FAQ: How are the RSSI and BGNDRSSI registers calculated?

Detailed info on the RSSI and BGNDRSSI register can be found on the AX5043 Programming Manual.

As shown in Figure 12 (page 18) the RSSI is evaluated by the receiver chain right after the AGC settles. The RSSI expressed in dBm is obtained from the AGCCOUNTER.

A first order RSSI Approximation is −AGCCOUNTER · 0.75dB − 38dBm. This approximation degrades at low S/N and low bitrates. A better RSSI approximation considers also the gain set in the digital domain: −AGCCOUNTER · 0.75dB + 20 log10(TRKAMPL)dB + constdBm .
The BGNDRSSI is calculated the same way, just that is done when there is no signal.

Thanks a lot for this information. Neither in the datasheet nor in the programming manual for this devices this formulas can be found and it is not clear, what the offset of -38dBm intends to represent or where it comes from. The same holds for “constdBm”. May you please give us further explanations about those parameters, how they have to be determined and what usefull values we should take for these? Are these board specific parameters? Further it would be very interesting how the calculated RSSI as an indicator can be used to draw conclusions on the effective signal strength at the receivers input.

Thank you in advance and have a nice day