DEEPER PI DETECTION DEPTH

[Davor]

Let us help you.
You won't regret it.

Aziz

I know, but some things are better left unseen until they are ready.

Anyway, I am toying with an idea of PI-VLF meld using Tayloe mixer as a SAH device, and anti-Tayloe it to a desired audio. I know, it sounds futile, but that's the way I see it.

To obtain a good sample I'll need 2Pi sinus or cosinus excitation, cosinus current being a bit more practical. Then I'll sample it several times in total of Pi duration which is enough for full 2Pi reconstruction. Then I'll do some magic with trivial signal processing, and voila a Franken-Pi-VLF. What I expect to happen is a design with ~ 100 pulses per second, and a significant power conservation. That's the plan.

Now, the first step is the exciter with more or less accurate cosinus current. The LTspice design is attached herewith, and I'd say it behaves. I made already an implementation with transistors (to some degree), but due to some flaws I'll not put it here just yet. Instead you'll see an idealised idea for a positive pulse (0-Pi). You'll find incredible similarities with PI pulse, so enjoy. The best part is that there is no stress on any of the active components or a power supply. Each pulse conserves most of the energy.

Check for coil voltage and current, and a capacitor voltage, also see the pulse duration.

[simonbaker]

Oh, but they do. The oscillator is DC shifted and amplitude modulation (say, you waving it against a mineralised soil) is translated to PWM, and quite linearly so.
For small signal e.g. near 0° sin(x)= x ... near 180° sin(x)= -x ... an ideal ramp - just perfect for PWM.

A synthesized oscillator as in induction heating example before could certainly do the trick. It would also enable some sexy approaches as Costas filtering (requires PLL), but that is completely OT.

The TGSL oscillator is over-driven so it isn't too susceptible to amplitude modulation I think. I don't see how it would be DC shifted that much either; how does that happen?

Let's say it is DC shifted -- would coupling the TX signal through the appropriate RC network to the SD sync pulse generator eliminate/reduce that source of PWM?

And because the sync pulse is zero crossing triggered, amplitude modulation (already small) would be mostly ignored?

-SB

[Davor]

...Of course this is the main strength of the design, that fluctuations in the oscillator are automatically tracked by the SD and for the most part do not create a phantom signal. However, a good math model of this process would help show what kind of errors can actually creep in and how severe. A more stable oscillator may be worth pursuing...

Oh, but they do. The oscillator is DC shifted and amplitude modulation (say, you waving it against a mineralised soil) is translated to PWM, and quite linearly so.
For small signal e.g. near 0° sin(x)= x ... near 180° sin(x)= -x ... an ideal ramp - just perfect for PWM.

A synthesized oscillator as in induction heating example before could certainly do the trick. It would also enable some sexy approaches as Costas filtering (requires PLL), but that is completely OT.

[Davor]

This principle is called "air combining", and it is expected that antennas are mutually orthogonal, or at least suppressed to some practical degree. It is widely used in 802.11n and OFDM in general, as it improves peak to average power ratio, and hence efficiency. It is one of principles (ab)used in MIMO Tx, and everyone in that trade will tell you how MIMO is different from air combining (and diversity in Rx), while practical thinker will see it as one and the same.

Trouble is that it complicates designs a lot. I am not too convinced about the practical side of it in metal detection. Among other things because of very limited information rate that is obtained by MD-s, and high correlation of signal. Additional spectrum will give you minimum new and uncorrelated information - such complications just don't make sense.

[simonbaker]

The main trick is not actually passing a high bandwidth pulse through the coil .... and I think alot here miss the point .... is exposing the target to a step function magnetic field transition.
The generation of a "stimulated resonance" at a single frequency is trivial ... see circuit below ... that particular one generates 10 amps peak to peak in the coil ....but draws only 350 ma ( with no coil loading ).

The next generation of detectors have a series of planar and spatial coil arrangements each driven by stimulated power resonance drivers. By arranging the oscillator amplitude phase and frequency the summation magnetic field ****at the target ***** generates the required field transition ... the step transition speed is determined by the the highest frequency in the combined fields and what you might call group delay caused by the ground being within the field path however this can be compensated by adjustment of the params of each oscillator.
The summation field is produced F0 + F3 + F5 + F7 etc .....( for a simple square wave field transition ) ... this is well known in signal processing fields.

moodz
[ATTACH]18591[/ATTACH]

To help me understand...

Is this equivalent to building all the harmonics of a "pulse" using separate oscillators and summing the harmonic waveforms?

I've always wondered about doing this. (stuff you can play with on the old Moog synthesizers...)

If that is what you are referring to -- is there a difficulty in summing the magnetic fields without coupling the coils and messing up their individual inductances?

-SB

[simonbaker]

Yes, the term is
v=L*di/dt, where v is an instantaneous voltage across the inductor - hence huge peaks on driving transistors
The least painful way of observing a MD system is by imagining targets as very loosely coupled transformers. And what transformers do with voltage? More voltage in (di/dt), more voltage out. Easy.

Right, I actually was potentially supporting your statement about current duration, where although Carl says the current "cuts off" in a couple of microseconds, it seemed plausible to me that the di/dt could persist longer.

Now about the previous questions...

Yes and no, depends on where you stick your probe. In case you put it on the very synchronous detector, you'll find a lot of garbage, and a trend which is hardly obvious if signal is weak. Real integration happens at low pass filters. If you stick a probe there, you'll see a nice line and no garbage. LPF BW is roughly 15Hz, and hence your integration period.

Agree (I think). I misread your post and thought you wrote 15 kHz, not 15 Hz. 15 Hz sounds more like an SD plus "integrator" action.

For this I expect your familiarity with PWM, and ... actually I can do it in simple lingo so that everyone can grasp it.

First off, imagine a perfect square signal, both positive and negative pulses are the same in duration, and Von is, say, 1V. The average voltage is 0V because Voff is -1V. Simple.
Now, imagine that you want a signal which has average voltage somewhat different than 0V. You could make Von variable, but what if you can't? You simple make a small difference in duration of on and off pulses, and there you have it. The principle is called pulse width modulation, or PWM.

Now, think of a mixer. It works as an analogue multiplier. In case you multiply a DC signal with the perfect local oscillator as explained above, you'll get a product that is 0DC and a HF component. In case of a synchronous detector it is just the other way around. In case of PWM shifted local oscillator, you'll get a DC component as well. In special case of quadrature synchronous mixing, you expect a DC component to be proportional to phase shift (and signal amplitude), but not the PWM induced component that lurks about.

Now for the above-mentioned TGSL. It extracts phase shifted local oscillator signal using comparators that are prone to picking up every signal variation and fire sooner or later than perfect. As this signal is derived from a directly coupled Tx oscillator, you may expect a nice PWM local oscillations that are related to Tx coil coupling, soil mineralisation, you name it.

In short, VLF-s could act less erratic if proper care is taken to local oscillator PWM elimination. Less erratic -> better sensitivity -> deeper detection.

Yes, I have wondered about modulation of the oscillator and its effect on the synchronous detector, since the SD is driven by the oscillator. Of course this is the main strength of the design, that fluctuations in the oscillator are automatically tracked by the SD and for the most part do not create a phantom signal. However, a good math model of this process would help show what kind of errors can actually creep in and how severe. A more stable oscillator may be worth pursuing.

Of course we all assume there are better MDs than the TGSL -- it is a very early design, and hopefully the MD manufacturers have not been completely blowing smoke over the last decades. The beauty of the TGSL is bang for the buck -- a heck of a lot of performance with minimal parts.

For me, ideas that add a boatload of parts for a tiny bit of improvement are not super exciting -- but I don't oppose it. One of the beneficial activities we do here, IMO, is reinventing the wheel for ourselves, so we really understand the inner workings of each step in making a better MD, and we can really play around intelligently instead of chasing myths.

-SB

[Aziz]

Hi Moodz,

really excellent idea and exciting possibilities. I must say, I like the transmitter much more now.

Comeon guys, share your ideas and we can optimize it further. Let's do a team work.

Aziz

[moodz]

Hi Moodz,

I like this kind of transmitter. It's very simple, flexible and versatile (narrow bandwidth/wide bandwidth with small modification).
The only flaw is the high voltage seen by the mosfet (break-down voltage). Hey we could use the auto-transformer coil to reduce the voltage across the mosfet. Or stacking the mosfet.
Oh yes, the forward biased diode could be an another mosfet (with the intrinsic diode), which can select the bandwidth characteristics (mode of operation).

Aziz

... hehe ... but thats not all as they say on TV.

Below is a TX circuit for a "wideband" VLF ..... we just use two frequencies
Arrange the two transmit coils for minimum coupling then a third receive coil with minimum coupling to two transmit coils and you have the tri coil wideband VLF....... the mag field from the two TX coils provides a "square wave" *****at the target**** through simple magnitude summation. The summation is shown in the red trace ... note the transition of the red trace appears faster than either of the individual frequencies ....LOL This two tone method gives target info at a low and high frequency. Because the TX is fed from a single pulse generator everything stays in lock. A PLL modification prevents detuning due to ground ( not shown here ) TX circuit is very simple and current draw is very low
moodz

[Aziz]

Hence, the step voltage excitation rules. Unfortunately only for dual coil systems.

I must rush now, and I don't want to post the actual schematic for the time being because of some stability issues - it is sub optimal in current design - works fine only with narrow tolerances - not at all to my liking. I'll play with it in a few hours. To make it worth a discussion here I'll create it using ideal parts e.g. voltage controlled voltage sources switches etc. I think you'll like it. Point is that with a coil as a king the rest of electronics is irrelevant - it must be made to behave.

Let us help you.
You won't regret it.

Aziz

[Aziz]

The main trick is not actually passing a high bandwidth pulse through the coil .... and I think alot here miss the point .... is exposing the target to a step function magnetic field transition.
The generation of a "stimulated resonance" at a single frequency is trivial ... see circuit below ... that particular one generates 10 amps peak to peak in the coil ....but draws only 350 ma ( with no coil loading ).

The next generation of detectors have a series of planar and spatial coil arrangements each driven by stimulated power resonance drivers. By arranging the oscillator amplitude phase and frequency the summation magnetic field ****at the target ***** generates the required field transition ... the step transition speed is determined by the the highest frequency in the combined fields and what you might call group delay caused by the ground being within the field path however this can be compensated by adjustment of the params of each oscillator.
The summation field is produced F0 + F3 + F5 + F7 etc .....( for a simple square wave field transition ) ... this is well known in signal processing fields.

moodz
[ATTACH]18591[/ATTACH]

Hi Moodz,

I like this kind of transmitter. It's very simple, flexible and versatile (narrow bandwidth/wide bandwidth with small modification).
The only flaw is the high voltage seen by the mosfet (break-down voltage). Hey we could use the auto-transformer coil to reduce the voltage across the mosfet. Or stacking the mosfet.
Oh yes, the forward biased diode could be an another mosfet (with the intrinsic diode), which can select the bandwidth characteristics (mode of operation).

Aziz

[Davor]

Hence, the step voltage excitation rules. Unfortunately only for dual coil systems.

I must rush now, and I don't want to post the actual schematic for the time being because of some stability issues - it is sub optimal in current design - works fine only with narrow tolerances - not at all to my liking. I'll play with it in a few hours. To make it worth a discussion here I'll create it using ideal parts e.g. voltage controlled voltage sources switches etc. I think you'll like it. Point is that with a coil as a king the rest of electronics is irrelevant - it must be made to behave.

[Aziz]

BTW,

Heinrich Hertz would have his 155. birthday today. It's a good time to remember the resonance systems.

The fact, that the impedance of the TX coil get's into a minimum ( X(LC)=0, the resonant tank capacitor reactance X(C) is subtracting the coils inductive reactance X(L) to zero ), resonance systems can really be ***** kicking exciting to the targets. All the spectral energy of the coil current pulse is focussed in a single frequency.

Well, if you can handle the high ground mineralization, you will have the best performing detector in the world.

No matter what you do - only dI/dt matters.
Aziz

[moodz]

Peak coil current.



Then we need to define a new term "Ampere-turns per us." Still, if a VLF can hit 1A-T/us then a PI can easily hit 20A-T/us.

The main trick is not actually passing a high bandwidth pulse through the coil .... and I think alot here miss the point .... is exposing the target to a step function magnetic field transition.
The generation of a "stimulated resonance" at a single frequency is trivial ... see circuit below ... that particular one generates 10 amps peak to peak in the coil ....but draws only 350 ma ( with no coil loading ).

The next generation of detectors have a series of planar and spatial coil arrangements each driven by stimulated power resonance drivers. By arranging the oscillator amplitude phase and frequency the summation magnetic field ****at the target ***** generates the required field transition ... the step transition speed is determined by the the highest frequency in the combined fields and what you might call group delay caused by the ground being within the field path however this can be compensated by adjustment of the params of each oscillator.
The summation field is produced F0 + F3 + F5 + F7 etc .....( for a simple square wave field transition ) ... this is well known in signal processing fields.

moodz

[Aziz]

Hi Davor,

show us your TX circuit and we show you, where it actually flaws.
(Which means, there isn't a "free lunch".)

To achieve a short decay time, we have to increase the TX-off time impedance. This can be done with impedance transformer too (converter).

Who remembers the split driven TX coil (auto transformer and center-tapped TX coil)? This is such a impedance transformer.
Low Z on-time (driven) - High Z off-time (damped).

Aziz

[Carl-NC]

What's happened to the damping resistor Rd?

It is the reason why we end up with that equation, instead of a different equation. In any case,