Hi Pawel,
receivers like the RTL dongles first use a mixer to downconvert your
signal, in this case from a center frequency of 107.5MHz +- 16kHz to a
complex baseband signal around 0Hz. That then only needs to be digitized
according the actual signal bandwidth. (In fact, if you can filter well
enough, you can even build direct receivers that just use aliasing and
don't need mixers. However, adjustable bandpass filtering is much harder
than building adjustable LOs and mixers)
Then, you're misinterpreting Nyquist: for the complex sampling case,
like used in direct conversion receivers like the RTL dongles, f_nyquist
= bandwidth = f_sample is sufficient; the double sampling rate applies
to real signals and real values coming from ADCs only.
Best regards,
Marcus
On 01/29/2016 12:44 PM, Paweł Tomaszewski wrote:
> Hello,
> In GnuRadio when I create some basic RTL-SDR receiver followed by FFT
> block I can set receiver signal frequency of my local FM station
> 107.5MHz and sampling frequency of 32kHz.
>
> With that settings I can correctly see on FFT display signal of my
> local station. But according to Nyquist–Shannon sampling theorem,
> sampling rate should be at least 2 times higher than signal frequency,
> so in my situation I should set at least 215MHz into sampling rate
> parameter.
>
> Why does it work correctly?
>
> Thanks,
> Paweł
>
>
>
>
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