# Modulate HF onto LF?

Discussion in 'Electronic Design' started by Klaus Jensen, Nov 30, 2006.

1. ### Klaus JensenGuest

I found an obscure reference which reads as follows. There is no
further information given.

"One person with whom I discussed this said you can use a lower
frequency carrier than the modulation. You can view the resulting
signal via a 'constellation diagram' such as used in quadrature
amplitude modulation and other techniques associated with modems, FSK

Is this bunk, or would someone care to venture how this might be
achieved?

I don't think they are referring to something as simple as
superimposing a HF ripple on the LF. It seems to equate to full AM of
lower frequency by a higher one, instead of the normal way around.

Klaus Jensen

2. ### Guest

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.

3. ### Frank RaffaeliGuest

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

4. ### J.A. LegrisGuest

See "Product-to-Sum Formulas" at
http://www.sosmath.com/trig/Trig5/trig5/trig5.html

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.

5. ### Don LancasterGuest

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.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552