## 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,

Project"  http://tinkeringcav=
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 =
Conklinlconklin@............

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
```