Hi Sean,

The method you describe is often used in direct digital detection
receivers. The IF (or tuned RF) output is in a known band
and an appropriate choice
of sample rate achieves a downconversion to 'base band' without
additional RF stages. I believe Plessey Semiconductor and/or Thomson
CSF had digital TV chip sets and app notes using this method.

It is actually quite instructive to take two function generators
(one as signal, the other as sampler) and feed the first through
an analog switch (4066) which is driven by the 2nd as a gate for
a sample and hold, to a scope. As one sweeps the frequency of the
input signal past Fs/2 one easily sees the changing wavelength
of the signal, and the folding effect (and zero beat as the frequency
approaches Fs).

Try it. It will make aliasing a very 'intuitive' quantity.
[10Khz is a nice sample rate which gives you stable bright traces]

Robert

"Sean H. Breheny" wrote:
>
> Hi Robert,
>
> Yes, you are right, I should have added the proviso that your sampling
> system needs to have a high enough bandwidth (and your sampling aperture
> needs to be very small and low jitter).
>
> I'm not talking about the progressive sampling method, though. I'm talking
> about just taking samples normally. As another poster pointed out, aliasing
> really only occurs when two "copies" of the signal overlap (in the
> frequency domain). This will only happen if you exceed the bandwidth
> allowed by the Nyquist Sampling Theorem, NOT just because the center
> frequency is very high. Sure, the output samples will be consistent with a
> much lower center frequency, but who cares if the modulation riding on top
> of them is the narrowband signal you are looking for? You really don't need
> to do any special processing (such as progressive sampling), as far as I
> can see. If you want to reconstruct the signal exactly, you will need to
> know what the original center frequency was, which you can obtain either by
> progressive sampling or by just using a frequency counter (like the
> Bitscope does).
>
> Sean
>
> At 11:05 PM 2/16/01 -0700, you wrote:
> >Bear in mind that the series resistance of the sampling switch (6k at
> >5V) and the value of the hold capacitor (51.2pf for a '76) will band
> >limit your signal to under 500khz.
> >However, with an external fast sample and hold, one could digitize video
> >at a 50khz rate (3 samples per video line). By sampling progressively
> >later in the line, one could acquire a 640x480 60Hz video frame in 3.8
> >seconds. (A fully analog version of this technique was used back in the
> >70's to convert full speed video to the 15Hz line/7.5S frame of slow
> >scan television used in the amateur radio bands (TV in a 3khz BW)).
> >
> >"Sean H. Breheny" wrote:
> > >
> > > Hi Don,
> > >
> > > For certain methods of accomplishing reconstruction, you are right, but I
> > > don't think that is true in general. You can take any signal bandlimited to
> > > delta_f, sample it at 2*delta_f, and reconstruct it from the samples. It
> > > doesn't matter what the center frequency is, or whether it is periodic, as
> > > long as it is bandlimited. So, for example, you could have a signal which
> > > filled the entire band from 0.9 to 1.1 MHz, sample it at only 0.4
> > > Megasamples per sec, and reconstruct it from that, as long as you had the
> > > additional piece of info that its center freq was 1 MHz.
> > >
> > > Sean
> > >
> > > At 07:41 PM 2/16/01 -0800, you wrote:
> > > >Hi,
> > > >
> > > >The signal must necessarily be periodic for this to work....
> > > >Lets not forget that restriction!
> > > >
> > > >Cheers,
> > > >Don
> > > >
> > > >--
> > > >http://www.piclist.com hint: PICList Posts must start with ONE topic:
> > > >[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
> > >
> > > --
> > > http://www.piclist.com hint: The list server can filter out subtopics
> > > (like ads or off topics) for you. See http://www.piclist.com/#topics
> >
> >--
> >http://www.piclist.com hint: The list server can filter out subtopics
> >(like ads or off topics) for you. See http://www.piclist.com/#topics
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics