In a message dated 18/02/2009, lconklin@............ writes:

In a  recent post, Chris Chapman commented "Coil + magnet systems detect  
velocity and are likely to suffer noise problems at very long  periods".
Could you elaborate a little on this?  What is the source of  the noise, 
and what sort of periods are we talking about?
Larry  Conklin
lconklin@............



Hi Larry,
 
    Apart from the background seismic noise, you have  to consider the input 
voltage and current noise of the amplifier, the intrinsic  noise of the input 
resistances on both inputs and the 1/f (flicker) noise of the  system. Note 
that a coil has noise determined by it's resistance. The 1/f noise  is likely to 
be a limiting factor below about a few Hz. It can be avoided by  using a 
chopper or a CAZ amplifier. Note that CAZ amplifiers like the  MAX420, MAX430, 
LTC1150 etc., still have much more noise than true chopper  amplifiers, although 
their drift and VLF noise is reduced when compared to  ordinary amplifiers. 
See AN-45 from _www.linear.com_ (http://www.linear.com)  
 
    The RMS voltage noise of a resistor =  SQRT(4.k.T.R.B) where k = 
1.38x10^-23 in J/K, T is the absolute temperature in  Deg Kelvin (Deg C + 273.14), R 
is the resistance on Ohms and B is the bandwidth  in Hz.
 
    Professional seismometers usually use variable  capacitor sensors and 
chopper amplifiers measuring position. See AN-87, p87 at  _www.linear.com_ 
(http://www.linear.com)  There is usually no  significant intrinsic noise associated 
with a capacitor, as there is with a  resistor or an inductor. These give the 
same output per mm of movement whether  this occurs over 1 or 1000 seconds.  
However, a coil + magnet velocity  detection system would only give 1/1000 the 
voltage output for the 1000 second  signal as compared to a 1 second signal, 
so you quickly reach the amplifier  noise limits as the period increases. 
 
    The seismic background noise is likely to be  greater than the 'ordinary' 
amplifier noise for periods up to ~ 30 seconds,  maybe quite a bit more. The 
complicating factor is the variation in the minimum  background seismic noise 
seen at different periods. 
    See 
_http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node28.html_ 
(http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node28.html) 
 
    I hope that this helps.
 
    Regards,
 
    Chris Chapman






In a message dated 18/02/2009, lconklin@............ writes:
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  recent post, Chris Chapman commented "Coil + magnet systems detect=20
  
velocity and are likely to suffer noise problems at very long=20
  periods".
Could you elaborate a little on this?  What is the sourc=
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  the noise, 
and what sort of periods are we talking about?
Larry=20
  Conklin
lconklin@............


Hi Larry,
 
    Apart from the background seismic noise, you ha=
ve=20
to consider the input voltage and current noise of the amplifier, the intrin=
sic=20
noise of the input resistances on both inputs and the 1/f (flicker) noise of=
 the=20
system. Note that a coil has noise determined by it's resistance. The 1/f no=
ise=20
is likely to be a limiting factor below about a few Hz. It can be avoided by=
=20
using a chopper or a CAZ amplifier. Note that CAZ amplifiers like the=20
MAX420, MAX430, LTC1150 etc., still have much more noise than true chopper=20
amplifiers, although their drift and VLF noise is reduced when compared to=20
ordinary amplifiers. See AN-45 from www.linear.com 
 
    The RMS voltage noise of a resistor =3D=20
SQRT(4.k.T.R.B) where k =3D 1.38x10^-23 in J/K, T is the absolute temperatur=
e in=20
Deg Kelvin (Deg C + 273.14), R is the resistance on Ohms and B is the bandwi=
dth=20
in Hz.
 
    Professional seismometers usually use variable=20
capacitor sensors and chopper amplifiers measuring position. See AN-87, p87=20=
at=20
www.linear.com There is usually no=20
significant intrinsic noise associated with a capacitor, as there is with a=20
resistor or an inductor. These give the same output per mm of movement wheth=
er=20
this occurs over 1 or 1000 seconds.  However, a coil + magnet velocity=20
detection system would only give 1/1000 the voltage output for the 1000 seco=
nd=20
signal as compared to a 1 second signal, so you quickly reach the ampli=
fier=20
noise limits as the period increases. 
 
    The seismic background noise is likely to be=20
greater than the 'ordinary' amplifier noise for periods up to ~ 30 seco=
nds,=20
maybe quite a bit more. The complicating factor is the variation in the mini=
mum=20
background seismic noise seen at different periods. 
    See http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/nod=
e28.html
 
    I hope that this helps.
 
    Regards,
 
    Chris Chapman