FPGA Central

On Tue, 8 Jul 2003 17:16:38 +0200, "Jesper Buch"
<[email protected]> wrote:

>---- Original Message -----
>From: "praveen" <[email protected]>
>Newsgroups: comp.dsp
>Sent: Tuesday, July 08, 2003 4:37 PM
>Subject: choosing a sampling rate lesser than nyquist rate(sub nyquist rate)
>
>
>> Hello,
>> I have a signal consisting of 4 harmonics (200k,400k,600k,and 800k Hz)
>
>You will never be able to produce an 800 Khz frequency with less than the
>double samplerate !

Gotta be careful here - Nyquist rate is related to the bandwidth of
the signal not the highest frequency and it assumes impulse sampling.

So if I have a 100GHz signal with a 1 Hz maximum bandwidth I only need
a slow ADC (2 or 3 Hz) but a very good sample and hold :-)

If I have a narrow band signal it can't deviate from the ideal
sinusiod by much in a short time interval can it? So I only need to
sample occasionally to sample the (low frequency) deviations from the
assumed "carrier". Hand-waving justification for sub-sampling.
Sub-sampling works when you have good apriori knowledge of a narrow
band signal.

The case in point here though is a signal with a bandwidth from DC to
800 kHz so that is the bandwidth. the ide though of dealing with DC
separately and then allowing folding (aliasing), as suggested by
others, is also what I was thinking. You still need a sample window
much much shorter than 1/1600kHz but the sample rate requirement is
relaxed.

Ian

Mattias Schick wrote:
> Ian wrote:
>
>>>You will never be able to produce an 800 Khz frequency with less than the
>>>double samplerate !
>>
>>Gotta be careful here - Nyquist rate is related to the bandwidth of
>>the signal not the highest frequency and it assumes impulse sampling.
>
>
> Gotta be even more careful here ;-)
> Think of a signal from 5kHz..20kHz (BW 15kHz) - when sampled
> with 30kHz the range from 15..20kHz is aliased to 15..10kHz,
> i.e. into the middle of of the source-band...
> Mattias

So you don't do that. You mix down, or complex sample, or....

Regards,
Steve

Steve Underwood wrote:
> Mattias Schick wrote:
>> Gotta be even more careful here ;-)
>> Think of a signal from 5kHz..20kHz (BW 15kHz) - when sampled
>> with 30kHz the range from 15..20kHz is aliased to 15..10kHz,
>> i.e. into the middle of of the source-band...
>> Mattias
>
> So you don't do that. You mix down, or complex sample, or....

Hi Steve,

clear. All i wanted to say is that he (praveen) cannot sample
his 600kHz-BW-Signal with 1.2MHz. Ok, we all knew that...

Mattias