If you had one what would you use it for
in [Design]
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From: Jan Panteltje on 21 Oct 2009 11:15 On a sunny day (Wed, 21 Oct 2009 06:21:02 -0700 (PDT)) it happened MooseFET <kensmith(a)rahul.net> wrote in <b2da0b94-f086-42e1-bf3a-28bb10499b56(a)x6g2000prc.googlegroups.com>: > >Imagine you had a sensor that could measure very small magnetic >fields. It measures with a noise floor of about 0.1fT. Unfortunately >the band width is only a few hundred Hz and it only works inside a >shield. > >What would you use such a sensor for? The best I've thought of is >detecting the flow of current in a PCB to find a short circuit. Brain current imaging?
From: John Larkin on 21 Oct 2009 12:08 On Wed, 21 Oct 2009 06:21:02 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: > >Imagine you had a sensor that could measure very small magnetic >fields. It measures with a noise floor of about 0.1fT. Unfortunately >the band width is only a few hundred Hz and it only works inside a >shield. > >What would you use such a sensor for? The best I've thought of is >detecting the flow of current in a PCB to find a short circuit. Interesting problem, detecting an AC field with a coil and amplifier. I suppose what you really detect is field*area, which is total flux, in webers. A coil with a given sense area makes voltage proportional to the number of turns. But turns add ohms, which adds Johnson noise. You need thinner wire to add turns given a reasonable space constraint. Cooling helps. A reasonable amplifier has moderately under 1 nV/rtHz wideband noise. Tuning helps if allowed by the application. If you have a huge uniform AC field, increasing coil diameter improves s/n without limit. Ohms go up with radius but sense area (sampled flux) goes up as r^2. I don't know if some sort of flux concentrator would help. It should, I'm guessing... you could pump the same flux through a coil of same turns but smaller diameter (less ohms). So, given the above, what's the noise floor of a practical room-temp pickup coil+amp at, say, 1 KHz and 1 cm^2 sense area? I'll do the math if I get time... gotta trudge off to work now. John
From: Rich Grise on 21 Oct 2009 13:57 On Wed, 21 Oct 2009 15:15:40 +0000, Jan Panteltje wrote: > On a sunny day (Wed, 21 Oct 2009 06:21:02 -0700 (PDT)) it happened MooseFET >> >>Imagine you had a sensor that could measure very small magnetic >>fields. It measures with a noise floor of about 0.1fT. Unfortunately >>the band width is only a few hundred Hz and it only works inside a >>shield. >> >>What would you use such a sensor for? The best I've thought of is >>detecting the flow of current in a PCB to find a short circuit. > > Brain current imaging? Detecting emotional response to situations, like a lie detector, a la GSR (Galvanic Skin Response) meters. http://en.wikipedia.org/wiki/Galvanic_skin_response Cheers! Rich
From: nuny on 21 Oct 2009 15:23 On Oct 21, 6:21 am, MooseFET <kensm...(a)rahul.net> wrote: > Imagine you had a sensor that could measure very small magnetic > fields. It measures with a noise floor of about 0.1fT. Unfortunately > the band width is only a few hundred Hz and it only works inside a > shield. Point-one FEMTO Tesla? Yipes! Um, at room temp? What's the spec on the shield? > What would you use such a sensor for? The best I've thought of is > detecting the flow of current in a PCB to find a short circuit. Hm. How fast does it respond, and what sort of output? It'd make a dandy "somebody opened the box with the DO NOT OPEN label" sensor, if nothing else. Mark L. Fergerson
From: Spehro Pefhany on 21 Oct 2009 16:39
On Wed, 21 Oct 2009 12:23:53 -0700 (PDT), "nuny(a)bid.nes" <alien8752(a)gmail.com> wrote: >On Oct 21, 6:21�am, MooseFET <kensm...(a)rahul.net> wrote: >> Imagine you had a sensor that could measure very small magnetic >> fields. �It measures with a noise floor of about 0.1fT. �Unfortunately >> the band width is only a few hundred Hz and it only works inside a >> shield. > > Point-one FEMTO Tesla? Yipes! > > Um, at room temp? > > What's the spec on the shield? Has to be superconducting. ;-) >> What would you use such a sensor for? �The best I've thought of is >> detecting the flow of current in a PCB to find a short circuit. > > Hm. How fast does it respond, and what sort of output? > > It'd make a dandy "somebody opened the box with the DO NOT OPEN >label" sensor, if nothing else. > > > Mark L. Fergerson |