RadioBanter - View Single Post - would like advice on idea for class E SSB Exciter

November 29th 03, 12:35 PM

(gudmundur) wrote in message ...

Base audio has no sidebands, it is monoband so to speak. It does not become
sideband until you modulate a carrier, remove the carrier, and possibly also
remove one sideband.

not true. consider that you are whistling a pure tone of 1KHz into an
SSB transmitter's input. If the SSB transmitter was tuned to 7.000MHz,
you would get just a carrier at 7.001MHz. Thus, a single tone SSB
signal is virtually indistinguishable from just a carrier.

now consider that the human voice is expressed as a number of
simultaneous pure tones superimposed over each other (easily obtained
by converting the time domain signal into frequeny domain).
Now imagine that for each of the tones, you have a separate
transmittter offsetted from the carrier by a frequency obtained by
frequency analysis and also with equivalent amplitude. this is in
effect, the SSB signal.

to rephrase, we need to modulate a carrier and then remove it because
that is seemingly the easiest way to generate SSB (but not a necessary
way).

Even if you simulate this process in a sound card, which
can be done, and you produce the 'garbly' sound, you still have 'audio' which
when impressed upon your transmit carrier will still create normal double
sideband modulation. The only way to get the result you are looking for would

there is an entirely different approach, we are advocating that we
split the signal into a number of component audio frequencies and
essentially run a carrier that is the equivalent of each of those
frequencies.

From the above example of a single tone, lets expand this to a two
tone ssb exciter. Lets imagine that such a signal can be represented
by two carriers that have relative amplitudes of the two tones and are
shifted from an imaginary carrier by amounts equivalent to their tone
frequencies.

a frequency modulated signal generates sidebands according to a bessel
function. Thus a correctly modulated FM signal will also result in two
tones being generated at two frequencies. In order to control multiple
sidebands, an envelop shaping will have to be applied to the FM
signal. The frequency modulation and the amplitude modulation can be
easily computed digitally.

What is really difficult is to accurately frequency modulate the
carrier. One way is to directly generate the carrier digitally using
DDS. I am not aware of any amateur effort in trying this method out. I
seem to have vaguely read about it in either EMRFD or the new ARRL
Handbook.

- farhan