Hello Chris,   So very happy to hear from you!
Ted in Idaho USA


----- Original Message ----- 
From: "Christopher Chapman" 
To: ; 
Sent: Wednesday, November 24, 2010 5:52 AM
Subject: Re: My AS-1 Type Sensor


I recently constructed an AS-1 type sensor. My station KCMA is in the
IRIS network.

I am using an amplifier/filter, designed by Andy Loomis, which works
quite well. The natural period of the sensor is 1.7 seconds. The cutoff
period of the filter is set at 1.4 seconds.

My questions are.... Does anyone have a amplifier/filter design that
will extend it to 10 to 15 seconds? I know the manufacturer of the AS-1
can provide the amp/filter, but it's a bit too pricey for my budget.

Hi Richard,

     The AS-1 electronics uses two single stages of a bass boost
amplifier in series to compensate for the f^2 sensor characteristic. It
then uses a 12 bit single polar ADC with an offset amplifier to
digitise the signal and this is directly fed to the computer.
     The specialist data recording and analysis program called Amaseis
is used. This has a range of possible ADC input capture programs. Check
them out? It also provides additional digital filtering, including a
period extension filter. But you can only apply the digital period
extension to an extracted trace, which means that you have first to
find the quake trace! This is a bit of a disadvantage!

     And, I believe somewhere, on the psn list, I recall someone who had
made improvement's to the AS-1 amp/filter. Any recommendation's?

     Have you visited John Lahr’s website? John and I did some
developments on the AS-1, but we failed to get Jeff Batten interested,
which was a great pity. The AS-1 as marketed, is not a very good
seismometer.  I replaced the oil damping with a copper plate + NdFeB
magnet damper, which greatly improved the performance. The AS-1 has a
compensated range only from about 1 to 5 seconds, which is OK for
recording microseisms, but otherwise is of limited use. I modified the
AS-1 electronics to extend the range to about 18 seconds, which brings
in the surface seismic waves very nicely. The other limitation is in
the high frequency response. You need 20 SPS to give a good trace of
local and regional P waves to 5 Hz, but the AS-1 is fixed at 6 SPS.

Additionally, my ADC is a Dataq 194RS which is only a 10 bit unit. The
original AS-1 "black box" operates at 12 bits. Is 10 bits satisfactory
or do I need a higher range?

     A 12 bit ADC is marginal for recording earthquakes. The relative
amplitudes of the P, S, L and R waves vary greatly. Dataq do market a
12 bit ADC, which they choose to call cheap. These work OK. I would
advise you to seriously consider buying a 16 bit ADC from Larry at PSN
net.

     The detected amplitude of the signal falls as f^2 below the
resonance point, so with a 1.7 second pendulum, a 20 second surface
wave will have 1/138 of their true amplitude.

    Regards,

    Chris Chapman


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