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.
Regards;
geoff
----- Original Message -----
From:
To:
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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