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.