Double-D tests and questions

[simonbaker]

Testing experimental Double-D coil, unshielded, 28 cm (Ivconic spec) on bench with targets to help understand phase.

What I did: I null the coil as flat as possible, no extra residue. Then I put target very close to coil in the overlap region of TX and RX coils.

My goal is to see the absolute phase shift of the RX signal by different targets. I put target extremely close to swamp the null residue signal. Then I measure on Scope the phase of RX signal relative to TX oscillator. I measure RX at pin 7 of first op amp like Max suggests.

TX frequency = 14.25 kHz

Results in degrees shift of RX to right of TX on oscilloscope:

<pre>
Copper 4 inch nail (dirty) (tiny signal)
Foil                        135 (signal too weak to be accurate)
US Nickel                   160
Pull-tab                    176
US Penny clad               189
British Half Penny old      194
US Dime clad                196
US Silver Dime              202
US Silver Half Dollar old   207
Beer bottle cap             207
Steel Washer Rusty          218
Steel Washer New            227
1 inch nut, rusty           233
Steel Nail new              248
</pre>

Relative Accuracy +/- 2.5 deg. Absolute accuracy +/- 10 deg.

The residual null when increased is at 207 degrees.

The coil really likes bigger coins. Signal amplitude drops a lot as coins are smaller.

I tried turning the bottle cap on edge - the signal inverted 180 degrees - appeared to turn upside down.

Moving target outside of overlap gets different phase, inverts as crosses into RX coil.

My main question is: I thought steel/iron was on other side of nickel. This doesn't look right for discrimination -- what is happening?

Regards,

-SB

[Tinkerer]

Bottle top target

Testing experimental Double-D coil, unshielded, 28 cm (Ivconic spec) on bench with targets to help understand phase.

What I did: I null the coil as flat as possible, no extra residue. Then I put target very close to coil in the overlap region of TX and RX coils.

My goal is to see the absolute phase shift of the RX signal by different targets. I put target extremely close to swamp the null residue signal. Then I measure on Scope the phase of RX signal relative to TX oscillator. I measure RX at pin 7 of first op amp like Max suggests.

TX frequency = 14.25 kHz

Results in degrees shift of RX to right of TX on oscilloscope:

<pre>
Copper 4 inch nail (dirty) (tiny signal)
Foil                        135 (signal too weak to be accurate)
US Nickel                   160
Pull-tab                    176
US Penny clad               189
British Half Penny old      194
US Dime clad                196
US Silver Dime              202
US Silver Half Dollar old   207
Beer bottle cap             207
Steel Washer Rusty          218
Steel Washer New            227
1 inch nut, rusty           233
Steel Nail new              248
</pre>

Relative Accuracy +/- 2.5 deg. Absolute accuracy +/- 10 deg.

The residual null when increased is at 207 degrees.

The coil really likes bigger coins. Signal amplitude drops a lot as coins are smaller.

I tried turning the bottle cap on edge - the signal inverted 180 degrees - appeared to turn upside down.

Moving target outside of overlap gets different phase, inverts as crosses into RX coil.

My main question is: I thought steel/iron was on other side of nickel. This doesn't look right for discrimination -- what is happening?

Regards,

-SB

You use a VLF, I use a PI. They are very different. It is interesting to see how the bottle top reacts on both metal detectors.

Tinkerer

[simonbaker]

You use a VLF, I use a PI. They are very different. It is interesting to see how the bottle top reacts on both metal detectors.

Tinkerer

What I don't know is how on edge would affect phase further from coil -- may not be same as close up, not sure. Catch-22 - I can't see phase unless I get very close, although if I ever get the Synchronous Detector calibrated I might get it from the voltage.

I wonder if I connected RX coil in correct phase, hard to see wire direction coming out of coil.

Regards,

-SB

[Götz von Berlichingen]

What I don't know is how on edge would affect phase further from coil -- may not be same as close up, not sure. Catch-22 - I can't see phase unless I get very close, although if I ever get the Synchronous Detector calibrated I might get it from the voltage.

I wonder if I connected RX coil in correct phase, hard to see wire direction coming out of coil.

Regards,

-SB

You can power the Coil with DC and use a Compass.

[Tinkerer]

What I don't know is how on edge would affect phase further from coil -- may not be same as close up, not sure. Catch-22 - I can't see phase unless I get very close, although if I ever get the Synchronous Detector calibrated I might get it from the voltage.

I wonder if I connected RX coil in correct phase, hard to see wire direction coming out of coil.

Regards,

-SB

A good place for testing is a distance equal the diameter of the coil.
The near field is different and not the part you use when detecting.

Tinkerer

[simonbaker]

You can power the Coil with DC and use a Compass.

Good idea!

[simonbaker]

A good place for testing is a distance equal the diameter of the coil.
The near field is different and not the part you use when detecting.

Tinkerer

Yes -- too bad signal too weak to see out there (with scope). I'll try to think of some way to do that.

-SB

[simonbaker]

You use a VLF, I use a PI. They are very different. It is interesting to see how the bottle top reacts on both metal detectors.

Tinkerer

I keep forgetting you're working on PI, with discrimination I think. Would be ultimate beach machine, I hope to work on one eventually. Just iron discrim would be good even. Will be interested in your progress.

Regards,

-SB