> Subject: Re: calibration info
> From:    ChrisAtUpw@.......
> Date:    Sun, 19 Nov 2000 18:32:01 EST
>
> In a message dated 19/11/2000, DSaum@............ writes:
>
> > Here is some noise and signal data from my first Lehman which I
> > am still debugging.  It looks like the noise is peaked near 6 seconds as
> > you suggest.  Does this look OK to you?
>
> Hi there Dave,
>
>     Just a few thoughts. What does the output fft look like if you a)
> substitute an equivalent resistor for the coil and b) then reduce the gain
of
> the input amplifier to unity? The noise below 0.05 Hz looks a bit high and
I
> was wondering if it was due to the low pass filter, or to ambient noise?
> Filters made using OpAmps may have a lower inherent noise.

I tried to do what you suggested and all I see is what looks to me to
be 3db/oct noise slope in both cases:

http://www.infiltec.com/seismo/nois10ka.gif   noise with 10k in place of
coil - amp
http://www.infiltec.com/seismo/nois10kf.gif    noise with 10k in place of
coil - fft
http://www.infiltec.com/seismo/nois1xa.gif        noise at unity gain, no
coil - amp
http://www.infiltec.com/seismo/nois1xf.gif         noise at unity gain, no
coil  - fft

So I guess my filter chip is not
doing anything and all I have is the RC filter that I was
using to get rid of the switching noise.  Hmmm.  Does
this mean I have to give back my diploma to the Radio
Shack school of electronics?

I find it interesting that a Lehman works pretty well
with only a single pole RC lowpass filter!
http://www.infiltec.com/seismo/001117x.gif
I guess my fallback for keeping my system simple is to
replace my switched cap filter chip with an opamp chip
configured as a 2 or 4 pole bessel filter.

I am not sure yet why my MAX7401 8 pole bessel
filter chip is not doing anything.  It could be a stupid
wiring problem on my board.  But on re-reading the
spec sheet it says "Maximum Capacitive load at OUT is
min 50 pfd, typical 500 pfd".  That could be my problem since my
RC filter cap the output is in the ufds.   I thought I set the Maxim
corner frequency to 4 Hz, so the sampling frequency would
be 100x=400 Hz.   I guess that when the spec sheet recommends RC
filtering for the sampling noise, it assumes that you have
much higher corner frequencies so you can limit your RC
capacitor size to pfds?  If this Maxim output capacitance is the problem
then only solution I can see is to put a unity gain amp
between the filter chip and the RC.  Ideas?

> > During the day the record shows many spikes
> > similar to what happens when someone walks near the pendulum and
> > deforms the slab. I get some pretty quiet data in the middle of the
night
> > when the Big Macs go home
>
>     Could be magnetic or mechanical or both. Did your fft represent the
> quietest period or was it average over the day?

My system noise record was from  9 UT or 4 AM local time.  Pretty quiet.
During the day it would be similar but riding on spikes every few
minutes.  These spikes seem to be related to slab tilting, since
they look like what happens when I walk over to the pendulum.
Here is an hour of system noise at noon local time.  The big spikes
may be me walking past the pendulum to get a Big Mac.

http://www.infiltec.com/seismo/nois-amp.gif     noise at 4am local - amp
http://www.infiltec.com/seismo/nois-fft.gif       noise at 4am local- fft
http://www.infiltec.com/seismo/nois12na.gif    noise at noon local - amp
http://www.infiltec.com/seismo/nois12nf.gif     noise at noon local - fft

Ciao,

Dave


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