You can work the passband against the SYNC detector by tuning off
center, in the opposite direction from the sideband chosen. This will
narrow the I. F. and lower splatter. It could use a 4 kHz filter for
AM/SYNC.

The main thing is getting ahold of a Murata CFJ-455I. Murata stopped
production on the higher performance filters back in 2002. I never use Sync
detection, so it is a non issue for me. A few years back, I designed an
Analog Devices AD607 based sync detector that didn't have the problems that
I see with other sync detectors. I've still got the prototype. I was going
to market the unit, but there are so many variables when interfacing such a
unit to a receiver that I decided against it. Also, there doesn't seem to be
much of a demand for an outboard unit. Sherwood Engineering has an outboard
unit, but I don't think that this device is a high volume seller.

Pete