Here's a technical question about seismometer electronics.  What phase 
sensitive detector design do you recommend for low noise?

I have built PSDs using an op amp and FET switches (the amp switches 
between gain of +1 and -1).  Analog Technology and National Semi 
describe these circuits in application notes.  And you can use a switch 
with an instrumentation amp.  But these switches produce huge high 
frequency noise pulses that are very difficult to shield out or filter out.

You could also use IC multipliers like the AD630 or a Gilbert cell 
multiplier.  I have not tried either of these and wonder if they are 
worth the cost and/or trouble of construction.

Any suggestions?

Thanks,

Chuck Burch


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