Hello,
We have about ten N320s and almost all are exhibiting issues regarding the LO
stability. It appears the LO is not locking at certain frequencies, or if it
does, it barely maintains lock. We can see this with either a gnuradio
flowgraph consisting of nothing more than a usrp_sink and a constant driving
it's input, or using the included UHD example "tx_waveforms". The problem
frequency I've been focusing on is 1112MHz. So, the following command
demonstrates the issue:
./tx_waveforms --freq=1112000000 --wave-type=CONST --wave-freq=0 --rate=1000000
--gain=40
Some of the N320s seem to lock ok, and you can see a reasonable tone at the
output. However, other N320's don't lock -- we will see a several MHz-wide
"blob" about 4 MHz lower than the requested frequency. Note that they aren't
reporting that the synthesizer isn't locked, but that is what it looks like. On
units where the LO appears to lock, if I look closely on a spectrum analyzer,
the phase noise often looks horrible, or I see large spurs around 106KHz off of
center that slowly move up and down by 30 to 40 dB. It looks like it's barely
maintaining lock.
This issue varies by N320 and also by channel ("A:0" vs "B:0") on the N320. It
doesn't matter if I use an internal or external reference -- the behavior might
be very slightly different, but not much. Gain settings, sample rates, etc.
don't seem to matter -- it appears to be an RF/synthesizer issue.
I also tried enabling "spur_dodging", since that changes LMX loop parameters
and that seemed to help in some cases (units/channels) but hurt in others.
I've been focusing on the TX path, but someone else in my office was mentioning
that they have seen the same type of thing when receiving -- the result was
that 1 out of several N320s he was using to simultaneously receive a signal
showed the same signal several MHz off of what the other N320s saw -- and it
looked very distorted. I'm guessing that what he saw was the result of the LO
not locking properly.
We're using UHD 4.1.0.5 and associated filesystem, FPGA image.
Any thoughts on this?
Thanks,
Jim
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