We are seeing burst noise at the baseband outputs of the ADRF6850. At our typical tuned frequency of 612MHz the burst appears at a rate of 3.5Hz for a duration of 2ms. If we change the frequency, the burst changes rate and duration.
The noise is very small and we were initially only able to see it on a scope with the 48dB of gain we have after the ADRF6850 at which point it was about 100mV peak to peak. Using an amplified speaker we can hear the burst noise at the baseband outputs of the ADRF6850.
We have made a few observations:
- If we reduce the charge pump current setting register from 7 to 0, the burst noise becomes immeasurable.
- On this latest board revision the loop filter was moved from being right at the CP pin to right at the VTUNE pin.
We are using the recommended loop filter components and Rset resistor of 4.7K from the datasheet.
Does it seem plausible that this noise could be caused by the charge pump?
Is it recommended to have the loop filter components as close as possible to the CP pin? Our reasoning for moving the components to the VTUNE pin was to filter any noise on the track to keep it from affecting the VCO.
Do you have any other suggestions for what could be causing the noise or solutions to fix it?