Here's what the ARRL's FCC Rule Book (10th edition) has on page 12-6:
Q&A -- Band Edges
Phone
Q. I'm an active HF phone DXer. How close to the band edge can I set my
VFO?
A. Many factors are involved. Amateurs commonly consider full-carrier,
double sideband AM signals to be about 6 kHz wide and single-sideband,
suppressed-carrier signals to be about 3 kHz wide. Thos bandwidths,
however, are usually only 6 dB down, and that isn't what the FCC worries
about. Thus, to determine where you may set your VFO in relation to the
band or subband edge for your class of license, you'll have to figure out
where your signal is attenuated by 40 dB. Don't assume that if your SSB
transmitter bandwidth is 3 kHz, that you can just add a few hundred Hz to be
safe. That's fine only if the shape of the bandwidth filter is sharp enough
to attenuate the signal to -40 dB at 3 kHz. If you're going to operate as
near as possible to a band edge, then do so only after carefully reviewing
your equipment specifications to see if such information is included.
Aother alternative is to carefully measure the attenuation if you have
access too top-notch lab equipment. If you can't do either, then allow a
larger margin of safety for possible inaccuracy of your frequency readout.
Remember, your carrier and all of your subbands must be confined within the
amateur bands and subbands as applicable.
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I seem to remember the Q&A in the FCC Rule Book being more specific several
years ago...
73 Tom
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