PSN-L Email List Message

Subject: Re: FILTERS
From: ChrisAtUpw@.......
Date: Mon, 2 Oct 2000 18:33:05 EDT

In a message dated 01/10/00 20:02:56 GMT Daylight Time, RADIOTEL@....... 
> Should it be a low pass filter set at 3 Hz, 10 Hz or 20 Hz? Or 
> do I need a separate short period seis. for local quakes?

Dear Jim Allen,

    Following Karl's letter, out of curiosity I put 10, 15 and 20 Hz Bessel 
Calculations in the Burr-Brown Filter programme Filter2.exe @ or call  then go to Application Bulletin #34
Press [F2] to plot and use the keyboard arrows to move the yellow X cursor.

    Apart from the seismograph signals, you may need to consider a) 
environmental noise of wind, traffic, etc. which can start around 20 Hz and 
also b) 60 Hz electricity, both magnetic field and vibrations. With a 6 pole 
Bessel Filter, 60 Hz is only 30.8 dB down for a 20 Hz filter, 45 dB down for 
a 15 Hz and 65 dB down for a 10 Hz. A 10 Hz filter is 6 dB down at 14 Hz, 14 
dB at 20 Hz and then drops away fast. The Bessel characteristic is much more 
'round shouldered' than the others, but it preserves the wave shape.

    You also need high A/D sample rates to record high input frequencies. If 
you are recording everything, this effects the storage space required. Please 
forgive me if I have been 'stating the obvious'.


    Chris Chapman

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Larry Cochrane <cochrane@..............>