The problem is the detection process. The solution would be a synchronous
detector.
With the conventional envelope detector, everything within the passband of
the receiver beats with everything else. The AM carrier beats not only with
the sidebands, but with every bit of noise. Noise components on every
freqency beat with all other noise components within the passband. And
every noise component beats separately with the upper sideband and the lower
sideband.
A synchonous detector is basically a product detector. With the product
detector, the only output signal components are the beat note products
between the carrier and the sidebands, and the carrier and the noise
components. Theoretically, there are no beat components between the noise
and the sideband components, or between various noise components. The
result is a better signal to noise ratio because of the absence of all the
intermodulation of sideband, carrier and noise components that occurs with
an envelope detector, such as the conventional diode detector.
The problem with attempting to copy AM using a SSB type product detector is
that in order for the demodulated upper and lower sideband components to
vectorially add in the proper manner, the re-inserted carrier must not only
be on the exact frequency as the original AM carrier; it must be exactly in
phase or exactly 180 degrees out of phase. In other words, the reinserted
carrier of the BFO must be in synchronism with the original AM carrier. One
way this can be accomplished is by using PLL technology.
The vectorial addition of the components of both sidebands actually gives
the demodulated signal an additional 6 dB boost in amplitude. But because
the DSB signal requires twice the receiver bandpass, twice the background
noise (3 more dB) is admitted, so the actual boost in s/n ratio is 3 dB
instead of 6. So with a product detector, the AM signal should be at least
as readable in the noise, if not more, than a SSB signal of equivalent
sideband power. The majority of the "advantage" that SSB is said to enjoy
over AM is due to the fact that SSB is demodulated with a more efficient
detector, the product detector. With a proper synchonous product detector,
the SSB advantage is far less. In addition, the carrier of the AM signal
can be reduced or even suppressed, since it is the re-inserted carrier that
demodulates the sidebands. With the envelope detector, the original AM
carrier has to do all the work. When the original carrier fades, you get
selective fading distortion with the envelope detector.
A particular configuration of the synchonous detector, called the Costas
Loop, can determine the frequency/phase of the original carrier strictly
from phase relationships between the two coherent sidebands, without using
the original carrier as a reference. Double-sideband suppressed carrier can
give a 3 dB improvement in s/n ratio over single-sideband suppressed
carrier, running the same total sideband power.
A PLL synchronous detector that uses the original AM carrier as reference
can allow for "exalted carrier" reception which allows DSB AM to function
at least at least 3 dB better, under noise conditions, as a SSB signal with
equivalent total sideband power. This type of detector works with DSB
reduced carrior AM. The AM carrier can be reduced without being totally
suppressed, as long as the residual, or "pilot" carrier is sufficent for the
synchronous detector to latch onto.
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