Hello,
I'm building a BLE receiver using an STM32WBA55CG.
The device should only receive BLE advertising packets, never transmit, so I've put it in a "central" role. The device will be used very close to sensitive equipment, so it is very important that the radio never transmit.
Despite being configured as a central, and having not called any functions which I would expect to transmit (such as scanning), I am seeing two different RF emission events from the radio:
- A "chirp" as soon as the device is powered on
- Multiple "blips" some time later at random intervals
The "chirp" is a fairly quiet burst, which seems to rise in frequency across the 2.4-2.5GHz ISM band. I assume this is some kind of calibration/set-up, as it's always emitted as soon as HOST_BLE_Init() is called, from what I can see. This is manageable, as I can block the RF path when powering on/initialising the receiver for a few seconds, which should be sufficient to prevent this being emitted over the air.
The "blips" are more troubling. They are higher amplitude than the "chirp", and look like advertising packets (there is a peak at BLE channels 37, 38 and 39, if I'm reading my instruments correctly, which are the advertising channels). There doesn't seem to be a predictable gap between emissions - I've seen intervals of 1-5 seconds, but also around 30 seconds, and sometimes 10 minutes or more.
These packets will cause us problems, especially as the intervals are unpredictable (so I can't mitigate by strategically blocking the RF path as a work-around).
Can you help me understand what these emissions are? And is there any way to ensure that the radio never transmits?
Thanks,
Andy
