u-Controller causing harmonics in PLL

Hello,


I have run into an odd problem and I don't know exactly why it's happening. I am using an Arduino UNO to program a ADF4153A which in turn is controlling a VCO HMC739. All are powered by the same bench top power supply. The ADF4153A has its own 3.3V voltage regulator. Above is a diagram of the PLL, the loop filter and register settings were created in ADIsimPLL and locks very well. However when its connected to the Aruino UNO board it produces a harmonic around the center frequency.

I suspect it's the arduino board causing this; after power up, and writing registers in the ADF4153A, I can un-power the arduino and the harmonics go away.


A fun bonus, when i have the USB cable attached to the Arduino (open ended or plunged in) the harmonics become quite pronounced. The cable is quite long, perhaps 1 meter.



Any idea why this is happening? A suggestion on how to suppress it? This will be an issue down the line if/when all of this gets integrated into a single unit.

Let me know if any you want any more details, and thanks in advance!
Sami

It is probably just supply line coupling but it could be genuine frequency shifting if your software is updating the PLL in a loop. If it is, change to software so it only updates when a change from the last programmed frequency is needed.

Brian.

VCO supply must be very clean and should be separated as much as possible.PLL IC may not be impacted very much due to its own PSRR but VCO Tuning Input and VCC are very sensitive against noise and interferers.