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 long believed to have been invented by US engineer Edwin Armstrong,

1. NAME :
ENROLL.:
SUB: E.COM
TOPIC : SUPER HETERODYNE
AM RECEIVER

2. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged

3.  A super- heterodyne circuit is used almost all the
transmitter radios (expecting a few used for
receiving local station).
 In this type of circuit the frequency of the incoming
signal is changed into a fixed frequency known as
Intermediate Frequency (IF). This permits higher
amplification per stage and better selectivity .
 It consist of following parts/ stages.
 1) RF Amplifier
 2)Mixer and oscillator
 3)IF amplifier
 4)Detector and AGC
 5)Audio Pre amplifier and driver
 6)Power amplifier

4. RF AMPLIFIER
 The first stage is a tuned RF Amplifier. The antenna
signal is fed to the RF amplifier
 The signal at the antenna has the lowest signal to
noise ratio . The main purpose of which is to
improve signal to noise ratio. It also gives some
improvement in RF selectivity and reduction of
oscillator re-radiation.
 However, it is usually omitted in cheaper receiver .
The output from the tuned RF stage is fed to the
signal input of an oscillator - mixer circuit in which
the heterodyne takes place.

5. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged

6. MIXER AND OSCILLATOR
 The output signal from the RF amplifier is coupled into
the mixer amplifier .
 The word mixer automatically indicates that there is
more than one signal fed in to section . Indeed, There
are two input signals, with the second signal coming fro
the local oscillator.
 The oscillator produces oscillation at a frequency , which
is higher than the frequency of the station receive.
 The difference between frequencies equal the
intermediate frequency (IF).Out of the signals
intercepted by the aerial, the signal from the desired
station are selected by the tuned circuit used at the input
of the mixer stage .These signals and the oscillations
from the oscillators are mixed by the mixer. The mixing
of these signals results in the production of their sum
(fo-fs) and (fo+fs)frequencies . The difference is IF
signal.

7. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged

8. IF AMPLIFIER
 It amplifies the IF signals . It provides a large part of
the gain and selectivity. This is one big advantage
of the heterodyne receiver.
 Two stages are used in this section to obtain the
required amplification . Tuned transformer knows as
IFT are used as a collector load and for the inter
stage coupling in the IF amplification . The
amplified signals are given to the detector.

9. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged

10. DETECTOR AND AGC
 The detector separates the audio signal from the modulated
carrier. There are several circuit for the detection of AM wave .
All use a diode rectifier and a low pass filter . The diode
rectifier recovers the negative half cycle of the modulated
wave in some cases and the positive half cycle in other cases.
 The sound received will depend on the strength of the signals
received at the antenna. The strength of the received signal
can vary widely due to the fading. This will cause variations in
sound , which can be annoying. Moreover the strength of the
signals can also be too large in close vicinity of MW
transmitter causing overloading of the second IF amplification.
 AGC is used to minimize the variation in sound with charges
in signal strength and prevent overloading. The AGC delivers
a varying bias signal which is proportional to the average
received signal strength and uses this bias t vary the gain of
one or more IF stages. When the average signal level
increases, the size of AGC bias increase and the gain of the
controlled stages decreases. When there is no signal, there is
a minimum AGC bias the amplifier produce maximum gain.

11. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged

12. AUDIO PRE AMPLIFIER AND DRIVER
 The audio signals received from the detector are
given to the pre amplification stage. This stage
amplifies these signals and gives to the driver
Stage .
 The Signal obtained from the pre - amplification are
not strong enough to drive the output stage. The
driver stage further amplifies these signals and
gives it to the output stages (power amplification).

13. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged

14. POWER AMPLIFIER
 The output stage amplifies the audio signals
obtained from the driver and develops sufficient
power to driver the loudspeaker . To obtain
maximum economy in battery consuption , push
pull output stages using two transistor in class B
are normally used.

15. RF
AMPLIFIE
R
LOCAL
OSCI.
MIXER
1ST IF
AMP.
2ND IF
AMP.
POWER
AMP
AUDIO
PRE AMP.
DETECTO
R AND
AGC
LOUDSPEAKER
ANTENNA
AGC
Ganged