is important.

The impedance of a sharp corners causes as much as 7.5% reflection.

It is a good idea to have smooth, rounded lines and constant line widths.

P C B Capacitance It was found that there is a large

amount of capacitance between power resistor R1 and the ground plane.

The ground is removed under the resistor to reduce the stray capacitance.

Resistor R1 is the output of the video amplifier. It has also been found that during tube ark conditions the voltage on R1 gets very high. A spark jumps through the side of R1 into the ground plane cracking the resistor’s insulation. The spark builds up a path of carbon leaving a conductive path from R1 to ground. If voltages can exceed 1,000 volts then it is a good idea to clear out the ground plane under carbon film and metal film resistors!

P C B Capacitance In this printed circuit board example the

dark color is the topside ground plane. The light color trace is the bottom side traces.

There is a signal that passes through R2, C2, D3, D4 and R4.

The ground plane has been cleared out around the traces in an attempt to reduce trace capacitance.

Care must be taken to keep the ground plane intact.

The second example shows a ground planes with a crosshatch pattern in the sensitive area. In this example the trace to ground capacitance is reduced to 50%. 0.007” traces set at 0.020” spacing that should have resulted in a 58% fill. The fill factor is typically 50%. Any other amounts of fill can be used (25% or 10%).

Oscilloscope Probes A typical oscilloscope probe

may have 10M resistance in parallel with 12pF capacitance.

That is 79 at 200MHz! The tube cathode capacitance

is another 12pF. Stray capacitance found in the

tube socket, arc protection circuitry, black level clamp, D.C. bias amp, wires and traces could add another 10pF.

The nominal loading of the video amplifier is 22pF.

With the probe the loading increases to 34pF.

The amplifier will not operate the same!

With a 30 volt peak to peak signal there may be as much as ¼ amp of current in the oscilloscope probe depending on the rise time of the signal! Choose your probe wisely.

Oscilloscope Probes The 12pf is the problem so we must

find a way of looking at the video signal without adding capacitance.

There are available several low capacitance probes.

They come in two types; active and passive.

The active probe I use has a built in amplifier. The problem is that it can not handle high voltage signals.

All probes have a problem handling high voltage signals.

When you mix high voltage and high frequency the probe’s A.C. plus D.C. voltage can easily be exceeded.

The passive probe is 1pf and 5000 ohms.

It works very will if the amplifier can handle the 5000 ohms load.

This probe can not handle more than 70 volts D.C. it typically needs to be A.C. coupled to look directly at the cathode of a CRT!

Home Made Probes Passive low capacitance probes can

easy be built. The oscilloscope must be switched to

50 ohms internal terminating impedance.

Use a short length of good 50 coax. On the end of the coax solder a 4950

or 450 ohm resistor. Any divider ratio can be used.

Watch the power rating on the resistor.

It is not uncommon to find 100 volts of bias on the cathode of a CRT tube.

You may want to A.C. couple the probe by adding a D.C. blocking capacitor in series with the 4950 resistor.

Keep all lead lengths s h o r t !!!!

Keep the ground lead length very short! Use non-inductive resistors!

Home Made Probes Many resistors will have a small

amount of stray capacitance that forms a peaking capacitor.

You may need to add the tiniest amount of peaking capacitor. Amateur Radio Operators commonly build high voltage sub-pf capacitors by twisting together insulated wire.

The first resistor in the photo has added three twists of insulated wire held together with clear heat shrink tubing. The wire can be cut shorter to reduce the capacitance. Twisting the wire tighter will increase the capacitance.

The second resistor has the two wires run in parallel inside clear tubing.

The wires can be pulled back to reduce the capacitance.

Home Made Probes 100 To get a low voltage version of

what is happening at the output of a video amplifier solder a 100:1 divider onto the amplifier.

Remember the leads must be very short.

To measure the rise and fall time of the amplifier an A.C. only divider can be made with two capacitors.

If the D.C. level is important then resistors must be added.

The 10M/12pf is the oscilloscope probe.

C1,C2,R1 and R2 are soldered onto the amplifier.

Handy Tips Sniff the Video (no direct connection)

Another option for viewing the output of a video amplifier without adding substantial loading;Wrap the cathode wire around the scope probe.

The wire to probe capacitance is only a fraction of a pico-Farad. The results is a divide by many thousand. The gain of the scope will have to be turned up all the way. The probe is A.C. coupled! No low-frequency effects can be seen.

This method is only good for looking at edges! Trise / Tfall

Handy Tips Proper Probe Use

When using high frequency probes their connection to the circuit is critical to accurate measurements

First remove the protective plastic “CLIP” cover. Make or buy small loops of bare wire (or spring wire) that hold the

probe in place and ground the probe. The ground ring on the probe makes an excellent connection

point.

BEWARE the probe tip ground may short the point being measured. BYE - BUY Amplifier!!!

Handy Tips Ground Lead First throw away the alligator clip ground

wire! A 10X oscilloscope probe that adds 10pF and

10M which might not cause trouble. A six inch ground lead has about 700nH of

inductance. You have just inserted a LC resonant circuit

into your amplifier (or should I say your oscillator)!

If the point you are looking at has fast edges it will cause the LC to ring.

The oscilloscope will see ring that is not there. Below is the frequency and phase response of a 10X probe with a 6” ground lead.

The actual ring may be different than this spice model. The ground lead is more complicated than a simple inductor and the capacitance is also complex. Move the path of the ground lead an inch and the look of the ring will change.

Handy Tips Ground Lead Here is the frequency and phase response of a 10X probe with a 6” ground lead.

Would you use a probe with this frequency response? Have you seen this ring before?

Handy Tips Without a Probe Do not trust your oscilloscope! The

important thing is to trust your eyes. That is how the end customer will judge the

product. The output of a video amplifier that drives a

CRT tube has a high level for black and a low level for white.

Normally the grid bias is set so that the high level makes black. 10% video ringing is almost impossible to see.

To see the details of the video set the bias very black; so just the tips of video will be beyond the black level.

Turn the room lights are turned out to see the imperfections in the video level.

In the above example; ringing becomes apparent.

Notice the slope to the video. It shows a low frequency response problem.

END Notes: