[Discuss-gnuradio] saturation with multi_fft.py

[ematlis]

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 to 
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 2 
V p-p. Any ideas?

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