PSN-L Email List Message

Subject: Re: detector design
From: Ron Thompson rlthompson@.................
Date: Sat, 08 Sep 2001 12:50:43 -0230

Thanks for the comments,

    I just got back from breakfast out and haven't given enough thought to the
reply that I received from Charles.

    Regarding the component count, I believe that the component count will not be
a problem.  Low noise op-amps should not be required until after the
phase/frequency comparator to drive the analogue output.

    I did look at the file for a design that used a capacitively coupled
radiating plate that detected motion by the comparing the field strength on two
receiving antennae on either side of the radiating plate.  This design had some
appeal, but I wanted to twist it around (just can't resist the desire to fix
something thats not broken - or to quote a Canadian comedian -' if it ain't broke
you're not trying').  My twist on this is to have the radiating plates on the
outside, the two plates 180 degrees out of phase with each other, and have the
receiving antenna on the arm in the null field position so that at rest no signal
is received.  A vibration of the receiving antenna would move it out of the null
position and the amplitude of the vibration would directly affect the amplitude
of the signal received, and by phase comparing it with a reference from the
oscillator the direction that the receiving antenna moved could also be
determined.  Take it one step further and use synchronous detection and
electrical noise received by the moving recieving antenna would be further

    I liked the concept of the Opto 2000 design, but I felt that the electronics
component count could be reduced.  And I wanted to see if a signal to noise ratio
improvement could be realized by using phase modulation / detection over analogue
detection of the Opto 2000 photo detection method.

    My personal goal, I guess, is to have a reasonably sensitive detector with an
acceptible noise immunity, with some longer term stability (thermal and
mechanical), a current drain low enought so that it could operate off a small
solar panel and battery, and be reasonably compact (like the Opto 2000).


CapAAVSO@....... wrote:

> Hello Ron and Charles,
> A coil moving in a magnetic field such as used on the Lehman seismometers
> most of us use is a simple motion detector but yet has adequate sensitivity
> to detect earthquakes. I believe before you replace this simple device with a
> more complicated one you should first try out the simple device to see just
> how much it is affected by the power lines you mention in your first letter.
> I have serious doubts that they will be as much of a problem as you
> anticipate.


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Larry Cochrane <cochrane@..............>