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[Discuss-gnuradio] saturation with multi_fft.py

From: ematlis
Subject: [Discuss-gnuradio] saturation with multi_fft.py
Date: Fri, 5 Oct 2007 17:19:38 -0400 (EDT)

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?


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

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