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Re: [Discuss-gnuradio] bandwidths< 400Khz no longer work with USRP2

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] bandwidths< 400Khz no longer work with USRP2
Date: Wed, 17 Nov 2010 02:33:05 -0500
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On 11/17/2010 02:20 AM, Matt Ettus wrote:
> Decimation is filtering.  When you decimate by 512 you are reducing
> noise by a factor of 512 (27dB).  Since you are using a BasicRX, there
> will be very little noise, and 27dB less after decimation.  In fact,
> there is so little noise that the output of the filters is a constant
> 0 once it is rounded to 16 bit ints.  That is why the FFT results
> essentially show negative infinity.
Well, OK, I'll buy that.  But there's a significant change below
decimation=256. A non-linear
  jump from "reasonable-looking data" to "negative infinity".

I'm seeing a jump from a level of around -20dB to "negative infinity" by
changing decimation from
  256 to 260, which is a noise bandwidth change of something like 0.04dB.

So while I'm totally willing to believe that a gross change from 400KHz
to 200KHz might cause a
  bit of weirdness, it seems highly counter-intuitive that a small
change as implied by
  decimation=256 to decimation=260 would cause a huge nonlinear leap in
filter output in
  the decimator.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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