runs obnormally at a certain decimation rate

I was in a test with ‘’ code in the gr-utils directory and found that the program is not running as I expected in certain conditions.
I was watching signals running the usrp2 as a spectrum analyzer with the code.

Test environment information

  • usrp2 + gnuradio 3.2.1 (or gnuradio 3.2.2)
  • usrp2 H/W version : rev. 3
  • usrp2 SD card image/code version : Cannot identify
  • RF RX Interface : basic RX board
  • Host PC O/S : ubuntu 9.04 Desktop (64bit version)


  • When I set the decimation rate to ‘20’, and open the RF input port(hooked up no signals), the noise level goes down to -380dBm and and flat. With the decimation rate of 18 or 22, it looks O.K. Noise level is around -90dBm and I can see the frequency domain envelope of white noise.
  • At that moment, the samples coming from usrp2 are all zero. I confirmed it with wireshark.
  • I figured out that it happens not only with the decimation rate of 20, but with 40, 60, etc.
    I was suspicous if it went like that with multiples of 20, but it happened with 50, 82, etc.
  • When I feed signal to the RF RX port, it goes back to normal. So, I can say that it happens when the input signal level(power) is lower that a certain level. However, it does not happen with other decimation rates like 4, 6, or 18.

I would like to know

  1. If this is a known issue. In that case, I also would like to know if there is any workaround.
  2. If this is a new issue, I would like to ask experts in this mailing list to reproduce the experiment and let me know why this happens and what I should do.

Best regards,