[AMRadio] SSB AM revisited


John Coleman wa5bxo at pctechref.com
Thu Mar 17 19:21:39 EST 2005


See the link at 
http://www.qsl.net/wa5bxo/amtech.html

There is a picture at the bottom of the graphical analysis that Don is
talking about.

John,
WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Donald Chester
Sent: Thursday, March 17, 2005 6:04 PM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] SSB AM revisited


>In thinking about it I was trying to come up with how we get the same PEP 
>power with SSB AM as we do with DSB AM.
>It turns out that you can run 4 times the side band power in a single side 
>band as you can when using double side band AM.
>
>With SSB AM 100% modulation the side band power equals the carrier power! 
>It is not 6 db down as each side band is with DSB AM.


But when you achieve near 100% modulation with SSB AM the quadrature 
distortion makes it almost unreadable.  You have to turn on the BFO on to 
demodulate it, just like with SSB suppressed carrier.  In fact, SSB AM is 
nothing more than SSB with poor carrier suppression.  With DSB AM, the 
quadrature distortion of one sideband nulls out the quadrature distortion of

the othe, so that the envelope detector can demodulate it with 
(theoretically) negligible distortion.

The only way SSB AM can be destortion free is to run a vely low percentage 
of modulation, or insert additional carrier (the BFO).  That's why, with an 
old buzzard receiver without product detector, you have to turn the rf gain 
way down to demodulate SSB.  Think of the BFO as the carrier, and the SSB 
signal as the sideband.  With the rf gain turned all the way up, the 
percentage of modulation is high, and the signal is highly distorted.  When 
you turn the rf gain down, you are effectively lowering the pecentage of 
modulation to the point that the quadrature distortion becomes negligible.

The entire process can easily be graphically demonstrated with vector 
diagrams.

The most effective use for SSB AM is for its carrier to serve as a "pilot" 
for a PLL-controlled  local oscillator to lock onto, maintaining zero tuning

error.

Don K4KYV


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