A Single Coil TEM field metal detector with simultaneous transmit and recieve.

[moodz]

Its time for a new detector topology in pulse induction.

The single coil ( AKA monocoil ) bipolar simultaneous transmit and recieve TEM metal detector has arrived.
No bucking coils / No figure 8s / No Induction balance / No bridges / no time domain multiplexing ... just a single coil of wire.

Today we put all the bits together ( not many ) and captured the special moment when the theory behind this was validated in a working circuit.

Below is the first ever photo of a true ( AFAIK ) bipolar monocoil with simulataneous TX and RX in action with the flyback ( a modest 190 volts peak - this is a prototype and we didnt want to blow it up on the first try) and RX signal.
But what is really important is the RX signal recovered from the coil - Its standard PI.
The HF AC ringing you see there is coming from the switchmode supply because after flyback the current in the monocoil has reversed .. the current has not gone to 0 as a standard PI would.
Transition time is around 700 nanoseconds for a 500 ma PP bipolar current in a 250 uH transmit coil ( it was the nearest to hand when the cct was first powered. )

Targets produce the expected response in the RX.
Obviously early days and lots more testing ...

What does this mean ? Well maybe physical balance ( bucking / IB ) systems are a thing of the past. Which means the monocoil is back and it means business

[Aziz]

[Altra]

Very Cool! Can you show a schematic for the tx?

[Detectorist#1]

More interesting is the scheme of the RX part that work w/o IB.

[moodz]

Before any "big reveal" ... you should all read up on this Minelab patent : https://ipsearch.ipaustralia.gov.au/patents/2023204592

Minelab are trying to lockup bipolar pulse induction till 2043.
What is interesting if you follow this document trail you will see at that site that Nokta has raised an objection to that patent since it would appear that Minelab is possibly trying to patent the idea of a controlled constant current bipolar transmit scheme.
Checkout the correspondence paper trail ( the lawyers are finding the gold here )
It is every companies right to protect their own IP however reading the patent seems to indicate they are patenting the method of producing bipolar constant current transmit scheme ( eg using feedback loops, current sensing, controlled high voltage and low voltage connections across a coil at various timings ...etc etc adapted and arranged. )
It is probable that the latest GPZ8000 model utilises this method. However it does require a separate recieve coil hence the horribly complex donut coil required on the GPZ8K.

Food for thought ...

[Aziz]

Hi Paul,

how dare you!, the big toilet seat coil is a big invention. You unbeliever!
It is performing better than the Super-D coil on the 7000. It is performing better than any DD-coil. The proof is in the pudding.

I'll continue with my "mono coil VLF" soon. The new coils made me quite busy.
But I'm looking forward to what you come up with.

[moodz]

...so here is a circuit that proved the idea works. ( the patent covers all the adapted and arranged workshop variations including bridge variations, centretap, balanced, single ended coil, current sources, differential sampling, etc etc )


V1 and L1 form a current source.
R1 is a damping resistor.
C1 is a DC block / Hi pass filter.
D1 keeps the current continuity from the current source happy during flyback ( current sources dont like open circuits )

The patent specifies more optimal embodiments .. however this illustrates the principle.

The experimental results indicate very good low noise target decay recoveries by the DSP chain.
Its a naturally low noise system because the current source is inherently low noise.
When L1 is very large the transmit coil current can be maintained to sub microamp levels of flatness in the TX coil.
Because L1 is so large the DC supply only sees a constant current draw ( no pulsing ... so noise is very low )
This circuit works effectively at 1.2 volts and is much more efficient than prior art.

White paper is attached below also. whitepaper_v1.pdf

[Aziz]

Very nice Paul.

So who is gonna make a working LTspice file?

[Aziz]

No LTspice simulation file yet?
Now you get LTspice simulation.

There is a parallel Cp connected to the TX coil. This is for reducing the flyback voltage for the case, that you don't have a convenient high voltage mosfet.

[Altra]

Amazing Paul! Thanks

[Detectorist#1]

This is the next idea in modern PI MD solutions. The competition with hybrid technology and sophisticated search coils with implemented "orthogonal RX and TX coil configuration" solutions will continue...

[Detectorist#1]

Hi Moodz,
It is interesting how you control upper two FETs (M1 and M2) - directly from some MOS drivers or via pulse transformers.

[Altra]

Moodz, skimming through the white paper I noticed some inventor name errors in the reference section. Here are four corrections, I didn't check the others (5)Douglas L. Johnson (7)David Emery (9)David E Johnson, (10)David E Johnson. Not nit picking but if this becomes legal it's good to be accurate.

[moodz]

Hi Moodz,
It is interesting how you control upper two FETs (M1 and M2) - directly from some MOS drivers or via pulse transformers.

In the prototype these high side mosfets are driven directly because the source voltage is low (< 2 volts ).
However gate driving is of course upto the person "skilled in the art" as they say in patents.
The circuit in the white paper is just a configuration .. not a detailed instruction.

[moodz]

Moodz, skimming through the white paper I noticed some inventor name errors in the reference section. Here are four corrections, I didn't check the others (5)Douglas L. Johnson (7)David Emery (9)David E Johnson, (10)David E Johnson. Not nit picking but if this becomes legal it's good to be accurate.

You have a good eye for detail Altra ! I cut the generated list from an AI summary ... my bad and apologies to all those guys.

Here is an updated document ... whitepaper_v1.pdf