Synchronous demodulation / detection

 

Synchronous demodulation / detection
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Today's radio receivers offer very high levels of performance and boast many facilities.
Many radio receivers incorporate memories, phase locked loops, direct digital synthesis,
digital signal processing and much more. One facility that can be very useful on the short
wave bands is synchronous detection or synchronous demodulation as this can give
much improved performance for receiving amplitude modulation (AM) transmissions.
Unfortunately little is written about this form of modulation, and often it is a matter of
accepting that it must be better than any normal options because it is included as a
feature in the receiver specification.
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Synchronous detection is used for the detection or demodulation of amplitude modulation
(AM). This form of modulation is still widely used for broadcasting on the long, medium
and short wave bands despite the fact that there are more efficient forms of modulation
that can be used today. The main reason for its use nowadays is that it is very well
established, and there are many millions of AM receivers around the world today.
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In any receiver a key element is the detector. Its purpose is to remove the modulation
from the carrier to give the audio frequency representation of the signal. This can be
amplified by the audio amplifier ready to be converted into audible sound by headphones
or a loudspeaker. Many receivers still use what is termed an envelope detector using a
semiconductor diode for demodulating AM. These detectors have a number of
disadvantages. The main one is that they are not particularly linear and distortion levels
may be high. Additionally their noise performance is not particularly good at low signal
levels.
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These detectors also do not perform very well when the signal undergoes selective
fading as often occurs on the short wave bands. An AM signal contains two sidebands
and the carrier. For the signal to be demodulated correctly the carrier should be present
at the required level. It can be seen that the signal covers a definite bandwidth, and the
effects of fading may result in the carrier and possibly one of the sidebands being
reduced in level. If this occurs then the received signal appears to be over-modulated
with the result that distortion occurs in the demodulation process.
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The spectrum of an amplitude modulated signal
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Diode envelope detector
In virtually every receiver a simple diode envelope detector is used. These circuits have
the advantage that they are very simple and give adequate performance in many
applications.
The circuit of a typical detector is shown in Figure 2. Here the diode first rectifies the
signal to leave only the positive or negative going side of the signal, and then a capacitor
removes any of the remaining radio frequency components to leave the demodulated
audio signal. Unfortunately diodes are not totally linear and this is the cause of the
distortion.
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An envelope detector for AM signals
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What is synchronous demodulation
Signals can be demodulated using a system known as synchronous detection or
demodulation. This is far superior to diode or envelope detection, but requires more
circuitry. Here a signal on exactly the same frequency as the carrier is mixed with the
incoming signal as shown in Figure 2. This has the effect of converting the frequency of
the signal directly down to audio frequencies where the sidebands appear as the
required audio signals in the audio frequency band.
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The crucial part of the synchronous detector is in the production a local oscillator signal
on exactly the same frequency as the carrier. Although it is possible to receive an AM
signal without the local oscillator frequency on exactly the same frequency as the carrier
this is the same as using the BFO in a receiver to resolve the signal. If the BFO is not
exactly on the same frequency as the carrier then the resultant audio is not very good.
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Synchronous demodulation
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Fortunately this is not too difficult to achieve and although there are a number of ways of
achieving this the most commonly used method is to pass some of the signal into a high
gain limiting amplifier. The gain of the amplifier is such that it limits, and thereby
removing all the modulation. This leaves a signal consisting only of the carrier and this
can be used as the local oscillator signal in the mixer as shown in Fig. 4. This is most
convenient, cheapest and certainly the most elegant method of producing synchronous
demodulation.
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A synchronous detector using a high gain-limiting amplifier to extract the carrier
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Advantages of synchronous detection
A synchronous detector is more expensive to make than an ordinary diode detector when
discrete components are used, although with integrated circuits being found in many
receivers today there is little or no noticeable cost associated with its use as the circuitry
is often included as part of an overall receiver IC.
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Synchronous detectors are used because they have several advantages over ordinary
diode detectors. Firstly the level of distortion is less. This can be an advantage if a better
level of quality is required but for many communications receivers this might not be a
problem. Instead the main advantages lie in their ability to improve reception under
adverse conditions, especially when selective fading occurs or when signal levels are low.
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Under conditions when the carrier level is reduced by selective fading, the receiver is
able to re-insert its own signal on the carrier frequency ensuring that the effects of
selective fading are removed. As a result the effects of selective fading can be removed
to greatly enhance reception.
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The other advantage is an improved signal to noise ratio at low signal levels. As the
demodulator is what is termed a coherent modulator it only sees the components of
noise that are in phase with the local oscillator. Consequently the noise level is reduced
and the signal to noise ratio is improved.
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Unfortunately synchronous detectors are only used in a limited number of receivers
because of their increased complexity. Where they are used a noticeable improvement in
receiver performance is seen and when choosing a receiver that will be used for short
wave broadcast reception it is worth considering whether a synchronous detector is one
of the facilities that is required.

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