mmediately after changing the CE pin from low to high level
When initializing registers with SPI
The PLL will be unlocked.
If you increase the time to start register initialization with SPI,
I lock it.
After CE pin goes high,
Immediately initializing registers with SPI
Is not it possible?
Make sure all voltages on the ADF4351 have settled to steady-state. If these haven't settled, then there is a risk that the wrong VCO band will be selected, which will cause unlock.
This was really helpful info!
I have been struggling with locking after reset (CE) for a long time, for most frequencies it worked fine, I would do SPI access after a ms to setup the PLL again. The band select logic sometimes went crazy, especially around the border between the mid and high VCO freq range. I would get a LD square wave at around 30 kHz.
I now measured Vcom/vref/vtemp time constants (I have 470 nF decoupling) and found that I have a 10 ms (tau) on Vtemp, and about 1 or 2 ms on the other two. Now I wait 30 ms after CE goes high to do my SPI sequence and all is good.
I think this info should really make it into the datasheet!
Regards -- Pontus