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Large spurs PFD frequency ADF5355

Question asked by Valimor on Dec 7, 2017

Hello everybody. In my project (custom board) I need to work on several frequencies 7315 MHz, 7500 MHz and 7815 MHz, at the same time the PFD frequency should be suppress by at least 60 dBc. I found a filter that theoretically should provide the necessary suppression. But in practice it is provided only at 7315 MHz where the suppression is 68 to 70 dB, at a frequency of 7,500 MHz the suppression is 59-60 dB, and at 7815 MHz is 53 dB. Why is that? this can be somehow related to the slope of the VCO characteristics?
and I have one more question, when working in fractional mode, I see strange "running" spurious. So it should be?

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