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ADRF6755 unexpected Fran-N minute frequency offset...

Question asked by pharbour on Mar 20, 2013
Latest reply on Mar 22, 2013 by pharbour



I am interested in using an ADRF6755 in a product under development.  I have been testing the eval board and I am seeing something I did not expect.  I am feeding a common REF between two PLL chips with Frac-N capability, the ADRF6755 and another chip.  When I program the two devices to the same frequency Int-N or Frac-N I am seeing that the output frequency of the two PLLs differ by a minute amount.  If I am perceiving this correctly I believe they are off by 1/(2^26) of the comparison frequency.  Can the ADRF6755 generate this small frequency offset from the programmed value?

As an example I have compared the other PLL Frac-N device with an alternate AD part (ADF4350) and it appears to synthesize a frequency that is accurate [exact] (Frac-N and Int-N).

Is there something I have configured in the ADRF6755 that will create this minute frequency programming error?  This seems like it has something to do with the 25 bit Frac-N modulator, but I cannot manually set the modulator to a value that is an exact frequency lock.  It is always either a minute frequency offset higher or lower at best.

Any insight on this would be helpful.