PSN-L Email List Message

Subject: Re: NEVADA EVENT ??? M2.7 ??? seen in Apache Junction ??
From: Geoffrey gmvoeth@...........
Date: Mon, 9 May 2011 14:26:31 +0000

Yes Chris,
I do understand this.

But to be verbose, I say the following:

With only shorting the HS10-1 I figure a damping or (h) of three
which should according to those calculations in the book
give me a flat velocity response between 6 seconds and 6 Hertz.
That is perfect for regional events only.

" I am not sure what a flat velocity response really is,
It means to me that given a steady vibrational amplitude and increasing
frequency the geophone will produce a voltage which increases with
the frequency, this means when it is a flat velocity profile, Output Voltage is directly proportional
to the frequency = = velocity. This is because in a magnet coil pair
voltage is proportional to velocity of the wires cutting the lines of force.
If you take the circumference of a circle and spin it at a given angular velocity
you produce some linear velocity about its circumference.
Angular velocity is what w or Omega is all about.It is related to linear velocity
at some radius or what you might also call voltage."

But when I try to use negative resistor damping I get this DC offset
on the output I am unable to deal with.
Something about that positive feedback seems to
cause an offset in the output voltage which
I am unable to deal with. possibly it is the
amplifier I am using OP177G, but not sure.

The best I can do is 6 Seconds to 6 Hertz.

If I go lower, I must accept the unusual response
by simply creating PREAMP/FILTER pair [Low pass filter of N=1]
to see signal down to 30 seconds. [One Meg resistor 5uf capacitor]
In whatever form the data results I can still see
things from 30 seconds to 10 Hz.
Be it Acceleration or Displacement.

I get my best results of seeing all kinds of signals
using this preamplifier pair.

If I am unable to zero an amplifier,
I do not wish to use that negative resistance design
with the positive feedback.

I think I really do understand what you say here,
But I am unable to comply with more complex
circuitry than the simple negative amplifier and
in addition the N=1 Active Low Pass filter.

Surely there must be another way of providing
the negative resistor which can be zeroed
by using a second op amp as a negative resistor ??

I am no Engineer and have great difficulty
figuring another way to do this same thing.

Can you show me other ways of electronic damping
which are not typically demonstrated to the public ?


-----Original Message----- 
From: Christopher Chapman
Sent: Monday, May 09, 2011 10:09 AM
To: psnlist@..............
Subject: Re: NEVADA EVENT ??? M2.7 ??? seen in Apache Junction ??

Subject: Re: NEVADA EVENT ??? M2.7 ??? seen in Apache Junction ??

It is odd but I am unable to get a flat velocity response cheaply.  I guess my waveform is displacement below 1 Hz and Acceleration 
Above 1Hz, Only right at 1Hz might you call it velocity.  What kind of seismometer is that ? Hi Geoff, If you put your 1 Hz geophone 
onto a high impedance non inverting amplifier input, you get a velocity response above the resonant frequency. You then increase the 
damping to ~critical by loading the geophone with a damping resistor. This is necessary to flatten out the high frequency response. 
Regards,Chris Chapman 


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