Rex,

I assume that you realize that the SG design can be configured as
a VBB system using its capacitive displacement detector, or by
replacing the latter with a VRDT.

For the STM-8, you might want to consider having the zero motor
control switch be a 3PDT with the extra pole activating the relay
to switch to the shorter period while zeroing.

Regarding the demodulator schematic: some things are still left over
from the tiltmeter application, but may be useful. Using numbers
from your schematics:

The jumper J1 at R23 and C12,13 is to provide about 10x gain for the
AC bridge amplifier. I have found that this makes the sensitivity too
high to work with for the VRDTs that I make. But there is an additional
10x gain in the DC displacement amplifier that could be omitted if the
demodulator gain is already 10xed. But a VRDT with 2500 mv/micron is
difficult to calibrate, even with a differential micrometer. On the other
hand, if the VRDT gap (between the coils) is large (like 3-5mm) the
added AC gain may be necessary to get a nominal output.

Switch S3 and C16,17 are a low pass filter. I am using 0.47uf, as well
as other values (as low as 0.001), to play with the loop response at 
high frequencies vs instabilities.

Also, the VRDT calibration switch S1 and resistors R12,14, 1nd 15 are
optional as far as the seis is concerned. The VBB seis is calibrated
with DC current in the auxilliary calibration coil. The values of the
resistors assume that the VRDT reactance change is 22 ohms / 10*microns,
and are handy for evaluating amplifier gains, etc. The same can be
said for the "zero-test" switch S2 and R16-18: it is very useful for
evaluating the circuit performance, but not used in the normal operation
of the seis. For this reason, these are mounted on the bridge amp board,
rather than being on the control box panel.

Regarding the seismic amplifier, the capacitors C11-14 are for power supply
filtering, and can be any handy value; they are in pairs at opposite
ends of the amp board. Also, the DC-blocking or 60/90 second high-pass
capacitors C17-20 and C21-24 are to remove the amplifier DC offset from
the telemetry VCO (voltage controlled oscillator) input to keep the FM
carriers from wandering around (we had only 90hz of "guard band" between
each channel). The 330uf capacitors are expensive, and they can be 
replaced with the jumpers at J1 and J2. Also, the gain switch(es)
can be replaced with  selected gain resistors to save money. An overall
gain of 60db (x1000) (S5 + S9) is a handy value, although a max of 90db 
(x 32768) (S7 + S10) may be necessary for a small geophone.

Regards,
Sean-Thomas


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