[USRP-users] LO offset in the N2X0

Marcus Müller marcus.mueller at ettus.com
Thu Apr 9 14:34:20 EDT 2015


Hi Venkatesh,

by default, UHD tunes the LO as close to the desired frequency as possible.
If you want to change that behavior in the GNU Radio Companion, you'd use
uhd.tune_request(1.8e9, 2e6)
to get the signal at 1800MHz.
UHD will do that by setting the LO as close as possible to 1.8GHz-2 MHz,
and compensating 2 MHz(+the eventual remaining LO offset of a few kHz
max) in the FPGA.

But: DBSRX2 has a maximum selectable (also the default) analog bandwidth
of 10MHz, so you must make sure that $f_\text{offset} +
\frac{f_\text{sample}}{2} \le 10MHz$; it's advisable to stay away from
the edge a bit.

Here's a bit of a sketch of how things look like. In this case, the
analog bandwidth that "reaches" the ADC (blue) is but a little bigger
than absolutely necessary to fully be able to capture the wanted signal
within the sampling (nyquist) bandwidth; however, it was possible by
means of offset tuning to move the LO out of the observed band:

SVG of the scenario
PNG of the scenario

Best regards,
Marcus

On 04/09/2015 05:34 PM, Venkatesh Sandilya via USRP-users wrote:
> Hello
>
> I am using a N210 with DBS-RX2 in receive mode. I have a question. Is
> there a certain frequency that the lo defaults to which is
> proportional to the centre frequency? If I am setting up the USRP in a
> flow graph in Gnuradio companion, is there a way to tune the lo frequency?
>
> Thanks
>
>
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

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