Re: [Discuss-gnuradio] Inconsistant Gain in X310

[Ben Hilburn]

Hi Andrew -

What you describe is very strange. There is nothing in the USRP that changes dynamically based on the input power level (there isn't even AGC). The only thing I can think of is that somehow the built-in DC offset calibration is going haywire. Are you setting the center frequency directly to the frequency of your input signal? If you tune with an LO offset, do you see the same behavior?

Another question: what is your signal source? Can you lock the USRP and your signal source to the same reference, and then tune such that your input signal is directly "over an FFT bin"? Do you still see different behavior between the two channels?

Also, the decimation is done in the FPGA, and must be the same for each channel.

Cheers,

Ben

On Fri, Nov 21, 2014 at 7:17 AM, Daigle, Andrew - 1008 - MITLL <address@hidden> wrote:

Hello,

 

I am running into a small issue with the X310 USRP. My basic setup involves receiving from two antennas (or at this point CW tones from signal generators) using two basic RX daughter cards.

 

My problem is that when I pump in a -50 dBm CW tone to channel A the FFT plot makes sense in that it is stable with an expected value (floor ~-117 dB and peak -62 dB). As the signal increases the peak increases accordingly (i/e for a -40 dBm CW tone I see a floor of ~-117 dB and a peak of -52 dB). This is consistent all the way up to -20 dBm (I didn’t want to get too close to the -15 dBm limit written on the outside of the USRP). When I move this signal into channel B its FFT mirrors channel A at an input of -50 dBm, but as soon the signal is increased past -45 dBm the noise floor starts jumping from ~117 dB to ~93 dB rapidly and the peak jumps and stays stable 14 dB higher (-38 dB vs -52 dB for the same -40 dBm). Below a -50 dBm input; however, everything is identical between the two channels. I was thinking maybe the decimation rate is changing on the second channel based on the input power (but for some reason not the first). The thing is I am using code loosely based on the rx_multi_samples example and I don’t know if there is a way to specify the decimation rate of each of the daughter-cards as if I was using GNU radio companion (or even if this is even the cause of my problem and I should be looking elsewhere).

 

Any thoughts? Thanks!

 

-Andrew

 

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