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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 15:20:43 -0500
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On 11/17/2010 03:05 PM, Steven Clark wrote:

On Wed, Nov 17, 2010 at 2:45 PM, Marcus D. Leech <address@hidden> wrote:
On 11/17/2010 12:43 PM, Marcus D. Leech wrote:

What I'm seeing is that the magnitudes (as seen in the number sink) coming off the source, even with roughly  75dB of gain ahead
 are roughly 0.002 to 0.003 when I'm using 400KHz sampling, and roughly 0.0006 to 0.0007 when the bandwidth is 250KHz.  If you
 process the numbers as voltages, then we're talking a roughly 10dB drop in apparent average power level by reducing the bandwidth
 by less than 3dB.  Both 400Khz and 250KHz use a decimation that is both even, and a multiple of 4, so they should be using exactly
 the same filter sequence in the decimator, correct?

Marcus, you're a blithering idiot who should routinely be denied air.  You have clearly conflated the decimation/bandwidth numbers and
 erroneously come to the conclusion that they should use the same half-band filter lineup.  They don't, you stupid, sorry excuse for
 an advanced lifeform you. God, can you even tie your shoes reliably?  Let's see, 250KHz uses a decimation of 400, which uses both
 half-bands in the FPGA because it's both even and a multiple of 4, whereas 400KHz uses a decimation of 250, which is even, but not a
 multiple of four, and so only uses a single half-band.  So *naturally*, the numbers won't "add up" between the two bandwidths.

Frikkin' hell man, get a clue would you?  Before I come over there and whack you upside the head with a gnarly-great clue-by-four.

:-) :-) :-)

Don't be so hard on yourself...many of us would have still been stumped :)
It's definitely not obvious/intuitive (to me, at least) that changing the decimation rate just slightly results in adding a whole 'nother additional set of filtering.

Shouldn't the half-band filters have unity-gain in the pass-band?

Yes, they definitely should have unity-gain within the passband, but they have a Bessel-like response
   as far as I recall. The edges have a 10dB roll-off, so my guess is that the convolution of the
   response of those (extended) edges gives us the apparent 8-10dB difference in average
   power level between 400KHz and 250KHz (or more generally, decimination-by-factor-of-four vs
   decimation-by-other-even factor).  The difference in detected power that one would "expect" for
   this bandwidth difference is roughly 2dB, all else being equal.  Clearly, "all else isn't equal".

Matt would know for sure.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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