"Ray Andraka" wrote in message
news:wZctf.58905$4l5.50283@dukeread05...
daestrom wrote:


So what you're saying is, *if* you know the carrier frequency and
band-width
of the signal imposed on that carrier, you can design a system that will
be
able to reproduce the imposed signal using a relatively low sample rate
(low
when compared to the carrier frequency).

But if the carrier frequency changes, then you need to modify the sample
rate to avoid a lot of aliasing issues. So in radio reception, the
sample
rate is adjusted along with tuning the receiver? Or is this done at the
intermediate frequency which is fixed so that sample rate adjustment is
fixed with the intermediate frequency? (do they even still use
superheterodyning in tuners?? ;-)

It's been a long time since I did any RF stuff. But A/D and D/A stuff
at AF
and lower has been quite a passion for me for some time. And the basic
Nyquist hasn't changed.

daestrom



The carrier frequency has nothing to do with it. What is important is
the bandwidth and the center frequency of the pass-band. Note that your
signal needn't take up the whole bandwidth, and in a typical radio
system the signal you are tuning to is a very small fraction of the
pass-band. In any case, the pass band is defined by the anti-alias
filter, so practically speaking, it is a fixed, known pass band.
Therefore your *if* is satisfied.

What subsampling buys you is a way to sample an IF that is at a higher
frequency than the sample rate of your system, which may be limited
either by the ADC or by your computational power.

BTW, I never said nyquist changed. I was simply stating that it is more
general than the commonly held belief that the sample rate has to be at
least 2x the highest frequency. The truth is, the sample rate has to be
at least 2x the bandwidth of the signal.

After proper filtering of the input.