4 quadrant VLF discrimination

[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

[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.

[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.]

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 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.

[Ferric Toes]

Back to coils for IB and PI, many years ago some IB detectors had stacked coaxial coils, i.e. TX sandwiched between two antiphase RX coils. I tried this on a PI discriminator back in the roaring '80s and it worked well on discrimination but failed if there was any ferrite mineralisation. The nice thing about this configuration is that the responses are simple with no negative going signals under the coil. Good emi cancellation too. Only downside is that the coil is considerably thicker. Garrett once used this type of coil for VLF units. In fact I used to buy formers and outer shells from him. Would this be worth looking at again in the light of the new IB technology being discussed.

Ferric.

[Davor]

A differential coil I'm thinking about is one with figure 8 Rx coil placed inside an O shaped Tx, or a bicycle style in-phase/anti-phase coils. Balance is incredibly simple to achieve, but a target under one Rx coil side gives response that is phase reversed under the other. So if you use a "normal" 2-quadrant VLF metal detector only one of the loops gives sensible response. Normal detectors do their thing only within 0-180° span. Using the counter phase as well would make these differential coils do superb job. Beside the far field EMI, EF, and what not, a ground response is cancelled as well. So it is a good thing.

The coaxial coil suffers from losing balance in proximity of ferrite laden ground. Coil closer to the ground gains in inductivity more than the one on top of it. That's why I expect much more from figure 8 configuration.

[Sergey_P]

A differential ... - ... coaxial coil

http://www.md4u.ru/download/file.php?id=5035
4/3*r0=r2-r1
( http://www.md4u.ru/viewtopic.php?f=77&t=3866 )

[Davor]

Yes, that one might work as well. I'm not certain if the radii calculation is accurate enough because field concentrates in proximity of wires (TX wires that is), and not dipole-like as in most of the calculations. I'm not experienced enough in such calculations, so - why not? It does not reverse phases as figure 8 coil does, at least not in the centre of the coil.

Maybe Aziz would care to test this coil in his nice software.

[Davor]

Hi all, just a small update.
I found two candidates for an analogue multiplier thing, one real, and the other one not so much, but with a great potential.

I first thought of a PWM modulator thing that would be a nice one to manipulate by logic ICs further on, and in a process I came up to a real treat, a direct way of producing PWM by means of the Armstrong PM modulator. Sounds like a rocket science, but it is not. I just wonder how come no one else didn't think of it before me. So what I get is a signal that produces almost perfect 4-quadrant product of two input signals using no log-antilog devices anywhere. Just analogue switches, comparators and EXORs. The output is in fact binary, but all you need to do to get analogue out is filter it. And instead of analogue switches I can do all multiplications by CMOS EXORs. Power consumption? Negligible.

However, a real treat is in similarity of these formulas that explain the second approach:

(a+b)^2 - (a-b)^2 = 4ab ...(1)
abs(a+b) - abs(a-b) = √((a+b)^2) - √((a-b)^2) ...(2)

Both these functions share the same behaviour at zero crossing of either a or b variables. To implement the second formula I only need some precision rectifiers configured by a few opamps, and the output is analogue.


I'll soon post some schematics of these, most probably in a form of "ABM blocks" just to show what it does. So far I prefer the latter approach because it preserves the low signals - the output is under the square root law - ideal for metal detecting.

[Davor]

So I wanted to have some nice audio masking function that would provide me with both classic GEB that cancels ferrites, plus another notch that would cancel salts. These are ~90° apart. In addition to this mask, I'd have one (like IDX) or more discrimination channels working in TRUE/FALSE way to establish 4-quadrant discrimination criteria. Tone is produced as TRUExMASKxCARRIER, where MASK is a level obtained from the audio masking function, and CARRIER is an arbitrary audible tone, say output from 555 or better.

My first idea was using analogue multiplication where a=GEB and b=Disc and Disc is ~90° from GEB. However, ab is not the best choice because a~b -> ab~a^2 hence for the small target response the multiplier response is ever so smaller. At Wolfram Alpha it is presented in 2D as this:



There is a huge plateau with nothing happening around the saddle point in the middle, and that means poor small signal response. So there is no point showing off with my super sexy and novel Armstrong modulator multiplier circuit with PWM output

There is an interesting twist to the ab multiplication. It can be expressed as: (a+b)^2 - (a-b)^2 = 4ab
It is incredibly similar to the following: abs(a+b)-abs(a-b) and while it has all the zeroes and maxima at the very same spots, it also has incredibly better small signal response. A bit pointy, but better. See Wolfram 2D representation, it has a distinctive diagonals:



A step forward is providing some compression to account for huge span of signal levels received from targets. A simple approach is using a limiter with antiparallel diodes, and it's transfer function would be like this:



Compressed abs(...) function thus has a formula and in 2D looks like this:



... while the compressed ab function has expression and looks like this:



The ab function after compression looks pretty, but on close inspection it still lacks a proper response for small signal. It could be abused as a sort of anti-chatter function.

In analogue world the ab function is obtained using the analogue multiplier, and the abs function is obtained by linearised full wave rectifier..

[Aziz]

Hey Davor,

don't you think, that changing into the digital domain (DSP) would be much easier?
You could do all this stuff in software very very elegant.
Aziz

[Davor]

True. However, analogue design has it's appeal. Having in mind that you have a vast span of signal levels at hand it is much easier to pre-process things in analogue and then feed it to some easy going micro. Otherwise you'd have to use power hungry ADC + DSP. Or a good ol' analogue design altogether. You only need a reasonable audio output for a good rig anyway. OK, maybe vibration too.

Whatever the means, the ends are the same, including the math that supports it.

So far I learned that you may find good analogue solutions for most of your needs if you only pay a little attention. Otherwise you have to drag some heavy batteries all the time.

[Tinkerer]

True. However, analogue design has it's appeal. Having in mind that you have a vast span of signal levels at hand it is much easier to pre-process things in analogue and then feed it to some easy going micro. Otherwise you'd have to use power hungry ADC + DSP. Or a good ol' analogue design altogether. You only need a reasonable audio output for a good rig anyway. OK, maybe vibration too.

Whatever the means, the ends are the same, including the math that supports it.

So far I learned that you may find good analogue solutions for most of your needs if you only pay a little attention. Otherwise you have to drag some heavy batteries all the time.

I like the idea for a vibrator as target indicator. However, I have not yet found a way to get a fast signal response out of it.
I consider a fast signal response to be lagging only a short time, 50ms, like 2" behind the sweep, at a standard sweep speed of 1 meter and a 10" coil. For good pinpointing.

How do you get that vibrator moving and stopping in this amount of time? At what RPM?

Tinkerer

[Davor]

When thinking of man-machine interfaces we often put the machine up front, which is wrong. It may be a good marketing, but nothing else. Interfaces should be mutually orthogonal, complementing in information they provide, and not competing with each other.
You may think of vibration as a perfect candidate for non-motion target response interface. You'll have no problem with fast response there, and it will not compete with your audible motion compensated detection or discrimination indication.