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Re: [Discuss-gnuradio] UHD Warning on ODD Interpolation

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] UHD Warning on ODD Interpolation
Date: Thu, 01 Aug 2013 08:28:02 -0400
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20111108 Fedora/3.1.16-1.fc14 Thunderbird/3.1.16

On 08/01/2013 08:16 AM, embyte wrote:
Hello list,

I have a graph consisting of a modulator mixed with a signal source, and a USRP sink.

I have it configured with a *sampling rate* of 720KHz.

When I run it, UHD fires the following warning (and transmission seems fine).

The requested interpolation is odd; the user should expect CIC rolloff. Select an even interpolation to ensure that a halfband filter is enabled. interpolation = dsp_rate/samp_rate -> 89 = (64.000000 MHz)/(0.719101 MHz)

What is this about? Thanks.

Discuss-gnuradio mailing list

Two problems:

(1) 720Khz is not an exact integer fraction of 64MHz (the sampling rate of the device)--so it rounded

(2) It's warning you that you'll get a "humped" frequency response with more rolloff than you'd like, due to the rounded-down sample rate
     requiring an interpolation rate that is not divisible by two.

Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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