Modulate HF onto LF?

This is might confusing verbage. Are you questioning if you can use a low side mixer instead of a high side mixer? If that is the question, then the answer is yes, but the spectrum is reversed. The effects for data can be compensated.

Modulation of two signals is like multiplication. If C= AxB then C=BxA

The result will be identical, main signal components at B +/- C which also = C +/- A (overly simplified). But the higher frequency carrier will have sidebands of the lower carrier just the same.

Frank

See "Product-to-Sum Formulas" at

For example, take a couple of sine waves that are functions of time and angular frequencies u & v. If we multiply them:

sin(ut) sin(vt) = 1/2 [(cos((u-v)t) - cos((u+v)t)]

The sum and difference frequencies are immediately apparent.

In the special case of AM, where the modulating signal, u, never goes negative, we get:

(1/2 sin (ut) +1) sin(vt) = 1/4 [(cos((u-v)t) - cos((u+v)t)] + sin(vt)

The first term is the sum and difference frequencies and the second term is the carrier. For example, if the carrier frequency is 100 and the modulation frquency is 10 we get the usual situation where the powerful carrier at 100 is flanked by a pair of weaker sidebands at 90 and 110.

But if the carrier frequency is 10 and the modulation frquency is 100 we get a powerful carrier at 10 while the weaker "sidebands" are still at 90 and 110.

-- Joe Legris

It sorta sounds like he was asking if you could put a wide bandwidth signal onto a narrower one by downconverting. Thus reducing the channel width for a given amount of transferred info.

The answer is that yes you can, but you totally demolish the info content in the process by unavoidable aliasing. There is no free lunch.

Alternately, he might have been talking about plain old trellis modulation.