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Re: [Discuss-gnuradio] low frequency oscope
From: |
Eric Blossom |
Subject: |
Re: [Discuss-gnuradio] low frequency oscope |
Date: |
Thu, 13 Sep 2007 11:35:51 -0700 |
User-agent: |
Mutt/1.5.9i |
On Thu, Sep 13, 2007 at 12:53:35PM -0400, address@hidden wrote:
> Hi all-
>
> I'm trying to understand the behavior of the USRP/LFRX when used as a low
> frequency oscilloscope. I am trying to measure a AM modulated waveform
> with carrier frequency 34 kHz, modulated at 4 Hz. The decimation in the
> FPGA is 250, with a 10 Hz low-pass fir filter with a decimation factor of
Do you really mean 10 Hz low-pass filter, or is this a typo?
> 64 giving an effective sampling rate of 4 kHz, and a high-pass filter at
> 50 Hz to remove the 0 Hz (DC) component created by the demodulation. I am
> using the std_4rx_0tx.rbf firmware to enable 4 channel reception.
> Contrary to my expectations, I can tune to USRP off the 34 kHz and still
> pickup the 4 Hz; in fact, it's better if I do tune away from 34 kHz! I
> can tune anywhere between 0 and 34.98 and I get a nice waveform. I I tune
> to exactly 34 kHz, I get a low frequency modulation at roughly 1.67 Hz on
> top of the 4 Hz I'm trying to measure. Here's a snippet of my
> (psuedo) code for the first channel:
>
> usrp --> dinterleaver --> (dinterleaver_ch0)magblock --> lpf --> hpf -->
> scope.
>
> Any thoughts?
>
> thanks!
> eric