[Discuss-gnuradio] low frequency oscope

[ematlis]

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 
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

************************************
Eric H. Matlis, Ph.D.
Aerospace & Mechanical Engineering Dept.
120 Hessert Center for Aerospace Research
University of Notre Dame
Notre Dame, IN 46556-5684
Phone: (574) 631-6054
Fax:   (574) 631-8355