Transcript Superhetrodyne

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Superheterodyne receiver uses frequency mixing
or heterodyning to convert a received signal to a
fixed intermediate frequency
More conveniently processed than the original radio carrier
frequency
Virtually all modern radio and television receivers use the
superheterodyne principle.
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The principle of operation of the superheterodyne
receiver depends on use of heterodyningor frequency
mixing.
The signal from the antenna is filtered sufficiently at
least to reject the image frequency and possibly
amplified.
A local oscillator in the receiver produces a sine wave
which mixes with that signal, shifting it to a
specific intermediate frequency(IF), usually a lower
frequency.
The IF signal is filtered and amplified and possibly
processed in additional ways. The demodulator uses
the IF signal rather than the original radio frequency to
recreate a copy of the original modulation (such as
audio).
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Block diagram of a typical superheterodyne receiver.
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The diagram at right shows the minimum requirements for a
single-conversion superheterodyne receiver design.
The following essential elements are common to all
superhet circuits:
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receiving antenna, a tuned stage which may optionally contain
amplification (RF amplifier),
a variable frequency local oscillator, a frequency mixer, a
band pass filter and intermediate frequency (IF) amplifer
a demodulator plus additional circuitry to amplify or process
the original audio signal (or other transmitted information).
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The received signal is now processed by
the demodulator stage where the audio signal is recovered
and then further amplified.
AM demodulation requires the simple rectification of the RF
signal (so-called envelope detection), and a simple RC low
pass filter to remove remnants of the intermediate
frequency.
FM signals may be detected using a discriminator, ratio
detector, or phase-locked loop.
 CW (morse code)and single sideband signals require
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a product detector using a so-called beat frequency
oscillator, and there are other techniques used for different
types of modulation.
The resulting audio signal (for instance) is then amplified
and drives a loudspeaker.
Called high-side injection has been used, where the local
oscillator is at a higher frequency than the received signal
(as is common), then the frequency spectrum of the original
signal will be reversed.
Must be taken into account by the demodulator (and in the
IF filtering) in the case of certain types of modulation such
as single sideband
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Early Autodyne receivers typically used IFs of only
150 kHz
Difficult to maintain reliable oscillation if higher
frequencies were used
Most Autodyne receivers needed quite elaborate
antenna tuning networks
often involving double-tuned coils, to avoid image
interference superhets used tubes especially designed
for oscillator/mixer use, which were able to work
reliably with much higher IFs, reducing the problem of
image interference and so allowing simpler and
cheaper aerial tuning circuitry.
For medium-wave AM radio, a variety of IFs have
been used, but usually 455 kHz is used.
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To work reliably with much higher Ifs
Reducing the problem of image interference and so allowing
simpler and cheaper aerial tuning circuitry.
For medium-wave AM radio, a variety of IFs have been
used, but usually 455 kHz is used.