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Lower ADF4351 phase noise or reduce ADF4355-2 lock time dilemma

Question asked by dmarcosgon on Nov 25, 2015
Latest reply on Sep 16, 2016 by rbrennan



I need an RF synthesizer to generate RF from 500 MHz to 4 GHz with chanel spacing of 10 KHz or less. The phase noise mask for the LO output is -90dBc @10 kHz, -110 dBc @ 100 kHz and -120 dBc @ 1MHz.


Frequency settling time after an arbitrary frequency jump has to be < 1ms worst case.


The reference clock @ 122,88 MHz (already designed and measured) can be modeled in ADIsimpll in point/floor mode using these parameters:

PN Floor -161 dBc/Hz

Phase Noise -115 dBc/Hz

at frequency 100 Hz

Flicker corner 200 Hz.

Whith these parameters the resulting phase noise matches very well the measurements.


I want to use an ADI part because of readily available drivers and support, and I found ADF4351 and ADF4355-2 as candidates.


I'm using ADIsimpll for designing the synthesizer and I've found that ADF4351 is more than fast enough  when changing frequency but I can't get the simulated phase noise low enough to be compliant in the whole frequency band.

On the other hand I can get the ADF4355-2 to be phase-noise compliant with even 10 dB of margin but lock times are too long due to the autocalibration process if I want to be within the datasheet limits of the parameters related to the autocalibration.


I would prefer to use the ADF4351 because it has lower power consumption, requires one less power rail, is cheaper and I've already used it succesfully in other designs.


Is there a way to reduce ADF4351 phase noise or ADF4355-2 lock times? I've played with loop filter bandwidth and topology, PFD frequency and such and I haven't been able.