DEEPER PI DETECTION DEPTH

[Aziz]

The half-cosine has discontinuities which results in higher broadband FD response. Interestingly, a half-cosine current has a half-sine voltage response, and vice-versa, so when you look at the FFT are you looking at the voltage or current? Does it matter?

In the far more interesting time domain, half-cosine has the advantage of a PI-like discontinuity; half-sine has the advantage of a di/dt=0 that is useful for GB. Personally, I would combine them.

- Carl

Hi Carl,

it doesn't matter, what you apply: current or voltage. The FFT isn't complaining at all. The current pulse is meant of course.

Well, the half-cosine pulse is DC free and has +6 dB more FD response due to +Max .. -Max range compared to the half-sine response (in the example +Max .. 0). If I would make a half-sine pulse over +Max .. -Max range, the whole half-sine FD response is shifted by +6 dB only.
But I like the wide discontinues of the half-cosine current pulse (wide band characteristics better than a half-sine pulse).

"half-sine has the advantage of a di/dt=0 that is useful for GB"

This requires an induction balance (IB) coil configuration (obviously). Unfortunately, the mono coil can't sample this in the half-sine pulse variant.

Cheers,
Aziz

[Carl-NC]

so what's the difference between a half sine and half cosine current pulse?

The half-cosine has discontinuities which results in higher broadband FD response. Interestingly, a half-cosine current has a half-sine voltage response, and vice-versa, so when you look at the FFT are you looking at the voltage or current? Does it matter?

In the far more interesting time domain, half-cosine has the advantage of a PI-like discontinuity; half-sine has the advantage of a di/dt=0 that is useful for GB. Personally, I would combine them.

- Carl

[Davor]

There is a distinct phase traversal at the middle which will have some saying in choice of a pulse duration in case you are into discrimination.

I guess we'll have to stick to a single convention here as to what we observe and why. I assumed a voltage as a more relevant (dI/dt and constant ampere-turns), but for all practical purposes - it is the same. I missed to include a library of a high voltage pnp in the schematics, so here it goes...

[Aziz]

Half Sine/Cosine Pulse FFT

Hi all,

so what's the difference between a half sine and half cosine current pulse?
See below in the order half sine and half cosine:
Color cyan represents the FFT magnitude response.

Cheers,
Aziz

[Davor]

What is a "slower target" mate?

Again: only dI/dt matters!

I'm afraid he just might be right. Responses to cosine edges (dI/dt) do tend to cancel each other for taus much longer than the stimulus duration, much like a gradient microphone does. So this idea could use some more thinking over. I think there is a potential in it, heck, gradient mikes are not bad at all. Maybe it will be perfect for gold - who knows?

I am still much more in favour of voltage step, but somehow got carried away with this. I've even put some flash around the bones, and not just one but two versions. Instead of a H-bridge which could prove better but much more complicated, I put just another coil to do the same thing in opposite direction with another diode. Again no stress to any of the components, and only plain vanilla parts. Simple. As MOSFET gate floats with voltage change, it required some exotic approach, and I assembled both opto coupler, and gate transformer. Since good transformation is obtained with somewhat too large inductivity to be found in normal stores, I made a simple solution with optos. Works as the concept or better, but I'll have to convince myself that there is any merit to it. Single coil is a tempting possibility though.
Here it goes...

[Tinkerer]

Hey Tinkerer, can you post the LTSpice file?

Regards,

-SB

Here is the zip with the traditional PI and the test_PI. I think that to put the result on the same graph, you need to re-draw one of the asc files onto the other one.

[simonbaker]

Using the usual resistive 200us and 15us TC targets.

Hey Tinkerer, can you post the LTSpice file?

Regards,

-SB

[Aziz]

I haven't thought this through, but there seems to be a fundamental difference between this "half-cycle" current pulse, and a traditional PI pulse which starts with a steady-state current and then suddenly switches off.

If you look at the di/dt of these two pulses, they are very different, are they not? I am wondering if the "half-cycle" works against itself by "unringing the bell" right after it strikes it. In other words, the di/dt has a quick "counter-pulse" -- could this partially cancel its effect on slower targets?

Just wondering... I guess we need to put it through the simulator.

-SB

What is a "slower target" mate?

Again: only dI/dt matters!
The half sine current pulse has it's maximum dI/dt at the beginning and at the end.
Both generate a wide band pulse. The only difference is the frequency response.

But the half sine current pulse has more advantages over the traditional PI pulse.

Aziz

[simonbaker]

Using the usual resistive 200us and 15us TC targets.

Nice - can we compare this with a traditional pulse on the same graph?

-SB

[Tinkerer]

I haven't thought this through, but there seems to be a fundamental difference between this "half-cycle" current pulse, and a traditional PI pulse which starts with a steady-state current and then suddenly switches off.

If you look at the di/dt of these two pulses, they are very different, are they not? I am wondering if the "half-cycle" works against itself by "unringing the bell" right after it strikes it. In other words, the di/dt has a quick "counter-pulse" -- could this partially cancel its effect on slower targets?

Just wondering... I guess we need to put it through the simulator.

-SB

Using the usual resistive 200us and 15us TC targets.

[simonbaker]

Hi all,

Davor's TX circuit principle does not differ too much from the discussed TEM transmitter. But the TEM transmitter has more benefits for its simplicity as it is definitely prior art.

It's just splitting the resonant mode into different phase sections. Davor's TX just resonates for a half cycle (cap -> coil -> back to cap with voltage reversal).
The zero reactance mode (resonant mode) has the benefit of being able to push incredible more power to the coil but it's time constant is defined by pi*sqrt(LC) (half cycle period of resonance frequency).

Split the full resonant mode into two half cycle sections and then you have it.

Aziz

I haven't thought this through, but there seems to be a fundamental difference between this "half-cycle" current pulse, and a traditional PI pulse which starts with a steady-state current and then suddenly switches off.

If you look at the di/dt of these two pulses, they are very different, are they not? I am wondering if the "half-cycle" works against itself by "unringing the bell" right after it strikes it. In other words, the di/dt has a quick "counter-pulse" -- could this partially cancel its effect on slower targets?

Just wondering... I guess we need to put it through the simulator.

-SB

[Aziz]

Hi all,

Davor's TX circuit principle does not differ too much from the discussed TEM transmitter. But the TEM transmitter has more benefits for its simplicity as it is definitely prior art.

It's just splitting the resonant mode into different phase sections. Davor's TX just resonates for a half cycle (cap -> coil -> back to cap with voltage reversal).
The zero reactance mode (resonant mode) has the benefit of being able to push incredible more power to the coil but it's time constant is defined by pi*sqrt(LC) (half cycle period of resonance frequency).

Split the full resonant mode into two half cycle sections and then you have it.

Aziz

[simonbaker]

@moodz, thank you for mentioning it. Time and again I wonder what else will get patented, and in case I ever wish to patent this exciter of mine I'll be sure to include this patent as a prior art.

Actually, the patent you mention is for a method of obtaining any kind of pulses via switched L and C arrangement, and it insists on "half-sine" - not cosine - and switches on two sides.

In any case, my exciter has one key component which makes it incredibly better than the patented solution, and it is a diode. It is an infallible solution for switch off at zero current.

Furthermore my exciter provides low-Z from beginning to the zero current, while with the patented solution it is a special case when a switch opens at the exact zero crossing, hence mine is far more reliable.

Sinc function may seem kind of sexy, but it just complicates things, and I don't need it. Perfect spectrum happens with loooong sampling times, and its spectrum goes by a popular name "brick wall" - it is exactly what I don't need, I'll be happy with periodicity in spectrum. What I do need is a perfect switch off to enable single coil operation, and diode provides. This solution is not a subset of a patented one because a diode is not some kind of a special case of a switch, but a component per se. I wanted positive pulses - I got positive pulses, a diode takes care of it. They insist on exact timings, durations etc. and I don't care about exact timings - in fact I intend to make this exciter happy as a little pig with any coil in wild range of inductances from at least 100uH to 10mH, and any kind of repetition just because I can. My solution is different from theirs, and so far I see that even as a concept mine is much better.

Lets make it official, here is a schematic of this concept where a simple diode does a timing trick. Now it is published, and for copyright information I can be contacted via e-mail or a private message on this forum. As far as I am concerned you are free to use it as ever you want for non-commercial purposes.

I'll fix the H-bridge capacitor reversal solution soon enough. It will reverse polarity to conserve the capacitor charge for subsequent pulses and reduce power consumption.

As Aziz says - patent trolls - keep off!

It will be interesting to see how fancy the circuit has to get to achieve continuous operation - and how easy to retain the diode in such a circuit. Good design challenge...

-SB

[simonbaker]

target models

How confident are we in our target models? It would seem that the target dynamics have some say in what is the ideal waveform to send at it.

-SB

[Davor]

@moodz, thank you for mentioning it. Time and again I wonder what else will get patented, and in case I ever wish to patent this exciter of mine I'll be sure to include this patent as a prior art.

Actually, the patent you mention is for a method of obtaining any kind of pulses via switched L and C arrangement, and it insists on "half-sine" - not cosine - and switches on two sides.

In any case, my exciter has one key component which makes it incredibly better than the patented solution, and it is a diode. It is an infallible solution for switch off at zero current.

Furthermore my exciter provides low-Z from beginning to the zero current, while with the patented solution it is a special case when a switch opens at the exact zero crossing, hence mine is far more reliable.

Sinc function may seem kind of sexy, but it just complicates things, and I don't need it. Perfect spectrum happens with loooong sampling times, and its spectrum goes by a popular name "brick wall" - it is exactly what I don't need, I'll be happy with periodicity in spectrum. What I do need is a perfect switch off to enable single coil operation, and diode provides. This solution is not a subset of a patented one because a diode is not some kind of a special case of a switch, but a component per se. I wanted positive pulses - I got positive pulses, a diode takes care of it. They insist on exact timings, durations etc. and I don't care about exact timings - in fact I intend to make this exciter happy as a little pig with any coil in wild range of inductances from at least 100uH to 10mH, and any kind of repetition just because I can. My solution is different from theirs, and so far I see that even as a concept mine is much better.

Lets make it official, here is a schematic of this concept where a simple diode does a timing trick. Now it is published, and for copyright information I can be contacted via e-mail or a private message on this forum. As far as I am concerned you are free to use it as ever you want for non-commercial purposes.

I'll fix the H-bridge capacitor reversal solution soon enough. It will reverse polarity to conserve the capacitor charge for subsequent pulses and reduce power consumption.

As Aziz says - patent trolls - keep off!