RADIO RECEIVERS

Radio receiver is an electronic equipment which

pick ups the desired signal, reject the unwanted
signal and demodulate the carrier signal to get
back the original modulating signal.
 Basic Functions of AM Receivers
i. Reception: Receiving antenna receives or picks up energy from the various EM waves
radiated by radio transmitter

ii. Selection: This consists in selecting or responding to desired radio wave with
the exclusion of all others.

iii. Detection or demodulation: The desired signal in the form of a modulated carrier
voltage is detected in a detector circuit to recover the original modulating voltage.

iv. Reproduction: This consists in feeding the detected signal to a loudspeaker or headphones
to reproduce the sound waves giving the original programme.
Design of Receiver

• The radio receiver has to be cost effective

• Requirements:
• Has to work according to application as for AM or FM
signals
• Tune to and amplify desired radio station
• Filter out all other stations
• Demodulator has to work with all radio stations
regardless of carrier frequency
 Features
i. Simplicity of Operation: These receivers are required to be handled by listeners who have
Little technical knowledge and hence simplicity of operation is essential.
ii. Good Fidelity: Fidelity of a receiver is it’s ability to reproduce exact replica of the
transmitted signals at the receiver output.
iii. Good Selectivity: The selectivity of an AM receiver is defined as it’s ability to accept or
select the desired band of frequency and reject all other unwanted frequencies which can be
interfering signals.

iv. Average Sensitivity: Sensitivity of a receiver is it’s ability to identify and amplify weak
signals at receiver output.

v. Adaptability to different types of aerials: The receiving aerial intercepts EM waves. A

broadcast receiver should be designed to operate satisfactory with any type of aerial.
 Classification of Radio Receivers

Depending upon application

• AM Receivers - receive broadcast of speech or music
from AM transmitters which operate on long wave,
medium wave or short wave bands.

• FM Receivers – receive broadcast programs from FM

transmitters which operate in VHF or UHF bands.
•Communication Receivers - used for reception
of telegraph and short wave telephone signals.

• Television Receivers - used to receive television

broadcast in VHF or UHF bands.

• Radar Receivers – used to receive radio

detection and ranging signals.
•Principles
•Frequency Spectrum Sharing
(many transmitters using one
medium)
•Demodulating desired signal
and rejecting other signals
transmitted at the same time
Receivers
•Radio system should be able to receive any type of
audio source simultaneously.

• Different stations with different sources transmit

signals simultaneously.

• Different listeners tune to different stations

simultaneously.
 Types of Receiver

1. Tuned Radio-Frequency (TRF) or Straight

Receivers (Operates in a straight forward
manner without frequency conversion)
2. Superheterodyne Receiver (To heterodyne
means to mix. Received RF signal is
converted to standard Inter-mediate (IF)
frequency before detection takes place)
 Tuned Radio-Frequency Receiver

Antenna
coupling RF RF RF
network amp. amp. amp.

Audio Audio
detector amplifier

Figure: Block Diagram of a TRF Receiver

Tuned Radio Frequency (TRF) Receiver:

• Composed of RF amplifiers and detectors.

• No frequency conversion

• It is not often used.

• Difficult to design tunable RF stages.

• Difficult to obtain high gain RF amplifiers

•Selectivity decreases as the carrier frequency

increases
 Homework
• For an AM receiver (TRF) commercial broad cast band receiver
(535KHz to 1.605MHz) with an input filter Q factor of 54 , determine
the bandwidth at the low and high ends of RF spectrum. Also explain
why band rejection takes place in TRF.
 f 540
Band width at low frequency B   10 KHz
Q Q

f 1600
Band width at high frequency B   29630 Hz
Q 54
-3dB band width at low frequency is 10KHz but at high frequency 3 times that of the
low frequencies.

Tuning at high end of the spectrum three stations would be received

simultaneously.

To achieve band width of 10KHz at high frequencies a Q of 160dB is required but

with a Q of 160 the band width at low frequencies is
f 540
B   3375Hz
Q 160
It is too selective and band rejection will takes place.
13
 SUPERHETERODYNE RECEIVER

A superheterodyne receiver, often shortened to superhet, is a type

of radio receiver that uses frequency mixing to convert a received
signal to a fixed intermediate frequency (IF) which can be more
conveniently processed than the original carrier frequency. It was
invented by US engineer Edwin Armstrong in 1918 during World
War I. Virtually all modern radio receivers use the super heterodyne
principle
"Superheterodyne" is a contraction of "supersonic heterodyne",
where "supersonic" indicates frequencies above the range of
human hearing. The word heterodyne is derived from the Greek
roots hetero- "different", and -dyne "power". In radio applications
the term derives from the "heterodyne detector" pioneered by
Canadian inventor Reginald Fessenden in 1905, describing his
proposed method of producing an audible signal from the Morse
code transmissions of the new continuous wave transmitters.
The French engineer Lucien Lévy filed a patent application for the
superheterodyne principle in August 1917. The American Edwin Howard
Armstrong also filed a patent in 1917. Levy filed his original disclosure about
seven months before Armstrong's. The German inventor Walter H. Schottky also
filed a patent in 1918. At first the US recognized Armstrong as the inventor, and
his US Patent 1,342,885 was issued on 8 June 1920. After various changes and
court hearings Levy was awarded a US patent No 1,734,938 that included seven
of the nine claims in Armstrong's application, while the two remaining claims
were granted to Alexanderson of GE and Kendall of AT&T.
 Mixer / Converter
Section

RF Section Mixer IF Section

Pre RF Band pass IF

selector amplifier filter Amplifier

IF signal
RF signal

Local
Oscillator

Gang tuning
Audio amplifier
Audio detector
Section
Section

Audio AM
speaker Amplifier Detector

Audio Frequencies
Different Sections of a Superhetrodyne Receiver:

• Downconvert RF signal to lower IF frequency

Main amplification takes place at IF
 A Radio Frequency (RF) section

 An RF-to-IF converter (mixer)

 An Intermediate Frequency (IF) section

 Demodulator

 Audio amplifier
 A constant frequency difference is maintained between the local oscillator and the RF
circuits, normally through capacitance tuning, in which all the capacitors are ganged together
and operated in unison by one control knob.

 Since the characteristics of the IF amplifier are independent of the frequency to which the
receiver is tuned, the selectivity and sensitivity of the superhet are usually uniform throughout
it’s tuning range and not subject to the variations that affects the TRF receiver
 RF Section
RF Section

Pre-selector Amplifier stage

It determines the sensitivity

Broad tuned band pass filter of the receiver.
with adjustable frequency
that is tuned to carrier
frequency RF amplifier is the
first active device in
the network it is the Receiver may have
primary contributor one or more RF
Provide initial band Reduces the noise to the noise. And it is amplifier depending
limiting to prevent bandwidth of the receiver the predominant on the desired
specific unwanted and provides the initial step factor in determining sensitivity.
radio frequency toward reducing the over all the noise figure.
called image Due to RF amplifier
receiver bandwidth to the
frequency from Greater gain and better sensitivity
minimum bandwidth
entering into Improved image frequency rejection
required to pass the
receiver. Better signal to noise ratio
information signal.
Better selectivity.
 Intermediate Frequencies and IF Amplifiers:

Choice of Frequency: The intermediate frequency (IF) of a

receiving system is usually a compromise, since there are reasons
why it should be neither low nor high, nor in a certain range
between the two. The following are the major factors influencing
the choice of the intermediate frequency in any particular system:

l . If the intermediate frequency is too high, poor selectivity and

poor adjacent channel rejection result unless sharp cutoff (e.g.,
crystal or mechanical) filters are used in the IF stages.

2. A high value of intermediate frequency increases tracking

difficulties.
3. As the intermediate frequency is lowered, image-
frequency rejection becomes poorer. It is showed that
rejection is improved as the ratio of image frequency to
signal frequency is increased; and this requires a high IF. It
is seen that image-frequency rejection becomes worse as
signal frequency is raised.

4. A very low intermediate frequency can make the

selectivity too sharp, cutting off the sidebands. This
problem arises because the Q must be low when the IF is
low, unless crystal or mechanical filters are used, and
therefore the gain per stage is low. A designer is more
likely to raise the Q than to increase the number of IF
amplifiers.
5. Lf the IF is very low, the frequency stability of the local
oscillator must be made correspondingly higher because any
frequency drift is now a larger proportion of the low IF than
of a high IF.

6. The intermediate frequency must not fall within the

tuning range of the receiver, or else instability will occur and
heterodyne whistles will be heard, making it impossible to
tune to the frequency band immediately adjacent to the
intermediate frequency.
Sensitivity: The sensitivity of a radio receiver is its ability to
amplify weak signals. It is often defined in terms of the voltage
that must be applied to the receiver input terminals to give a
standard output power, measured at the output terminals.

For AM broadcast receivers, several of the relevant quantities

have been standardized. Sensitivity is often expressed in micro
volts or in decibels below 1 V and measured at three points
along the tuning range when a production receiver is lined up. It
is seen from the sensitivity curve in Fig. That sensitivity varies
over the tuning band.

The most important factors determining the sensitivity of a

super heterodyne receiver are-the gain of the IF amplifier(s) and
that of the RF amplifier. It is obvious that the noise figure plays
an important part.
Fig: sensitivity curve for good domestic receiver
Selectivity:

The selectivity of a receiver is its ability to reject

unwanted signals. It is expressed as a curve, such as the
one of following Fig., which shows the attenuation that
the receiver offers to signals at frequencies near to the
one to which it is tuned. Selectivity is measured at the
end of a sensitivity test with conditions the same as for
sensitivity, except that now the frequency of the
generator is varied to either side of the frequency to
which the receiver is tuned. Looking at the curve, we see
that at 20 kHz below the receiver tuned frequency, an
interfering signal would have to be 60 dB greater than
the wanted signal to come out with the same amplitude.
 Fig: Typical selectivity curve

It should be noted that selectivity determines the adjacent-channel rejection

of a receiver.