Mixer Frequency of RFX900

Hi, All,

Kind of confusing about the IF of RFX900.

According to what Eric said (
http://lists.gnu.org/archive/html/discuss-gnuradio/2010-06/msg00113.html)
the RFX boards directly converts UHF to baseband signal. Can it be
understood as usrp.tune(f,…) sets the mixer frequency in the RFX board
to
be f.
Then, why do we need DDC? On the other hand, if the mixer frequency in
the
RFX board is set to be f1, while mixer in DDC is set to be f2, where
f1+f2=f. How can we calculate/control f1 and f2?

On 10/16/2011 12:19 PM, Raullen Chai wrote:

the RFX boardis set to be f1, while mixer in DDC is set to be f2,
where f1+f2=f. How can we calculate/control f1 and f2?

Best Regards
(Raullen) Qi Chai
With UHD, you get to either just pass the desired frequency, and let UHD
take care of tuning the RF and DDC parts, or you can construct
your own “tune_request_t”, using uhd.tune_request(). This is
available within recent GRC as well.

In general, PLL synthesizers have a finitely-small tuning granularity,
so if the target frequency isn’t a multiple of the resolution of the
PLL synthesizer, the DDC is used to arrange for the desired target
frequency to appear at baseband, otherwise the DDC frequency
is zero.

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