[USRP-users] LO leakage on receiver when tuning the transmitter

Stefano Speretta s.speretta at isispace.nl
Thu Dec 4 04:14:56 EST 2014

Hello all,
I am using a USRP B200 in full duplex mode and I need to keep the 
receiver to a constant frequency but tune every few seconds the 
transmitter to a different frequency. When I tune the transmitter I see 
for a moment a strong LO leakage on the receiver which disturbs normal 
operations. The transmitter is not transmitting, I am just tuning it in 
this test, and I do not see any signal coming out of the transmitter 
port with a spectrum analyzer.

The leakage is only present during the tuning of the transmitter and 
then it disappears and, if I never tune the transmitter, it is never 
there. The test was done using UHD 3.8 compiled by me from the tag and 
it was run on Debian 7.3 (x86_64).

I managed to reproduce the issue using rx_ascii_art_dft.cpp if I apply 
the attached patch to the code. This patch tunes the transmitter to a 
different frequency every few seconds (the glitch does not show up if I 
tune the transmitter multiple times to the same frequency).  After 
compiling, I run the modified rx_ascii_art_dft.cpp with the following 
command line:

./rx_ascii_art_dft --freq 145000000 --rate 125000 --ant RX2 --gain 40 
--ref-lvl -50

and I clearly see  glitch appear every time I tune the transmitter. The 
amplitude of the glitch, according to the FFT, is between -55 and -65. 
The same file was run with a USRP B100 and a USRP1 using a WBX 
daughter-board but the glitch never appears. In the patch I also added a 
message handler to stop displaying messages because sometimes using a 
USRP B100 there were some messages which were disrupting the FFT plot.

Best regards,

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