4 quadrant VLF discrimination

[Davor]

So here is the thing. The schematic is the reused small project I used for qualitative research on coil shielding, and I needed a phase-wise response of a system over full 360° range. The easiest way to do that is using the oscillators 1Hz apart and setting transient time at 1s. Easy.

The legend on the picture is just for indication purposes, and not explaining what happens in the circuit. The very circuit gives the response of a piece of Al foil (L/R=10u) and i use it here simply because of the null close to the zero time on the diagram. So in fact everything has to be shifted a bit to the left.

This picture shows only the 2 quadrant part for brevity, but in 4 quadrant it is just duplicated. Mask does not stop beyond copper.

Sure, notch sharpness depends on the signal amplitude, and it can be optimised for, say inch above sea surface.

[Dave J.]

My favorite searchcoil was the unshielded mono coil I wound on the rim of an 8 inch (20 cm) diameter wicker basket for the CodFisher, a pulse induction machine I built in a wooden box used to package dried codfish. No telescoping rods for this, I bolted everything to a tree branch left on the ground in the aftermath of some sloppy city street maintenance. It was "conceptual art". Don't remember the exact year, think it was about 1985.

The performance was for real by the standards of that time. It ran on a regular zinc-carbon 9 volt "transistor battery" and could detect US coins out to about 8 inches (20 cm) distance. And the flyback voltage was about 3.5 volts, the highest voltage in the entire system was the battery.


--Dave J.

[Davor]

So you are the guy with whitish beard and the employer is First Texas Products & Fisher Research Labs?

Increasingly my faith to fair employment policies is restored when I see such companies like White's and First Texas taking good care of their people.

So we have Ferric, Carl and Dave here. I must be onto something

My big picture is a detector with unshielded, preferably differential coil so simple to produce that it could be hand woven by Peruvian virgins, let alone a hobbyist, dual proportional tone discrimination with possible overlapping, and as intrinsic as possible. Big chew for a little mouth? Maybe, but the spoils of the development could be shared in just about any other project as well.

[Dave J.]

I do efficient high-stability low-distortion temperature-compensated precision phase referenced balanced-drive untapped VLF transmitter coil circuits with only one drive transistor being needed, and nothing special about the drive transistor either. (Sorry, my employer owns the IP, I'm not at liberty to disclose the details.) It's not a textbook design, I was about 30 years in this business before I discovered this topology.

I'm posting for the purpose of lending credibility to the statements of Aziz, who is often 'way ahead of most folks here.

--Dave J.

[Aziz]

Ok, I'm really happy now.
The kiss-principle has kissed me again and I have a new kiss-masterpiece of more magic (ultra high power efficiency, ultra simple circuit and other ultra features).
More to come in our own osc thread.
Aziz

[Aziz]

Hi Davor,

but I'm not happy with the npn-transistors power loss. They get warmer than the pnp ones (10 times more power loss). Nevertheless, the center-tapped TX produces a lot more bang to the targets and the differential voltage (tp2-tp1) goes up.
The switching transitions could be better though.
Aziz

[Davor]

You must have switched to something stronger lately

Hi friends, would you be so kind and go in Land? I hate salt sea moisture in my shoes.

What of benefits we can expect from 4th quadrant VLF discrimination in Land? Is this feature applicable to VLF IB (coil) technology only?

4-quadrant processing will be able to accommodate any IB coil you can think of, but the main advantage is in applying the differential coils that do not make much sense in nowadays 2-quadrant rigs. These cancel any far field EMI and all non-differential signals, including ground.
The sea water masking thing is my idea of a cure for the remaining obstacle in providing a nice ground balance for VLFs. Multi frequency rigs use it already by means of subtraction of the sea water response at different frequencies.

[Aziz]

...
There is nothing wrong with VLFs except that they don't accept differential coils, and a 4-quadrant Rx will remedy that.

I'll make a sim of this masking system. I also have some interesting multiplier on my mind to make it kick.

Our magnificent(tm) and magic(tm) cross-coupled balanced LC oscillator can do it. That's a bang master to the targets.
L: Center-tapped TX coil, center tap connected to gnd
C: Parallel connected to the whole TX coil or splitted (left+right cap connected to gnd).

But be careful. The differential voltage can go well beyond the transistor specification limits (use high voltage transistors or clip the high voltage at the expense of additional power consumption). And you need two emitter inductors (not coupled, individual toroid cores) to get better efficiency.

RX coil can be any (single ended or differential).

Aziz

[WM6]

Hi friends, would you be so kind and go in Land? I hate salt sea moisture in my shoes.

What of benefits we can expect from 4th quadrant VLF discrimination in Land? Is this feature applicable to VLF IB (coil) technology only?

[Davor]

I built a test unit for beach work that sampled at 5uS after the initiation of the TX switch off and it was almost unuseable on wet sand due to the water conductivity, unless you were very careful about holding the coil parallel at a set height to the sand.

Precisely! It is very same with regular VLFs all the time
PI has a time smearing of the strong responses so if it is deaf to sea water, it is surely deaf to small gold too. Ground balancing schemes are normally done for larger delays. Guess this sea water balancing would require some very early sample to make it kick?

VLFs tend to have neat phase responses that do not smear. I think that is the ground for pursuing this 4-quadrant Rx thing with differential coils and perhaps a masking system for notching salts and ferrites. It seem to me that it is an approach with better energy efficiency, discrimination, and perhaps even depth once this masking system kicks in.

Given opportunity a VLF Tx can supply fields comparable to any PI, yet in a very energy efficient way. Noise-wise VLFs can also outperform PIs by high margin on the bandwidth grounds. So far only QED has done something about it in PI by means of integration. There is nothing wrong with VLFs except that they don't accept differential coils, and a 4-quadrant Rx will remedy that.

I'll make a sim of this masking system. I also have some interesting multiplier on my mind to make it kick.

[Davor]

Multiply: X*p ("magnetic susceptibility" * "conductivity") ~ τ (tau target): sea water, gold - very little. PI such targets - cuts off (small times)

Small target - a small phase shift ...
The difference of sea water and the target - the conductivity! The response is proportional to ~ p*w^2 ("conductivity"*"square of the frequency") - increase the frequency of.

More or less correct. Small gold taus are related to the flake thickness because it is smaller than the skin thickness for gold. There sure exist a flake with a tau that is so small it overlaps with sea water ... so you'll not be able to detect it by any detector with non-differential coil on the shore. Due to the irregularities on the sea bed and sea surface you can't expect even a differential coil to wipe the sea signal entirely.

Anyway, point taken, I'll make the sea water masking optional. Multiplying a real "all metal" signal with "1" instead of the imaginary does the trick.

[Ferric Toes]

PIs are for example deaf to such short taus.

Yes, existing commercial PIs for beach and water hunting are deaf to short taus because of the overwhelming response from seawater due to it's huge effective volume. However, PI as a principle can be almost as sensitive as a high frequency induction balance detector. I built a test unit for beach work that sampled at 5uS after the initiation of the TX switch off and it was almost unuseable on wet sand due to the water conductivity, unless you were very careful about holding the coil parallel at a set height to the sand. 10uS delay is about as short as you can go for beach hunting and with the coil in about a foot of water. For diving work 30uS would be the limit for a 10in coil and as coil size increased you would need to back off the delay still further. I've considered ways of reducing the water signal by using a differential coil arrangement similar to that used for cancelling low frequency em noise, but never tried it. Maybe a notch system could be designed such as is used in PI for ground cancelling. Still much to do.

Ferric.

[Sergey_P]

Multiply: X*p ("magnetic susceptibility" * "conductivity") ~ τ (tau target): sea water, gold - very little. PI such targets - cuts off (small times)

Small target - a small phase shift ...
The difference of sea water and the target - the conductivity! The response is proportional to ~ p*w^2 ("conductivity"*"square of the frequency") - increase the frequency of.

[Davor]

Sure, there are some differences in phase angle related to the salinity, but sea water is still a very bad conductor, worse than the worst metal, with meter thick skin depth, diamagnetic, and hence the resulting water response vectors are tightly grouped in a very narrow span of angles. I guess when excited by some higher frequency you could observe some little heeling of the vector with salinity, yet still quite apart from, say, gold flakes.

Making a clipping window phase - you cut a small gold.

The idea is not to make it a window but a real sharp notch, and that's precisely why I need a nice analogue amplifier. But sure, if I overdo the signal disappearing trick it would affect small gold in dry areas too, but on the beaches such small gold signal is overwhelmed by sea water response anyway. Only the detecting technologies with sea water disappearing tricks do the business there. PIs are for example deaf to such short taus.

[Ferric Toes]

I'm not sure how a VLF machine as described would cope with sea water. Experiments that I did in the past with PI showed that both amplitude and time constant changed considerably depending whether you were just on a wet beach, up to your waist, or diving at 50ft. Water in the Mediterranian Sea is also saltier, with a higher conductivity, and hence longer time constant. Near a river estuary the opposite situation applies. It also varies with different search coil diameters. Changes in time constant and amplitude would surely indicate that phase angle would also be changing in the frequency domain. One good thing about iron mineralised ground is that the decay is always the same (give or take a small percentage) and it is only the amplitude that changes.


Ferric.