So here is some technical info to munch on if you are interested
in making the broadband fedback seismometer.

The displacement transducer requires an excitation oscillator that
is stabilized in both frequency and amplitude. In the schematics
that have been posted so far, the oscillator is drawn with both
a simple RC (resistor-capacitor) circuit using a 4047, and a crystal
stabilized oscillator using the Exar PXO-1000 oscillator/divider. 
But the Exar device is no longer available, and other options take
too much power (for making a battery operated system)..

A solution is to use a different oscillator/divider IC that also
supports a crystal oscillator, the 74HC4060. Using a 5.0688 mhz CPU
crystal (which is readily available) and the IC's divide by 1024 (2^10)
output, 4950 hz is obtained for input to the 8-step sine converter, 
which then has an output of 618.75hz. This method maintains the low power
provision of the original design. Of course, other crystal frequency
and divider combos are possible; I think the "74HC" design is good
to 20 mhz. (THe parts are available from Mouser or Jameco for $0.69
and $0.65).

I'm not sure when I will get the new schematic posted on the web site,
but the connections to the 4060 are simple: The crystal oscillator is
connected across an internal clock inverter at pins 10 and 11. Pin 10 has a 
10 megohm (use 2 x 5.1meg) resistor connected to pin 11, and a 3k ohm
resistor connected to one end of the crystal. This same end of the 
crystal has a 33 pf mica capacitor connected to common. The other end
of the crystal is also connected to pin 11, as is a 15 pf mica capacitor
to common. The 1/(2^10) divider output is at pin 15. Vcc (5 volts) is
connected to pin 16, and pins 8 and 12 (common and reset) are connected
to ground.

There are also other options for the stabilized operating voltage for
the oscillator and the 4018 sine generator. I am currently using the 
MAX6250, an 8-pin IC that provides 15 ma at 5.000 volts, so no additional 
components are required.

Regards,
Sean-Thomas

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