Re: Modulate HF onto LF?
- From: Don Lancaster <don@xxxxxxxxxx>
- Date: Thu, 30 Nov 2006 08:43:53 -0700
J.A. Legris wrote:
Klaus Jensen wrote: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.
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
and PSk radio."
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.
Awaiting your invocations.
Klaus Jensen
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.
--
Joe Legris
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
rss: http://www.tinaja.com/whtnu.xml email: don@xxxxxxxxxx
Please visit my GURU's LAIR web site at http://www.tinaja.com
.
- References:
- Modulate HF onto LF?
- From: Klaus Jensen
- Re: Modulate HF onto LF?
- From: J.A. Legris
- Modulate HF onto LF?
- Prev by Date: Re: Modulate HF onto LF?
- Next by Date: Re: Discrete opamp bjt front end
- Previous by thread: Re: Modulate HF onto LF?
- Next by thread: high voltage discharge emi problem..
- Index(es):
Relevant Pages
|
