I finally got the AD607 sync detector running very well. I was able to widen
the loop bandwidth and gain a good phase margin. The original appnote used
an earlier version of this chip that had an AGC detector. The RSSI pin was
decoupled with a 1.5uF cap. I didn't realize that this cap would be
necessary with the newer chip. Anyway, I decided to give this measure a try
this morning. The howling noises vanished, leaving a clean audio signal. One
of the Philips support people stopped over today, along with the Arrow rep.
He was impressed with the adjacent channel selectivity. This sync detector
circuit adds another 8 poles of selectivity at 455kHz, giving the complete
system 14 poles at 455kHz. Once you tune a local station out of the I.F.
passband, it isn't there anymore.
Compared to the TDA1572's low distortion envelope detector, this detector is
smoother sounding. I think I've got a winner her. In the next couple of
days, I should be hearing from the Philips folks............they may be able
to help me with the implementation of the 1572's on-board oscillator, using
a fundamental mode crystal.
The boss (software engineer) was working on the interface circuitry over the
weekend, and came along with quite a few ideas. We have decided to add an
RS232 interface to the receiver; this way, folks have the additional option
of computer control. We should have a pretty cool GUI to go along with it.
Stay tuned!

Pete