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

[Marcus D. Leech]

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.
>
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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
http://www.sbrac.org