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[Discuss-gnuradio] Re: about TVRX board IF frequency

From: Matt Ettus
Subject: [Discuss-gnuradio] Re: about TVRX board IF frequency
Date: Thu, 28 Jun 2007 12:13:54 -0700
User-agent: Thunderbird (X11/20070326)

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 ‘usrp_fft.py’ 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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