In a message dated 26/01/2006, jonfr500@......... writes:

I have  been wondering about low pass filter and
high pass filter, I do know  where I can detect the earthquakes. But that is
from 2Hz to 10Hz that  can work for me. But I don't know if they
low pass freq cut-off is 2Hz  or 4Hz and other way around regarding the
high pass freq. cut-off. I have  been reading help for winsdr regarding that,
but I don't understand  what is what.



Hi Jon,
 
    A low pass filter is one which passes all  frequencies up to about the 
cut-off frequency without loss. The amplitude of any  signal above the cut-off 
frequency drops very quickly and it is depends  on the number of 'poles' or 
filter sections.
    A high pass filter passes frequencies above the  cut-off frequency.
    The low pass filter on your amplifier board should  be 10 Hz and have 8 
poles. In practice, a signal at 13.4 Hz will be reduced to  1/10 and one at 18 
Hz to 1/100.
    The amplifier has a high pass filter set at 0.05 Hz  = 20 sec., which is 
well below the lowest response of your geophones.
 
    The low frequency response is set by the  geophone, 4.5 Hz in this case. 
From 4.5 to 60 Hz, the response is  nearly constant. Below 4.5 Hz the response 
falls off roughly proportional to the  square of the frequency ratio. So a 2 
Hz signal can give ~1/5 the  amplitude, but you may still see it. A 1 Hz 
signal will only give ~1/20 the  amplitude and this is likely to be lost in the 
noise. A 0.5 Hz  signal will give ~1/100 the amplitude.
 
    So, putting this signal into your system,  you will see a flat response 
between 4.5 an 10 Hz. The response falls  very quickly for signals above 10 Hz, 
but much less quickly for signals  below 4.5 Hz. 
 
    The local quakes which you are likely to observe  have a lot of higher 
frequency components, certainly to 10 Hz and maybe to  over 40 Hz, which is good 
for your system. Unfortunately the environmental noise  also increases with 
frequency in this range, which will limit your  ability to detect the quakes. 
Choosing the quietest place for your  geophones is important. This is best done 
by taking several 24 hr  recordings, observing the weather and wind strength 
and estimating how the noise  signal changes with time. There are likely to be 
higher noise levels when  residents leave for work in the morning and return 
in the evening. Snow  clearance vehicles and heavy lorries can make quite a 
noise. The wind noise is  likely to increase with the square of the wind speed. 
Buildings are shaken by  the wind and the vibrations are coupled into the 
ground nearby. Placing the  geophone on a basement floor, but away from the 
outside walls may  help.
 
    Hope that this is of help. Please say if there is  anything you don't 
understand.
 
    Chris Chapman
 
 






In a message dated 26/01/2006, jonfr500@......... writes:
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  been wondering about low pass filter and
high pass filter, I do kn=
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  where I can detect the earthquakes. But that is
from 2Hz to 10Hz t=
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  can work for me. But I don't know if they
low pass freq cut-off is=
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  or 4Hz and other way around regarding the
high pass freq. cut-off. I ha=
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  been reading help for winsdr regarding that,
but I don't understan=
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  what is what.


Hi Jon,
 
    A low pass filter is one which passes all=20
frequencies up to about the cut-off frequency without loss. The amplitude of=
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signal above the cut-off frequency drops very quickly and it is depends=20
on the number of 'poles' or filter sections.
    A high pass filter passes frequencies above the=
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cut-off frequency.
    The low pass filter on your amplifier board sho=
uld=20
be 10 Hz and have 8 poles. In practice, a signal at 13.4 Hz will be reduced=20=
to=20
1/10 and one at 18 Hz to 1/100.
    The amplifier has a high pass filter set at 0.0=
5 Hz=20
=3D 20 sec., which is well below the lowest response of your geophones.
 
    The low frequency response is set by the=20
geophone, 4.5 Hz in this case. From 4.5 to 60 Hz, the response is=20
nearly constant. Below 4.5 Hz the response falls off roughly proportional to=
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square of the frequency ratio. So a 2 Hz signal can give ~1/5=
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amplitude, but you may still see it. A 1 Hz signal will only give ~1/20=
 the=20
amplitude and this is likely to be lost in the noise. A 0.5 H=
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signal will give ~1/100 the amplitude.
 
    So, putting this signal into your system,=20
you will see a flat response between 4.5 an 10 Hz. The response fa=
lls=20
very quickly for signals above 10 Hz, but much less quickly for signals=
=20
below 4.5 Hz. 
 
    The local quakes which you are likely to observ=
e=20
have a lot of higher frequency components, certainly to 10 Hz and maybe=
 to=20
over 40 Hz, which is good for your system. Unfortunately the environmental n=
oise=20
also increases with frequency in this range, which will limit your=20
ability to detect the quakes. Choosing the quietest place for your=20
geophones is important. This is best done by taking several 24 hr=20
recordings, observing the weather and wind strength and estimating how the n=
oise=20
signal changes with time. There are likely to be higher noise levels when=20
residents leave for work in the morning and return in the evening. Snow=20
clearance vehicles and heavy lorries can make quite a noise. The wind noise=20=
is=20
likely to increase with the square of the wind speed. Buildings are shaken b=
y=20
the wind and the vibrations are coupled into the ground nearby. Placing the=20
geophone on a basement floor, but away from the outside walls may=20
help.
 
    Hope that this is of help. Please say if there=20=
is=20
anything you don't understand.
 
    Chris Chapman