On Thu, 5 Feb 2004 22:13:05 -0500, "Tom Holden"
wrote:

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

Perhaps an example is the best way to explain this. Assume that you
have a nominal 9 MHz crystal filter, with a 2.0 KHz BW at the -6 dB
points.

The -6 dB frequencies are thus 8.999 and 9.001 MHz. Forgetting the
bandwidth restrictions on the incoming signal, let's say you would
like the recovered audio signal to be 6 dB down at 500 and 2500 Hz
respectively.

For an upper sideband signal, the BFO should be 500 Hz below the lower
cutoff frequency, i.e. 8.9985 MHz. For lower sideband, the BFO should
be 500 Hz above the upper cutoff frequency, 9.0015 MHz.

Where these two frequencies land on the skirts is dependent on the
shape factor. These "rules" that say the BFO should be so many dB
down are really "rules of thumb" that work with typical shape factors.

Wes Stewart N7WS