Re: Bandpass signal sampling


"Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message
news:nuydnfRdPIqYEy3anZ2dnUVZ_qOknZ2d@xxxxxxxxxxxxxxx
Dedicated wrote:
Hi,

I have a problem.
If anyone may help , I would be appreciated.

There are signals at 70 MHz IF output of a DDC.
and there are equispace signals which should be demodulated.
I am trying to find the best sampling frequency for this purpose.

I know about the bandpass sampling theorem which says:

Fs >= 2*B (1)

(2Fc-B)/m >= fs >= (2*Fc + B)/(m+1) (2)

Where Fs = Sampling frequency
Fc = Center Frequency B = Bandwidth
M any integer which satisfies the condition (1)


But that is for one signal which is at Fc with BW = B

But what about many signals around the Signal of Interest ?

If you could not find the correct Fs, There may occur aliasing between
the
other signals as well.

I'm trying to sort out what you mean, and I'm not getting it.

I _can_ tell that you are trying to go beyond a rule without understanding
the reasoning behind it. When you find yourself doing this, the best
thing to do is to study the rule, understand what goes into it, then take
the pieces and make your own rule. This article may help, in a tangential
way: http://www.wescottdesign.com/articles/Sampling/sampling.html.

It seems that you want to sample a signal and interference without
filtering out the interference before sampling, and you want to know how
fast to sample? If that's the case then take Fred's advise -- or do some
filtering before you hit your sampler.

When I hit a problem like this I make sure to make a little diagram, like
the ones you see in textbooks, that show the spectrum of the signal before
sampling, all the folding frequencies due to sampling, then a separate
sketch of the signal (fortuitously sampled to within my sampling rate, I
hope) and all of it's aliases (which don't interfere with the part of the
signal that I want, I hope). This aids in my understanding, so I have
confidence I'm not blindly (and possibly incorrectly) applying a rule that
I don't understand.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com


I sure agree with the diagrams ... I call 'em "cartoons".

Fred


.

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