# Low frequency oscope

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

On Thu, Sep 13, 2007 at 12:53:35PM -0400, [email protected] 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.

Yes, it’s 10 Hz. And a 50 Hz high-pass; I originally didn’t have that
in
place, thinking it would filter off the 4 Hz, but in fact it doesn’t
seem
to, although it does remove most of the DC. Could this problem be an
aliasing problem? Or could it be related to the frequency resolution of
the local oscillator?

thanks,
eric