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

[Matt Ettus]

Decimation involves filtering.  Filtering reduces the noise power, which 
can cause you to have many zero samples in a row, which will result in 
what you saw.  Different combinations of the CIC and halfband filters 
will result in different noise behavior.  What you are seeing is 
perfectly normal, and won't happen when there is a real signal there.

Matt

ILKYOUNG KWOUN wrote:
> 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.
>
>
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