RSSI measured values for NRF52832

[Pradeep Chl]

I have an NRF52832 Receiver chipset board that detects a tag that is transmitting 10 bytes of payload every second.

I have an attenuator on the board before the antenna of the Receiver so that I can give required attenuation level.(gains).

I have gains of 0db, -4db, -8db, -12db, -16db, -20db, -24db and -28db. If I give a gain of 0db to the attenuator, I get a maximum distance of 20 meters. The tag gets detected in the 20 meter range. If I give a gain of -28db to the attenuator, I get a maximum distance of 3 meters.

For 0db gain, I am getting an RSSI value of 90 at 20 meters. For -28db gain, I am getting an RSSI value of 90 at 3 meters. For any gain I set at the attenuator, I am getting RSSI of about 90 at its maximum distance.

Why is the RSSI value dependent on the Gain of the attenuator?

I was under the impression that RSSI value will change as per the distance between the Receiver and the transmitter.

Rgds,

[Wojtek]

RSSI is not sent by transmitter (how would it know how far the receiver is?) - it is measured on the receiver. So, if you lower the gain by attenuator -> receiver measures power -> you get lower RSSI. As far as i know, nRF52 is sensitive for up to -96db (or something about that value), so you don't see packets with lower RSSI... Isn't that what attenuator is used for?

[awneil]

RSSI = Received Signal Strength Indication - so it is the signal strength seen at the receiver, by the receiver.

So you should expect exactly the results that you see: as you attenuate the transmitter's output, the receiver's received signal level will reduce correspondingly!

I was under the impression that RSSI value will change as per the distance

Other things being equal, that is true - the greater the distance between receiver & transmitter, the lower will be the received signal strength at the receiver. But, if you change the transmitted signal level (which is what your attenuator does), then the "other things" are no longer equal!

Google "link budget" ...

[awneil]

nRF52 is sensitive for up (sic?) to -96db

Or, perhaps it would be clearer to say that it's sensitive down to -96dB - below that, the receiver just can't "hear" it.

[Wojtek]

ah yes, you are completely right :)