PSN-L Email List Message

Subject: Re: tin cries - dithering
From: Brett Nordgren brett3nt@.............
Date: Tue, 16 Aug 2011 13:10:50 -0400

Hi Chris,

Thanks for your suggestion.  I understand your approach, though 
implementing it will be a bit difficult.

Normally, to get a decent idea of long-period noise we have to record 
for at least a day, and preferably several.  And to separate 
instrument noise unambiguously from true ground noise we would need 
to record three instruments and use correlation techniques to 
distinguish their individual noise contributions.
Sleeman, Wettum and Trampert
"Three-Channel Correlation Analysis: A New Technique to Measure 
Instrumental Noise of Digitizers and Seismic Sensors"
BSSA v96 n1 p258

As it stands, at the lowest frequencies, the noise spectrum we 
normally see rises smoothly as 1/f and looks almost exactly like the 
best instruments except, of course, it's not as low.  It is entirely 
possible that some of the noise we see may be true ground noise.  We 
just don't have a quiet enough location to tell without playing the 
correlation game, which could be a pain.

If anyone could suggest what is the magnitude of the effect on the 
spring characteristics, I could at least model the instrument 
response to such stepwise force changes.  Someone must have studied 
this and come up with some real numbers.

Wouldn't the six-second microseisms be an effective dither 
mechanism?  They're usually large compared with everything else.


At 10:01 AM 8/16/2011, you wrote:
>Hi Brett,
>     I would suggest a purely practical approach. Add another small 
> winding to force feed
>back coil. Drive it from a variable AC sinewave current source at 5x 
>to 10x the cutoff
>frequency of your Low Pass fllter, so that you can't see any 
>oscillatory response. Then
>monitor how the very long period background noise detected changes, 
>as the drive
>current is increased ?
>     Regards,
>     Chris
>>From: Brett Nordgren brett3nt@.............
>>Subject: Re: tin cries
>>What amount of dither do you think would be adequate?  I suppose it
>>could best be expressed as variation in the spring strain, or in
>>whatever other way you suggest.


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