Re: Relationship between filter roll off and response time

Fred Bloggs will probably have something to say about this - he has
been critical of Larry Brasfield's responses in the past, and this
should defintiely engage Fred's critical faculty.

Effectively, the original poster is proposing to amplitude modulate a
10kHz carrier, and wants to know what a high pass filter will do to the
modulated signal.

What the OP needs to know is that AM multiplies the carrier waveform by
the modulating waveform, producing a waveform whose Fourier transform
contain the carrier frequency and upper and lower sidebands separated
from the carrier frequency by the modulating frequency.

If the modulating waveform is a pure sinusoid, there will be only one
upper and one lower side band. If it is a square wave, there will be
extra sidebands corresponding to the odd harmonics of the frequency of
the square wave, but the amplitudes of the sidebands decline in
proportion to the harmonic number.

IIRR 100% modulation suppresses the carrier frequency. There a way of
suppressing either the upper or the lower side-bands - search on
"single-side-band" - but that isn't relevant here.

So the OP should be able to modulate the 10kHz carrier at up to a few
kHz before the sidebands get anywhere near the cut-off of a high-pass
filter for blocking 60Hz.

-----------
Bill Sloman, Nijmegen

.

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