Re: [Discuss-gnuradio] Sampling rate lower than Nyquist-Shannon frequency?

[Marcus Müller]

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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