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ADF5355 reference spurs non-deterministic?

Question asked by SQ7JHV on Jun 15, 2015
Latest reply on Jul 3, 2015 by Kato

Hello,

I want to minimize reference spur level in the output signal of the ADF5355. I've designed the loop filter according to ADIsim PLL - I think this was pretty much successful. However, I noticed that reference spur level depends somehow on the programming process. It means that when I program exactly the same binary sequence to ADF5355 (all 13 registers, from "12" down to "0") several times, in a result I get different spectrums of the output signal after each iteration. Example screenshots I present below.

Screen_0000.png

Screen_0001.png

Screen_0002.png

Each of pictures above I've got by programming exactly the same sequence to ADF5355. After each programming sequence the spectrum is similar to one of those 3 pictures, but selection seems to be random.

 

The measurement was performed on a custom PCB, but you can notice this effect also on the EVAL-ADF5355 board. Spur level depends on the selected output frequency, however this random effect can be observed at almost any frequency.

 

Main settings used to configure the PLL:

screen_settings.png

f_ref = 100 MHz

f_pfd = 10 MHz

f_out = 4 GHz

Integer N

 

Questions:

1. Why the ADF5355 behaves in a non-deterministic way?

2. Is there any way to chose the setup, which gives lowest spur level?

3. After several attempts, I've found a workaround: to change frequency I perform the sequence: change frequency to 6 GHz, wait few miliseconds, finally change frequency to desired one. It is not a big issue for my application, because I change frequency rarely. However this procedure seems a bit strange to me. Can it be simplified to just a single frequency change?

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