In a message dated 21/09/2006, tchannel@.............. writes:

2  Have you set up the damping to near 0.7 critical? No damp  made yet, but 
hope to use magnets  I know the importance of this but was  hoping to prove the 
amp and filter first.

Hi Ted, 
 
    Without any damping the arm should swing freely for  maybe 20 mins. You 
time one complete oscillation to ge the period
 
    I have put designs for NdFeB magnetic dampers and  high output sensors at 
_http://jclahr.com./science/psn/chapman/lehman/index.html_ 
(http://jclahr.com./science/psn/chapman/lehman/index.html)  Both  relay type coils and flat 
rectangular high linearity coils are considered. The  dampers use a horizontally 
mounted plate. Copper is preferable, but soft Al will  work OK.
 
    What construction and suspension systems are you  using? 
 
    It is vastly preferable to mount the whole  apparatus on one base frame 
which can then be adjusted and levelled. See the  photo. You need only make the 
arm about 2 ft long

3  2.5 Hz lowpass is on the low side.
  The amp/filter is still on the breadboard so I can change the  values,  Do 
you think I should use 10Hz instead of  2.5Hz?

    I would be inclined to use at least 5 Hz. You can  download a free filter 
design program and application notes from 
_http://focus.ti.com/docs/toolsw/folders/print/filterpro.html_ 
(http://focus.ti.com/docs/toolsw/folders/print/filterpro.html) 
    I usually use a 5 pole Bessel design, with the  first pole on the 
feedback resistor of the low noise input opamp. For a 10 Hz  bandwidth, I set the 
filter rolloff at 7 Hz.
    You can also use a 10 Hz Butterworth filter, but  they give about a 10% 
overshoot and have extended delays at the roll off  frequency.
 
    Environmental noise, which you don't usually want  to record, tends to 
get serious above about 15 Hz, maybe less in an urban  environment, so the 
filter needs to reduce the amplitude of these signals quite  drastically.
 
Sorry I don't know how many poles,  It goes from a op amp "the  filter" into 
another op amp "the amp" with a gain of 100 I think.
 
    What low noise input opamp are you using? 
 
    What circuit diagram? You need to build in a  variable gain from maybe 
200 up to several thousand.
 
    I suggest that you check on the frequency and  gain specification of 
Larry's amplifiers at _http://psn.quake.net_ (http://psn.quake.net) 

and the output voltage is min. 1.4v and max 7.5 volts. I adjust it  to 2vs 
into an AD Converter DATAQ 194 which requires 0 to  10v.

    Have you modified your Dataq, or something? The  standard DI-194 has 
+/-10 V input, but it has only 10 bits resolution. Used in  the way you describe, 
it will only have a range of about 300 counts!! This is  seriously inadequate.
 
    Other experimenters PLEASE NOTE that Dataq are  still selling off their 
DI-154 +/-10V 12 bit ADCs for $39 at the  moment. See _www.dataq.com_ 
(http://www.dataq.com)  This is about  the minimum resolution which is useful for 
seismic recording, which  requires a considerable dynamic range. They are supported 
by Amaseis. 16  bit ADCs are preferable.
 
    I am puzzled as to why your are doing this? 
    You need +/-12 V stabilised supply rails for  your opamps!!

4  Sorry I don't understand this part. I am in Idaho hundreds  of miles from 
the ocean, could I still see these?
 You get a microseism background from the ocean between 4 and 8 sec,  so you 
may want to filter above and below this. Can you take an unfiltered  trace of 
background and call up an FFT plot of amplitude versus frequency? The  ocean 
background peak should show up clearly. The period is a bit different  for the 
west and east coasts. In the middle of the USA, you may see two  peaks.


    The microseismic background can be observed  anywhere on Earth and has an 
amplitude of 0.5 to 15 microns, more  typically about 1 to 2 microns. The 
Pacific coast ones may have shorter periods  of 4 to 6 seconds, whereas the 
Atlantic ones are often 6 to 8 seconds. You will  likely see both in Idaho.
 
    Regards,
 
    Chris Chapman





In a message dated 21/09/2006, tchannel@.............. writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
2  Have you set up the damping to near 0.7 critical? No dam=
p=20
  made yet, but hope to use magnets  I know the importance of this but=20=
was=20
  hoping to prove the amp and filter first.
Hi Ted, 
 
    Without any damping the arm should swing freely=
 for=20
maybe 20 mins. You time one complete oscillation to ge the period
 
    I have put designs for NdFeB magnetic dampers a=
nd=20
high output sensors at http://jcl=
ahr.com./science/psn/chapman/lehman/index.html Both=20
relay type coils and flat rectangular high linearity coils are considered. T=
he=20
dampers use a horizontally mounted plate. Copper is preferable, but soft Al=20=
will=20
work OK.
 
    What construction and suspension systems are yo=
u=20
using? 
 
    It is vastly preferable to mount the who=
le=20
apparatus on one base frame which can then be adjusted and levelled. See the=
=20
photo. You need only make the arm about 2 ft long
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
3  2.5 Hz lowpass is on the low side.
  
  The amp/filter is still on the breadboard so I can change the=20
  values,  Do you think I should use 10Hz instead of=20
2.5Hz?
    I would be inclined to use at least 5 Hz. You c=
an=20
download a free filter design program and application notes from http:/=
/focus.ti.com/docs/toolsw/folders/print/filterpro.html
    I usually use a 5 pole Bessel design, with the=20
first pole on the feedback resistor of the low noise input opamp. For a 10 H=
z=20
bandwidth, I set the filter rolloff at 7 Hz.
    You can also use a 10 Hz Butterworth filter, bu=
t=20
they give about a 10% overshoot and have extended delays at the roll off=20
frequency.
 
    Environmental noise, which you don't usually wa=
nt=20
to record, tends to get serious above about 15 Hz, maybe less in an urban=20
environment, so the filter needs to reduce the amplitude of these signals qu=
ite=20
drastically.
 
Sorry I don't know how many poles,  It goes from a op amp "the=
=20
filter" into another op amp "the amp" with a gain of 100 I think.
 
    What low noise input opamp are you using? 
 
    What circuit diagram? You need to build in a=20
variable gain from maybe 200 up to several thousand.
 
    I suggest that you check on the frequency=20=
and=20
gain specification of Larry's amplifiers at http://psn.quake.net
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
and the output voltage is min. 1.4v and max 7.5 volts. I adjust=20=
it=20
  to 2vs into an AD Converter DATAQ 194 which requires 0 to=20
  10v.
    Have you modified your Dataq, or something? The=
=20
standard DI-194 has +/-10 V input, but it has only 10 bits resolution. Used=20=
in=20
the way you describe, it will only have a range of about 300 counts!! This i=
s=20
seriously inadequate.
 
    Other experimenters PLEASE NOTE that Dataq are=20
still selling off their DI-154 +/-10V 12 bit ADCs for $39 at the=20
moment. See www.dataq.com This is a=
bout=20
the minimum resolution which is useful for seismic recording, which=20
requires a considerable dynamic range. They are supported by Amaseis. 1=
6=20
bit ADCs are preferable.
 
    I am puzzled as to why your are doing this? 
    You need +/-12 V stabilised supply rails for=
=20
your opamps!!
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
4  Sorry I don't understand this part. I am in Idaho hundre=
ds=20
  of miles from the ocean, could I still see these?
  
 You get a microseism background from the ocean between 4 and 8=20=
sec,=20
  so you may want to filter above and below this. Can you take an unfiltered=
=20
  trace of background and call up an FFT plot of amplitude versus frequency?=
 The=20
  ocean background peak should show up clearly. The period is a bit differen=
t=20
  for the west and east coasts. In the middle of the USA, you may see two=20
  peaks.

    The microseismic background can be observed=20
anywhere on Earth and has an amplitude of 0.5 to 15 microns, more=20
typically about 1 to 2 microns. The Pacific coast ones may have shorter peri=
ods=20
of 4 to 6 seconds, whereas the Atlantic ones are often 6 to 8 seconds. You w=
ill=20
likely see both in Idaho.
 
    Regards,
 
    Chris Chapman