capacitance bridge sample rate v.s. noise
 

All,
I am trying to use a wheatstone capacitance bridge to measure a very
special capacitor that will change value in some frequency. To be
precise, the gap between the capacitor plates will shake back and forth
in a known frequency. I need some knowledge about the noise and sample
rate of the capicitance bridge, so that I know I got a good data. Any
idea about what reference book I should read. Thank you.

UC Berkeley
Low temperature lab
LiHong Herman

On 14 Jun 2005 23:39:00 -0700, wrote:
I am trying to use a wheatstone capacitance bridge to measure a very
special capacitor that will change value in some frequency. To be
precise, the gap between the capacitor plates will shake back and forth
in a known frequency. I need some knowledge about the noise and sample
rate of the capicitance bridge, so that I know I got a good data. Any
idea about what reference book I should read. Thank you.

Hi Herman

You working with Phonon Polaritons?

A Wheatstone Bridge is not the best capacitance measuring bridge,
however, you may not have that option. A bridge of any sort is not
sample rate limited, the detector (or indirectly the method of
excitation) is. By and large, a bridge is a linear device and if you
want to use it as a demodulator, then you want to run the source
frequency of the excitation at least ten times faster. Nyquist would
demand at least twice, but that is not very useful to run at the
ragged edge if you want "good data."

Depending upon this excitation frequency, the utility of the common
Wheatstone configuration may degrade. It all depends on what you
consider to be accuracy.

73's
Richard Clark, KB7QHC

Conect your special cap into an oscillator circuit and measure frequency
change. A wheatstone bridge will have to be rebalanced with each change in
capacitance.

I am assuming that your "special cap" will be moving a bit faster than you
can balance; specifically since you mentioned frequency.

Once frequency is measured, you can calculate the capacitance. What is the
range of your cap?



wrote in message
oups.com...
All,
I am trying to use a wheatstone capacitance bridge to measure a very
special capacitor that will change value in some frequency. To be
precise, the gap between the capacitor plates will shake back and forth
in a known frequency. I need some knowledge about the noise and sample
rate of the capicitance bridge, so that I know I got a good data. Any
idea about what reference book I should read. Thank you.

UC Berkeley
Low temperature lab
LiHong Herman

At what frequency do you expect the plates to move? Is the motion a
result of external forces, or of electrostatic forces caused by charge
on the plates of the capacitor? What is the capacitance? Is the Q
reasonably high? Is the leakage resistance very high? Answers to
those questions may influence how you make the measurement.

I think Fred's idea of including the capacitor as a frequency control
element in an oscillator is a good one. You can feed the oscillator
output to an FM demodulator, or perhaps even use counting techniques to
monitor the period of the waveform generated. It's possible to
digitally demodulate quite accurately.

Another idea: use the capacitance like a capacitor microphone. That
is, if the capacitance has very good insulation resistance, put a
charge on it, and note that i=C*dv/dt+v*dC/dt; if i=0 (or very nearly
so), then dv/dt= -(v/C)*dC/dt. (Alternatively, q=C*V.) If the
frequency is high compared with 1/(R*C), where C is the nominal
capacitance and R is the net leakage resistance, then i is practically
zero. Research capacitor microphone circuits for further ideas.

Normally a bridge with a capacitor in one arm is not called a
Wheatstone bridge...there are many different capacitor bridge circuits.
See, for example, "Reference Data for Engineers" pub. by H. Sams for a
chapter on bridge circuits and measurements.

If these ideas are not enough for you, do a search of journals. I'd
especially recommend "Review of Scientific Instruments," which over the
years has published myriad ways of measuring about any physical
quantity you can think of. And if you come up with a new way, consider
submitting it to them.

Cheers,
Tom