Receivers are designed to receive electromagnetic waves transmitted from transmitters. The received signal passes through input circuits where it is filtered, amplified, and demodulated. It is then sent to output circuits like speakers or screens depending on the receiver's purpose. Receivers can be grouped as professional systems, specialized systems, or broadcasting receivers. They can also be classified by their schematic as direct amplification or superheterodyne receivers. The superheterodyne receiver mixes the input RF signal with a local oscillator to produce an intermediate frequency signal that is amplified before detection. This design makes superheterodyne receivers more sensitive and selective than other receiver types.

1. 8.1 Introduction
Receivers are designed to receive electromagnetic waves. The waves originate from
transmitters in a modulated form. The received wave travels at the speed of light. On
arriving at receiver antenna they are fed into input circuits (filters). The signal is
amplified and the modulated waves are demodulated. The required signal is fed into
output circuits of the receiver system. Receiver output transducers include speakers,
telephones, television, screens and printers. The type of equipment at
the receiver output depends on the purpose for which the system is meant.
Receivers can be grouped as:
(i) Professional systems on special assignment such as point-to point communication
radio receivers used in water and electricity utilities
(ii) Specialized receivers which include those used in radio location, radar, navigation,
satellite, aeroplane and meteorological predictions.
(iii) Broadcasting receivers for both radio and televisions. Receivers can be classified
according to their schematic block diagrams:
(a) Direct amplification receivers and
(b) Superheterodyne receivers.
8.2 Technical Receiver Parameters
Sensitivity
This is the ability to receive the most minimum signal level and produce the rated
output. The most sensitive receiver is the one which is able to produce the required
signal level at the most minimum signal level at the receiver input. For instance,
a receiver produces 5 w at the output and the input signal level is 0.35 × 10-6
V, and
another receiver produces the same (5 w) at the output while the input signal level is
0.5 × 10-6 V. The first receiver is more sensitive than the second.
Selectivity

2. This is the ability of a receiver to antenuate disturbing stations (signals) and amplify
the required stations (the signal to which the receiverhas been tuned to). Selectivity
(S) is by how much the disturbing station signal has been attenuated as compared to
the required signal level.
8.3 Receiver Block Diagrams
Detector receiver is the simplest. The receiver input circuit consists of a resonant
tuned circuit. This selects the required modulated wave among all incoming stations.
The signal is fed into a detector. The output of this detector is an audio frequency
signal. The block diagram of a detector receiver is shown in figure 8.1 below.
Figure 8.1: Direct receiver
Another receiver structure is the direct amplification. This type of receiver includes
radio frequency (RF) amplifier and audio frequency (AF) amplifier. The required
signal obtained from the antenna by the input resonant circuits is amplified by the RF
before being fed to the detector. After signal detection into audio frequency the signal
is amplified by AF before being fed into a transducer such as speaker, telephone,
television, etc. The block diagram is shown in figure 8.2 below.

3. Figure 8.2: Detector receiver block diagram
Most receivers are designed on super heterodyne type. The block diagram is shown in
figure 8.3 below.
Figure 8.3: Superheterodyne receiver
The superheterodyne receiver includes frequency mixing cascades. A local oscillator
generates a frequency which is mixed together with the amplified input RF signal. The
mixer products is a signal at intermediate frequency. This IF is lower than the RF and
higher than AF. The intermediate frequency amplified feeds into detector circuit. At
the detector the output AF is amplified and fed to the transducer such as (speaker,
telephone, etc.)

4. Each receiver has its own intermediate frequency. This can be 455 kHz or 465 kHz.
High frequency mixing can be done twice in some heterodyne receiversystems to
obtain lower intermediate frequency. This mixing can start with an IF pf
10.7 mHz and the second IF of 465 or 455 kHz. The local oscillator frequencies must
be properly chosen. In the IF amplifier, high signal without distortions is possible.
This cascade operates on fixed frequency and its circuits do not need tuning. Hence
the intermediate frequency amplifier makes the superheterodyne to be more sensitive
and selective.
There is an intensive modification of the receiver blocks in recent years. The size of
the receiver system is decreasing to integration and reduction of the blocks of
the receiver. In specialized and professional communication trans-receivers frequency
synthesizers are employed in channel division. This has reduced the size and number
of blocks in multichannel systems.
8.6 Intermediate Frequency Amplifier (IFA)
Intermediate frequency amplifier is a resonant circuit operating on one
fixed frequency. The signal received is mixed with a local oscillator frequency to
obtain an immediate frequency. An IFA on single resonant contour is shown in figure
8.7 below.
Figure 8.7: IFA circuit on single contour and doulbe network

5. (a) Single contour
(b) Doulbe network
IFA can be of two resonant circuits (striped filter) as shown in figure 8.7 above.
Since IFA operates on a fixed resonant frequency, more than one contours can be
used. This makes the resonance curve closer to reality and more selective. IFA with
semi-coupled resonant circuit is commonly used in radio receiver systems.
The resultant resonance curve is shown in figure 8.8 below. The curve is wide
enough.

6. Figure 8.8: Semi-coupled resonant curves
As the number of contours increase the resonance curve tends to a rectangular form.
The load to the double contour intermediate frequency amplifier can be filters with
concentrated selection. These can be three or four bandpass filters tuned
at intermediate frequency. IFA with selective filter as is shown in figure 8.9 below.
Figure 8.9: IFA with selective/liters
The form of two contour IFA resonance curve depends on the degree coupling under
coupled contour has a rather flat resonance curve and less in amplitude as compared to

7. critical coupled. Critical coupled has an optimum resonance curve. Overcoupled has a
resonance curve with two maximum at different as shown in figure 8.10 below.
Figure 8.10: Double contour IFA curves
Resonance amplification factor for IFA cascade can be obtained using the formula,
, where 8.03
 is the coupling factor. For critical conditions,  = 1,  < 1 for undercoupled and  >
1 for overcoupled. R is the equivalent resistance of the contour at resonance (Ohms).
S is the transistor characteristic gradient
, m1 and m2 are connection coefficients of the contour to the transistor. Output and
to the next cascade respectively. This value can be chosen to be between (0.2 - 0.4).
The choice of IF must consider that higher frequency decrease the
IF amplifier frequency stability. Most domestic receivers have the
following frequencyband.
Band fmin (kHz) fmax (kHz)
Long waves 150 410
Medium waves 530 1610

8. Short waves 3960 12110
The IF in most amplitude modulated receivers at Long and Short waves is either 400
or 500 kHz. In ultra high frequencies UHF and very high frequencies, VHF. The IF is
6.0-8.0 MHz.
In professional industrial radio communication systems, double mixing is necessary to
have two intermediate frequencies. The first is chosen to be higher (e.g. 10.7 MHz) so
as to achieve high selectivity relative to adjacent channels. The second IF is chosen to
be lower (e.g. 465 kHz) so that a high stable amplification factor and high channel
selectivity is achieved.
8.12 Automatic Gain Control (AGC)
Figure 8.28: Automatic Gain Control (AGC) and Automatic Frequency Control
(AFC) connection
Signal reception depends on receiver sensitivity, transmitter power and distance to the
receiver. The volume of the signal received is regulated using volume control
resistance. A strong signal from a nearby transmitter to a high power amplification
receiver would affect the amplifying components of a receiver. Electromagnetic wave
intensity is never constant due to feeding. This affects reception. The receiver must
therefore automatically control the output to the speaker. AGC includes all blocks
within high frequency track. AGC circuit is shown in figure 8.29 below.

9. Figure 8.29: AGC Circuit connection
A constant voltage from the detector is fed back to the RF amplifier and intermediate
frequency amplifier through filter components.
The audio frequency voltage at the detector output is prohibited from being fed to the
base of the transistors by the filters. Audio frequency voltage feedback to the units
under control causes secondary modulation. This secondary modulation results into
frequency and non-linear distortions in the receiver. The AGC is subject to weaken
signals at switching moment. To avoid this, AGC with delay circuit is used. AGC-
delay starts to operate after a certain input signal level has been reached (Vi) as shown
in figure 8.30 below.
Delayed AGC should have its own delayed detector in which a delayed voltage is fed.
The delayed detector remains closed until fed voltage reaches the value of the delayed
AGC voltage. Hence, at voltages less than the delayed AGC voltage Vi, the receiver
operates without AGC. An ideal AGC would keep the output at an increasing input
signal (3). A receiver without AGC amplifies continuously as the input signal
increases (1).
The receiver sound quality performance depends on the delayed AGC quality.

10. Figure 8.30: AGC Characteristics