[AMRadio] Audio response and Long distant QSO's (was: Suggestions Please)

[Donald Chester]

>From: Gary Schafer <garyschafer at attbi.com>

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

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