Chuck,

The biggest drawback that I can see is that they don't seem to do too well 
with higher drive frequencies.  I usually see them run at 5-6kHz or so, 
while some other sensor types are happy at much higher frequencies.  After 
you demodulate the signal, it contains a large second harmonic component 
along with the higher even harmonics, and you need to filter it to recover 
the baseband seismic signal.  But in a feedback seismograph, that filter 
can introduce phase shifts which tend to make the loop oscillate.

If you move your carrier up to 20kHz, you can design a filter which cuts 
off at a higher frequency and which, as a result, doesn't add as much phase 
shift in the 10-30 Hz region where loop oscillations are an issue.  The 
STS-2 uses a 20kHz carrier and a 3rd order Bessel filter designed to cut 
off at 1600 Hz.  That filter adds less than two degrees of phase at 
30Hz--practically nothing, but it reduces the 40kHz 2nd harmonic to below 
0.018% of its starting value.

With a 5kHz carrier it becomes harder to fit the filter between the high 
frequency corner at 10-30Hz (gain crossover) and the 10khz second 
harmonic.  So I am not that fond of magnetic transducers.

If you do want to go that route and use 5kHz, a 3rd order filter that cuts 
off somewhere around 400Hz should be about optimum.  It would add about 5 
degrees at 20Hz, not too bad, and it should nicely filter the harmonics.

Does anyone know of a nice 400Hz 3rd order, low noise, Bessel filter design 
to use here?

Regards,
Brett



At 12:52 AM 9/19/2008 -0600, you wrote:

>All,
>
>Does anyone have experience with Variable Reluctance Displacement Transducers
>(VRDTs)?  Morrissey appears to have used them extensively and they seem to 
>have
>a number of advantages.  They are inexpensive and easy to fabricate (so 
>Morrissey
>says).  They are sensitive and have a low impedance output so parasitic 
>capacitance
>wouldn't be a problem.
>
>It's true that they do produce heat, of course.  Any other drawbacks?
>
>Chuck Burch


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