Re: [Discuss-gnuradio] low frequency oscope

[Eric Blossom]

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