From: ChrisAtUpw@.......

Date: Wed, 18 Feb 2009 19:29:37 EST

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 ChapmanIn a message dated 18/02/2009, lconklin@............ writes:<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>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 sourc= e of=20 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.comThe 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.I hope that this helps.Regards,Chris Chapman