PSN-L Email List Message
Subject: microseism filter
From: "Dave Nelson" davefnelson@.......
Date: Thu, 24 Feb 2005 17:09:48 -0800
> I am going to document the filter as soon as possible. I will send the
> info to anyone who requests it . I also will give it to John Lahr for his
> In short-- The filter is based on the National Semiconductor LMF100 dual
> switched capacitor filter operating as bandpass in "mode 3" Both sections
> are operated in parallel with the clocks 180 degrees out of phase to
> cancel clock noise feedthrough. Both filer outputs are then subtracted
> from the original signal in an op amp to produce the notch . I am ac
> coupling with long time constants into the op amp summer to avoid offset
> voltages. This may not be necessary if an offset adjustment is included at
> the summing point. The 15 hz clock comes from a 30 hz CMOS gate
> oscillator and a divide by two counter to get two phases with 50% duty
> cycle.It could also come from the 60 hz power line divided by 4. The notch
> center frequency is then 15/100 hz or 0.15 hz.
> The filter IC runs on +_ 5 volts and the op amp at +- 12 or 15 volts to
> give the full swing for the a/d ( Dataq DI 154RS in my case).
> The microeseism signature is virtualy gone in the helicorder display and
> the spectrum shows a notch where there was a peak. Nothing else seems
> I'll get going with the documentation.
> The ASMET 1V is nothing more than a can about the size of a quart paint
> can with a connector at the top. I have it covered with insulation and a
> plastic dust cover so I can't get at it to get a photo--I'll do it next
> time I go into the box..
> I am running two copies of Amasies simultaneosly -- one short period
> vertical and the other the wideband vertical. I like the program very much
> but am always looking to try others that may be availabe . I am really new
> at this so have a lot to learn.
> Dave Nelson
> Palos Verdes Peninsula California
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