Module:4

Radio Receivers

4.1 a) Characteristics of radio receivers:

When the signals reach the destination i.e. at the
receiver end, then the signal strength will be very less. This weak signal is amplified with the
help of other signals. After amplification this signal is filtered from the other signals which were
used earlier to modify it. When the signal becomes ready for demodulation process, then the
below steps are performed for demodulation.
These steps are basically the functions of the receiver:
1. Demodulating and amplifying the received signal.
2. Filtering of the original received signal from the non necessary signals.
3. Proper display of the received signal after the completion of demodulation process.

The characteristics of the AM Receivers are Sensitivity, Selectivity, Fidelity, Image frequency
rejection etc. some of which are explained below:

1. Selectively

• The selectivity of an AM receiver is defined as its ability to accept or select the desired
band of frequency and reject all other unwanted frequencies which can be interfering
signals.
• Adjacent channel rejection of the receiver can be obtained from the selectivity parameter.
• Response of IF section, mixer and RF section considerably contribute towards selectivity.
• The signal bandwidth should be narrow for better selectivity.

2. Fidelity

• Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals
at the receiver output.
• For better fidelity, the amplifier must pass high bandwidth signals to amplify the
frequencies of the outermost sidebands, while for better selectivity the signal should have
narrow bandwidth. Thus a trade off is made between selectivity and fidelity.
• Low frequency response of IF amplifier determines fidelity at the lower modulating
frequencies while high frequency response of the IF amplifier determines fidelity at the
higher modulating frequencies.

3. Sensitivity

• Sensitivity of a receiver is its ability to identify and amplify weak signals at the receiver
output.
• It is often defined in terms of voltage that must be applied to the input terminals of the
receiver to produce a standard output power which is measured at the output terminals.

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 • The higher value of receiver gain ensures smaller input signal necessary to produce the
desired output power.
• Thus a receiver with good sensitivity will detect minimum RF signal at the input and still
produce utilizable demodulated signal.
• Sensitivity is also known as receiver threshold.
• It is expressed in microvolt or decibels.
• Sensitivity of the receiver mostly depends on the gain of IF amplifier.
• It can be improved by reducing the noise level and bandwidth of the receiver.

4. Double spotting

• Double spotting is a condition where the same desired signal is detected at two nearby
points on the receiver tuning dial.
• One point is the desired point while the other is called the spurious or image point.
• It can be used to determine the IF of an unknown receiver.
• Poor front-end selectivity and inadequate image frequency rejection leads to double
spotting.
• Double spotting is undesirable since the strong signal might mask and overpower the
weak signal at the spurious point in the frequency spectrum.
• Double spotting can be counter acted by improving the selectivity of RF amplifier and
increasing the value of IF.
• Consider an incoming strong signal of 1000 kHz and local oscillator tuned at 1455 kHz.
Thus a signal of 455 kHz is produced at the output of the mixer which is the IF
frequency.
Now consider the same signal but with 545kHz tuned local oscillator. Again we get 455
kHz signal at the output.
Therefore the same 1000 kHz signal will appear at 1455 kHz as well as 545 kHz on the
receiver dial and the image will not get rejected. This is known as Double spotting
phenomenon.
• It is also known as Adjacent channel selectivity.

5. Noise
All receivers generate a certain amount of noise, which you must take into account when
measuring sensitivity. Receiver noise may originate from the atmosphere (lightning) or
from internal components(transistors, tubes). Noise is the limiting factor of sensitivity.
You will find sensitivity is the value of input carrier voltage (in microvolt) that must be
applied from the signal generator to the receiver input to develop a specified output
power.
TYPES OF RECEIVERS
Receivers are basically available in two types:
1. Tuned Radio Frequency Receiver.
2. Super Heterodyne Receiver.

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4.1 b) TRF(Tuned Radio Frequency) receiver:
The definition of the tuned radio frequency, TRF receiver is
a receiver where the tuning, i.e. selectivity is provided by the radio frequency stages. A tuned
radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or
more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to
extract the audio signal and usually an audio frequency amplifier. This type of receiver was
popular in the 1920s.

RF Tuner Section- The modulated signal received by the antenna is first passed to the tuner
circuit through a transformer. The tuner circuit is nothing but a LC circuit, which is also called
as resonant or tank circuit. It selects the frequency, desired by the radio receiver. It also tunes
the local oscillator and the RF filter at the same time.

RF Mixer- The signal from the tuner output is given to the RF-IF converter, which acts as a
mixer.It has a local oscillator, which produces a constant frequency. The mixing process is done
here, having the received signal as one input and the local oscillator frequency as the other
input. The resultant output is a mixture of two frequencies [(𝐟𝟏 + 𝐟𝟐), (𝐟𝟏 − 𝐟𝟐)] produced by
the mixer, which is called as the Intermediate Frequency (IF).
The production of IF helps in the demodulation of any station signal having any carrier
frequency. Hence, all signals are translated to a fixed carrier frequency for adequate
selectivity.

IF Filter- Intermediate frequency filter is a band pass filter, which passes the desired frequency.
It eliminates any unwanted higher frequency components present in it as well as the noise.
IF filter helps in improving the Signal to Noise Ratio (SNR).

Demodulator- The received modulated signal is now demodulated with the same process used at
the transmitter side. The frequency discrimination is generally used for FM detection.

Audio Amplifier- This is the power amplifier stage which is used to amplify the detected audio
signal. The processed signal is given strength to be effective. This signal is passed on to the
loudspeaker to get the original sound signal.
This super heterodyne receiver is well used because of its advantages such as better SNR,
sensitivity and selectivity.

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 Advantages of TRF Receiver
▪ Simplest type of receiver since it does not involve mixing and IF operation.
▪ Very much suitable to receive single frequency.
▪ TRF receivers have good sensitivity.
Disadvantages of TRF Receiver
▪ Bandwidth changes with center frequency, when the TRF receiver is used to receive a wide
range of frequencies.
▪ Multiple stages of RF amplifiers are used. This can lead to instability since they all are tuned to
the same center frequency. Hence stagger tuning is to be used.
▪ The gain of the TRF receiver is not uniform over a wide range of frequencies.
Application of TRF Receiver
▪ It was used in the early days of wire-less technology but it is rarely used today as other
techniques offering much better performance are available.
Limitations of TRF Receiver
▪ It suffers from variations in BW over the tuning range ( s 40 – 1650 kHz).
▪ The gain of TRF RX is not uniform over the tuning range.
▪ The TRF is unstable at high frequency.
▪ Gang tuning of more capacitors simultaneously is difficult.

4.1 c) Super - heterodyne receiver:

Superheterodyne receiver works on the principle of
heterodyning which simply means mixing. A superheterodyne receiver (or superhet) is a radio
receiver that combines (or mixes) a locally generated frequency with the carrier frequency to
produce a lower-frequency signal that is easier to demodulate than the original modulated carrier.
The output of mixer provides a lower fixed frequency also known as intermediate frequency.
These receivers are called Superheterodyne receivers as the frequency of the signal generated
by the local oscillator is more than the frequency of the received signal.

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• Receiving antenna: The receiving antenna receives the signal which was sent by the
transmitter. It sends the received signal for further processing.
• RF amplifier: The received signal is fed to the RF amplifier stage so as to amplify it, as
the signal gets attenuated during long-distance transmission. It is tuned in such a way that it
can choose the desired carrier frequency and amplify it.
• Local Oscillator: This circuit basically generates a signal with a fixed frequency and the
output is then fed to the mixer. When we talk about AM broadcast system, the
intermediate frequency is 455 KHz that simply means that local oscillator should select
such a frequency which is 455 KHz above the incoming signal frequency.
• Mixer: A mixer simply mixes the carrier frequency with the frequency of the signal
generated by the local oscillator.

Here, two different frequencies are to be mixed so as to have another frequency component
of lower value. Now the thing that first comes to our mind is why the mixer produces a
lower frequency value, which is the difference between the two frequencies. The
summation of the carrier and local oscillator frequency at the output of the mixer will give
rise to image frequency which is treated as a type of noise or distortion in the signal. This is
the reason why the mixer generates a frequency difference at its output. This difference
frequency is a constant value irrespective of the variations in the input, known as
the intermediate frequency.

The constant frequency at its output is gained by capacitance tuning. In capacitance

tuning, several capacitances are arranged together and operated by a controlling knob. It
doesn’t matter what the incoming signal frequency is, the RF amplifier and local oscillator
must be tuned to it.

• IF amplifier: This section basically amplifies the output of the mixer. IF amplifier provides
sensitivity(gain) and selectivity (bandwidth requirement) to the receiver. As it consists of
several transformers consisting of pairs of the tuned circuit. Here, the sensitivity and
selectivity are uniform and does not show variations as in case of TRF receivers because IF
amplifier’s characteristics are independent of that of the received signal frequency as
it works on the intermediate frequency.

Due to this, the system design is quite easy so as to provide constant bandwidth along with
high gain. This section has narrow bandwidth and due to its lower bandwidth, it rejects all
other frequency so as to reduce the risk generated from interference. The lower bandwidth
accepting nature supports Superheterodyne receivers to give much better performance than
other types of receivers.
• Demodulator: Demodulator is placed exactly after the IF amplifier so that the constant
frequency signal is demodulated and the message signal can be extracted from it.
• Audio amplifier: The original signal is fed to the audio amplifier which does not hold
distortion or noise so that it can amplify audio signal to a particular level.

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 • Power amplifier: Here, the signal is further amplified to a particular power level which
can activate the loudspeaker. The amplified signal is finally fed to the loudspeaker circuit
which converts the electrical form of the signal into an audio sound signal which can be
heard by the listeners.
Following factors determines the sensitivity of Superheterodyne receiver:

1. The gain of RF amplifier

2. The gain of IF amplifier
3. Noise factor of receiver
Selectivity: It is the ability of any system that decides whether the signal is desired or
undesired. At high-frequency selectivity is poor, better selectivity is achieved at low
frequency.

Fidelity: It is the system’s ability to produce an original signal without distortion. It is

essential to produce a good quality signal. It ensures that the reproduced signal must be
an exact copy of the original signal.

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Advantages:

• It operates at low signal level.

• The mixer provides fixed frequency operations.
• Provides excellent selectivity and sensitivity.

Disadvantage:

• Overall system cost is increased as additional circuits are used.

Applications:
• Superheterodyne receivers find its use in various places as in Television, Radio receiver,
commercial radios

4.1 d) Tracking and choice of IF:

The process where the local oscillator frequency follows the
signal frequency to maintain the correct intermediate frequency is known as tracking in receiver.
The choice of the intermediate frequency (IF) for a radio receiver is influenced by several factors:

1. Selectivity: A higher IF can provide better selectivity due to the availability of narrow bandpass
filters, which can more effectively discriminate between adjacent channels.
2. Image Frequency Rejection: The IF frequency should be chosen to minimize the effects of
image frequencies. The image frequency is a spurious signal that can interfere with the desired
signal. The IF should be far enough from the RF frequency to allow for effective filtering.
3. Mixer Performance: The choice of IF affects the performance of the mixer used in the receiver.
Different mixers have varying characteristics and performance metrics at different frequencies.
4. Component Availability: The availability of components, such as filters and amplifiers
designed for specific frequency ranges, can influence the choice of IF. Designers often select an
IF that aligns with the most readily available components.
5. Cost and Complexity: Higher IFs may necessitate more complex circuitry and components,
which can increase costs. A balance must be struck between performance and cost.
6. Noise Figure: The noise performance of the receiver can be impacted by the choice of IF.
Different frequencies may have different noise figures due to the characteristics of the
components used.
7. Bandwidth Requirements: The desired bandwidth of the received signal can influence the IF
selection. A wider bandwidth may require a different IF to accommodate the filter
characteristics.
8. Frequency Stability: Certain IFs may provide better frequency stability, which is crucial for
maintaining the quality of reception over time and under varying conditions.
9. Heterodyne Effects: The chosen IF must consider the effects of heterodyning, where two
frequencies mix to produce new frequencies, which can complicate the design if not properly
managed.
10. Compatibility with Digital Processing: In modern receivers, especially software-defined
radios (SDRs), the choice of IF can affect how well the signal can be processed digitally. An IF
that is easy to sample and process can be advantageous.

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In summary, the choice of intermediate frequency for a radio receiver is a complex decision that balances
performance, cost, and practicality, taking into account the specific application and design goals of the
receiver system.

4.1 e) AGC (automatic gain control) and its types:

AGC is a system that controls the increase
in the amplitude of an electrical signal from the original input to the amplified output,
automatically.

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4.1 f) Communication receiver:

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4.2 a & b) FM receiver block diagram & comparison with AM receiver:
The FM receiver is the whole unit which takes the modulated signal as input and
produces the original audio signal as an output. Radio amateurs are the initial radio
receivers.
However, they have drawbacks such as poor sensitivity and selectivity.
Selectivity is the selection of a particular signal while rejecting the others. Sensitivity is
the capacity of detecting a RF signal and demodulating it, while at the lowest power level.

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