I’m trying to understand the behavior of the USRP/LFRX when used as a
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
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
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
to exactly 34 kHz, I get a low frequency modulation at roughly 1.67 Hz
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 -->
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