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.
You need high spatial resolution for a general-purpose board short finder. Magnetically, you can do it with a pulsed current source and a really tiny ferrite-core pickup coil. I do it lately with a DC source and a thermal imager, so you can *see* the current path.
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.
Sometimes when in a ham radio contest and the plates of the notoriously under-dimensioned output side variable caps fused together I'd occasionally key the transmitter CW at full legal limit ... tunggg ...
*PHOOMP* ... problem fixed, some ozone and weld shop smell wafting through the air.
Reminds of this morning, put the grandson on the school bus, breeze was such that I could smell fumes from the exhaust... very much like Clorox??? Anyone know diesels, and why/how such a smell? ...Jim Thompson
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
IMHO sloppy design/fabrication. Domestic bus engine mfgs need a serious dose of education. I find it rather odd that states slap all sorts of extreme smog regs on cars while school buses can just keep belching out dirt. Probably the quality of Diesel in the US ain't that great either but that's no excuse, the Setra buses from Germany I've seen here didn't belch stuff and did not smell. They were huge tour buses and run on the same Diesel as the school buses. So it can be done.
I just finished an injector design (but for aerospace). Initially I was looking for dedicated chips. What I found was very archaic and some app note circuits almost made me sick. So I rolled my own, much better precision than the, ahem, "professional" solutions of the trade.
Not chlorine, NO2. Running lean, I would suppose. Excess O2 and hot combustion burns a little N2. No cat =3D NOx and unburned hydrocarbons pass through unadulterated, hence the belch of soot on takeoff as well.
I would guess they use a very old engine design. Maybe one with fuel control instead of throttle (hence the huge span of mixture). Oxygen sensor, engine computer? Who needs 'em.