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RSSI, 900mhz Vs 2.4ghz Xbees?

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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.

asked Apr 25, 2013 in NDS-based Gateways by RobDrizzle New to the Community (0 points)
recategorized Apr 24, 2013 by RobDrizzle

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1 Answer

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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!
answered May 3, 2013 by MikeN Community Contributor (87 points)
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.