From: "GMV" gmvoeth@...........

Date: Thu, 19 Feb 2009 00:15:09 -0700

That publication seems to be referencing db to one meter per second squared ( acceleration) and I was just wondering if it would be calculated 10(log(n/(1m/s^2))) or 20(log(n/(1m/s^2))) n being what you want to measure against the chart ? anybody know what they are using here ? thanx geoff ----- Original Message ----- From:To: Sent: Wednesday, February 18, 2009 5:29 PM Subject: Re: Coil + magnet sensors = long period noise? > > 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 > __________________________________________________________ Public Seismic Network Mailing List (PSN-L)