PSN-L Email List Message

Subject: Re: Coil + magnet sensors = long period noise?
From: "tchannel" tchannel@............
Date: Wed, 18 Feb 2009 17:47:57 -0700


Hi Chris,  Earlier I posted this question about this "Jar Project"  =
http://tinkeringcaveman.googlepages.com/home  On his posting he used an =
LED and phototransistor, instead of a coil and magnet.   Could you or =
anyone explain the benefits of each?   The coil and magnet are more =
common, is it better?

Thanks, Ted







----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Wednesday, February 18, 2009 5:29 PM
  Subject: Re: Coil + magnet sensors =3D long period noise?


  In a message dated 18/02/2009, lconklin@............ writes:
    In a recent post, Chris Chapman commented "Coil + magnet systems =
detect=20
    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,=20
    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=20

      The RMS voltage noise of a resistor =3D SQRT(4.k.T.R.B) where k =
=3D 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 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.=20

      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.=20
      See =
http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node28.ht=
ml

      I hope that this helps.

      Regards,








Hi Chris,  Earlier I posted this question about this "Jar=20 Project"  http://tinkeringcav= eman.googlepages.com/home =20 On his posting he used an LED and phototransistor, instead of a coil and = magnet.   Could you or anyone explain the benefits of=20 each?   The coil and magnet are more common, is it = better?
 
Thanks, Ted
 
 
 
 
 
 
 
----- Original Message -----
From:=20 ChrisAtUpw@.......
To: psn-l@..............
Sent: Wednesday, February 18, = 2009 5:29=20 PM
Subject: Re: Coil + magnet = sensors =3D long=20 period noise?

In a message dated 18/02/2009, lconklin@............ = writes:
In a=20 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 = source=20 of the noise,
and what sort of periods are we talking = about?
Larry=20 Conklin
lconklin@............
Hi Larry,
 
    Apart from the background seismic noise, = you have=20 to consider the input voltage and current noise of the amplifier, the=20 intrinsic noise of the input resistances on both inputs and the 1/f = (flicker)=20 noise of the system. Note that a coil has noise determined by it's = resistance.=20 The 1/f noise is likely to be a limiting factor below about a few Hz. = It can=20 be avoided by using a chopper or a CAZ amplifier. Note that CAZ=20 amplifiers like the MAX420, MAX430, LTC1150 etc., still have much more = noise=20 than true chopper amplifiers, although their drift and VLF noise is = reduced=20 when compared to 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 = temperature in=20 Deg Kelvin (Deg C + 273.14), R is the resistance on Ohms and B is the=20 bandwidth in Hz.
 
    Professional seismometers usually use = variable=20 capacitor sensors and chopper amplifiers measuring position. See = AN-87, p87 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 = whether=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 second=20 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=20 greater than the 'ordinary' amplifier noise for periods up to ~ = 30=20 seconds, maybe quite a bit more. The complicating factor is the = variation in=20 the minimum background seismic noise seen at different periods.
    See http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_ht= ml/node28.html
 
    I hope that this helps.
 
    Regards,
 
    Chris=20 Chapman

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