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@.......Sent:Wednesday, February 18, = 2009 5:29=20 PMSubject: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.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 = 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.I hope that this helps.Regards,Chris=20 Chapman