BLE Long Range RSSI measurement

[Bard]

Hi

I'm testing the range of a custom sensor (nRF52840) by monitoring the RSSI. I use two receivers (the custom sensor and a nRF840 DK) and monitor the received advertising packages. The receivers are stationary, while the sensor is moved around an open field. The received rssi is plotted vs. time below (DK: orange x, Custom: blue +).

The lower threshold seems to be i bit higher than stated in the datasheet of nrf52840 (-103 dBm sensitivity in 125 kbps BLE mode (long range)). The lowest received rssi is actually -99dBm. Why don't i get values closer to -103dBm? 

[Andreas]

Hi,

 

As you can see in the datasheet, the linear range for RSSI is between -20 and -90 dBm. Outside this you will get less accuracy than what is specified, meaning as the received power decreases below -90dBm the reported value will be increasingly offset from the actual value. At - 103 dBm the reported RSSI should be 4-5 dB higher.

 

Best regards,

Andreas

[Bard]

Thanks a lot for your answer. I should have seen the specifications in the datasheet.

But do you have information on typical accuracy to expect below -90dBm, since you estimate: "At - 103 dBm the reported RSSI should be 4-5 dB higher."