[Discuss-gnuradio] Problems with the USRP/LFRX at lower frequencies

[dan s]

Hello,

 

I'm having a ton of trouble with the USRP/LFRX daughterboard setup.  I am trying to calibrate the counts that are stored as integers when I use the usrp_rx_cfile.py script.

 

I am using an HP waveform driver with 50 ohm output impedance to feed single frequency sinewaves to the LFRX channel A.  I am generating sinewaves with an amplitude of 50 mV peak to peak.  I tried measuring several frequencies from 5 to 250 kHz.  I am using a decimation of 128 (500 kHz) and the default gain of 10dB.

 

I recorded the data for each frequency and used matlab to perform an FFT (normalized) to receive my peak counts for each case.   The number of counts I measure is not the same for all the frequencies.  I noticed the number of counts stays constant from 5 to 20 khz, then drops from 20 to 150 khz, then the number of counts rises slightly from 150 to 240 kHz.

 

I'm puzzled why the number of counts I measure is not constant even though I have a constant amplitude sinewave signal fed into the LFRX.

 

I also tried performing the same tests with an HP spectrum analyzer.  The analyzer measured a response of 16 mVrms (which corresponds to 50mV peak to peak) at each of the frequencies.  I am 100 percent confident the signal I am feeding into the LFRX is perfect.

 

Thanks,

Dan Sura