On 02/11/2022 08:11, Jim Palladino wrote:
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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Also, if you try sending a tone at other than DC (perhaps 10kHz), do you
get different results? This might just be the
DC-offset removal algorithm producing results that look like lack of
synthesizer lock.
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