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ADF4351 Intermittent Glitches

Question asked by djohnston on Oct 29, 2012
Latest reply on Aug 26, 2014 by JUST_ANOTHER_RF_GUY

We are having problems with the ADF4351 PLL chip's amplitude/phase/frequency accuracy in a very intermittent way. We are using two PLL's in a ping pong type architecture that tunes from 640MHz to 3400MHz in 20MHz steps every 10mS, yielding a lock time of about 40-50us. Occasionally, one of the tuned frequencies starts acting strange for awhile. For example, PLL 1 is hitting 1420MHz just fine for over 5 minutes straight, then a batch of glitches occurs for a few seconds, then goes away. All the other tuned frequencies are fine. Now, PLL2 has a problem at, say 2000MHz, same things occurs. It works fine for a long time then a batch of glitches pop up. We have swapped the PLL's and the frequency glitch occurs at a different frequency. It appears the "glitch" follows the particular chip. I can change the settings of the chip,i.e. have another way to determine the same frequency, and the problem still occurs at the same frequency.

 

 

Below is the technical set up:

No speedup enabled

Int-N configuration: FRAC=0

Prescaler=8/9

Charge pump current: 5mA

PLL Loop Bandwidth: 223kHz, calculated from ADIsim

PLL Phase Margin: 50 degrees, calculated from ADIsim

Reference frequency: 20MHz

Phase detector frequency: 10MHz, reference divider enabled, R=1

Band select clock divider:20

Loop filter values below, all caps NP0/C0G, R's are 1%:

C1=75pF

R1=1.21k

C2=2.2nF

R2=12.1k

C3=18pF

 

Any input would be greatly appreciated,

David

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