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Re: [Discuss-gnuradio] [BULK] Rx/Tx gain

From: Sean Nowlan
Subject: Re: [Discuss-gnuradio] [BULK] Rx/Tx gain
Date: Thu, 14 Mar 2013 09:46:31 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130221 Thunderbird/17.0.3

On 03/14/2013 07:41 AM, Nada ABDELKADER wrote:

Would you plz tell me the best way to set the gain values for Rx/Tx USRP with benchmark_tx and benchmark_rx at command line. How to adjust these values?


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The process is trial-and-error and involves a lot of things, including but not limited to:
1) free space loss between the transmitter and receiver
2) whether the transmitter and receiver are so close that you're coupling signals on the front-end unintentionally
3) noise

My recommendations:
- Open up an FFT plot using uhd_fft on the receiver and set the gain slider somewhere in the middle (tune it to Rx freq, obviously) - Run benchmark_tx.py on the transmitter with default gain and tx-amplitude settings - See how strong the signal looks on uhd_fft. I've found you need 25 or 30+ dB SNR to make it work (I don't remember the exact numbers) - If you have a board with no front-end gain, such as RFX2400, increase the tx-amplitude parameter in small steps and see if you get better results. If you see clipping/compression - you'll see a big, wide, ugly signal on uhd_fft - you've gone too far. Keep in mind that the gain is specified in the range [0, 1.0] inclusive, but the lower and upper regions don't produce a clean, linear signal. You should stay in the 0.1-0.2 range but your mileage may vary. - If your board does have front-end gain, leave the tx-amplitude at its default value and change the gain only. - You may have to adjust the uhd_fft Rx gain, but again, beware of clipping on the Rx side. - Once you think you have the numbers right, try running benchmark_rx.py with the same Rx gain setting you found with uhd_fft.py and the same Tx gain settings you found for benchmark_tx.py.



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