All,

I got a chance yesterday to run some tests on my instrument.

Here's what I did:

I opened the loop and drove the force balance coil directly with a HP 
function generator ( Model 3310B). I placed a 1000 ohm resistor in series 
with the coil. I took my input signal from across the coil so I had a 
sensitivity of 1000 volts/amp. The output was taken at the output of the 
cap gauge circuit, after a 2 pole 35 Hz filter that would have been 
inconvenient to remove. The output device was a HP 54603B memory scope. The 
driven mass is pretty close to 500 grams.

I attempted to take phase and P-P position data at constant acceleration 
from 0.1 Hz to 10 Hz while holding the current through the coil at 94 
microamps, i. e. 94 millivolts across 1000 ohms. Why that value? Because it 
was the maximum that I could use at 0.1 Hz and still maintain linearity of 
the cap gauge signal.

Results:

The force balance coil/magnet combination have a transfer function of 0.26 
Newtons/Amp.

I lost the signal in the noise at about 6 Hz. Obviously, I could go higher 
with a larger input signal.

The phase between acceleration and displacement was less than 20 degrees 
throughout the range.

The P-P displacement was about 60 nanometers at 6 Hz.

The laws of physics still apply, as the P-P position signal was inversely 
proportional to the square of the frequency.

There was no sign of roll-off due to damping from the cap gauge, but then 
the amplitudes were pretty low at the higher frequencies.

Comment: My low pass filters are set to roll off at 5 Hz so data beyond 
that point is academic.

Dave ...

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