PSN-L Email List Message

Subject: Re: Mass deflection weight test
From: "Geoff" gmvoeth@...........
Date: Thu, 1 Nov 2007 22:45:15 -0700

Hello Chris;

Affirmative, I like that idea.

I just found while playing with different
configurations what I thought to be
a bad geophone. Nothing I would do
would get me a good signal from the
geophone. After a couple days of considering
digging it up I decided to run the signal straight through
from the geophone to the A/D converter.

I found there to be in the ground a
high frequency > 10 Hz that was
overdriving my preamplifier essentially
"jamming" the amplifier. I solved this problem
simply by filtering harshly with a LPF at about 3 seconds
0.3333... Hz. This will allow me to equalize the
signal down to this freq.

Conquering the noise also has allowed me to
achieve a stable baseline something I have been
fretting over the past several years :-(

Now I have at a gain of 10,000 and down to DC
the ability to see signals between DC and 3 or 4 Hz.

If I can ever get back to playing around with
that positive feedback arrangement I will let
you know what happens.

At the moment I had to go back to using the
Diff amp to solve the noise problem and get back online.

I cant tell you how terribly noisy my location is.

With a gain of only about X1000 the geophone was putting
out a human artifact that drove the amplifier way beyond its limits.
When I put the N=4 LPF with 2.0 Hz Fc The noise was greatly
reduced indicating it was above that. When A high freq does this
all other signals become distorted or so I found.

It seems solving the noise makes everything behave
in a more predictable fashion.

That was how i spent Halloween between the amplifier and
monster shows on tv.

Hope you had a nicer holiday.

One Other Thing,

I have found there to be a common mode capacitance
between the shield and wires to the geophone of about
6.74 e-9 farads and would like to hear your comments
about proper grounding to the amplifier as it relates to
this common mode figure. By common mode i mean
if you short the two geophone wires together then measure
between the two wires and the shield between the geophone
and the house this 6.74nf is what i get after subtracting
maybe 0.04nf of meter ( DMM ) capacitance. I know this
is important but have no idea the proper way to deal
with it.
The geophone cable is about 35 to 40 feet long and is
100% shielded with copper foil and infused wuth what looks
to be silicone grease for underground buriel.

----- Original Message ----- 
Sent: Thursday, November 01, 2007 4:50 PM
Subject: Re: Mass deflection weight test

> In a message dated 01/11/2007, gmvoeth@........... writes:
> I have  the best way to simulate this with smaller geophones is to pulse it 
> with a  battery limiting
> the current flow ( resistor ) to whatever produces 100 or  so microvolts 
> across the geophone.
> Hi Geoff,
>    One other way to do it is to use a capacitor with a  resistor in series. 
> You charge up the capacitor to a known voltage and simply  short it across the 
> geophone, seismometer etc. This supplies a known charge I x  T to the system 
> and the resistor damps any oscillations. 
>    Regards,
>    Chris Chapman

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