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

[Sean Nowlan]

On 03/14/2013 07:41 AM, Nada ABDELKADER wrote:
> Hi,
>
> 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?
>
> Thanks.
>
>
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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.

HTH.

--sean