On Wed, 18 Aug 2004 12:18:36 GMT, Liberals for Guns
wrote:

Can someone help me understand something about IF vs synch. I've read
that IF shifting can get rid of an interference that is occurring
slightly lower or higher than the desired frequency. So, if you've
got noise within the filter range and it's on the lower side of the
frequency than you simply shift everything up so the noise is out of
the "window". My question is that I'm not sure why you just couldn't
use sync in to get rid of the interference. By just listening to the
upper sideband the noise wouldn't be there. Right? Do I need to pay
for an advanced feature like IF shift when I could get away with
simply using a sync detector function?

One of the values of synchronous detection is that it can be effective
even on signals within the passband. Shifting the passband can result
in 'blocking'. In effect the intefereing signal may be outside the
passband, but is still seen, and drives the AGC, effectively reducing
the level of the desired signal.

It is also rare in most receivers for the skirts on the passband to be
sharp enough for this to work well. Usually a Q multiplier set on null
is a better approach, you just put a black hole in the passband at the
desired spot.

Synch other attraction, and perhaps the most attractive features is
something that passband tuning cannot help with. Selective fading of
the carrier. Sometimes it isn't really interference it is selective
fading causing the sidebands to have more energy than the carrier,
causing severe distortion. Synch detection insures adequate carrier
energy.