On 11/24/2014 10:26 AM, Daigle, Andrew - 1008 - MITLL wrote:
Ben,
I have tried injecting both a 50.0 MHz and a 50.1 MHz tone and setting
the USRP with a center frequency of 50 MHz and a sampling rate of 1
MSps. The signal generator I am using is clocked with the same 10 MHz
reference as the USRP (octoclock). I am currently setting the digital
gain to zero (though I have tried a number of different values with no
noticeable differences in terms of this non-linearity to input power).
I have also tried inserting a LO offset, but haven’t had any real
success with that either (outside of removing the small peak at DC). I
tried disabling the DC offset but that didn’t do anything either.
Looking in the time domain at the IQ for each of the channels I see
two things which appear strange to me. First, when I set the USRP
center frequency to the same frequency as my tone (50 MHz) I would
expect a flat line, but instead I see a triangular wave with a
non-zero amplitude (actually it is quite large). When I offset the
frequencies I see a very noisy sinusoid which has the wave on it. As
the power increases, the channels scale evenly in both noise and
intensity, but at a threshold power level (~45 dBm) it appears as
though leakage occurs between the main sinusoidal wave and this
secondary carrier signal.
Thoughts? Thanks for taking the time to look at this.
-Andrew
Andrew:
What does the following test flow-graph yield? With the same signal
input to both sides (via a splitter):
http://www.sbrac.org/files/test_x310_basicrx.grc
If you're injecting a -50dBm test tone at 50.1MHz, you should see it at
100kHz in the FFT display of each half, and within 1dB of each other.
*From:*Ben Hilburn [mailto:b...@ettus.com]
*Sent:* Friday, November 21, 2014 7:36 PM
*To:* Daigle, Andrew - 1008 - MITLL
*Cc:* discuss-gnuradio@gnu.org
*Subject:* Re: [Discuss-gnuradio] Inconsistant Gain in X310
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
<andrew.dai...@ll.mit.edu <mailto:andrew.dai...@ll.mit.edu>> 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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Shirleys Bay Radio Astronomy Consortium
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