Electronic-Nulled Loops

[Mario-j23]

I've already done it, but I didn't recognize all the elements :-) [ATTACH]n418028[/ATTACH]

[Altra]

Thanks for posting Mario! It will make a good study.

The digipot center taps labeled red and bla go to the preamp pcb in the coil. Where one goes to a series cap and the other to a series resistor. These are both connected to the rx coil entering the preamp. A small micro controller could probably replace most of the comparators and op amps.

[dbanner]

Thanks for sharing your excellent work!

The preamp being inside the coil housing naturally implies more wires in the coil cable and more pins on the coil connectors to the control box. I much prefer the preamp to be back in the control box.

Nautilus dmc 2b and 2ba were very good precisely because it allowed the operator to manually (2b) or auto (2ba) balance the search head before hunting for targets. And so the coils were always perfectly balanced. The auto balance feature made things a whole lot easier for the operator.

I see you made a clone of the 2b. I hope you can get it working.

[Mario-j23]

There is no +5v path to CD 4051 in the top Autotune diagram! hidden under the layout and I couldn't track it.

The preamplifier in the coil is an advantage (less noise) and the number of wires is not a problem.

This is how I made a DMC clone. The detector "detects metal", unfortunately I cannot cope with BG, it does not remove ferrite and DISC does not work well.
I don't know what frequency the receiving coil should be tuned to because it probably causes incorrect operation of the BG/DISC

[dbanner]

I would try the tesoro silver sabre Tx and Rx resonant frequencies. I can't remember them but you can compute them from the tesoro schematic. The Rx resonant frequency should be tuned higher than the Tx frequency as in the tesoro machine.

I think the silver sabre Tx was somewhere in the 12khz range.

[Altra]

The major reason the Nautilus 2b utilized coil nulling was the tx voltage averaged 48-52Vpp. Nulling the coils to an acceptable range was difficult without the added circuit. There was also the benefit keeping aging coils in range.

What moodz and Carl, I think are implying. Is to use controlled nulling for removing ground offsets. This would allow higher gains in the pre-amp.

[ivconic]

I think it is much more important to refine the RX, to reduce/remove the offset, than to insist on "power" on the TX side.
There is much to learn from the technology of advanced "minesweepers".
But at the same time, this technology is also a closely guarded secret.
I pointed that out before.
Unfortunately, we don't have many open projects on that topic.
And even less concrete schematics of existing devices.
Maybe that's a direction to think in... (I don't think I said anything new)

[dbanner]

The major reason the Nautilus 2b utilized coil nulling was the tx voltage averaged 48-52Vpp. Nulling the coils to an acceptable range was difficult without the added circuit. There was also the benefit keeping aging coils in range.

What moodz and Carl, I think are implying. Is to use controlled nulling for removing ground offsets. This would allow higher gains in the pre-amp.

And also:

The Nautilus patent explicitly states "by providing adjustable, phase related controlled feedback between the transmitting coil and receiving coil for offsetting the unwanted phase and amplitude signals received in a manner which algebraically adds sufficiently to these signals to eliminate unwanted signals coupled from the GROUND or from unwanted junk objects."

[Altra]

And also:

The Nautilus patent explicitly states "by providing adjustable, phase related controlled feedback between the transmitting coil and receiving coil for offsetting the unwanted phase and amplitude signals received in a manner which algebraically adds sufficiently to these signals to eliminate unwanted signals coupled from the GROUND or from unwanted junk objects."

I think the patent you are referring is an early design, not the Nautilus. Before phase adjusted syncronis demodulators were used. The old tr detectors used all kinds of tricks to gb and disc. I was friends with Jerry Tyndall, we used to talk often.

[dbanner]

Yes of course. You are right. Some years ago I did make a centre tapped Tx coil and Hartley oscillator. The amplitude of the Tx waveform was quite high. I imagine there might be significant leakage between Tx coil and Rx coil even after mechanical fine balance. As the coil aged, I imagine the leakage due to drift could become intolerable due to the high Tx amplitude, rendering the coil useless. So you are quite right.
Then I recalled the circuit had a pot to control the power of the Tx. This could be incorporated into an AIB or e- nulling system as well, depending on the ground conditions. By changing the power level one would have to manually recalibrate the feedback pots. AIB could do this.
Point is that a mutually coupled center tapped coil running off a Hartley arrangement is a winner for deep targets. The effects of exaggerated ground mineralization is dealt with by the AIB system along with power control.

[dbanner]

A target inhibit function would be essential. You wouldn't want AIB to be on while passing over a target. It should pause and just hold the last known setting.

[Altra]

A target inhibit function would be essential. You wouldn't want AIB to be on while passing over a target. It should pause and just hold the last known setting.

Yes that's the obstacle to auto gb and nulling. Not to tune the target out.

[dbanner]

Target inhibit function should be quite straightforward.
the Rx signal is fed through a synchronous sampling gate and digitized. The software controls the sampling duty cycle.
The μC determines the zero crossing point of the sampled waveform's trailing gain section and phase compares this to the reference signal (Tx) zero crossing point.
Any phase shift beyond a prescribed angle will trigger a hold and inhibit for AIB in the software until the phase angle drops below the prescribed amount. Then AIB resumes.

[dbanner]

Block

[dbanner]

Mario-j23, pin 6 and 2 of 555 IC should be connected.(Not visible on photos as it is connected beneath the chip). Configured as astable. This drives the INC pin 1 on the X9Cxxx

Now I think the auto-SLB push button is a normally closed type.