[Discuss-gnuradio] RFX transceiver board receive VCO offset automated calibration

[Richard Clarke]

Hi All,

Before I go ahead and try to implement an auto calibration routine for 
the Rx frequency offset of the receive subsystem on the RFX400/USRP, I 
was wondering if anyone else had already implemented something similar, 
or had any pointers for me. Basically what I want to do is create a 
calibration routine that will determine the frequency offset between the 
RFX400/USRP receive subsystem (nominally already set to equal the 
frequency from a transmit source (e.g sig gen) and the transmit source, 
and then nudge the tune frequency to null that offset. I find that my 
RFX400 receiver after time to stabilise is around 3.2KHz in error, which 
is well within it's spec I know, however I want to automatically 
determine this offset and null it out. I guess the actual fine frequency 
adjustment will actually be occurring in the DDC, would that be correct?

One way to measure the frequency offset is to look at either the I or Q 
waveform of the (unmodulated) downconverted RF signal. When an offset is 
present between Tx source carrier and Rx subsystem the in-phase or 
quadrature signal is a sinusoidal waveform with a frequency 
corresponding to the magnitude of the offset involved. Use this to then 
fine tune the RFX400/USRP.

So, has anyone done something similar before? Do you have any code you'd 
be willing to share?

Thanks very much.

Cheers
Richard

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