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ADF5355 Unstable output

Question asked by rubenz on Sep 1, 2017
Latest reply on Oct 11, 2017 by MRichardson
  • Operating frequency: 3 GHz
  • Ref frequency: 20 MHz
  • Charge Pump: High Current mode
  • Integer-N Mode

PLL locks at first, then gradually spectrum spreads out only in higher frequency direction and goes out of lock.

Checking:

  • Ref divider output is good
  • N divider output is quite low
  • Charge pump is railed low
  • VCO is sourcing current!

We have two boards exhibiting this behavior, each boards has 2x ADF5355 chips installed. Replaced the chips --- observed the same issue.

 

Attached are circuit diagram and programming code.

Thanks.

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