How are RSSI measurements made?

[Thavish]

Hi,

I'm  trying to understand how the nRF52840 makes RSSI measurements. From what I understand, while other manufacturers implement non-standard RSSI systems, the nRF52840 reports RSSI in decibels relative to milliwatts (dBm), which is typically referred to as RX power. I found the following in its product documentation (6.20.8):

What exactly is this "mechanism for measuring the power"? I've tried looking at the following answers, but am still unsure.

 - https://devzone.nordicsemi.com/f/nordic-q-a/314/how-to-translate-the-result-from-register-rssisample-to-the-real-rssi/1650#1650

 - https://devzone.nordicsemi.com/f/nordic-q-a/28380/how-is-rssi-measured

Just to clarify, my question isn't on the reliability of RSSI measurements, but on the mechanism/formula used to generate them. Any help would be much appreciated.

[ketiljo]

The RSSI is measured with a ADC that measures the voltage in the receiver, just before the demodulator. So it's measuring the power that comes from the antenna. 

[Thavish]

Thanks. Is there a document that describes how the signal is passed between the antenna, ADC, demodulator, etc.? Can I also check that the following steps are right?

1. A signal is received by the antenna (either inbuilt into the nRF52840 or external).
2. The signal is passed to the ADC in the nrF52840.
3. The ADC measures voltage and returns a value for signal power.
4. The signal continues to the demodulator.

[ketiljo]

Yes, that's basically what happens.