Absender:	"Erhard Wielandt" <320062183964-0001@T-Online.de>
Empfänger:	"Bob Hutt" <bhutt@usgs.gov>
Datum:		01. Okt 2009 14:56
Betreff:	Re: GST document, 28 Sep 2009 version

Dear Bob:
I know that you are busy, nevertheless you may be
interested in the attached three plots. They show
how sensor nonlinearity is seen by CALEX and DISPCAL.

The experiment was made during a lecture at Potsdam
in the lecture room; the audience is responsible for
occasional bursts of noise in the data. The seismometer 
was a Mark L4-3D passive, 1 Hz, damped electromagnetic 
sensor.

gfzsweep.png: a sweep calibration. The signal coil
was used for the stimulus as well, so the output
signal is a mixture of input and output, and the
resonance of the sensor at 1 Hz causes only a slight
increase of the amplitude. The residual exhibits
some misfit at high frequencies (5 Hz), a better fit
between 2 and 1 Hz, and the second harmonic of the 
stimulus at lower frequencies (where the mass 
displacement is largest). 

gfzstep.png: the same with a stepped calibration current.
We sent a positive, zero or negative current through
the coil, as is apparent from the input trace (top).
Now look at the polarity of the residual: it has
opposite sign when going from -current to zero, and
from zero to +current. Of course, for a linear system
the residual should be the same. CALEX fits the average
step and then one of the steps is smaller and the other 
larger.

gfzsteptable.png: the same sensor on the step table. The 
upward steps are slower than the downward steps because 
the motor has to work harder. Nevertheless the final
displacement must be the same. The last trace
in the plot shows the deviation of the step size from
the average. (The level after each step indicates the
deviation; the plot clips at 1%.) The downward steps 
are systematically smaller than average by about 2% and 
the upward steps are larger by 2%. You may read these
numbers from the protocol file dispotstep.out or 
from disptable.lst. After removing the average 
discrepancy between up-steps and down-steps, the 
generator constant scatters by only +-0.2%. So the test 
does in fact show sensor nonlinearity very clearly. We 
also had a Guralp (60 sec) broadband sensor on the same 
table and saw no nonlinearity.

Kind regards, Erhard

Anlagen:	ptable.png,
		psweep.png,
		pstep.png,
		disptable.lst,
		dispotstep.out