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the phase noise get worse when ADF4351 use external reference clock

Question asked by 星魂wyj on Apr 9, 2018

when ADF4351 use 10M crystal oscillator on board the output phase noise is good, but use external reference clock the output phase noise get 30dB worse. External reference clock use Keysight signal generator, the output phase noise is as good as crystal oscillator on board. I had added a 50ohm resistor between REFin and gnd, but there is on help. where is the problem? How to solve it? Thank you! The PCB is shown below

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