[USRP-users] Complex FFT display of Frequency offset.

Martin Braun martin.braun at ettus.com
Wed Jul 5 17:44:39 EDT 2017

On 06/22/2017 06:34 PM, Walter Maguire via USRP-users wrote:
> Hi all.
> We were performing some basic Receiver tests on the E310 in RFNoC mode.
> A snapshot of my simple grc and associated outputs are shown in the
> attached screen shots.
> The RX frequency of the E310 is set to 836.5MHz.  The sample rate is set
> to 1MHz. The center frequency on the QT Frequency Sink is set to 836.5
> MHz.   If I apply a carrier only frequency at 836.5MHz then it appears
> as expected in the center of the frequency display.  However, if I
> change the frequency of the carrier by +100KHz to 836.6MHz I see the
> carrier displayed at +300kHz offset or 836.8MHz.  This does not seem to
> be correct.  We would expect it to be at 836.6MHz   I might expect a
> halving or doubling due to complex I/Q signals being 1/2 the BW.  I
> don't understand why we see times 3 for the offset.
> I tested the QTGUI display using a complex sine wave and it works as
> expected that is at 0Hz it is in the center and the frequency displayed
> on the FFT track the frequency of the complex sine wave. Therefore I
> think the issue (or understanding) is in the E310.

For the record: This was resolved in the following thread.

-- M

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