PSN-L Email List Message

Subject: Re: Equipment: SG vs. Lehman...Hybrid
From: Thomas W Leiper twleiper@........
Date: Fri, 25 May 2001 16:27:18 -0400


Actually, that was a link to Carl's site embedded in the earlier
post(s) I was responding to. I can point you to a picture of
my magnet/coil sensor within my ancient article on Larry's
site:  http://www.seismicnet.com/leiper/Image1.gif , or you
can read the whole article on that system at:
http://www.seismicnet.com/leiper/seismograph.html

The only difference with the current scheme is that the coil
is used for feedback instead of detection, and there is a
little oscillator box with a capacitor plate on the side that
sits to the left of the magnet core in the plan (overhead)
view with a gap of about .050". That 250 Khz oscillator
is phase locked to a 1 Mhz oven crystal oscillator standard
by running the outputs of each into a divide by 512 and
divide by 2048 resettable counter strings respectively.
This results in a "sample" rate of about 500 samples per
second (488 actually). Whichever counter reaches the
maximum/minimum first either increments or decrements
a 16 bit counter feeding a 16 bit D/A chip whose analog
output (via some tailoring circuitry) drives the coil...thus
moving the magnet and changing the capacitance gap and
frequency of the variable oscillator.

The only "response" adjustment is how you apply the
digital counter error. The way the counter scheme works
is kind of like a race between the two oscillators. Both counters
are reset, and begin counting (actually "down") from their
maximum. Whichever one gets to "zero" first begins clocking
the A/D counter (up or down depending on which counter
is the "winner") until the other counter reaches minimum.

I have experimented with several increment/decrement schemes.
The first was to simply increment or decrement by one count
and immediatley reset both counters (regardless the actual
difference) because the "sample" rate was sufficiently high
to "move" my first ( 8 bit) A/D quickly enough to get good
feedback response. Later (here comes the fanaticism) I switched
to a 16 bit A/D and counter/driver and began to clock them
with the "difference count" between the two counters reaching
their minimums. When the first oscillator reaches minimum
it gates increments or decrements (from any of the counter/divider
outputs I choose) to the A/D counter until the other counter
reaches minimum...at which time both counters are reset. This
allows the increment rates and decrement rates to be independently
adjusted if necessary. This will be especially useful for my next
version of this experiment, which will utilize two "opposed" VFOs
and get rid of the Lab standard oscillator. This will eliminate
the temperature drift in the VFO (relative to the Standard) since
both VFOs will be affected similarly by such changes.

Overall, it is an extremely sensitive method of detection that
shows great promise, but there are a few more things to do...

Tom



On Fri, 25 May 2001 13:05:19 -0500 "Bryan&Regina Goss"
 writes:
Thomas, Do you have any pictures of your sensor. the link you sent is







Actually, that was a link to Carl's site embedded in the earlier
post(s) I was responding to. I can point you to a picture of
my magnet/coil sensor within my ancient article on Larry's
site:  http://www.seismicnet.= com/leiper/Image1.gif ,=20 or you
can read the whole article on that system at:
http://www.= seismicnet.com/leiper/seismograph.html
 
The only difference with the current scheme is that the coil
is used for feedback instead of detection, and there is a
little oscillator box with a capacitor plate on the side that
sits to the left of the magnet core in the plan (overhead)
view with a gap of about .050". That 250 Khz oscillator
is phase locked to a 1 Mhz oven crystal oscillator standard
by running the outputs of each into a divide by 512 and
divide by 2048 resettable counter strings respectively.
This results in a "sample" rate of about 500 samples per
second (488 actually). Whichever counter reaches the
maximum/minimum first either increments or decrements
a 16 bit counter feeding a 16 bit D/A chip whose analog
output (via some tailoring circuitry) drives the coil...thus
moving the magnet and changing the capacitance gap and
frequency of the variable oscillator.
 
The only "response" adjustment is how you apply the
digital counter error. The way the counter scheme works
is kind of like a race between the two oscillators. Both counters
are reset, and begin counting (actually "down") from their
maximum. Whichever one gets to "zero" first begins clocking
the A/D counter (up or down depending on which counter
is the "winner") until the other counter reaches minimum.
 
I have experimented with several increment/decrement schemes.
The first was to simply increment or decrement by one count
and immediatley reset both counters (regardless the actual
difference) because the "sample" rate was sufficiently high
to "move" my first ( 8 bit) A/D quickly enough to get good
feedback response. Later (here comes the fanaticism) I switched
to a 16 bit A/D and counter/driver and began to clock them
with the "difference count" between the two counters reaching
their minimums. When the first oscillator reaches minimum
it gates increments or decrements (from any of the=20 counter/divider
outputs I choose) to the A/D counter until the other counter
reaches minimum...at which time both counters are reset. This
allows the increment rates and decrement rates to be independently
adjusted if necessary. This will be especially useful for my next
version of this experiment, which will utilize two "opposed" VFOs
and get rid of the Lab standard oscillator. This will=20 eliminate
the temperature drift in the VFO (relative to the Standard) since
both VFOs will be affected similarly by such changes.
 
Overall, it is an extremely sensitive method of detection that
shows great promise, but there are a few more things to do...
 
Tom
 
 
 
On Fri, 25 May 2001 13:05:19 -0500 "Bryan&Regina Goss" <bgoss@..................>=20 writes:
Thomas, Do you have any = pictures of=20 your sensor. the link you sent is broke.
=

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