Jim,

Congratulations on building your vertical sensor. With a small
resistor (like 10 ohms) across the (speaker) coil to damp it,, it 
does work nicely as a conventional moving coil sensor. If you have
something to record it on or with, it will record larger events.

But for a sensitivity to all events of interest, the VBB response
is necessary. And the VBB output signal is obtained from the
displacement detector when the triple feedback is connected.
But the displacement data can come from any type of detector, 
including a capacitive detector similar to the S-G design.

I had previously thought of suggesting the use of the S-G sensor, 
but hesitated to suggest it because I have no information on the 
sensitivity and particularly the linearity of the S-G capacitive sensor. 
There is also a question of the stability of the tuned circuits
involved, which can seriously affect the linearity. (synchronous
demodulators avoid this problem). THere is also a concern with the
physical design and the capacitive change with distance; this could
probably be improved by making the outer plates significantly larger
than the center plate. The VBB feedback, because if its broad
frequency response, does require a fairly linear displacement
response. So I don't know if the S-G sensor will work with the
triple VBB circuit, but it probably will work as for the horizontal
S-G design with Larrys' circuit.. I also think that the VRDT electronics
design will work with the capacitive sensor if the frequency is
shifted up to 100khz or more (limited by the speed of the demodulator
switch).

Other options are to get someone to make the VRDT and electronics
for you. I have been pursuing several options to make the electronics
available. BUT FIRST I have been devoting considerable effort to 
tidying up all the electronics and making a truly final and detailed
version of the schematics of all the electronics. This is why I have
held the SASE envelopes until I am 99% confident that what is drawn
is what is working. Toward this end I have assembled 3 complete
multi-period school seismometers that all perform exactly the same,
even when the circuit cards are swapped around.  Some simplifications
have worked out well, and the total number of amplifiers (op-amps)
is only 10, 5 of which are in the displacement detector demodulator.
I tried to update the schematics on the web site two weeks ago, but
was cut short by a lightning power surge that smoked the 5-Volt, 
200-amp power supply of our main spark server, so the schematics
are still stuck in the scanner work station. (although they have since
been updated again)..

I am currently getting information on having the printed-circuit boards
laid out and printed in limited quantities. But I am still unsure as to 
when these circuit cards and/or working (assembled) electronics might be 
available. Maybe by Christmas?.

Regards,
Sean-THomas

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