Target signals

[green]

Trying to learn PI. Do the signals make sense? Channel 2 (top trace) target, channel 1 (bottom trace) coil volts. Target sequence_ no target, nickel, quarter, aluminum foil, nail. 5 inch DD coil. Held target near coil. Amplifier gain 10. The last five traces, the coil volts should be 50/ div not 5.

[Altra]

Looks like a good test. All you need is a piece ferrite to test. See how the signal reacts during the on period and off period.

Add: Another test is run a nail horizontal to the coil and then vertical. Note the difference.

[green]

I tried some ferrite beads. Looks like the quarter. Which target is closest to ground? The nail parallel signal was almost flat. The nail above was vertical.

[mikebg]

I tried some ferrite beads. Looks like the quarter. Which target is closest to ground? The nail parallel signal was almost flat. The nail above was vertical.

The phase angle of ferrite is almost opposite to phase angle of the quarter, but this is in frequency domain. The phase difference between ferrite and vertical nail is small in frequency domain.
Can you show the current in TX coil (pulse width and period of repetition)? Connect as sensor a piece thin wire in series to TX coil.

[Carl-NC]

All your responses look reasonably correct. During the ON time, ferrite should have a response opposite to a quarter. During the OFF time, it will be very weak.

[green]

The bottom trace is the volts across a 1 ohm resistor in series with the coil. I repeated the nail traces because I mixed vertical with horizontal. The ferrite is four Fair Rite beads #2664021801. No target, nail vertical, nail horizontal, quarter, Fair Rite beads

[green]

I looked at the return signal with the quarter and the ferrite beads with the damping resistor across the coil. The same response, no return signal at turn off. How do you find quarters with a fast turn off ? Does a mono coil respond different than a DD coil ?

[Altra]

Another fundimental test for observation. Take a three inch copper wire and run it across the coil.
Not much response

Now form a loop and short the two ends togeather. Run it across the coil. Same amount of metal and
note the difference in response.

If you have and extra coil laying around short the leads and move it close to your experimental detector

[Altra]

I looked at the return signal with the quarter and the ferrite beads with the damping resistor across the coil. The same response, no return signal at turn off. How do you find quarters with a fast turn off ? Does a mono coil respond different than a DD coil ?

It's hard to say we would need to see a schematic of your test setup and where your measuring points are.

[green]

Does anyone look at the target at turn off with a damping resistor, maybe 1/2 usec after ? The signal strength is higher. No target, nickel, quarter, aluminum foil, nail, Fair Rite beads. The shorted wire looks like the quarter.

[Altra]

No, most circuits ignore the the first 8-15 uS. That is the beauty of the PI concept. Ground signals-
Resistive minerals such as salt and reactive iron oxides(ferrite) will give false signals. It would be
hard to seperate the good from the bad during this period

[green]

Using some components that I have to play with. 4093 drives a IRF740 . OP27 amplifier pins 2 and 3, 400 ohm connected to ground. Rx coil ground to pin 3. 4000 ohm resistor pin 2 to pin 6. 400 ohm resistor or a diode in series with a 5 volt zener across the coil. The first plots measured the coil volts. Forgot to change scope scale to 50 volts when changing the probe to x10 with the 400 ohm damping resistor. The latter plots measure the volts across a 1 ohm resistor in series with the coil (coil current). Signal measured at pin 6 of the OP27. You are saying no one measures the signal during charge or discharge of the coil ? The quarter and aluminum foil have lost almost if not all signal strength at 8-15 usec. I can't argue with success but I'm having a hard time seeing how you can detect a quarter unless my measurements aren't correct.

[Carl-NC]

Most PI detectors don't look at the ON time, and only look at the OFF time after 8-20us. If you simply look at the preamp response you are right, the visible response @ 15us or so is practically invisible. You need to run it through a sampling integrator to see it.

Your turn-off time seems slow (~12us), should be 1-2us unless you are doing energy harvesting. This will further weaken the turn-off response.

Your ON time ferrite response in post #6 doesn't look right. Ferrite gives an opposite response to e.g. quarters.

[green]

Carl, some of the first plots the coil dumped across a diode in series with a zener. The last set dumped across a 400 ohm resistor. I don't know why the ferrite looks the same as the quarter. Does some ferrite behave different. The target was 4 ferrite beads (Fair Rite p/n 2664021801).

[green]

Does someone have a PI with a separate Tx and Rx coil sampling at about 15 usec that could compare detecting distance of a quarter and a nickel. I would be interested in the mono coil difference also. From my tests with separate coils the nickel should test much better. Don't know about the mono coil. If they test the same I'm going to find what is wrong with my test fixture.
Thanks green