Communication Systems Assignment

TOPIC: Receiver performance measures

Submitted By

Akshay kumar.V

12BEI0001

Slot : A2

Contents

Selectivity: .................................................................................................................................................... 3

Sensitivity ..................................................................................................................................................... 4

Image frequency Rejection: ......................................................................................................................... 6

Fidelity: ......................................................................................................................................................... 8

Intermediate frequencies: ........................................................................................................................... 8

Frequency Stability: .................................................................................................................................... 10

Signal to Noise Ratio (SNR): ....................................................................................................................... 11

Concept of signal to noise ratio SNR ........................................................................................... 11

Signal to noise ratio formula ........................................................................................................... 12

Effect of bandwidth on SNR ............................................................................................................ 12

Signal to noise ratio specifications ............................................................................................... 13

Points to note when measuring signal to noise ratio ............................................................... 13

Bandwidth: ................................................................................................................................................. 14

Ideal receiver selectivity: ........................................................................................................................... 14

Amplitude Limiter: ................................................................................................................................. 19

Dynamic range: ........................................................................................................................................... 20

Selectivity:

Selectivity is a measure of the performance of a radio receiver to respond only

to the radio signal it is tuned to (such as a radio station) and reject other signals

nearby infrequency, such as another broadcast on an adjacent channel.

Selectivity is usually measured as a ratio in decibels (dBs), comparing the signal

strength received against that of a similar signal on another frequency. If the

signal is at the adjacent channel of the selected signal, this measurement is also

known as adjacent-channel rejection ratio (ACRR).

Selectivity also provides some immunity to blanketing interference.

LC circuits are often used as filters; the Q ("Quality" factor) determines

the bandwidth of each LC tuned circuit in the radio. The L/C ratio, in turn,

determines their Q and so their selectivity, because the rest of the circuit - the

aerial or amplifier feeding the tuned circuit for example - will contain present

resistance. For a series resonant circuit, the higher the inductance and the lower

the capacitance, the narrower the filter bandwidth (meaning the reactance of the

inductance, L, and the capacitance, C, at resonant frequency will be relatively

high compared with the series source/load resistances). For a parallel resonant

circuit the opposite applies; small inductances reduce the damping of external

circuitry.

There are practical limits to the increase in selectivity with changing L/C ratio:

tuning capacitors of large values can be difficult to construct

stray capacitance, and capacitance within the transistors or valves of

associated circuitry, may become significant (and vary with time)

the series resistance internal to the wire in the coil, may be significant (for

parallel tuned circuits especially)

Large inductances imply physically large (and expensive coils) and/or thinner

wire (hence worse internal resistance).

Therefore other methods may be used to increase selectivity, such as Q

multiplier circuits and regenerative receivers. Super heterodyne receivers allow

use one or more fixed intermediate tuned circuits for selectivity. Fixed tuning

eliminates the requirement that multiple tuning stages accurately match while

being adjusted.

Sensitivity

Receiver sensitivity or RF sensitivity is one of the key specifications of any radio receiver whether it is used for Wi-Fi, cellular telecommunications broadcast or any other form of wireless communications.

The ability of the radio receiver to pick up the required level of radio signals will enable it to operate more effectively within its application.

The two main requirements of any radio receiver are that it should be able to separate one station from another, i.e. selectivity, and signals should be amplified so that they can be brought to a sufficient level to be heard. As a result receiver designers battle with many elements to make sure that these requirements are fulfilled

Methods of specifying sensitivity performance

As the RF sensitivity performance of any receiver is of paramount importance it is necessary to be able to specify it in a meaningful way. A number of methods and figures of merit are used dependent upon the application envisaged:

Signal to noise ratio: This is a straightforward comparison ratio of a given signal level to the noise within the system.

SINAD: This receiver sensitivity measurement is slightly more formalised, and it also includes distortion as well as the noise.

Noise factor : This RF receiver measurement compares the noise added by a unit - this could be an amplifier or other unit within the system or it could be a complete receiver.

Noise figure: The noise figure, or NF of a unit or system is the logarithmic version of the noise factor. It is widely used for specifications of sensitivity and noise performance of a receiver, element within a system, or the whole system.

Carrier to noise ratio, CNR: The carrier-to-noise ratio is the signal-to-noise ratio (SNR) of a modulated signal. This term is less widely used than SNR, but may be used when there is a need to distinguish between the performance with regards to the radio frequency pass-band signal and the analogue base band message signal after demodulation.

Minimum discernable signal, MDS: The Minimum detectable or minimum discernable signal is the smallest signal level that can be detected by a radio receiver, i.e. one that can be processed by its analogue and digital signal chain and demodulated by the receiver to provide usable information at the output.

Error vector magnitude, EVM: Error vector magnitude, EVM is a measure that can be used to quantify the performance of a digital radio transmitter or receiver. There various points on the constellation diagram set to identify various digital states. In an ideal link, the transmitter should generate the digital data such that it falls as close to these points as possible - the link should not degrade the signal such that the actual received data does not fall onto these points, and the receiver should also not degrade these positions. In reality, noise enters the system and the received data does not fall exactly onto these positions. The error vector magnitude is a measure of how far from the ideal positions the actual received data elements are. Some times EVM may also be known as the Receive Constellation Error, RCE

Bit error rate, BER: Bit error rate is a form of measurement used for digital systems. As the signal level falls or the link quality degrades, so the number of errors in the transmission - bit errors - increases. Measuring the bit error rate gives an indication of the signal to noise ratio, but in a format that is often more useful for the digital domain.

All the receiver sensitivity specification methods use the fact that the limiting factor of the sensitivity of a radio receiver is not the level of amplification available, but the levels of noise that are present, whether they are generated within the radio receiver or outside.

Image frequency Rejection:

An image frequency is an undesired input frequency equal to the station

frequency plus twice the intermediate frequency. The image frequency results in

two stations being received at the same time, thus producing interference. Image

frequencies can be eliminated by sufficient attenuation on the incoming signal by

the RF amplifier filter of the super-heterodyne receiver.

For example, an AM broadcast station at 580 kHz is tuned on a receiver with

a 455 kHz IF. The local oscillator is tuned to 580 + 455 = 1035 kHz. But a

signal at580 + 455 + 455 = 1490 kHz is also 455 kHz away from the local

oscillator; so both the desired signal and the image, when mixed with the local

oscillator, will also appear at the intermediate frequency. This image

frequency is within the AM broadcast band. Practical receivers have a tuning

stage before the converter, to greatly reduce the amplitude of image

frequency signals; additionally, broadcasting stations in the same area have

their frequencies assigned to avoid such images.

The unwanted frequency is called the image of the wanted frequency,

because it is the "mirror image" of the desired frequency reflected . A

receiver with inadequate filtering at its input will pick up signals at two different

frequencies simultaneously: the desired frequency and the image frequency.

Any noise or random radio station at the image frequency can interfere with

reception of the desired signal.

Early Autodyne receivers typically used IFs of only 150 kHz or so, as it was

difficult to maintain reliable oscillation if higher frequencies were used. As a

consequence, most Autodyne receivers needed quite elaborate antenna

tuning networks, often involving double-tuned coils, to avoid image

interference. Later superhets used tubes especially designed for

oscillator/mixer use, which were able to work reliably with much higher IFs,

reducing the problem of image interference and so allowing simpler and

cheaper aerial tuning circuitry.

Sensitivity to the image frequency can be minimised only by (1) a filter that

precedes the mixer or (2) a more complex mixer circuit [1] that suppresses

the image. In most receivers this is accomplished by a bandpass filter in

the RF front end. In many tunable receivers, the bandpass filter is tuned in

tandem with the local oscillator.

Image rejection is an important factor in choosing the intermediate frequency

of a receiver. The farther apart the bandpass frequency and the image

frequency are, the more the bandpass filter will attenuate any interfering

image signal. Since the frequency separation between the bandpass and the

image frequency is , a higher intermediate frequency improves image

rejection. It may be possible to use a high enough first IF that a fixed-tuned

RF stage can reject any image signals.

The ability of a receiver to reject interfering signals at the image frequency is

measured by the image rejection ratio. This is the ratio (in decibels) of the

output of the receiver from a signal at the received frequency, to its output for

an equal-strength signal at the image frequency.

Fidelity:

The fidelity of a receiver is its ability to accurately reproduce, in its output, the

signal that appears at its input. You will usually find the broader the band passed

by frequency selection circuits, the greater your fidelity. You may measure fidelity

by modulating an input frequency with a series of audio frequencies; you then

plot the output measurements at each step against the audio input frequencies.

The resulting curve will show the limits of reproduction.

You should remember that good selectivity requires that a receiver pass a narrow

frequency band. Good fidelity requires that the receiver pass a broader band to

amplify the outermost frequencies of the side bands. Receivers you find in

general use are a compromise between good selectivity and high fidelity.

Intermediate frequencies:

In communications and electronic engineering, an intermediate frequency (IF) is

a frequency to which a carrier frequency is shifted as an intermediate step

in transmission or reception.[1] The intermediate frequency is created by mixing the

carrier signal with a local oscillator signal in a process called heterodyning, resulting in a

signal at the difference or beat frequency. Intermediate frequencies are used

in superheterodyne radio receivers, in which an incoming signal is shifted to an IF

for amplification before final detection is done.

Conversion to an intermediate frequency is useful for several reasons. When several

stages of filters are used, they can all be set to a fixed frequency, which makes them

easier to build and to tune. Lower frequency transistors generally have higher gains so

fewer stages are required. It's easier to make sharply selective filters at lower fixed

frequencies.

There may be several such stages of intermediate frequency in a superheterodyne

receiver; two or three stages are called double or tripleconversion.

Intermediate frequencies are used for three general reasons. At very high (gigahertz)

frequencies, signal processing circuitry performs poorly. Active devices such

as transistorscannot deliver much amplification (gain).[1][2] Ordinary circuits

using capacitors and inductors must be replaced with cumbersome high frequency

techniques such as striplinesand waveguides. So a high frequency signal is converted

to a lower IF for more convenient processing. For example, in satellite dishes, the

microwave downlink signal received by the dish is converted to a much lower IF at the

dish, to allow a relatively inexpensive coaxial cable to carry the signal to the receiver

inside the building. Bringing the signal in at the original microwave frequency would

require an expensive waveguide.

A second reason, in receivers that can be tuned to different frequencies, is to convert

the various different frequencies of the stations to a common frequency for processing.

It is difficult to build multistage amplifiers, filters, and detectors that can have all stages

track in tuning different frequencies, but it is comparatively easy to build

tunable oscillators. Superheterodyne receivers tune in different frequencies by adjusting

the frequency of the local oscillator on the input stage, and all processing after that is

done at the same fixed frequency, the IF. Without using an IF, all the complicated filters

and detectors in a radio or television would have to be tuned in unison each time the

frequency was changed, as was necessary in the early tuned radio frequency receivers.

The main reason for using an intermediate frequency is to improve

frequency selectivity.[1] In communication circuits, a very common task is to separate out

or extract signals or components of a signal that are close together in frequency. This is

called filtering. Some examples are, picking up a radio station among several that are

close in frequency, or extracting the chrominance subcarrier from a TV signal. With all

known filtering techniques the filter's bandwidth increases proportionately with the

frequency. So a narrower bandwidth and more selectivity can be achieved by converting

the signal to a lower IF and performing the filtering at that frequency.

Perhaps the most commonly used intermediate frequencies for broadcast receivers are

around 455 kHz for AM receivers and 10.7 MHz for FM receivers. In special purpose

receivers other frequencies can be used. A dual-conversion receiver may have two

intermediate frequencies, a higher one to improve image rejection and a second, lower

one, for desired selectivity. A first intermediate frequency may even be higher than the

input signal, so that all undesired responses can be easily filtered out by a fixed-tuned

RF stage.[3]

In a digital receiver, the analog to digital converter (ADC) operates at low sampling

rates, so input RF must be mixed down to IF to be processed. Intermediate frequency

tends to be lower frequency range compared to the transmitted RF frequency. However,

the choices for the IF are most dependent on the available components such as mixer,

filters, amplifiers and others that can operate at lower frequency. There are other factors

involved in deciding the IF frequency, because lower IF is susceptible to noise and

higher IF can cause clock jitters.

Modern satellite television receivers use several intermediate frequencies.[4] The 500

television channels of a typical system are transmitted from the satellite to subscribers

in theKu microwave band, in two subbands of 10.7 - 11.7 and 11.7 - 12.75 GHz. The

downlink signal is received by a satellite dish. In the box at the focus of the dish, called

a low-noise block downconverter (LNB), each block of frequencies is converted to the IF

range of 950 - 2150 MHz by two fixed frequency local oscillators at 9.75 and 10.6 GHz.

One of the two blocks is selected by a control signal from the set top box inside, which

switches on one of the local oscillators. This IF is carried into the building to the

television receiver on a coaxial cable. At the cable company's set top box, the signal is

converted to a lower IF of 480 MHz for filtering, by a variable frequency oscillator.[4] This

is sent through a 30 MHz bandpass filter, which selects the signal from one of

the transponders on the satellite, which carries several channels. Further processing

selects the channel desired, demodulates it and sends the signal to the television.

Frequency Stability:

Frequency drift is an unintended and generally arbitrary offset of

an oscillator from its nominal frequency. Causes may include component

aging,[1] changes in temperature that alter the piezoelectric effect in a crystal

oscillator, or problems with a voltage regulator which controls the bias voltage to

the oscillator. Frequency drift is traditionally measured in Hz/s. Frequency

stability can be regarded as the absence (or a very low level) of frequency drift.

On a radio transmitter, frequency drift can cause a radio station to drift into

an adjacent channel, causing illegal interference. Because of this, Frequency

allocation regulations specify the allowed tolerance for such oscillators in a type-

accepted device. A temperature-compensated, voltage-controlled crystal

oscillator (TCVCXO) is normally used for frequency modulation.

On the receiver side, frequency drift was mainly a problem in early tuners,

particularly for analog dial tuning, and especially on FM, which exhibits a capture

effect. However, the use of a phase-locked loop (PLL) essentially eliminates the

drift issue. For transmitters, a numerically controlled oscillator (NCO) also does

not have problems with drift.

Drift differs from Doppler shift, which is a perceived difference in frequency due

to motion of the source or receiver, even though the source is still producing the

same wavelength. It also differs from frequency deviation, which is the inherent

and necessary result of modulation in both FM and phase modulation.

Signal to Noise Ratio (SNR): The noise performance and hence the signal to noise ratio is a key parameter for any radio receiver. The signal to noise ratio, or SNR as it is often termed is a measure of the sensitivity performance of a receiver. This is of prime importance in all applications from simple broadcast receivers to those used in cellular or wireless communications as well as in fixed or mobile radio communications, two way radio communications systems, satellite radio and more.

There are a number of ways in which the noise performance, and hence the sensitivity of a radio receiver can be measured. The most obvious method is to compare the signal and noise levels for a known signal level, i.e. the signal to noise (S/N) ratio or SNR. Obviously the greater the difference between the signal and the unwanted noise, i.e. the greater the S/N ratio or SNR, the better the radio receiver sensitivity performance.

As with any sensitivity measurement, the performance of the overall radio receiver is determined by the performance of the front end RF amplifier stage. Any noise introduced by the first RF amplifier will be added to the signal and amplified by subsequent amplifiers in the receiver. As the noise introduced by the first RF amplifier will be amplified the most, this RF amplifier becomes the most critical in terms of radio receiver sensitivity performance. Thus the first amplifier of any radio receiver should be a low noise amplifier.

Concept of signal to noise ratio SNR

Although there are many ways of measuring the sensitivity performance of a radio receiver, the S/N ratio or SNR is one of the most straightforward and it is used in a variety of applications. However it has a number of limitations, and although it is widely used, other methods including noise figure are often used as well. Nevertheless the S/N ratio or SNR is an important specification, and is widely used as a measure of receiver sensitivity

Signal to noise ratio for a radio receiver

The difference is normally shown as a ratio between the signal and the noise (S/N) and it is normally expressed in decibels. As the signal input level obviously has an effect on this ratio, the input signal level must be given. This is usually expressed in microvolts. Typically a certain input level required to give a 10 dB signal to noise ratio is specified.

Signal to noise ratio formula

The signal to noise ratio is the ratio between the wanted signal and the unwanted background noise.

It is more usual to see a signal to noise ratio expressed in a logarithmic basis using decibels:

If all levels are expressed in decibels, then the formula can be simplified to:

The power levels may be expressed in levels such as dBm (decibels relative to a milliwatt, or to some other standard by which the levels can be compared.

Effect of bandwidth on SNR

A number of other factors apart from the basic performance of the set can affect the signal to noise ratio, SNR specification. The first is the actual bandwidth of the receiver. As the noise spreads out over all frequencies it is found that the wider the bandwidth of the receiver, the greater the level of the noise. Accordingly the receiver bandwidth needs to be stated.

Additionally it is found that when using AM the level of modulation has an effect. The greater the level of modulation, the higher the audio output from the receiver. When measuring the noise performance the audio output from the receiver is measured and accordingly the modulation level of the AM has an effect. Usually a modulation level of 30% is chosen for this measurement.

Signal to noise ratio specifications

This method of measuring the performance is most commonly used for HF communications receivers. Typically one might expect to see a figure in the region of 0.5 microvolts for a 10 dB S/N in a 3 kHz bandwidth for SSB or Morse. For AM a figure of 1.5 microvolts for a 10 dB S/N in a 6 kHz bandwidth at 30% modulation for AM might be seen.

Points to note when measuring signal to noise ratio

SNR is a very convenient method of quantifying the sensitivity of a receiver, but there are some points to note when interpreting and measuring signal to noise ratio. To investigate these it is necessary to look at the way the measurements of signal to noise ratio, SNR are made. A calibrated RF signal generator is used as a signal source for the receiver. It must have an accurate method of setting the output level down to very low signal levels. Then at the output of the receiver a true RMS AC voltmeter is used to measure the output level.

S/N and (S+N)/N When measuring signal to noise ratio there are two basic elements to the measurement. One is the noise level and the other is the signal. As a result of the way measurements are made, often the signal measurement also includes noise as well, i.e. it is a signal plus noise measurement. This is not normally too much of a problem because the signal level is assumed to be much larger than the noise. In view of this some receiver manufacturers will specify a slightly different ratio: namely signal plus noise to noise (S+N/N). In practice the difference is not large, but the S+N/N ratio is more correct.

PD and EMF Occasionally the signal generator level in the specification will mention that it is either PD or EMF. This is actually very important because there is a factor of 2:1 between the two levels. For example 1 microvolt EMF. and 0.5 microvolt PD are the same. The EMF (electro-motive force) is the open circuit voltage, whereas the PD (potential difference) is measured when the generator is loaded. As a result of the way in which the generator level circuitry works it assumes that a correct (50 Ohm) load has

been applied. If the load is not this value then there will be an error. Despite this most equipment will assume values in PD unless otherwise stated.

While there are many parameters that are used for specifying the sensitivity performance of radio receivers, the signal to noise ratio is one of the most basic and easy to comprehend. It is therefore widely used for many radio receivers used in applications ranging from broadcast reception to fixed or mobile radio communications.

Bandwidth:

Bandwidth is the difference between the upper and lower frequencies in a

continuous set of frequencies. It is typically measured in hertz, and may

sometimes refer to passband bandwidth, sometimes to baseband bandwidth,

depending on context. Passband bandwidth is the difference between the

upper and lower cutoff frequencies of, for example, a bandpass filter,

a communication channel, or a signal spectrum. In the case of a low-pass

filter or baseband signal, the bandwidth is equal to its upper cutoff frequency.

Bandwidth in hertz is a central concept in many fields,

including electronics, information theory, digital communications, radio

communications, signal processing, and spectroscopy and is one of the

determinants of the capacity of a given communication channel.

A key characteristic of bandwidth is that any band of a given width can carry the

same amount of information, regardless of where that band is located in

the frequency spectrum. For example, a 3 kHz band can carry a telephone

conversation whether that band is at baseband (as in a POTS telephone line)

or modulated to some higher frequency.

Ideal receiver selectivity:

Selectivity is one of the major specifications of any receiver. Whilst the sensitivity is important to ensure that it can pick up the signals and receive them at a sufficient strength, the selectivity is also very important. It is this parameter that determines whether the receiver is able to pick out the wanted signal from all the other ones around it. The filters used in receivers these days have very high levels of performance and enable receivers to select out individual signals even on today's crowded bands.

Superhet principle

Most of the receivers that are used today are superhet radios. In these sets the incoming signal is converted down to a fixed intermediate frequency. It is within the IF stages that the main filters are to be found. It is the filter in the IF stages that defines the selectivity performance of the whole set, and as a result the receiver selectivity specification is virtually that of the filter itself.

Figure 1 Block diagram of a basic superhet receiver

In some receivers simple LC filters may be used, although ceramic filters are better and are used more widely nowadays. For the highest performance crystal or mechanical filters may be used, although they are naturally more costly and this means they are only found in high performance sets.

Filter parameters

There are two main areas of interest for a filter, the pass band where it accepts signals and allows them through, and the stop band where it rejects them. In an ideal world a filter would have a response something like that shown in Figure 2. Here it can be seen that there is an immediate transition between the pass band and the stop band. Also in the pass band the filter does not introduce any loss and in the stop band no signal is allowed through.

Figure 2 The response of an ideal filter

In reality it is not possible to realise a filter with these characteristics and a typical response more like that shown in Figure 3. It is fairly obvious from the diagram that there are a number of differences. The first is that there is some loss in the pass band. Secondly the response does not fall away infinitely fast. Thirdly the stop band attenuation is not infinite, even though it is very large. Finally it will be noticed that there is some in band ripple.

Figure 3 Typical response of a real filter

In most filters the attenuation in the pass band is normally relatively small. For a typical crystal filter figures of 2 - 3 dB are fairly typical. However it is found that very narrow band filters like those used for Morse reception may be higher than this. Fortunately it is quite easy to counteract this loss simply by adding a little extra amplification in the intermediate frequency stages and this factor is not quoted as part of the receiver specification.

It can be seen that the filter response does not fall away infinitely fast, and it is necessary to define the points between which the pass band lies. For receivers

the pass band is taken to be the bandwidth between the points where the response has fallen by 6 dB, i.e. where it is 6 dB down or -6 dB.

A stop band is also defined. For most receiver filters this is taken to start at the point where the response has fallen by 60 dB, although the specification for the filter should be checked this as some filters may not be as good. Sometimes a filter may have the stop band defined for a 50 dB attenuation rather than 60 dB.

Shape factor

It can be seen that it is very important for the filter to achieve its final level of rejection as quickly as possible once outside the pass band. In other words the response should fall as quickly as possible. To put a measure on this, a figure known as the shape factor is used. This is simply a ratio of the bandwidths of the pass band and the stop band. Thus a filter with a pass band of 3 kHz at -6dB and a figure of 6 kHz at -60 dB for the stop band would have a shape factor of 2:1. For this figure to have real meaning the two attenuation figures should also be quoted. As a result the full shape factor specification should be 2:1 at 6/60 dB.

Filter types

There is a variety of different types of filter that can be used in a receiver. The older broadcast sets used LC filters. The IF transformers in the receiver were tuned and it was possible to adjust the resonant frequency of each transformer using an adjustable ferrite core.

Today ceramic filters are more widely used. Their operation is based on the piezoelectric effect. The incoming electrical signal is converted into mechanical vibrations by the piezoelectric effect. These vibrations are then affected by the mechanical resonances of the ceramic crystal. As the mechanical vibrations are then linked back to the electric signal, the overall effect is that the mechanical resonances of the ceramic crystal affect the electrical signal. The mechanical resonances of the ceramic exhibit a high level of Q and this is reflected in its performance as an electrical filter. In this way a high Q filter can be manufactured very easily.

Ceramic filters can be very cheap, some costing only a few cents. However higher performance ones are also available, and these are likely to be found in scanners and many other receivers.

For really high levels of filter performance crystal filters are used. Crystals are made from quartz, a naturally occurring form of silicon, although today's components are made from synthetically grown quartz. These crystals also use

the piezoelectric effect and operate in the same way as ceramic filters but they exhibit much higher levels of Q and offer far superior degrees of selectivity. Being a resonant element they are used in many areas where an LC resonant element might be found. They are used in oscillators - many computers have crystal oscillators in them, but they are also widely used in high performance filters.

Normally crystal filters are made from a number of individual crystals. The most commonly used configuration is called the half lattice filter as shown in Figure 4. Further sections can be added to the filter to improve the performance. Often a filter will be quoted as having a certain number of poles. There is one pole per crystal, so a six pole crystal filter would contain six crystals and so forth. Many filters used in amateur communications receivers will contain either six or eight poles.

Figure 4 A basic half lattice crystal filter section

Choosing the right bandwidth

It is important to choose the correct bandwidth for a give type of signal. It is obviously necessary to ensure that it is not too wide, otherwise unwanted off-channel signals will be able to pass though the filter. Conversely if the filter is too narrow then some of the wanted signal will be rejected and distortion will occur. As different types of transmission occupy different amounts of spectrum bandwidth it is necessary to tailor the filter bandwidth to the type of transmission being received. As a result many receivers switch in different filters for different types of transmission. This may be done either automatically as part of a mode switch, or using a separate filter switch. Typically a filter for AM reception on the short wave bands will have a bandwidth of around 6 kHz, and one for SSB will be

approximately 2.5 kHz. For Morse reception 500 and 250 Hz filters are often used.

Summary

Selectivity is particularly important on today's crowded bands, and it is necessary to ensure that any receiver is able to select the wanted signal as well as it can. Obviously when signals occupy the same frequency there is little that can be done, but by having a good filter it is possible to ensure that you have the best chance or receiving and being able to copy the signal you want.

Amplitude Limiter: Amplitude Limiters and FM Thresholding The vast majority of terrestrial FM radio communications systems use conventional noncoherent demodulation because most standard frequency discriminators use envelope detection to remove the intelligence from the FM waveform. Unfortunately, envelope detectors (including ratio detectors) will demodulate incidental amplitude variations as well as frequency variations. Transmission noise and interference add to the signal and produce unwanted amplitude variations. Also. frequency modulation is generally accompanied by small amounts of residual amplitude modulation. In the receiver, the unwanted AM and random noise interference are demodulated along with the signal and produce unwanted distortion in the recovered information signal. The noise is more prevalent at the peaks of the FM waveform and relatively in ignificant during the zero crossings. A limiter is a circuit that produces aconstant-amplitude output for all input signals above a prescribed minimum input level, which is often called the threshald,quieting. or capture level. Limiters are required in most FM receivers because many of the demodulators discussed earlier in this chapter demodulate amplitude as well as frequency varia-tions. With amplitude limiters, the signal-to-noise ratio at the output of the demodulator (postdetection) can be improved by as much as 20 dB or more over the input (predetection) signal to noise. Essentially, an amplitude limiter is an additional IF amplifier that is overdriven. Limiting begins when the IF signal is sufficiently large that it drives the amplifier alternately into saturation and cutoff. Figures 8-9a and b show the input and output waveforms for a typical limiter. In Figure 8-9b. it can be seen that for IF signals that are below threshold, the AM noise is not reduced. and for IP signals above threshold, there is a huge reduction in the AM noise level. The purpose of the limiter is to remove all amplitude variations from the IF signal. Figure 8-10a shows the limiter output when.the noise is greater than the signal (i.c.. the noise has captured the limiter). The irregular widths of the serrations are caused by noise impulses satu-rating the limiter. Figure 8-10b shows the limiter output when the signal is sufficiently greater than the noise (the signal has captured the limiter). The peaks of the signal have the limiter so far into sat-

uration that the weaker noise is totally eliminated. The improvement in the SIN ratio is called FM thresholding, FM quieting, or the FM capture effect. Three criteria must be satisfied before FM thresholding can occur.

The pre detection signal-to-noise ratio must he I0 dB or greater. The IF signal must be sufficiently amplified to overdrive the

limiter. The signal must have a modulation index equal to or greater than

unity

Dynamic range: The dynamic range of a radio receiver is essentially the range of signal levels over which it can operate. The low end of the range is governed by its sensitivity

whilst at the high end it is governed by its overload or strong signal handling performance. Specifications generally use figures based on either the inter-modulation performance or the blocking performance. Unfortunately it is not

always possible to compare one set with another because dynamic range like many other parameters can be quoted in a number of ways. However to gain an

idea of exactly what the dynamic range of a radio receiver means it is worth looking at the ways in which the measurements are made to determine the range

of the radio receiver.

The overall dynamic range of the receiver is very important because it is just as important for a set to be able to handle strong signals well as it is to be able to

pick up weak ones. This becomes very important when trying to pick up weak signals in the presence of nearby strong ones. Under these circumstances a set with a poor dynamic range may not be able to hear the weak stations picked up

by a less sensitive set with a better dynamic range. Problems like blocking, inter-modulation distortion and the like within the receiver may mask out the weak

signals, despite the set having a very good level of sensitivity. These parameters are obviously important when determining what equipment should be used in a radio communications system.

When looking at dynamic range specifications, care must be taken when interpreting them. The MDS at the low signal end should be viewed carefully, but the limiting factors at the top end show a much greater variation tin the way they are specified. Where blocking is used a reduction of 3 dB sensitivity is normally specified, but in some cases may be 1 dB used. Where the inter-modulation products are chosen as the limiting point the input signal level for them to be the same as the MDS is often taken. However whatever specification is given, care should be taken to interpret the figures as they may be subtlety different in the way they are measured from one receiver to the next.

To gain a feel for the figures which may be obtained where inter-modulation is the limiting factor figures of between 80 and 90 dB range are typical, and where blocking is the limiting factor figures around 115 dB are generally achieved in a good radio receiver used for professional radio communications applications.