Can anybody tell me why the multi_* versions of the fft and scope
applications (found in the multi-antenna directory) seem to have such a
large gain on the input signal for high decimation values (eg 250),
relative to the non-multi versions (at same -g gain setting) of the fft
and scope aps? I’m saturating the inputs for anything above .3 V p-p
using a function generator, whereas the non-multi versions work fine up
the 2 V p-p limit of the A/D.
I’m putting in frequencies of 10 kHz and tuning to 0 Hz. I also noticed
that the magnitudes of the values displayed by the multi_* programs is
dependent on the decimation. At lower decimation values the magnitudes
are much less. This variation is not as pronounced on the non-multi
versions, although there is a ‘dip’ in the response. To give you some
idea of the magnitude difference:
for a .3 V p-p 10 kHz sine-wave input, here are the peak values:
decimation usrp_fft.py (with -S) multi_scope.py
64 1750 1750
128 1750 1750
144 1750 2900
188 1000 8000
200 1200 10000
250 1750 25000
So for decimations greater than 128 the mult_* applications cannot take
V p-p. Any ideas?
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