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ADF4351 has spurs when updating registers via SPI?!?

Question asked by pklos on Oct 6, 2016
Latest reply on Nov 2, 2016 by klostech

We are using an ADF4351 in our design to tune our LO.  We've recently discovered that the ADF4351 seems to be generating a spur whenever we write to the registers via the SPI bus.  Here is what the spurs look like on a spectrum analyzer:


Spikes during tuning


If I increase how often we write the registers, the spurs increase.  If I decrease how often we write the registers, the spurs decrease.  If I program the registers once and stop, there are no spurs at all.


I have a reference frequency of 10 MHz and am setting the target frequency to 3.800 GHz.  (INT = 380, FRAC = 0, MOD = 4000, phase = 1, band select clock divider = 80, clock divider = 1, R = 1, reference doubler disabled, reference divide-by-2 disabled)


Is there some method we can use to program the registers that will prevent these spurs from happening?  I've tried "Low spur mode", "Mute till lock detect", various clock divider modes, both high and low band select clock modes, but nothing seems to get rid of these spurs.