Hi,
 
  I note the recent interest in sensors for seismic use. I have  used full 
bridge capacitive sensors for a vertical broadband unit which though  work well, 
are time consuming to put together and test as well as being rather  bulky. 
The quadrant photo cell is a better ready made precision device. A  spot of 
light from an LED modulated at 10KHz is projected onto the cell and  
diametrically opposite quadrants are phase inverted to produce an X-Y signal  that change 
phase and falls to zero as they cross the null point. The output  signals are 
processed by phase sensitive detectors. Since these sorts of sensors  are high 
impedance, the first stage needs a device with a high noise equivalent  
resistance such as an FET. Unfortunate the flicker noise appears in the  noise 
voltage generator which is equal to the shot noise at 1 or 2 KHz. By  operating at 
10KHz and a narrow band filter ( phase lock loop time  constant) the flicker 
noise becomes insignificant. The output in volt/  meter depend a lot on the 
support electronics but in general the quad photo  cell approach seems to offer 
a high output. The LED is somewhat temperature  sensitive (approx. -0.7% C ) 
but by summing the four quadrant and applying this  as feedback this 
temperature dependence can be made negligible.  For a  single axis, pairs of quadrants 
could be used.  
This technique is more if a development that a new idea but  might be of 
interest. 
 
 
Martin



   





Hi,
 
  I note the recent interest in sensors for seismic use. I ha=
ve=20
used full bridge capacitive sensors for a vertical broadband unit which thou=
gh=20
work well, are time consuming to put together and test as well as being rath=
er=20
bulky. The quadrant photo cell is a better ready made precision device. =
;A=20
spot of light from an LED modulated at 10KHz is projected onto the cell and=20
diametrically opposite quadrants are phase inverted to produce an X-Y signal=
=20
that change phase and falls to zero as they cross the null point. The output=
=20
signals are processed by phase sensitive detectors. Since these sorts of sen=
sors=20
are high impedance, the first stage needs a device with a high noise equival=
ent=20
resistance such as an FET. Unfortunate the flicker noise appears in the=
=20
noise voltage generator which is equal to the shot noise at 1 or 2 KHz. By=20
operating at 10KHz and a narrow band filter ( phase lock loop time=20
constant) the flicker noise becomes insignificant. The output in volt/=20
meter depend a lot on the support electronics but in general the quad p=
hoto=20
cell approach seems to offer a high output. The LED is somewhat temperature=20
sensitive (approx. -0.7% C ) but by summing the four quadrant and applying t=
his=20
as feedback this temperature dependence can be made negligible.  For a=20
single axis, pairs of quadrants could be used.=20
      
  This technique is more if a development that a new idea but=
=20
might be of interest. 
 
 
Martin