--- Charlie Chisholm <no7up@yahoo.com> wrote:
> OK, here's a question.
> Let's say you have a 10kHz signal of a certain
> amplitude within an electrical circuit.
> I assume it is traveling at the speed of light minus
> the velocity factor in copper.
> Take that signal and connect it to a giant antenna.
> It will radiate RF, not sound, right?
> Take the identical signal and send it to a speaker.
> It will radiate sound, not RF, right?
Yupp, it really just boils down to whether the energy
is being transferred into an electromagnetic load vs.
a mechanical one (sound waves).
> It's the same signal within the electrical circuit.
> Is
> it possible when engineers are talking about radio
> FREQUENCY or audio FREQUENCY, they are just talking
> about a certain FREQUENCY, not whether it is sound
> or
> RF? Can't the same signal be either sound OR RF?
> Maybe they call the lower frequencies AF because
> they
> are low enough to be CONSIDERED audio - because that
> is the way they have traditionally been used.
I view it as simply being which it is most commonly
associated with, even though there is no rigid
delineation between the two. Ultrasound as used in
medicine operate upwards of 3MHz, not what you would
normally associate with "audio".
I
> think
> possibly the new DSP situation renders the old
> nomenclature of RF vs. AF obsolete.
>
I don't think it has any really bearing one way or
another regarding RF vs. AF, the long-standing
practical definitions will always apply. It's the
relevance of "IF" that I wonder about.
To see what I mean ponder the Qudrature Sampling
Detector. It may make the notion of needing an
"intermediate" frequency (IF) obsolete. It is
described in a recent series of 4 QEX articles that
you can find at:
http://www.flex-radio.com/
There are 4 "Software Defined Radio for the Masses"
(~1MB PDF) files in the "SDR Articles" section that
describe the workings of the Qudrature Sampling
Detector. Some really fascinating reading can be found
there. In a nutshell the QSD "samples" the desired
frequency at 4X, this effectively generates the 0, 90,
180, and 270 degree components needed to create the I
(in phase) and Q (quadrature) signals for the DSP to
chew on. As far as I know all existing DSP IF radios
already use the I and Q concept as their basis. The
key difference being that all existing DSP IF radios
that I'm aware of use the "phasing method" to generate
the I and Q signals rather than directly "sampling"
like the QSD.
Some major advantages I see to this approach:
1. Eliminates a number of IF stages (mixers), -
remember that each time you add a mixer it increases
both loss and provides an opportunity to introduce
distortions.
2. Shear simplicity, uses just 1 LO, not 2, 3, 4 or
more.
3. Uses inexpensive and easier to create high
performance A to D's, they operate at "audio"
frequencies.
4. The QSD is a linear device; all traditional
detectors (mixers) are non-linear and are therefore
more prone to introducing distortions.
It seems to me that the QSD has the potential to
really throw a bucket full of gravel into the gears of
traditional radio RF design thinking. Hopefully it
will find its way into more radio designs whether they
are "direct conversion" or other perhaps some other
single conversion designs.
Duane
N9DG
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