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Problems with LMH0356 reclocker

Richard Woodham1
Prodigy 210 points
Other Parts Discussed in Thread: LMH0376, LMH0356

We have a fibre-optic receiver board receiving HD-SDI (SMPTE292) video data.  The output of the fibre transceiver is fed directly into channel 0 of the '356.  The output goes to a buffer amplifier then to a BNC connector.  The eye diagram I get from the output is poor (jitter is ~250ps after 5ns).  If I feed the output to the SD3GDA eval board the LMH0376 chip cleans the signal up a treat.

What could possibly be wrong with the '356?  We thought originally that the bypass pin might have been inadvertently pulled high, but no.  The LOCK DETECT is high.  SD/HD_n is low.  The 27MHz crystal appears to be working OK.  It's as if it were in BYPASS mode - but it isn't.

Originally our board was fitted with loop filter capacitor 560p (don't know why).  We changed to the recommended 56n - no observable improvement.

Any thoughts?

Regards,

Richard

  • 0
    TI__Guru* 88835 points
    Hi Richard,

    First please put LMH0356 LOCK DETECT on scope to make sure it is not toggling. I don't think it is but just to be sure. Also, since LMH0376 output is fine it tells me LMH0356 rate select pins are set correctly. Also, LMH0356 can be optioned to have line rate clock on the 2nd output and if the device is locked at HD you should see 1.485GHz clock on the second output.

    I could be wrong but since LMH0376 cleans up the signal this tells me there is some ISI that is being introduced between the output of the LMH0356 and the input of the LMH0376. If indeed the LMH0356 is locked, at the output you should see a crisp wide open eye. If your scope has 100 ohm termination you can cut the trace from the output of the LMH0356 and put your scope right on the output of the LMH0356. As long as, LMH0356 is locked you should see a wide open eye on the scope. So please check to make sure there is enough bandwidth for components or traces between the output of the LMH0356 and the input of the LMH0376. Lack of bandwidth could cause ISI(Inter Symbol Interference) that is being cleaned up by the LMH0376.

    Regards,,nasser