In a message dated 26/01/2006, jonfr500@......... writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I have=20
been wondering about low pass filter and
high pass filter, I do kn=
ow=20
where I can detect the earthquakes. But that is
from 2Hz to 10Hz t=
hat=20
can work for me. But I don't know if they
low pass freq cut-off is=
2Hz=20
or 4Hz and other way around regarding the
high pass freq. cut-off. I ha=
ve=20
been reading help for winsdr regarding that,
but I don't understan=
d=20
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=
any=20
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=
=20
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=
the=20
square of the frequency ratio. So a 2 Hz signal can give ~1/5=
the=20
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=
z=20
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