I have an issue that I can't understand and looking for your help to give me some insight to solve it. I'm using a module named dw1000 that is used to measure distance using UWB at 6.4Ghz, 3.95 Ghz...etc.
The IC block diagram looks like this:
The pins VDDPA1 and VDDPA2 on the left side of the block are used to power directly the Tx/Rx power amplifier and since the module is power hangery ( 150mA @ 3.6V) plus the noise that this amplifier generates. 3 decoupling capacitors are recommanded on each of these pins (12pF,330pF and 100nF) and they should be placed as close as possible to these pins as any decoupling capacitors.
I followed the recommanded values, placements, etc. The module has a continuous transmisison mode where the central frenquency is observed and tuned so I use this mode to check out if my device is working or not.
PCB layout of the decoupling capacitors:
First tests gave me the following RF ouput:
As you can see in the image above there is no central frequency and it's obviously very noisy.
So I put the 3 decoupling capacitors on top of each other:
To get them as close as possible to the pins and it did work for one board or 2 but not all.
Then I added a ferrite(BLM15AG601SN1D) on each pin just before the piled capacitors and like magic this fixed the problem in all boards.
The output signal when everything is clean and working looks like this:
Is there an explantation for this and how can I fix it?