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severe clock noise on FMCOMMS1

Question asked by douzzer on Feb 11, 2013
Latest reply on Apr 23, 2013 by jay_d

I'm seeing intermittent junk-o-rama on the output from the FMCOMMS1 we just received.  Sometimes it's perfectly clean with negligible residual FM, but presently it's a royal mess, with the test tone output spread 300khz wide.  Probing the clock, I see a signal added in with a fundamental that wanders around 1.5MHz and an amplitude about 25% of the clock's.  It's pretty obvious this explains the spreading I'm seeing at the output.  Yesterday when I looked at the same signals, both were clean, with negligible residual FM at the output and under 100ps n-cycle jitter on the clock (and none of the 1.5MHz junk).

 

Game over for the board?  Needs replacement?  (Update from end of the day: this is just some sort of intermittent connection problem on the PCB, so yes it needs replacement, which Analog is working on.  The symptoms seem like a DC-DC converter filter cap with a bad joint, but I only see one in the design, U18, an ADP2323, and it maxes out at 1.2MHz, so eh, who knows.  In any case, pressing lightly on the underside of the board around U13-U15 cleans up the signals momentarily.  I looked at that area under a microscope and nothing jumped out, so I'm baggin' it.)

 

fmcomms1_spectrum.gif/fmcomms1_clock.gif

 

Below is what an IQ plot looks like with the noise on the clock.  The input signal is <1hz rms residual FM, set to match the LO frequency (2.3999995GHZ works best).  The sample window is maxed out at 10000 to show the angle slewing.  When I did this yesterday they came up as dots.  Not so much, today!  I should add that on Friday when I first fired up the RF section, it was junked up like it is today.

 

IQ.png

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