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

From: embyte
Subject: Re: [Discuss-gnuradio] UHD Warning on ODD Interpolation
Date: Fri, 02 Aug 2013 15:28:51 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130701 Icedove/17.0.7

Dear Marcus,

> How wide is your signal?

My signal is 25 KHz wide, centered at 25 KHz. (I mix the baseband signal with a sin function at 25 KHz)

Then find the nearest sample rate > your_bandwidth that satisfies the
desired outcome that the computed
   interpolation rate  (64.0e6 / desired_rate) is an even number.
If the signal has artefacts that "spill" outside the required sample
rate, then, yet, filter it first.

While experimenting, I discovered that with 870 KHz sampling rate works, i.e. without the half-band filter problem. However, I get the baseband signal mirrored at 50 KHz lower than the center frequency configured in the USRP. It seems like related to the sampling theorem (I mean aliasing, 2*Fmax=Fs)

I can't figure out how the USRP works internally, and why moves the baseband signal to the transmitting frequency (Yes, I'm not an RF/DSP expert, just intermediate :) Looking on-line,I found out that the function block diagram is something like: CIC (Intepolation) -> half-band filter -> DAC -> TX. Can you confirm it?

I appreciate.

on Aug 01, 2013, *embyte* <address@hidden> wrote:

    Hey Marcus, thanks for answering.

     > (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.

    If I understood correctly, this it means that the transmitted signal
    gets filtered more on the edges than expected?

    Note that by setting a sampling rate of 720KHz, the transmission and
    receiving work fine.

    That is, can I fix the sampling rate warning and preserve the
    transmitted signal by modifying the sampling rate (e.g. to 480KHz) and
    by adding a band-pass filter before the USRP sink block?

    Thanks for your insights. I appreciate.

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