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

[Matt Ettus]

The short answer is that to receive very weak signals, you want to 
decimate LESS in the FPGA since it is fixed-point.  Then do more 
decimation in your software on the host computer in floating point. 
Your optimal performance will come with a decimation of 4 in the USRP2, 
and all further decimation and filtering done in the host computer in 
floating point.

Remember, full scale on the ADC is about 10 dBm.  It is a 14-bit ADC and 
thus max signals would be +/- 8192.  A +/-1 signal is thus about 80dB 
lower, or -70dBm.  In order to receive signals weaker than that, you 
need to filter to get processing gain.  But this can only be done in 
floating point.

Matt

ILKYOUNG KWOUN wrote:
> Matt,
>  
> Thank you for your reply.
>  
> Actually, I repeated the experiment again with a HP signal generator to 
> find that it happens when the signal level was not quite much low( I 
> don't remember the number.). Since I don't remember the exact level that 
> the simptom appears, I will do the experiment again and let you know the 
> result.
> Since I am trying to detect very low level signals with USRP2, this 
> issue is rather critical for me.
>  
> By the way, to work it around, I should do either
> 1) Amplifier input signal before AD Converting. (Put an LNA before RF 
> input port, activate amplifier in the ADC chip, etc)
> 2) Enhance filters' signal suppression performance (Don't know if I can 
> manage this, though... )
>  
> Do I get it right?
>  
> Regards,
>  
> ILKYOUNG.
>
>  
> 2009/10/6 Matt Ettus <address@hidden <mailto:address@hidden>>
>
>
>
>     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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