Rx/Tx gain

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.

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

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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

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