PSN-L Email List Message

Subject: microseism filter
From: "Dave Nelson" davefnelson@.......
Date: Thu, 24 Feb 2005 17:09:48 -0800

>Hi Steve
> 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 
> website.
> 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 
> effected..
> 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.
> Best,
> Dave Nelson
> Palos Verdes Peninsula California


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