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Receiver Primer
There seems to be a lot of question about receivers in general. Most of the information can be found on the Internet and also in the various forums online. However, some of the information can be misleading and in some cases totally wrong.Basic PrinciplesEarly receivers consisted on a tuning coil and a detector with the output being fed to a high impedance headphones. Technology has advanced somewhat since those early days but in reality the principle remains the same.Receivers are now, to give it its proper title, Superhet Receivers. Superhet derives from the full description of Supersonic Heterodyne. Supersonic meaning "Of or relating to sound waves beyond human audibility." and Heterodyne meaning "of or relating to the the beat produced by heterodyning two oscillations v : combine (a radio frequency wave) with a locally generated wave of a different frequency so as to produce a new frequency equal to the sum or the difference between the two" |
The diagram shown is a simple receiver, the
plug in crystal runs at 34.675 MHz which is the correct frequency
for channel 73 (35.130 MHz). This forms part of the Local oscillator
which feeds into a Mixer Circuit. An incoming signal at 35.130
MHz is amplified by the RF Amp and is fed to the other input
to the Mixer. The mixer generates the Sum and Difference signals,
plus various other signals but these are not really relevant
at the moment. The Sum signal is at 69.805 MHz and the Difference
Signal is at 0.455 MHz (455 KHz). This frequency is the one
that we need ! This signal is then amplified and filtered before
being fed to the Detector and Decoder stage. The output of the
Decoder stage are the Servo Signals.
Very simple but quite complicated in some respects.
There are various difficulties to overcome in designing receivers,
one of which is how do we achieve good selectivity (the ability
to only respond to our chosen channel and not the next channel
that is in use). The design calls for multiple tuned circuits
in order to achieve selectivity at the IF level, this is so
much easier if the IF is fixed and filters are used. At higher
RF frequencies, it is very difficult and expensive to make narrow
filters for the narrow band signals that we are dealing with.
Another question is why do we have such an odd Intermediate
Frequency ? Industry has rationalised certain frequencies as
standard IF's, these are 455KHz, 1.4 MHz, 10.7 MHz, 45 MHz and
70 MHz. Using standard frequencies means that ceramic filters
are low cost and easily available. Using non-standard frequencies
and bandwidths would be much more expensive to make.
The mixer is used to convert from one frequency
to another, but these create unwanted frequencies. As stated
above, the Sum and Diff are just 2 of the frequencies created
apart from the IF that we are needing. Various harmonics are
created and effectively a noisy mess is created at the output
of the mixer. The IF Amplifier will be tuned to accept the 455
KHz but some of the noise will get through. The ceramic/crystal
filter has a narrow bandwidth of typically 6 KHz and will remove
virtually all of the unwanted signals before feeding the 455
KHz signal to the detector.
However, as the mixer mixes everything and
anything, suppose there was another signal at 34.220 MHz. This
would be mixed with the Local Oscillator frequency (34.675 )
and will produce a signal at.... 455 KHz ! This frequency is
called the "Image Frequency" and is something that
we do NOT want to happen !
There are ways to prevent this from happening,
if a larger IF is used the suppression of the image signal is
easier but this makes the selectivity difficult to achieve.
One way to overcome these obstacles is by using what is known
as a Dual Conversion Receiver. A High first IF will give good
image rejection and a low second IF will give good selectivity.
As can be seen above, the new Dual Conversion
crystal is now running at 24.430 MHz a full 10.7MHz below the
wanted signal of 35.130 MHz (35.130 - 24.430 = 10.7). This crystal
frequency is fed into the Mixer Mix1 where the resulting output
is filtered by IF1. This will be a 10.7 MHz ceramic filter so
ONLY the 10.7 MHz signal will be passed through. This is in
turn fed to Mixer Mix2 which has its other signal being derived
from an onboard fixed crystal running at 11.155 MHz (11.155
- 10.7 = 0.455). The output of Mix2 is fed into IF2 which will
have a 455 KHz ceramic filter and the output will only be the
455 KHz wanted signal. this is now passed on for processing
and decoding.
The diagram shown is a simple receiver, the plug in crystal runs at 34.675 MHz which is the correct frequency for channel 73 (35.130 MHz). This forms part of the Local oscillator which feeds into a Mixer Circuit. An incoming signal at 35.130 MHz is amplified by the RF Amp and is fed to the other input to the Mixer. The mixer generates the Sum and Difference signals, plus various other signals but these are not really relevant at the moment. The Sum signal is at 69.805 MHz and the Difference Signal is at 0.455 MHz (455 KHz). This frequency is the one that we need ! This signal is then amplified and filtered before being fed to the Detector and Decoder stage. The output of the Decoder stage are the Servo Signals.Very simple but quite complicated in some respects. There are various difficulties to overcome in designing receivers, one of which is how do we achieve good selectivity (the ability to only respond to our chosen channel and not the next channel that is in use). The design calls for multiple tuned circuits in order to achieve selectivity at the IF level, this is so much easier if the IF is fixed and filters are used. At higher RF frequencies, it is very difficult and expensive to make narrow filters for the narrow band signals that we are dealing with. Another question is why do we have such an odd Intermediate Frequency ? Industry has rationalised certain frequencies as standard IF's, these are 455KHz, 1.4 MHz, 10.7 MHz, 45 MHz and 70 MHz. Using standard frequencies means that ceramic filters are low cost and easily available. Using non-standard frequencies and bandwidths would be much more expensive to make.The mixer is used to convert from one frequency to another, but these create unwanted frequencies. As stated above, the Sum and Diff are just 2 of the frequencies created apart from the IF that we are needing. Various harmonics are created and effectively a noisy mess is created at the output of the mixer. The IF Amplifier will be tuned to accept the 455 KHz but some of the noise will get through. The ceramic/crystal filter has a narrow bandwidth of typically 6 KHz and will remove virtually all of the unwanted signals before feeding the 455 KHz signal to the detector.However, as the mixer mixes everything and anything, suppose there was another signal at 34.220 MHz. This would be mixed with the Local Oscillator frequency (34.675 ) and will produce a signal at.... 455 KHz ! This frequency is called the "Image Frequency" and is something that we do NOT want to happen !There are ways to prevent this from happening, if a larger IF is used the suppression of the image signal is easier but this makes the selectivity difficult to achieve. One way to overcome these obstacles is by using what is known as a Dual Conversion Receiver. A High first IF will give good image rejection and a low second IF will give good selectivity. |
As can be seen above, the new Dual Conversion crystal is now running at 24.430 MHz a full 10.7MHz below the wanted signal of 35.130 MHz (35.130 - 24.430 = 10.7). This crystal frequency is fed into the Mixer Mix1 where the resulting output is filtered by IF1. This will be a 10.7 MHz ceramic filter so ONLY the 10.7 MHz signal will be passed through. This is in turn fed to Mixer Mix2 which has its other signal being derived from an onboard fixed crystal running at 11.155 MHz (11.155 - 10.7 = 0.455). The output of Mix2 is fed into IF2 which will have a 455 KHz ceramic filter and the output will only be the 455 KHz wanted signal. this is now passed on for processing and decoding. |