From: Gary Schafer <[EMAIL PROTECTED]>
When you tune off to one side you cut off the other side
band and you loose 1/2 of the power. That drops the signal down into the
noise
on a weak signal.
The alleged superiority of SSB over AM is the fact that SSB uses a product
detector. With an envelope detector, every component in the passband mixes
with every other component. With the product detector, the only mixing
action is between the BFO and each component. The result is better
readability and less noise in the presence of interference.
The Costas synchronous detector solves this problem by allowing you to
receive double sideband AM with a product detector. The problem with DSB
reception is that the injected carrier from the BFO must be exactly on the
same frequency and be EXACTLY IN PHASE with the original carrier, in order
for the products of each sideband to reinforce each other. There are
several ways to make a sync detector work; most use PLL technology.
The "synchronous detectors" on some commercial receivers are not true
synchronous detectors because they phase out one sideband and the result is
simply SSB reception of an AM signal. The only difference si that it is
done by the phasing method rather than with the more common method of using
a sharp i.f. filter.
With double sideband, you get 6 dB better signal-to-noise ratio than with
SSB under identical conditions. First of all, with DSB there is 3 dB more
sideband power. With SSB however, you gain 3 dB because the receiver
operates at half the bandwidth and thus rejects 3 dB of noise. These
effects cancel, so it would seem that there is no advantage to DSB. However,
the coherent detection of two sidebands gives twice the voltage output at
the detector than what you would get with only one sideband. Twice the
voltage = 4 times the power, or 6 dB improvement.
Don K4KYV
_________________________________________________________________
MSN Photos is the easiest way to share and print your photos:
http://photos.msn.com/support/worldwide.aspx
