SUPERHETERODYNE
RECEIVER:
A superheterodyne receiver(often shortened to superhet) uses
frequency mixing to convert a received signal to a fixed intermediate frequency
(IF) which can be more conveniently processed than the original radio carrier
frequency.
ü Basic
Superheterodyne Block Diagram and Functionality:
The basic
block diagram of a basic superhet receiver is shown below. This details the
most basic form of the receiver and serves to illustrate the basic blocks and
their function.
The way
in which the receiver works can be seen by following the signal as is passes
through the receiver.
·
Front end amplifier and tuning block: Signals enter
the front end circuitry from the antenna. This circuit block performs two main
functions:
o Tuning: Broadband tuning is applied to
the RF stage. The purpose of this is to reject
the signals on the image frequency and accept those on the wanted frequency. It
must also be able to track the local oscillator so that as the receiver is
tuned, so the RF tuning remains on the required frequency. Typically the
selectivity provided at this stage is not high. Its main purpose is to reject
signals on the image frequency which is at a frequency equal to twice that of
the IF away from the wanted frequency. As the tuning within this block provides
all the rejection for the image response, it must be at a sufficiently sharp to
reduce the image to an acceptable level. However the RF tuning may also help in
preventing strong off-channel signals from entering the receiver and
overloading elements of the receiver, in particular the mixer or possibly even
the RF amplifier.
Amplification: In terms of amplification, the
level is carefully chosen so that it does
not overload the mixer when strong signals are present, but enables the signals
to be amplified sufficiently to ensure a good signal to noise ratio is
achieved. The amplifier must also be a low noise design. Any noise introduced
in this block will be amplified later in the receiver.
·
Mixer /
frequency translator block: The tuned and amplified signal
then enters one port of the mixer.
The local oscillator signal enters the other port. The performance of the mixer
is crucial to many elements of the overall receiver performance. It should eb
as linear as possible. If not, then spurious signals will be generated and
these may appear as 'phantom' received signals.
·
Local
oscillator: The local oscillator may consist of a variable
frequency oscillator that can be
tuned by altering the setting on a variable capacitor. Alternatively it may be
a frequency synthesizer that will enable greater levels of stability and
setting accuracy.
·
Intermediate
frequency amplifier, IF block : Once the signals leave the mixer
they enter the IF stages. These
stages contain most of the amplification in the receiver as well as the
filtering that enables signals on one frequency to be separated from those on
the next. Filters may consist simply of LC tuned transformers providing
inter-stage coupling, or they may be much higher performance ceramic or even
crystal filters, dependent upon what is required.
·
Detector
/ demodulator stage: Once the signals have passed through the IF stages
of the superheterodyne receiver,
they need to be demodulated. Different demodulators are required for different
types of transmission, and as a result some receivers may have a variety of
demodulators that can be switched in to accommodate the different types of
transmission that are to be encountered. Different demodulators used may
include:
o AM diode detector: This is
the most basic form of detector and this circuit block would simple consist of a diode and possibly a small capacitor to
remove any remaining RF. The detector is cheap and its performance is adequate,
requiring a sufficient voltage to overcome the diode forward drop. It is also
not particularly linear,
and finally it is subject to the effects of selective fading that can be
apparent, especially on the HF bands.
Synchronous AM detector: This form
of AM detector block is used in where improved
performance is needed. It mixes the incoming AM signal with another on the same
frequency as the carrier. This second signal can be developed by passing the
whole signal through a squaring amplifier. The advantages of the synchronous AM
detector are that it provides a far more linear demodulation performance and it
is far less subject to the problems of selective fading.
o
SSB
product detector: The SSB product detector block consists of a mixer
and a local oscillator, often termed
a beat frequency oscillator, BFO or carrier insertion oscillator, CIO. This
form of detector is used for Morse code transmissions where the BFO is used to
create an audible tone in line with the on-off keying of the transmitted
carrier. Without this the carrier without modulation is difficult to detect.
For SSB, the CIO re-inserts the carrier to make the modulation comprehensible.
o Basic FM detector: As an FM
signal carries no amplitude variations a
demodulator block that senses frequency variations is required. It should
also be insensitive to amplitude variations as these could add extra noise.
Simple FM
detectors
such as the Foster Seeley or ratio detectors can be made from discrete components
although they do require the use of transformers.
o PLL FM detector: A phase locked loop can be used to make a very good FM demodulator. The incoming FM signal can be fed into the reference input, and the VCO drive voltage used to provide the detected audio output.
o
Quadrature FM detector: This form of FM detector
block is widely used within ICs. IT is simple to implement and provides a good
linear output.
·
Audio
amplifier: The output from the demodulator is the recovered
audio. This is passed into the audio
stages where they are amplified and presented to the headphones or loudspeaker.
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