In a message dated 04/04/2009, gmvoeth@........... writes:

I  believe that increasing the mass will increase the signal to noise ratio 
but  not sure.
I used an old 500Lb. seismometer once 
Hi Geoff,
 
    You can get away fine with 500gm to 1kg mass.  Anything more just adds 
cost and construction problems. You only needed  massive weights to drive a 
mechanical amplifier + a pen on chart paper. This  went out with photographic and 
later with direct electronic recording.
 
I believe that using a balanced sensor will make the electronics easier  to 
build by providing a proper input for a op amp diff amp.
 
    You should NOT be limited by electronic noise. It  is background seismic 
noise which is the problem. A single ended stage is quite  satisfactory. If 
you are troubled by electronic noise, redesign your amplifier,  or copy a known 
good circuit.
    You can wind a bifilar coil, but differential input  opamps do not have 
the very low noise that you can get from single opamps. You  can also buy 
bifilar insulated equipment wire. The INA118 is one of the  best of the 
differential amplifier types.

A  balanced sensor is two coils/magnets wired in additive fashion with the 
center  point grounded
making a single three wire sensor. But both must be  matched as close as 
possible to being
identical. Both sensors can use the  same mass or be two separate single 
devices on the same
foundation  (platform). 
    Do it if it makes you happy, but it is not  necessary

Cable  should be 100% shielded and possibly the preamp located at the sensor 
itself  with maybe 50Ft of cable at most.
    Short, buried cable runs are to be preferred. Use  cable with a woven 
braid shield. You also need to consider lightning  protection for long cable runs.

The  shield should probably be cut just before reaching the sensor but not 
sure. If  everything is insulated from EARTH you might surround even the sensor 
in the  electrical ground. Grounding is important to eliminate static and RFI 
noise as  well as the 60hz power hum that might originate
from power  lines.
    Earth the sensor frame through the coaxial shield  braid. Earth the cable 
at the amplifier input.

Use  magnetic damping separate from the sensor and you will get better output 
from  the sensor.
    This will lower the noise.

IF you  could place the mass in a vacuum there would be no buoyancy effects 
for  changes in air density on the mass.
    You can either place the sensor in a sealed  container or compensate the 
arm with a sealed balance wolume. A vacuum is not  essential, but it can 
eliminate any convection noise.

The  greater the A/D Resolution the lower the overall gain needs to be in the 
 electronics.
    Preferably use an ADC with 16 bit +/-1/2 LSB  resolution. Some of the 
Sigma-Delta converters give this, but with old fast  ADCs you may need to use 
signal averaging to remove the converter noise,  sometines three bits.

Filter  Low Pass the output so that the Aliasing Freq occurs at 0Db or less 
of  electrical gain if possible.
    Set the loss at the Aliasing frequency to be at  least 20 dB, preferably 
30 to 40 dB

Match  all your components as closely as possible in the preamp to avoid 
baseline  drifting and common mode problems (same signal on the two main signal  
leads).
    Use 1% Metal Film resistors only, never carbon. You  may want to buy 5% 
capacitors and then pick out near matching pairs for filters.  Use polyester 
coupling capacitors, NOT Alumium or Tantalum  Electrolytics.  

Double  regulate the power supply with at least three + volts between the 
output and  the source.
So if you got 5 volts out you need at very least 8 volts  in.
    Using 15V and then 12V regulators in series will  reduce noise and drift.

Anything  at all that varies could affect the input
into your A/D converter. And all  you want is pure signal.

The best of all would possibly be a three axis  signal that is mathematically 
combined to create a single magnitude. But then  you are not concerning 
yourself with the three dimensions but only phases and  first time of arrival which 
is what interests myself the  most.
    You usually want to know the amplitudes and phases  of the three 
components separately. This gives you the directional  information. 

Some  people believe that the sensor magnet is fixed to the physical ground 
is the  best way to go but not sure here since I have had excellent results 
with short  period devices the other way around.
    DO NOT put magnets on the arm, unless they are  totally shielded and even 
this is inadvisable. DO NOT use ferromagnetic  components on the arm unless 
absloutely necessary. A Brass mass is preferable to  Lead. It is easier to 
machine, physically stronger and you can drill and  tap holes in it. Lead is so 
soft that it can relax under a clamp, but you can  soft solder it to brass 
fixture. 

If you  could string 100 devices together 50 on each side of Signal ground 
And locate  them on the same platform just think of the sensitivity that might 
produce.  But who has that kind of monies
    This would be an array. They are principally used  in direction finding, 
CTBT Arays, Volcanoes and for reducing local seismic  noise. 

I think  you are not interested in the electronics but not sure about that. 
Need the  tools. Expensive.
(designing/Making Your Own or building premade kits) But  you can do exactly 
what you want this way. I do not recommend this unless you  have great amount 
time on your hands.
    Either buy an amplifier from Larry, build a similar  type from John's 
website, or get interested in electronics. Some systems like  the above require a 
period extending amplifier.
 
 
 
Take a look at Dewayne Hill's vertical at 
_http://jclahr.com/science/psn/hill/index.html_ (http://jclahr.com/science/psn/hill/index.html) 

 
    Then add quad magnetic block + plate damping. DON'T  bother to try out 
oil damping!
 
    Remove the knife edge suspension and substitute two  flex wires in 
tension. This reduces noise.
 
    Clamp both ends of the suspension spring. This  reduces noise.
 
    Add a quad magnet block + rectangular sensor coil  to give a greatly 
increased signal output, probably > 20x.
 
    Fit a 1/f^2 to linear amplifier and you  can sense quakes from 5 Hz to 20 
seconds very nicely. 
 
    This avoids the problems associated with the high  thermal coefficient of 
steel springs / providing Ni-SpanC springs. 
 
    Aim for a mass of about 1 lb and cut the spring  length to suit. 
 
    Regards,
 
    Chris  Chapman






In a message dated 04/04/2009, gmvoeth@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I=20
  believe that increasing the mass will increase the signal to noise ratio b=
ut=20
  not sure.
I used an old 500Lb. seismometer once 
Hi Geoff,
 
    You can get away fine with 500gm to 1kg mass.=20
Anything more just adds cost and construction problems. You only needed=
=20
massive weights to drive a mechanical amplifier + a pen on chart paper. This=
=20
went out with photographic and later with direct electronic recording.
 
I believe that using a balanced sensor will make the electronics ea=
sier=20
to build by providing a proper input for a op amp diff amp.
 
    You should NOT be limited by electronic noise.=20=
It=20
is background seismic noise which is the problem. A single ended stage is qu=
ite=20
satisfactory. If you are troubled by electronic noise, redesign your amplifi=
er,=20
or copy a known good circuit.
    You can wind a bifilar coil, but differential i=
nput=20
opamps do not have the very low noise that you can get from single opamps. Y=
ou=20
can also buy bifilar insulated equipment wire. The INA118 is one of the=
=20
best of the differential amplifier types.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>A=20
  balanced sensor is two coils/magnets wired in additive fashion with the ce=
nter=20
  point grounded
making a single three wire sensor. But both must be=
=20
  matched as close as possible to being
identical. Both sensors can use t=
he=20
  same mass or be two separate single devices on the same
foundation=20
  (platform). 
    Do it if it makes you happy, but it is not=20
necessary
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Cable=20
  should be 100% shielded and possibly the preamp located at the sensor itse=
lf=20
  with maybe 50Ft of cable at most.
    Short, buried cable runs are to be preferred. U=
se=20
cable with a woven braid shield. You also need to consider lightning=20
protection for long cable runs.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>The=20
  shield should probably be cut just before reaching the sensor but not sure=
.. If=20
  everything is insulated from EARTH you might surround even the sensor in t=
he=20
  electrical ground. Grounding is important to eliminate static and RFI nois=
e as=20
  well as the 60hz power hum that might originate
from power=20
lines.
    Earth the sensor frame through the coaxial shie=
ld=20
braid. Earth the cable at the amplifier input.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Use=20
  magnetic damping separate from the sensor and you will get better output f=
rom=20
  the sensor.
    This will lower the noise.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>IF you=20
  could place the mass in a vacuum there would be no buoyancy effects for=20
  changes in air density on the mass.
    You can either place the sensor in a sealed=20
container or compensate the arm with a sealed balance wolume. A vacuum is no=
t=20
essential, but it can eliminate any convection noise.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>The=20
  greater the A/D Resolution the lower the overall gain needs to be in the=20
  electronics.
    Preferably use an ADC with 16 bit +/-1/2 LSB=20
resolution. Some of the Sigma-Delta converters give this, but with old=20=
fast=20
ADCs you may need to use signal averaging to remove the converter noise,=20
sometines three bits.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Filter=20
  Low Pass the output so that the Aliasing Freq occurs at 0Db or less of=20
  electrical gain if possible.
    Set the loss at the Aliasing frequency to be at=
=20
least 20 dB, preferably 30 to 40 dB
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Match=20
  all your components as closely as possible in the preamp to avoid baseline=
=20
  drifting and common mode problems (same signal on the two main signal=20
  leads).
    Use 1% Metal Film resistors only, never carbon.=
 You=20
may want to buy 5% capacitors and then pick out near matching pairs for filt=
ers.=20
Use polyester coupling capacitors, NOT Alumium or Tantalum=20
Electrolytics.  
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Double=20
  regulate the power supply with at least three + volts between the output a=
nd=20
  the source.
So if you got 5 volts out you need at very least 8 volts=20
  in.
    Using 15V and then 12V regulators in series wil=
l=20
reduce noise and drift.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Anything=20
  at all that varies could affect the input
into your A/D converter. And=20=
all=20
  you want is pure signal.

The best of all would possibly be a three=20=
axis=20
  signal that is mathematically combined to create a single magnitude. But t=
hen=20
  you are not concerning yourself with the three dimensions but only phases=20=
and=20
  first time of arrival which is what interests myself the=20
most.
    You usually want to know the amplitudes and pha=
ses=20
of the three components separately. This gives you the directional=20
information. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Some=20
  people believe that the sensor magnet is fixed to the physical ground is t=
he=20
  best way to go but not sure here since I have had excellent results with s=
hort=20
  period devices the other way around.
    DO NOT put magnets on the arm, unless they are=20
totally shielded and even this is inadvisable. DO NOT use ferromagnetic=20
components on the arm unless absloutely necessary. A Brass mass is preferabl=
e to=20
Lead. It is easier to machine, physically stronger and you can drill an=
d=20
tap holes in it. Lead is so soft that it can relax under a clamp, but you ca=
n=20
soft solder it to brass fixture. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>If you=20
  could string 100 devices together 50 on each side of Signal ground And loc=
ate=20
  them on the same platform just think of the sensitivity that might produce=
..=20
  But who has that kind of monies
    This would be an array. They are principally us=
ed=20
in direction finding, CTBT Arays, Volcanoes and for reducing local seis=
mic=20
noise. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I think=20
  you are not interested in the electronics but not sure about that. Need th=
e=20
  tools. Expensive.
(designing/Making Your Own or building premade kits)=20=
But=20
  you can do exactly what you want this way. I do not recommend this unless=20=
you=20
  have great amount time on your hands.
    Either buy an amplifier from Larry, build a sim=
ilar=20
type from John's website, or get interested in electronics. Some systems lik=
e=20
the above require a period extending amplifier.
 


    Take a look at Dewayne Hill's vertical at http://jclahr.com/sci=
ence/psn/hill/index.html
 
    Then add quad magnetic block + plate damping. D=
ON'T=20
bother to try out oil damping!
 
    Remove the knife edge suspension and substitute=
 two=20
flex wires in tension. This reduces noise.
 
    Clamp both ends of the suspension spring. This=20
reduces noise.
 
    Add a quad magnet block + rectangular sensor co=
il=20
to give a greatly increased signal output, probably > 20x.
 
    Fit a 1/f^2 to linear amplifier and you=20
can sense quakes from 5 Hz to 20 seconds very nicely. 
 
    This avoids the problems associated with the hi=
gh=20
thermal coefficient of steel springs / providing Ni-SpanC springs. 
 
    Aim for a mass of about 1 lb and cut the spring=
=20
length to suit. 
 
    Regards,
 
    Chris=20
Chapman