Elements Of

Telecommunication
CHAPTER 9
RADIO RECEIVER
Aim
To equip participants with the fundamental knowledge of radio
receivers so that they can be able to design and troubleshoot a
basic radio system.
Objectives
At the end of the chapter participants should be able to:

• Identify the components that makes up a basic radio transmitter.

• Analyze basic receiver.
• Construct and explain the superheterodyne receiver
 Introduction
• A radio receiver is an electronic device that receives radio waves,
converts the information carried by them to a usable form.
• The antenna intercepts radio waves (electromagnetic waves)
• The EM wave is converted to alternating current which is applied
to the receiver.
• The receiver extracts the desired information.
• The receiver uses electronic filters to separate the desired radio
frequency signal from all the other signals picked up by the
antenna
• An electronic amplifier to increase the power of the signal for
further processing.
• Finally it recovers the desired information through
demodulation.
Function of a radio receiver

• Select the wanted signal from all the signals picked up by

the antenna, whilst rejecting all others.
• Extract the intelligence contained in the modulated
signal
• Produce an audio-frequency output of sufficient power
to operate the loudspeaker or other receiving device
through the amplification of such a signal.
Receiver Principles

• A wanted frequency can be selected from a number or frequencies by utilizing the

Selectivity characteristics of a parallel resonant circuit
• In this receiver selection is obtained by adjusting capacitor C1 to give resonance at the
wanted signal frequency.
• The diode D1 acts as a non-linear detector
• It extracts the audio-frequency signal, plus a number of other components, which is
passed through the earphones for conversion into sound.
• A loudspeaker cannot be used because there is insufficient power.
• It may often be considered desirable to prevent the r.f components of the detector
output passing through the earphones; this is easily achieved by shunting the
earphones; with a suitable capacitor
• The wanted signal is selected by the parallel-resonant
circuit L2, C1
• C2 bypasses the r.f components of the detected output.
• Capacitor C3 presents the d.c. components of the
detector output reaching the amplifier and upsetting the
bias arrangements.
• Further increase in the sound power delivered by the
receiver could possibly be provided if a second and
perhaps a third stage of a.f. gain were employed.
• but there are two snags with such a proposal.
• The output signal-to-noise ratio may be rather poor.
• Distortion of the output waveform could be reduced if a linear
diode detector were used instead of the non-linear detector.
• For a diode detector to work efficiently, with little
distortion, the r.f voltage applied to its terminals should
have a peak value of about 1V.
Tuned-Radio-Frequency
(T.R.F.)

• A better method of increasing the a.f. power output is to

employ one or more stages of radio-frequency gain.
T.R.F
• The wanted signal frequency is selected by the tuned
circuits in the R.F. amplifier, amplified and applied to the
detector stage.
• If sufficient R.F. gain is provided, a diode detector can be
employed; if not, some form of non-linear detection will
still be required.
• The detected output is amplified by the A.F. amplifier to
the level necessary to operate the loudspeaker.
• Carrier frequencies in the medium and long wavebands
are spaced at intervals of only 9kHz
• A radio receiver must be capable of selecting one carrier
frequency whilst rejecting the two immediately adjacent
to it.
Short falls of T.R.Fs
• It is then important to note that T.R.Fs suffer from poor
adjacent signal selectivity
• because the Q of tuned circuits changes with frequency.
• This problem is compounded by the fact that t.r.fs are
simply a chain of amplifiers each tuned to the same
frequency followed by a detector.
• During the study of R.F amplifiers; we found out that the
use of several stages in cascade results in bandwidth
shrinkage.
The Superheterodyne Radio Receiver
• The problems associated with the t.r.f radio receiver are
easier to overcome if most of the necessary gain and
selectivity could be provided at a fixed frequency.
• This is in fact the principle of the SUPERHETERODYNE
radio receiver.
Superhet Radio History

• The superhet radio, or to use its full name, the

superheterodyne radio was developed during the First World
War.
• Its development arose from the need for much greater levels
of performance both in terms of selectivity and sensitivity.
• However the additional valves (tubes) it used meant that its
use did not become more common until the 1930s when the
technology required became cheaper and the levels of
performance it provided became a necessity.
FM/AM Superheterodyne
Receiver
Practical Superheterodyne
Receiver

• In a superheterodyne radio receiver the wanted signal

frequency is converted into a constant frequency known
as the intermediate frequency at which most of the gain
and the seIectivity of the receiver is provided.
Mixing and the superhet
receiver
• The idea of the superheterodyne receiver revolves around the
process of mixing.
• Here RF mixers are used to multiply two signals together. (
• When two signals are multiplied together the output is the
product of the instantaneous level of the signal at one input
and the instantaneous level of the signal at the other input.
• It is found that the output contains signals at frequencies
other than the two input frequencies.
• New signals are seen at frequencies that are the sum and
difference of the two input signals, i.e. if the two input
frequencies are f1 and f2, then new signals are seen at
frequencies of (f1+f2) and (f1-f2).
Example 1

Two signals, one at a frequency of 5 MHz and another at a

frequency of 6 MHz are mixed together calculate new signals.
Concept of the superheterodyne
receiver
• In the superhet radio, the received signal enters one inputs of
the mixer.
• A locally generated signal (local oscillator signal) is fed into the
other.
• The result is that new signals are generated.
• These are applied to a fixed frequency intermediate frequency
(IF) amplifier and filter.
• Any signals that are converted down and then fall within the
pass-band of the IF amplifier will be amplified and passed on
to the next stages.
• Those that fall outside the pass-band of the IF are rejected.
• Tuning is accomplished very simply by varying the frequency
of the local oscillator.
The Radio-frequency (RF)
Stage
• The radio frequency stage of a superheterodyne radio
receiver must perform the following functions:
• Efficiently couple the antenna to the receiver
• Suppress signals at or near the image and the intermediate
frequencies.
• Provide gain at frequencies in excess of about 3MHz.
• Operate linearly to avoid the production of cross-modulation.
• Have sufficient selectivity to minimize the number of frequencies
appearing at the input to the mixer that could result in inter-
modulation products lying within the pass-band of the i.f
amplifier.
RF Stage
• At frequencies up to about 3MHz or so, the noise picked up by an
antenna is larger than the noise generated within the receiver.
• An r.f. amplifier will amplify the antenna noise as well for the signal
and produce little, if any, improvement in the output signal-to-noise
ratio.
• At higher frequencies the noise picked up by the antenna falls and
the constant-level receiver noise becomes predominant; the use of
r.f gain will then improve the output signal-to-noise ratio.
• An r.f amplifier also permits the use of two or more tuned circuits in
cascade, with a consequent improvement in the image response
ratio.
Mixer Stage
• The function of the mixer stage is to convert the wanted signal
frequency into the intermediate frequency of the receiver.
• The wanted signal, at frequency fs is passed together with other
unwanted frequencies by the radio-frequency stage to the mixer
(or frequency changer).
• The r.f. stage is not provided to select the wanted signaI but
chiefly to prevent certain particularly troublesome frequencies
reaching the mixer stage.
• This process is carried by mixing the signal frequency with the
output of the local oscillator and selecting the resultant
difference frequency.
• Amongst the newly generated frequencies are components at the
sum and the difference of the wanted signal and the local
oscillator frequencies. i.e.
Local Oscillator Stage
• The local oscillator must be capable of tuning to any frequency in the band
to which the receiver is tuned pIus the intermediate frequency,

• The ability of a receiver to remain tuned to a particular frequency without

drifting depends upon the frequency stability of its local oscillator.
• Receivers operating at one or more fixed frequencies can use a crystal
oscillator.
• L-C oscillator with Automatic frequency control or a frequency synthesizer
can be used,
• The frequency Stability of an i.s.b/s.s.b. receiver should be good enough to
ensure that the tuning of the receiver will not drift from its nominal value
• This is necessary because any change in the local oscillator frequency will
cause a corresponding shift in the frequency of the output signal.
• Generally, the long-term frequency stability of an h.f. communications
receiver is better than 1 part in 107
• ISB is a compromise between double sideband (DSB) and single sideband
(SSB)
Intermediate (If) Stage
• The purpose of the if amplifier in a superheterodyne radio receiver is to
provide most of the gain and selectivity of the receiver.
• The difference frequency (f o - fs) is known as the intermediate frequency and
is selected by the intermediate frequency (IF) amplifier.
• The intermediate frequency is a fixed frequency and this means that when a
receiver is tuned to receive a signal at a particular frequency, the local
oscillator frequency is adjusted so that the correct difference is obtained.
• The amplified output of the i.f. amplifier is applied to the detector stage and
it is here that the information in the modulated signal is recovered.
• Most broadcast receivers utilize the impedance/frequency characteristics of
single or double-tuned circuits to obtain the required selectivity.
• The use of ceramic or crystal filter provide the selectivity of a radio receiver
offers a number of advantages over the use of L-C networks
• A very narrow bandwidth can be obtained.
• The selectivity of the receiver does not depend upon the correct alignment of the i.f
amplifier.
• The selectivity of the filter is not affected by the application of automatic gain control
to the receiver.
The Detector Stage
• The function of the detector stage in a radio receiver is to
recover the information modulation modulated onto the
carrier wave appearing at the output of the i.f amplifier.
• Most a.m. broadcast receivers use the diode detector because
of its simplicity and good performance but IC versions often
use the transistor detector.
• The transistor detector is not often used in discrete form for
broadcast receivers
• Most f.m. broadcast receivers may use the ratio detector but
high-quality broadcast receiver may use the Foster-Seely
circuit.
• The detector must be preceded by a limiter stage.
The Audio-frequency Stage
• The function of the audio-frequency stage of a radio receiver is to
develop sufficient a.f power to operate a loudspeaker or other
receiving apparatus.
• The a.f. stage will include a volume control and sometimes treble
and bass controls.
• The a.f. stage may also include a squelch or muting facility.
• A sensitive receiver will produce a considerable output noise level
when there is no input signal because there will then be no a.g.c
voltage developed to limit the gain of the receiver.
• The noise unavoidably present at the input terminals of the receiver
then receivers maximum amplification.
• This noise output can cause considerable annoyance to the operator
of the receiver and, to reduce or eliminate this annoyance, a
SQUELCH circuit is fitted which disconnects, or severely attenuates,
the gain of the amplifier whenever there is no input signal present.
Local Oscillator stage
Choice Of Local Oscillator Frequency
• The intermediate frequency of a superheterodyne radio
receiver is the difference between the wanted signal
frequency and the local oscillator frequency.
• Two possibilities exist:
• the local oscillator frequency call be higher than the signal
frequency, or
• the local oscillator frequency call be lower than the signal
frequency
• Consider a receiver with an intermediate frequency of 470 kHz
that is tunable over the band from 525 kHz to 1605 kHz.
Frequency of the local oscillator is higher than
the wanted signal
• If the frequency of the local oscillator is higher than the
wanted signal frequency
• The oscillator must be tunable from

(525 + 470) = 995 kHz to (1605 + 470) = 2075 kHz

• A frequency ratio of 2075/995, or 2.085:1.

• Such a frequency ratio would require the use of a variable
capacitor having a ratio maximum-capacitance/ minimum--
capacitance of
:1, or 4.35: 1
• Such a capacitance ratio is easily obtained.
signal frequency higher than
the local oscillator frequency
• The oscillator frequency must then be variable from:

• This is a frequency ratio of 1135/55, or 20.64:1

• A capacitance ratio of (20.64)2, or 425.9: 1.
• Such a large capacitance ratio could be obtained with a single
variable capacitor and so tuning would not be as easy or
cheap to achieve.
• It is therefore, usual to make the local oscillator frequency
higher than the wanted signal frequency, i.e.
Automatic Gain Control
• The field strength of the wanted signal at the antenna is not constant
but fluctuates widely because of changes in propagation conditions.
• Automatic gain control (a.g.c,) is applied to a radio receiver to
maintain the carrier level at the input to the detector at a more or
less constant value even though the level at the antenna may vary
considerably.
• A.G.C. ensures that the audio output of the receiver varies only as a
fraction of the modulation of the carrier and not with the carrier level
itself.
• The use of a.g.c. also ensures that a large receiver gain can be made
available for the reception of weak signals without causing
overloading of the r.f amplifier stages with consequent distortion, by
strong signals.
• Further, a reasonably constant output level is obtained as the receiver
is tuned from one station to another.
• In an f m receiver automatic gain control is often fitted
to ensure
• That the signal arriving at the input terminals of the limiter is
large enough for the limiting to take place.
• That overloading of the r.f. and i.f. amplifier stage does not occur.
In some case the automatic gain control of an f.m receiver may
mean switching into the r.f. stage of one or more stages of an
attenuator.
• The polarity of the a.g.c. voltage should be chosen so that an
increase in the carrier level, which will produce an increase in
the a.g.c. voltage, will reduce the gain of each stage.
• This will in turn, reduce the overall gain of the receiver and
tend to restore the carrier Level at the detector to its original
value.
• Conversely, of course if the carrier level should Fall, the gain of
the receiver will be increased to tend to keep the level at the
detector very nearly constant.
• Another a.g.c loop known as auxiliary a.g.c, is often provided
to give extra control of the gain of the receiver and to limit the
amplitude of strong input signals to prevent overloading of the
r.f. amplifier and the consequent distortion and cross·
modulation.
Main A.G.C.

• Automatic gain control systems are either of the simple or the delayed
type.
• In a SIMPLE A.G.C. SYSTEM the a.g.c. Voltage is developed
immediately a carrier voltage appears at the output of the i.f amplifier.
• This means that the gain of the receiver is reduced below its
maximum value when the wanted signal is weak and the full receiver
gain is really wanted.
• This disadvantage of the simple a.g.c. system can be overcome by
arranging that the a.g.c. voltage will not be developed until the carrier
level at the detector has reached some pre-determined value
• Generally that at which the full audio- frequency power output can be
developed. Such a system is known as a delayed a.g.c system.
Automatic Frequency Control
(AFC)
• The intermediate frequency bandwidth of a communication
receiver operating in the u.h.f. band is only a small percentage
of the carrier frequency.
• A relatively small percentage error in the frequency of the
local oscillator may lead to the wanted signal being wholly or
partly rejected by the selectivity of the i.f. amplifier.
• Some of the necessary frequency stability can, however, be
obtained by a suitable choice of the type of oscillator to be
used but the most stable types of oscillator cannot be tuned
to different frequencies.
• The required frequency stability can be obtained by the use of
AUTOMATIC FREQUENCY CONTROL (a.f.c).
Interferences
Image Channel Interference
• No matter what frequency a superheterodyne receiver is tuned
to there is always another frequency that will also produce the
intermediate frequency.
• This other frequency is known as the image frequency.
• The image signal has a frequency Fim such that the difference
between it and the local oscillator frequency is equal to the
intermediate frequency, fi, i.e.

• Substituting for fo from equation

• Or
Image channel interference
• The image signal is thus separated from the wanted signal by twice the intermediate
frequency.
• The image signal must be prevented from reaching the mixer or it will produce an
interference signal which, since it is at the intermediate frequency, cannot be eliminated
by the selectivity of the i.f. amplifier.
• The r.f. stage must include a resonant circuit with sufficient selectivity to reject the image
signal when tuned to the wanted signal frequency.
• Tuning is necessary because the wanted signal frequency, and hence the image signal
frequency will vary.
• It is not difficult to obtain a resonant circuit with good enough selectivity to accept the
wanted signal and reject the image signal when their separation is an appreciable fraction
of the wanted signal frequency.
• As the signal frequency is increased, the fractional frequency separation becomes smaller
and the image rejection less efficient.
• Any vestige of the image signal reaching the mixer will produce a signal appearing as
crosstalk at the output of the receiver.
• If a signal at a few kilohertz away from the image signal should reach the mixer, the two
i.f. signals produced would beat together to produce a whistle at the output of the
receiver.
• The image response ratio is the ratio, in decibels, of the voltages at the wanted signal and
image signal frequencies necessary at the receiver input terminal to produce the same
audio output.
Example
A superheterodyne radio receiver has an intermediate frequency
of 470 kHz and is tuned to 1065kHz. Calculate:

a) The frequency of the local oscillator, and

b) The frequency of the image signal.
Example
A superheterodyne radio receiver has an intermediate frequency
of 10.7 MHz and is tuned to 97.3MHz. Calculate:

a) the frequency of the local oscillator and

b) The image channel frequency.
I.F. Breakthrough
• If a signal at the intermediate frequency is picked up by an
antenna and allowed to reach the mixer, it will reach the i.f.
amplifier and interfere with the wanted signal.
• Such a signal must therefore be suppressed in the r.f. stage by an
i.f. trap.
• The i.f. trap consists of either a parallel-resonant circuit, tuned to
the intermediate frequency, connected in series with the antenna
lead, or a series-resonant circuit, also tuned to the intermediate
frequency, connected between the antenna lead and earth.
• In the first circuit the i.f. trap has a high impedance and blocks the
passage of the unwanted Lf. signal;
• in the second circuit the i.f. trap has a low impedance and shunts
the unwanted signal to earth.
• al.
Example
• A superheterodyne radio receiver has an intermediate
frequency of 465 kHz and is tuned to receive an unmodulated
carrier at 1200 kHz. Calculate the frequency of the audio
output signal if present at the mixer input there are also

a) a 1208 kHz, and

b) a 462 kHz sinusoidal signal
Co-channel Interference
• This is due to another signal at the same frequency and
cannot be eliminated by the receiver itself.
• When it occurs it is the result of unusual propagation
conditions making it possible for transmissions from a distant
(geographically) station to be picked up by the antenna.
• Harmonic of the local oscillator frequency may- combine with
unwanted station or with harmonic produced by the mixer to
produce various difference frequency components
• some of which may fall within the passband of the i.f.
amplifier.
• It is also possible for two r.f signals arriving at the input to the
mixer to beat together and produce a component at the
intermediate frequency
Selectivity
• The SELECTIVITY of a radio receiver is its ability to discriminate
between the wanted signal and all the other signals picked up
by the aerial, particularly the adjacent-channel signals.
• The selectivity of a receiver is usually quoted by means of a
graph showing the output of the receiver, in dB relative to the
maximum output.
• For example, the selectivity of an h.f. receiver may be quoted
as -6 dB at 3 kHz bandwidth and -60 dB at 12 kHz bandwidth.
Sensitivity
• The SENSITIVITY of a radio receiver is the smallest input signal voltage that is
required to produce a specified output power with a specified signal-to-noise
ratio.
• For amplitude modulation receivers, the specified output power is usually 50 mW
with a signal-to-noise ratio of 20 dB and the input signal modulated 30% at 1000
Hz (or 400 Hz).
• For an f.m. receiver a signal-to-noise ratio of 40 dB is required with the input signal
modulated by a 1000 Hz signal to give 30% modulation.
• It is necessary to include signal-to-noise ratio in the definition of sensitivity,
otherwise the output power could consist mainly of noise and be of little use.
• The sensitivity of a radio receiver is determined by
• (a) The overall voltage gain of its individual stages.
• (b) The gain/frequency characteristic of the d. stage.
• (c) The noise generated by thermal agitation in its input stages.
• This means that the sensitivity is directly related to the noise figure of the receiver.
• Typical figures for sensitivity are (a) a.m. broadcast receiver 50 µV, (b) f.m.
broadcast receiver 2µV, and (c) s.s.b. receiver 1µV.
The Double Superheterodyne
Radio Receiver
• To obtain good adjacent channel selectivity, the intermediate frequency of a
superheterodyne radio receiver should be as low as possible
• To maximize the image channel rejection the intermediate frequency must be as high
as possible.
• For receivers operating in the low and medium frequency bands it is possible to
choose a reasonable compromise frequency.
• In the h.f. band it may prove difficult to select a suitable frequency and for this reason
many receivers use two, or more rarely, three or four different intermediate
frequencies.
• The first intermediate frequency is chosen to give good image channel rejection ratio
• The second frequency is chosen for good adjacent channel selectivity.
• Typically, the first intermediate frequency might be 3 MHz and the second
intermediate frequency 100 kHz, although in modern receivers there is a tendency to
use a very high first intermediate frequency, such as 35 MHz, to give a very good
image rejection (usually the second intermediate frequency is then about 1 MHz).
• The disadvantages of the double superheterodyne principle are the extra cost and
complexity involved and the generation of extra spurious frequencies because there
are two stages of mixing. The most serious of these new frequencies is the
second/image channel frequency.