I'm experiencing weird ripple(I'm not sure should I call it ripple..) when using nfc module(pn532) with esp32
My circuit is simple, connect PN532 (GND, SDA, SCL, 3.3V)<-> ESP32-C3 (GND, SDA, SCL, VCC5V) using i2c
and as I know PN532 has internal LDO, it is ok to put 3.3V in VBAT(=VCC5V) https://www.nxp.com/docs/en/nxp/data-sheets/PN532_C1.pdf
Circuit diagrams are below

<PICO C3>
<elechouse PN532>



problem is here, whenever I turn on nfc scan, there are ripple(?) in VCC 5V <-> GND (Yellow color in Osciloscope) which has 13.55MHz frequency (it seems rfid frequency)
and due to ripple in VCC, wifi in esp32-c3 become unstable (slow or drop ping)
after turn off nfc scan, wifi works flawlessly



so Question is..

- why does ripple happend since there are many 0.1uf, 10uf capacitor which act as high frequency filter?

- 10uf, 13.56Mhz -> Capacitor Impedance 0.0012? https://www.allaboutcircuits.com/tools/capacitor-impedance-calculator/​

- how can I prevent ripple?

- I didn't expect ripple since I'm using component as complete module (elechouse pn532, wemos pico c3) not IC nor custom pcb​

- is it common situation?, how can I know problem happen before soldering module? and how can I prevent this situation?​

 

It might be a ground loop that you are looking at, or that the bypass capacitors are not positioned close enough to the ripple current source,

If your module is not defective then an answer to your question might be found by asking the manufacturer or reading the data sheet for clues.

 

It might be a ground loop that you are looking at, or that the bypass capacitors are not positioned close enough to the ripple current source,

If your module is not defective then an answer to your question might be found by asking the manufacturer or reading the data sheet for clues.

Thanks for reply,

I read about ground loop but my circuit don't have as I know..
I don't know how close enough capacitor should be, but I don't think it isn't far enough
Adding additional Electrolytic capacitor(0.47uf) fix the problem.
I'm sure module itself is not defective since I tested another board (maybe module design is defective..)

so.. only option left seems asking manufacturer...

 

Ripple is influenced by a number of things including the ESR (Equivalent Series Resistance) of filter and bypass capacitors.