RSSI, 900mhz Vs 2.4ghz Xbees?

I have this little bit of code written for a copter telemetry system and find that it works really well, however, I did notice that the RSSI strength works very well with the 2.4ghz Xbee Pro radios and not well at all with the 900mhz Pro radios. The 2.4s range from 100% to about 20% before completely dropping out and gets weaker as I walk farther from the transmitting radio. The 900s tend to hang about 46% (with the antennas within inches of each other) instead of 100% and the signal does not show that it gets all that much weak as I walk away. I get about 700ft away and the signal will begin to flicker between 46-35% and then drops.

Does anyone know what the difference is between the two radios and the way that they transmit RSSI?

I was using:
rssiDur = pulseIn(19, HIGH, 100); // Measuring Pulse Len
rssiPercent = 100 * rssiDur / 64; // 64us = 100%

For the meter calculation.


The older 900 MHZ devices, not the new XB900HP, have a powerful front end LNA that changes how the RSSI function works. End results is that only the last 20% of the RSSI range is valid. The rest is a constant value. However, that LNA sure give the radio nice range!

Thank you Mike for the information! One last question, by “the last 20%”, you mean the pulse width from 12.8us to 0us correct?

Thanks again!

Sort of, watch the output. It will stay constant for 80% of the range and then it will start to vary. Kind of a step function. I have not actually measured the PWM output so I am not sure of the exact points.