RadioBanter - View Single Post - Another Dallas Lankford article on synch detectors

Jake Brodsky · August 1st 06, 11:17 PM posted to rec.radio.shortwave

wrote:
To add more fuel ot the fi
http://www.kongsfjord.no/dl/Audio/AM%20Synchronous%20Detector%20Experiences.pdf

The date on this addition is July 19, and either it was just placed in
the archive, or I just noticd it. At least it gives us something to
think about.

The problem with synchronous detectors is not that they aren't a good
idea; it's that they're a hard to do well enough to improve upon a good
envelope detector. Even when they work well, they expose poor
performance in the rest of the receiver.

I built the 1993 Synchronous detector from the QST article. It works
well on groundwave signals. However, it's very hard to keep the thing
locked when going through a fade. To stay locked, one needs more
drastic measures. A Costas loop circuit could do this. So could a
second harmonic IF dedicated to locking up on the carrier.

Another major problem is the pathetic dynamic range one finds in typical
IF sections. Many engineers use AGC to keep things within a fairly
narrow range. The dynamic range of two in-band carriers isn't something
typically measured in most receivers. Good receivers have in-band
dynamic ranges in the 80 dB range.

A good in-band dynamic range, a clean LO, and a solid synchronous
detector design will sound very good. Anything less will not be much of
an improvement over conventional envelope detectors. This guy happened
to pick the cheaper designs.

It's as if he's passing judgment on all sushi by eating the cheap, less
than fresh stuff.

DE AB3A