Hi Chris,

Have you considered using a mixer with local oscillator 
frequency that puts the unwanted image above the range of the 
sound card.

It inverts the spectrum but that's no problem with computers as 
fast as we have.

Gordon Couger
Stillwater, OK
www.couger.com/gcouger

ChrisAtUpw@....... wrote:
 > In a message dated 18/07/2005 19:31:08 GMT Daylight Time,
 > mike8s2@......... writes:
 >
 >     installed some larger-capacitance-value coupling 
capacitors on my
 >     24-bit sound card so I could resolve lower frequencies. 
What is a
 >     good datalogging app to
 >     use with this?
 >
 > Hi Greg,
 >
 >     I suggest that you look at Larry's website at
 > http://psn.quake.net/ and read some of the articles. Look at 
other
 > websites on http://psn.quake.net/dave/map.htm
 >     A soundcard by itself has a frequency range of about 10 
Hz to 24 KHz
 > and it is basically an AC only device. The 10 Hz lower 
frequency is
 > limited by the input C + R circuit. The standard PC drivers 
set the
 > sample rate from 8,000 to 48,000 sps. Using this, you end up 
with simply
 > massive files on a daily basis and no quick way of monitoring 
them. I
 > haven't seen an application which enables you to use the 
soundcard ADC
 > at really low sample rates, say 20 / sec and return them to a 
file.
 >     Assuming that you are in the States, you can buy a $25 10 
bit ADC
 > from Dataq, but it is usual to use 12 to 16 bit ADCs. 
Following on a
 > geophone, you will need a low noise amplifier and a filter. See
 > http://users.viawest.net/~aloomis/seismom.htm or
 > http://www.jclahr.com/science/psn/amp_filt/index.html
 >     It is usual to limit amateur seismic sensors to 
frequencies of less
 > than 10 Hz. This cuts out most of the urban traffic and 
environmental
 > noise, which is of no interest to most of us and may swamp 
everything
 > else above 20 Hz.
 >     You also need a timing system which is good to better 
than 1 sec.
 > Unfortunately, most computers are fitted with something called a
 > 'software clock', which can vary by minutes per day. The 
first seismic P
 > waves travel at maybe 10 km / sec, so a 1 minute error would 
give a
 > location error of ~380 miles. This really is useless for 
seismic work
 > and why you need a dedicated data recording program which 
takes this
 > into account.
 >     The P and S waves are of most interest to us, since they 
enable you
 > to determine the distance of the quake from your station. 
Long distance
 > P waves are at about 1 Hz and S waves are at about 0.5 Hz, 
but nearby
 > quakes have higher frequency components.
 >     The 'cheap' 4.5 Hz geophones can be extended down to 
about 0.5 Hz
 > with a special amplifier, but they will also pick up local 
and near
 > regional quakes on their own.
 >     It is also possible to make a really cheap vibration 
detector /
 > seismometer using piezo disks and added weights. I use one 
from about
 > 0.25 Hz to 10 Hz, but I need a FET input opamp and good 
screened cable
 > to do this.
 >     *Do 'read up' about earthquakes and seismometers before 
you start.
 > There is a lot to learn!*
 >     To go beyond this very basic advice, I would need to know 
where you
 > are located, your knowledge of electronics, what tools / 
constructional
 > skills you have and how much you are prepared to spend. I am 
not being
 > inquisitive, merely practical. Commercial seismometer systems 
may cost
 > several 10's of thousands of $, but amateur systems may be 
made from a
 > few $100 upwards. You can exchange construction time + skills 
for ready
 > made equipment, but the effort may be considerable. The real 
art lies in
 > not making too many mistakes.....
 >
 >     I hope that this hslps...
 >
 >     Regards,
 >
 >     Chris Chapman
 >

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