"daestrom" wrote in message
...
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).

It's a litle more general than that -- you only need to know that your signal
lies inbetween some lower and upper frequencies and that bandwidth is
(generally) less than 1/2 of the sample rate of the ADC.

But if the carrier frequency changes, then you need to modify the sample
rate to avoid a lot of aliasing issues.

Assuming all the "information" (the carrier and whatever sideband(s) you care
about) is still within your bandpass frequencies, you've lost nothing and
there is no aliasing with any non-zero signals.

So in radio reception, the sample rate is adjusted along with tuning the
receiver?

Not usually, although there are so many ways to build 'a radio,' I'm sure this
approach has been implemented at some point in time.

It pretty common to digitize significantly more of a radio band than the
bandwidth of the signal you're interested in and then just digitally track &
demodulate the one signal you need from the many that are present. This is
popular because none of the 'fundamental' settings of the system (local
oscillator frequencies, IF frequencies, ADC sample rate, anti-alias filters,
etc.) change; this makes the architecture inexpensive and highly flexible.
The downside is that sensitivity can be poor if there are other, stronger
sides in the band that you've digitized but aren't really interested in... A
common fix for this problem is to stick an adjustable notch filter somewhere
in the analog path, but of course that adds cost again... etc, etc, etc... we
sit around all day making these tradeoffs. :-) Another common fix is to
switch to frequency hopping spread spectrum modulation like Bluetooth uses.
(From a certain point of view, people like the cell phone carriers have it
easy in that they _own_ the spectrum they're operating in and know _exactly_
what signals should be present, their power levels, etc. -- That makes their
radio designs noticeably simpler and cheaper than "general purpose" wideband
receivers that are used by, e.g., the military, hams, etc.)

Or is this done at the intermediate frequency which is fixed so that sample
rate adjustment is fixed with the intermediate frequency?

This is quite common.

(do they even still use superheterodyning in tuners?? ;-)

Superheterodyning is still common to get the RF down to an IF that can be
digitized directly. As Ray mentioned earlier, the problem with trying to
digitize, say, a narrowband 900MHz signal using a 5Msps ADC is that the effect
of any clock jitter going into the ADC gets multiplied by the 900/5, so at
some point obtaining a decent oscillator becomes impractically expensive.

---Joel