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

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] bandwidths< 400Khz no longer work with USRP2
Date: Tue, 16 Nov 2010 23:20:29 -0800
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101027 Fedora/3.0.10-1.fc12 Lightning/1.0b2pre Thunderbird/3.0.10

On 11/16/2010 11:13 PM, Marcus D. Leech wrote:
On 11/17/2010 02:00 AM, Tom Rondeau wrote:

WBX with latest firmware downloaded from Ettus Monday.

Hmmm, I'm using Basic_Rx.  That should make *zero* difference.

I discovered that there's a "magic" break at any sample rate that
requires a decimation>256.
   So *somewhere* is having a hard time with greater-than-eight-bit integers.

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.

The solution is to decimate less in hardware and more in software, or to use more amplification.


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