## PSN-L Email List Message

Subject: Re: Filter Time delay
From: ChrisAtUpw@.......
Date: Sun, 8 May 2005 13:57:51 EDT

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In a message dated 08/05/2005, lcochrane@.............. writes:

Hi  Chris,

> Hi Larry,
>     Do you have any  figures, or preferably measurements, for the time

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.

Hi Larry,

The calculated figure for your old board, which  used a 10 Hz 6 pole
Butterworth filter, is a bit over 50  milli sec, which agrees with your old
measurements.

However, your current boards for a Lehman  use a 5 Hz 8 pole Butterworth
filter. Reducing the  frequency to 5 Hz and adding a couple of stages pushes
up the calculated  value to ~160 milli sec as above.

The calculated value for the 10 Hz 8 pole geophone  amplifier is ~75
milli sec.

Is there any chance of getting approximate  measurements for your current
boards, please? 0.16 sec is an appreciable  correction.
Have you considered using Bessel filters, which  give a flat response
with no transient impulse peaking?

Regards,

Chris  Chapman

=20

In a message dated 08/05/2005, lcochrane@.............. writes:
<=
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000>Hi=20
Chris,> Hi Larry,>     Do you have any=20
figures, or preferably measurements, for the time > delay produced=20=
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time ago. The delay is about 30 to 50 ms depending on the cut-off=20
frequency. I did the measurement by feeding in a square-wave and looking a=
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the output with a o-scope.   >    =
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Putting in the values for an 8th order 5 Hz Butterworth low pass >=20
filter suggest that the delay is likely to be ~160 milli sec below 2 Hz,=20
> peaking to about 280 milli sec at 5 Hz.

Hi Larry,

The calculated figure for your old board, which=
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used a 10 Hz 6 pole Butterworth filter, is a bit ove=
r 50=20
milli sec, which agrees with your old measurements.

However, your current boards for a Le=
hman=20
use a 5 Hz 8 pole Butterworth filter. Reducing the=20
frequency to 5 Hz and adding a couple of stages pushes up the calculate=
d=20
value to ~160 milli sec as above.

The calculated value for the 10 Hz 8 pole geoph=
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amplifier is ~75 milli sec.

Is there any chance of getting approximate=20