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I have been scouring the internet trying to find this answer, I've found hints and ideas, but nothing concrete, like an actual theory of operation.

I'm making a fox and hound wire tracer for my final project in ECE 202. The signal generation isn't hard, I get that concept, but I'm having trouble figuring out the theory behind the "hound," that picks up the signal from the wire.

I had always thought they used inductance to pick up the signal from the wire, and found one such design here. I may go with that, but I still want to know how the more common (and likely less bulky) capacitve probes work.

I found these three schematics, all of which seem to be a variance on the same principle. I see a capacitor connected to a high impedance before going into the amplifiers. The fancier ones appear to have some sort of bandpass filtering circuit as well, but I know what to research there.

I just don't understand the principle which is allowing these to pick up the signal.

Is it just an antenna? If so, what is the purpose of the capacitor? I vaguely recall something about electrically lengthening an antenna with inductors and shortening with capacitors, but that doesn't seem like what's going on here.

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The 2nd design is an inductive coupled example using an AM radio. The other two pick up an alternating electric field just like your body picks up AC alternating electric fields as discovered when you touch the input of a speaker amplifier and you get a low hum sound through your speaker accompanied by higher frequencies if you are in the vicinity of other circuits such as switch mode power supplies.

For a simple AC power example, the electric field lines exit a power conductor tangentially and "connect" or "end" at anything that represents an "earth". Normally these field lines are quite short because the neutral wire in AC power wiring represents "earth".

However, you can make a higher frequency oscillator that puts a signal onto a plate/wire and the capacitive connection that the person holding the transmitter makes with ground will cause electric fields to emanate. The receiver, likewise has a plate/probe/wire and can intercept these fields. It has an amplifier to boost the small signal it picks up and this is heard thru the speaker.

Both transmitter and receiver rely on a person or large object making a capacitive ground connection unless the equipment is powered from AC and earthed already.

That's my take on it anyway.

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  • \$\begingroup\$ I ... kind of understand. I think. Doesn't completely make sense though, why would it need the little wire with the capacitor if I was the "antenna"? In the case of my speaker, I have to physically touch the input for it to pick up those frequencies you're referring to, and it has plenty of amplification in it, but these devices have plastic casings; they make no contact with the input whatsoever, nor with me. \$\endgroup\$ – Daniel B. Jan 31 '14 at 4:45
  • \$\begingroup\$ The capacitor is there, I suspect to act as a high pass filter so that ac power frequencies don't easily get picked up but, your test frequency, which is much higher, does. A plastic case is an insulator just like the dielectric in a capacitor and this means your hand and body are offering the return path to gnd for the signal. \$\endgroup\$ – Andy aka Jan 31 '14 at 10:39
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I designed such a line tracer once. The spec was that the hand held unit had to detect a 1kHz 1Vp-p signal on a line stretched in the open air from a distance of 1 metre. Essentially, it was just a very high gain high Q bandpass amplifier whose input was a brass electrode tip into a high impedance op amp circuit. The tip (IIRC) was diode clamped to prevent static damage.

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  • \$\begingroup\$ So it's basically an active antenna tuned to the toner frequency? \$\endgroup\$ – Daniel B. Feb 4 '14 at 21:35
  • \$\begingroup\$ Yes, but the coupling is mainly by the electric field (I assume) \$\endgroup\$ – user32885 Feb 6 '14 at 11:29

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