PSN-L Email List Message
Subject: Re: Filters
From: ChrisAtUpw@.......
Date: Sun, 7 Jan 2007 22:19:51 EST
In a message dated 07/01/2007, tchannel@.............. writes:
Hi Folks, One issue where I seem to need help, is how to filter. I have
keep notes on the Low and High Pass filter values used by others.
I also know that the filter values will change with all the variables. Let
us assume, using a vertical similar to the AS1, what would be good starting
points for Local, Regional, and Teleseismic events?
Hi Ted,
If you are using an AS-1, the sample rate is set to 6 sps, so the max
frequency that can be detected is 3 Hz. You have no choice.
However, if you are using a Lehman, you might wish to set the low pass
at 20 Hz for Local / Volcanic events, 10 Hz for Regional events or 5 Hz for
Teleseismic events. The high pass filter should be set at or a bit above the
set period of the sensor - lower in frequency.
But you are usually presented with an amplifier with built in frequency
/ period values. The filters are used to remove man made / environmental
noise, which is usually serious at frequencies of 20 Hz and above and VLF 1/f
noise.
Also could someone explain what the poles numbers do?
The corner frequency is set by the capacitor values. Increasing the
number of poles (stages) increases the cut-off slope by 6 dB per octave per pole
above the corner frequency and this also requires an increase in the number
of opamps. It is usual to assign two poles (RC stages) per opamp. However,
each stage adds to the signal delay through the filter. A five or six stage
filter is usually adequate.
Bessel filters have a constant delay over the passband and this figure
may be entered into the SDR program to compensate. The signal 'shape' is
fairly well maintained. Butterworth filters have a large peak delay at the edge of
the passband and the signal 'shape' may be seriously distorted if any
frequency component is near the edge of the passband.
Regards,
Chris Chapman
In a message dated 07/01/2007, tchannel@.............. writes:
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
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Hi Folks, One issue where I seem to=
need=20
help, is how to filter. I have keep notes on the Low and High Pass filter=20
values used by others.
I also know that the filter values will c=
hange=20
with all the variables. Let us assume, using a vertical similar to t=
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AS1, what would be good starting points for Local, Regional, and Teleseism=
ic=20
events?
Hi Ted,
If you are using an AS-1, the sample rate is se=
t to=20
6 sps, so the max frequency that can be detected is 3 Hz. You have no=20
choice.
However, if you are using a Lehman, you might w=
ish=20
to set the low pass at 20 Hz for Local / Volcanic events, 10 Hz for Regional=
=20
events or 5 Hz for Teleseismic events. The high pass filter should be set at=
or=20
a bit above the set period of the sensor - lower in frequency.
But you are usually presented with an amplifier=
=20
with built in frequency / period values. The filters are used to remove=
man=20
made / environmental noise, which is usually serious at frequencies of =
20=20
Hz and above and VLF 1/f noise.
Also could someone explain what the p=
oles=20
numbers do?
The corner frequency is set by the capacitor=20
values. Increasing the number of poles (stages) increases the cut-off slope=20=
by 6=20
dB per octave per pole above the corner frequency and this also requires=20
an increase in the number of opamps. It is usual=20
to assign two poles (RC stages) per opamp. However, each stage add=
s to=20
the signal delay through the filter. A five or six stage filter is usually=20
adequate.
Bessel filters have a constant delay over the=20
passband and this figure may be entered into the SDR program to compensate.=20=
The=20
signal 'shape' is fairly well maintained. Butterworth filters have a la=
rge=20
peak delay at the edge of the passband and the signal 'shape' may be serious=
ly=20
distorted if any frequency component is near the edge of the passband.
Regards,
Chris Chapman
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