Forum: GNU Radio Re: about TVRX board IF frequency

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Matt E. (Guest)
on 2007-06-28 23:16
(Received via mailing list)
melanio wrote:
> Dears,
> In data sheet of Microtune 4937 DI5(or in other doc.), the IF
> frequency of a tuner comes out to 5.75MHz.
> But according to it directly measures(pin17,18) with spectrum
> analyzer, the IF frequency is 44MHz.

Various versions of the tuner operate on different IF frequencies.  The
one you have is at 44 MHz.

> Moreover, DDC values comes out to -20M if ‘’ is carried out.
> So the ADC is 64MSPS, I don’t know why DDC is -20M and why IF
> frequency is 44MHz.. is it possible figure?
> I think 44MHz is so high(It is in contravention of the Nyquist
> theorem) and I don’t know why DDC is -20M

If the ADC samples at 64 MHz, Nyquist is 32 MHz.  In a bandpass sampled
system, 44 MHz will appear at 20 MHz.  We set the tuner for -20MHz to
bring that 20 MHz down to zero.

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