[Discuss-gnuradio] usrp2_fft.py hehaves abnormally at a certain decimation rate

[ILKYOUNG KWOUN]

I was in a test with 'usrp2_fft.py' 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)
 
Symptom
    - 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,
 
ILKYOUNG.