Centered tapped coil_Bipolar pulsing

[green]

Been searching bipolar pulsing and centered tapped coils lately. Don't know if I saw this option. Would it work to cancel EF and if I did see it could someone point me to the thread.

[Carl-NC]

It's been discussed but I don't know where. Yes, it will cancel Earth field. You want pretty good matching between the windings so for best results use bifilar wire.

[green]

Thanks for the reply.

[green]

Thinking of winding a centered coil. If a mono coil takes 30 turns should the centered tapped coil take 15 turns of each half?

[Silver Dollar]

Yes with 15 turns the whole coil inductance will be 300uh but one half will be 80uh.
You would transmit using one half for fast turn off and receive with a differential
amplifier. You can use a few more windings to get better receive...

The windings would need to be bifilar (IE 2 wires wound at the same time) or mutual
inductance will change the readings so both sides are not the same. I tried center tapping
a coil made with centronics cable which was 19 twisted pairs but couldn't find the inductive
center...

[Chet]

Been searching bipolar pulsing and centered tapped coils lately. Don't know if I saw this option. Would it work to cancel EF and if I did see it could someone point me to the thread.

Hi green; searched for an example of a bipolar circuit but could not find one.


My understanding is that the earth’s magnetic field provides a constant polarized signal as we swing the coil through it. By alternating the polarity of the flyback pulse the earth’s field adds to the receiver signal on one pulse. Then on the next pulse of opposite polarity the earth’s field subtracts from the receiver signal.


The alternating opposing earth field signal values are then cancelled out by the receiver integration stage. The output of the Integration stage then provides the integrated signal for target detection minus the earth’s field effect.


This method should provide stronger target signal strength. The normal manner of sampling and subtracting loses some of the target strength.


Your simulation circuit in post #1 appears to be good. Silver Dollar already addressed the inductance values. Some consideration should be giving to the autotransformer effect of the center tapped coil. The collapsing field is across the entire coil so the fall time is still related to the overall inductance. Maybe the overall end to end coil inductance should be kept at 300 uh?


The two damping resistors raise a few questions; could they through careful value selection be used to fine balance small differences in center tap inaccuracies? Is it better to have two damping resistors or one resistor across the entire coil?

[green]

[The two damping resistors raise a few questions; could they through careful value selection be used to fine balance small differences in center tap inaccuracies? Is it better to have two damping resistors or one resistor across the entire coil?]

Hi Chet, Just trying to learn something about centered tapped coils. Don't know what the benefits and disadvantages are. Made two coils, single layer tapped in center and a spider web ZZ with the two wires wrapped at the same time going around the tooth picks opposite each other. Including scope shots. Not damped enough but show the problems. The single layer doesn't couple. The ZZ couples better but it's slow. Maybe someone could say how close the coupling should be.

[Chet]

Hi green, I'm not familiar with your test set up, are you pulsing one half or across both windings? It looks like the natural frequency of the left scope shot is much higher frequency than the one on the right. What is the inductance of each winding?

[green]

Hi green, I'm not familiar with your test set up, are you pulsing one half or across both windings? It looks like the natural frequency of the left scope shot is much higher frequency than the one on the right. What is the inductance of each winding?

Just pulsing the one side, haven't made the two side circuit yet. The left scope shot is the single layer coil tapped in the center. The right the ZZ coil. Haven't measured the inductance yet.