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[Discuss-gnuradio] received signal strength circuit

From: Ben Olsen
Subject: [Discuss-gnuradio] received signal strength circuit
Date: Fri, 8 Sep 2006 21:43:28 -0500


I'm analyzing transmission between two Ettus USRP boards using the benchmark_gmsk_tx and _rx.py scripts and I have a question regarding the rssi (received signal strength indicator) circuits, because i've noticed some strange behavior.  Let's say I transmit at a center frequency of 2440 Mhz, with a bitrate of 1Mb/s.  On the wireless spectral analyzer, I see a spike of appropriate width centered at 2440 MHz, but at the receiver when i use the u.read_aux_adc function to see the received signal strength from the USRP it gives me a value of >4000 within 10-20 MHz of the center frequency!  I have the boards about 3 meters apart.  My questions are:

First, generally, how are the rssi values measured?

Second, is it possible that the antennas that I'm using have a gain that is too large?  One possible explanation may be that the antennas transmit at unreasonably high power, saturating the rssi circuits at the receiver for a large bandwidth.  However, the spectral analyzer doesn't seem to corroborate this, since the spike it sees does not seem to be enormous in amplitude compared to the bubbling sea of noise that surrounds it. 

It boggles me that I can see the 1Mb/s transmission still 10MHz away from the center frequency at the receiver - is there something I'm missing?

Thanks very much.

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