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ADF4350 not locking in 4040-4100 range on some boards.

Question asked by Stuart on Jan 13, 2011
Latest reply on Feb 4, 2011 by icollins

I am using a pair of ADF4350s on a single board.  Normally they work fine over the complete range of 138-4400MHz.  I have found that there is a frequency range from around 4040-4100 where the the PLL will not lock.  Naturally this means there will be no output on 2020, 1010, 505, and 252MHz etc.  This doesn't always occur however, out of 25 boards we've built about half of them will exhibit this problem on one synthesizer or the other sometimes both.


I'm using a single-ended output with just a 50ohm pullup resistor.  The output is buffered using a MMIC amplifier.


Any help?


Message was edited by: Stuart I did find the problem after some investigation.  Turns out the original prototypes were designed to use a 26MHz TCXO.  The loop filter was then designed for a 26MHz PFD.  The actual product used a 10MHz TXCO but the loop filter was never changed.  The reason this was difficult to find was because the symptoms do not look like a loop stability problem.  The synthesizer would only be unlocked over a narrow frequency span.  Sometimes just every other MHz or so in the range would drop lock.  The spectrum didn't exhibit the typical "horns" that a PLL will display when the loop if approaching an unstable condition.  More over, frequencies beyond the unstable point were fine.  Bottom line, double check your loop filter design...sometimes its just the simple things that can cause problems.