PSN-L Email List Message

Subject: Re: Time
From: Larry Cochrane lcochrane@..............
Date: Sat, 07 May 2005 18:28:04 -0700

Hi Chris,

> Hi Larry,
>     Do you have any figures, or preferably measurements, for the time 
> delay produced by the filters on your amplifier boards, please?

I measured this a long time ago. The delay is about 30 to 50 ms depending on the 
cut-off frequency. I did the measurement by feeding in a square-wave and looking at 
the output with a o-scope.

>     Putting in the values for an 8th order 5 Hz Butterworth low pass 
> filter suggest that the delay is likely to be ~160 milli sec below 2 Hz, 
> peaking to about 280 milli sec at 5 Hz.
>     These delays, while swamping any timing errors, are unlikely to give 
> significant errors in general Earthquake location. However, since an 8 
> pole 1.5 Hz filter can produce a lag of over 1/2 second, we may 
> sometimes need to take filter delays into account, more particularly for 
> local events.

I ran into this problem when I was first setting up my system. I was using my 
telemetry demodulator board to record data from one of the USGS sensor sites. I 
noticed that there was a constant ~200 ms offset from the data I recorded and the 
data the USGS recorded for the same sensor. It turned out to be the two 8-pole switch 
capacitor filters I was using on the demodulator board. Each chip was delaying the 
data by about 100 ms. I did two things to correct for the delay. One was to remove 
one of the filter chips since there was no need for a 16 pole filter, and the other 
thing I did was add a delay parameter to SDR. The delay number in milliseconds was 
subtracted from the event file start time when the program saved an event file. Doing 
these two things corrected the problem.

Larry Cochrane
Redwood City, PSN


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