To the feedback seismo designers:

Check out  

They are selling Bafco model 911A2D Frequency Response Analyzers.

These cover a frequency range of 0.01 hz - 10 khz.

The analyzer generates a test signal at its output and then tunes for that 
exact frequency return-signal at the input terminals.  Digital readout is 
in db amplitude and degrees phase of the measured signal vs the output 
signal.  Should be perfect for checking out low frequency feedback systems 
(feedback seismometers).

They want $275 plus priority mail shipping from Las Vegas (was $47 to New 
Jersey).  Originally these were several thousand dollars.  Mine appears to 
have not been un-packed since it was shipped to the Navy in 1991, and came 
complete with manual and all accessories.

If this is within your budget and you're serious about building feedback 
seismo's you might want to have a look.

Good luck,

Brett


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