|ARRL Handbook says that the BFO frequency should be at the -20dB point
down
|the skirt of the SSB filter. Pete, KE9OA, says -24dB.
|1. What is the reasoning behind these choices?
|2. How critical is it that the BFO be so positioned?
|3. Wouldn't the shape factor have some bearing on where it should be?

I think you understand. The filter is placed in relation to the AUDIO or
sideband that it has to pass. The filter shape factor then determines where
on the skirt the BFO happend to fall. On the type of filter (commonly)
used, it appears to be the -20dB point.

Someof the other reaponses are saying the same thing in various ways.

|4. Is linear interpolation between the -6 and -60dB points accurate
enough
|to determine the BFO freq?

That's probablu not to bad, but the answer above says that this is going
at it the wrong way. Place the filter pass band where it needs to be and
the BFO goes on the carrier freq, wherever it is. If you want the filter to
cut off some of the highs, then put the BFO further down the skirt -
visa-versa.

If I actually answer your question and consider a "typical" (in my mind
anyway) filter shape, linear interpolation will put the BFO a little too
close (less than -20dB down -- as in like -18dB down) to the bandpass.
--
Steve N, K,9;d, c. i My email has no u's.