2. A receiver is an electronic equipment which picks
up any desired radio frequency signal.
Amplifying the desired signal to the requisite level.
Demodulating RF signal.
Feeding modulating voltage to an indicator (loud
3. Classification of Radio Receivers
Based on the Principle of Operation
Straight or TRF Receivers
Operation is straight forward without any frequency conversion or
Super heterodyne receivers
Incoming signal frequency is converted into standard
intermediate frequency (IF) with the help of frequency converter
4. Classification of Radio Receivers
According to the application (type of traffic they are
designed to handle)
Receives speech or music radiated from AM transmitter operating on
long wave , medium wave (broadcast band) or short wave
Receives signals from FM transmitters operating in VHF or UHF bands
Receives television broadcast in VHF or UHF bands
To receive telephony and telegraphic signals
Receives radar signals
5. Characteristics of Radio Receivers
• Ability to receive weak signals
• Ability to select desired signal and reject all other
• Faithful reproduction of original signal
Signal to Noise ratio
• A good receiver should have high SNR
6. TRF (Tuned Radio Frequency )Receiver
A tuned radio frequency receiver actually
tunes the receiver on the true radio frequency
without any frequency conversion
7. TRF (Tuned Radio Frequency )Receiver
Selects the desired station and amplifies weak antenna
signal(Normally a wideband amplifier tunable from approx 535
kHZ to 1650 KHZ)(standard commercial AM band)
Two or three stages of RF amplifiers are required to filter and
amplify the received signal to a level sufficient to drive the
8. TRF (Tuned Radio Frequency )Receiver
Demodulates the AM wave and converts it to the original
It used a form of detection called envelope detection and used a
diode to rectify the signal.
9. TRF (Tuned Radio Frequency )Receiver
Used to amplify the recovered signal
To drive the speaker
10. TRF receiver
• TRF receivers are simple and have a relatively high
• Easy to design at broadcast frequency 535 kHz to
22. Local Oscillator
Variable oscillator capable of generating a signal from 0.995 MHz to
What should be the frequency of the local oscillator used for
translation from RF to IF?
fLO = fs + fIF (up-conversion)
or fLO = fs - fIF (down-conversion)
Tuning ratio = fLO, max / fLO, min
Up-Conversion: (1600 + 455) / (530+455) ≈ 2
Down-Conversion: (1600–455) / (530–455) ≈ 12
Easier to design oscillator with small tuning ratio.
28. Choice of IF Frequency
Image is 2x IF away from the wanted frequency
◦ Larger IF frequency makes suppression of image easier
◦ Too low an IF and the RF input filters are too difficult
Tuning range of receiver cannot cross the IF
◦ Hence HF receivers often have a very high 1st IF, >60MHz
Realistic RF filtering usually forces the choice of 1st IF.
◦ This may not be good for selectivity!
Hence a second lower IF is often used – DUAL CONVERSION
◦ High 1st IF gives good image rejection
◦ Low 2nd IF gives good selectivity
NBFM (2.5kHz dev) demodulation also requires a low IF, 455kHz
◦ For WBFM (75kHz dev) it can be greater, 10.7MHz
31. Superheterodyne Receivers
Ganged tuning is the process used to tune two or more
circuits with a single control.
In our example, when you change the frequency of the
receiver all three stages change by the same amount.
There is a fixed difference in frequency between the local
oscillator and the RF amplifier at all times.
This difference in frequency is the IF.
This fixed difference and ganged tuning ensures a constant
IF over the frequency range of the receiver.
33. Superheterodyne Receivers
• The superheterodyne receiver offers superior sensitivity, frequency
stability and selectivity.
• Compared with the tuned radio frequency receiver (TRF) design,
superheterodyne offer better stability.
34. Superheterodyne Receivers
• One major disadvantage to the superheterodyne receiver is the problem
of image frequency.
• In heterodyne receivers, an image frequency is an undesired input
frequency equal to the station frequency plus twice the intermediate
• 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 superheterodyne
37. Amplitude Limiter
• Most frequency discriminators use envelope detection to extract the
intelligence from the FM wave form
• Envelope detection will demodulate incident amplitude variations as
well as frequency variation
• Transmission noise and interference add to the signal to produce
unwanted amplitude variations
38. Amplitude Limiter
• In the receiver, unwanted AM and random noise are demodulated
along with the signal: unwanted distortion is produced
• A limiter circuit is used to produce a constant amplitude output for all
input signal above a specified threshold level
39. AM Vs FM
Radio AM Radio FM
Carrier range RF 535 – 1605 KHz 88 – 108 MHz
IF 455 kHz 10.7 MHz
Bandwidth 10 kHz 200 kHz