Geophone Box  Notes
                    -----------------------

                                             Initial Rev.  30-Nov-2021
                                             Current Rev.   1-Dec-2021


This file is a set of notes about the Geophone Box.


 - The Geophone sensor that is currently in this box is the one
   that Niyaz lent me about 2 months ago at the start of this
   project.  I do not know its part number or characteristics.
   It is a sensor that detects vertical motion.

 - The box includes a 2.4k Ohm load resistor on the geophone
   which should get rid of most of the low frequency peaking.
   2.4k Ohm is at the high end of the typical recommended value
   of load resistor for critical damping of sensor of this type
   but even a factor of 2 high still gets rid of most of the low
   frequency peaking.  The loaded sensitivity of the geophone
   will be about 70% to 80% of its specified open circuit
   sensitivity.

 - This Geophone Box includes a low noise amplifier with a gain
   of 100 and a high frequency roll off set at 7.2 kHz.  Driving
   a high impedance load  (i.e. 10k Ohm or higher)  the output of
   this amplifier can swing at least +- 7 Volts.

 - With the Geophone Box sitting on my work bench  (i.e. no
   special precautions to control mechanical vibrations)  I see
   a noise level of about 50 mV rms, about 150 mV pk-pk  at its
   BNC output connector.  On the scope the bulk of this noise
   has a period of about 25 msec to 35 msec - perhaps the natural
   vibration frequency of the work bench.  Dropping a small 6-32
   screw onto the work bench from a height of 2 or 3 inches
   causes an output signal of about 670 mV pk-pk in the same
   frequency range.

- The batteries that power this amplifier should have an
   operating life of about 60 hours.

 - The box is made of steel to provide some magnetic shielding
   for the geophone sensor.  To increase the weight of the box,
   to give it good mechanical contact to the surface where you
   set it,  I have included an additional 1/4" thick steel plate
   on the bottom of the box.

 - All electronics inside the box uses an isolated analog common
   that connects to the shell of the isolated BNC signal output
   connector.  This leaves the question of how to "ground" the
   metal box itself so that it can provide shielding against
   pickup of electrical noise.  We don't want to make a ground
   loop with this connection to the box.  Two choices are
   provided for:

     Connect the metal box itself to the internal isolated analog
     common and put a sheet of insulating material on the bottom
     of the box so that it does not make an electrical connection
     (and thus a possible ground loop)  with the surface where you
     set the box.  This is how the Geophone Box is setup
     currently.

     Remove the link between the box and internal isolated analog
     common  and instead  proved an external "ground" connection
     to the box that is at the same noise free potential as the
     "ground" at the input to your DAQ system.  It's easy for me
     to open this box ground connection link if you want me to.

 - I've provided some thin very flexible 15 foot long BNC cables
   that you can use to connect the Geophone Box to the input of
   your DAQ system.  The intent is to limit the amount of
   mechanical vibration that is conducted to the box by its BNC
   output cable.  Because these BNC cables are thin and flexible
   they are relatively delicate.