Hi Chris,
The seismometer is not a Lehman but a single=20
pendulum with a natural period of 1 second. It is supported by two pairs of=20=
leaf=20
suspensions arranged in a gimble form and give the pendulum two degrees of=20
freedom but no rotation about it's axis. The LED and projection optics=20=
are=20
mounted on the lower end and projects a 4mm spot of light modulated at 10KHz=
=20
onto the quadrant photo cell. The quadrant detector produces the X & Y=20
components of motion which in turn, via phase-sensitive detectors and=20
appropriate feedback to orthogonal force transducers, hold the pendulum in a=
=20
fixed position. A two axis broadband in fact but with a period of 60=20
seconds. The two axis output signals are of course the restraining forc=
e in=20
volts/meter/second. Unlike the vertical broadband, temperature correction is=
=20
unnecessary and tilt errors are compensated by giving the integrator a large=
=20
dynamic range.
The capacitance does not directly generate noise but modifies the=20
source impedance which in turn change the ratio of the noise voltage to nois=
e=20
current contributions to the total noise. The photo current of 0.7 micr=
o=20
amps is large compared with the FET gate leakage current and is the dominate=
=20
noise source.
Narrower bandwidths of lock-in systems will improve the S/N ratio but m=
ay=20
compromise the transient response, it's a question of optimisation.
The important characteristics of the LED are uniformity of the light sp=
ot=20
and linearity. The LED control feedback is dynamic i.e. both amplitude and w=
ave=20
form are controlled, but there is a limit to how much the loop can=20
correct non-linearity before the onset of instability.
The theoretical analysis of Force Balance Feedback is covered in Thomas=
=20
Forbriger and Erhard Wielandt papers, the former is in German but the more=20
practical aspects are well documented by Sean-Thomas Morrissey and Allen=20
Coleman.
I should perhaps point out that large quadrant silicon photo=20
detectors are a little expensive to say the least and I would not have use t=
his=20
approach if I had not been given a handful.
Regards Martin