DEEPER PI DETECTION DEPTH

[Tinkerer]

Here is the PI_TRADITIONAL switch OFF transient FFT

[Davor]

If you do not like toroid, you can use ferrite pot too.

Instead of real toroid you may break it in half and use as a "snap-on" ferrite for cables RFI. Of course, it will be prudent to check if snap-ons are any good by themselves because they are dirt cheap and come with a handy enclosure too. At the very moment I'm laboratory-challenged, so I can't test it myself

[Davor]

Here is the PI_TRADITIONAL switch OFF transient FFT

Please note that many more interesting things happen in frequencies up to 1MHz (micro-seconds).

[WM6]

Instead of real toroid you may break it in half and use as a "snap-on" ferrite for cables RFI. Of course, it will be prudent to check if snap-ons are any good by themselves because they are dirt cheap and come with a handy enclosure too. At the very moment I'm laboratory-challenged, so I can't test it myself

Davor, thanks for idea. I will try.

[Tinkerer]

Please note that many more interesting things happen in frequencies up to 1MHz (micro-seconds).

The way I see it, it is if little use to stimulate the targets with higher than 300KHz. A traditional PI is not capable of detecting targets with a TC if 1us.

With the TEM method I can sample any time so there is maybe an advantage with higher frequency stimulation.

The FFT shows 2 stimulation frequency bands.

Low frequency with +2.5dB at 5KHz and -20dB at 66KHz.

High frequency stimulation at a different time, +5.75dB at 70Khz and -20dB at 1Mhz.

Probably -20 dB is way beyond any usefulness, maybe we should put the limit of interest a lot higher.

Ideas and comments?

[Davor]

There are two points to emphasize here. First is magnitude, and the other is phase.
When you see magnitude falling uniformly by 20dB per decade you know it is a first order filter. (no surprise)
Second, you know from the bode plot of any first order low pass filter that all the fun with phase happens one decade lower than with magnitude. Any magnitude ripple will tell you that something also happens with phase, and in this case it is the on-ramp to worry about. Or not. That would depend on how long you wish your PI to be deaf for anything but its Tx coil.

IMHO it would be beneficial to be able to sample as soon as possible.

[simonbaker]

The way I see it, it is if little use to stimulate the targets with higher than 300KHz. A traditional PI is not capable of detecting targets with a TC if 1us.

With the TEM method I can sample any time so there is maybe an advantage with higher frequency stimulation.

The FFT shows 2 stimulation frequency bands.

Low frequency with +2.5dB at 5KHz and -20dB at 66KHz.

High frequency stimulation at a different time, +5.75dB at 70Khz and -20dB at 1Mhz.

Probably -20 dB is way beyond any usefulness, maybe we should put the limit of interest a lot higher.

Ideas and comments?

If you plan to implement target detection in the time domain, it's probably just easier to look at each target's time domain response and work on strategies to detect it and maybe make discrimination judgments.

Then vary the things you can control, such as the TX pulse shape and the RX receive circuit (response characteristics) and see what makes your life easier for detection and discrimination.

As Davor indicated, the TX pulse seems to look like a pure delta function passed through a low pass filter, so someone could probably do a fairly good paper and pencil analysis that would be equivalent to simulations. But sims are by far the easiest way to tinker.

A good set of target response graphs would be useful to look at and discuss.

-SB

[Aziz]

Hi guys,

I am happy now, that you all are dealing with and talking about the frequency domain to understand, what is happening there.


Regarding the TX coil current and the 20 dB roll-off of it:
The higher the (stimulation) frequency, the lower the TX coil currents gets due to the impedance (reactance XL) of the TX coil. But this doesn't matter as the dI/dt increases proportional with the frequency and the induction law compensates this in the RX coil. This is the reason, why the RX response tends to go to the same flat level (see the AC response I'm still talking about). High TC targets go in the low frequency region to this level and low TC targets go in the high frequency region.
Interesting to know, that in the high frequency region, all targets respond with the same level of response. Whereas in the low frequency region, low TC targets will be missed due to low response.

So if you want to detect the low TC targets, you need some higher bandwidth (covering the high frequency region). Higher bandwidth is achieved with fast amplifiers and fast coils (see bbsailor's article).

Aziz

[simonbaker]

Hi guys,

I am happy now, that you all are dealing with and talking about the frequency domain to understand, what is happening there.


Regarding the TX coil current and the 20 dB roll-off of it:
The higher the (stimulation) frequency, the lower the TX coil currents gets due to the impedance (reactance XL) of the TX coil. But this doesn't matter as the dI/dt increases proportional with the frequency and the induction law compensates this in the RX coil. This is the reason, why the RX response tends to go to the same flat level (see the AC response I'm still talking about). High TC targets go in the low frequency region to this level and low TC targets go in the high frequency region.
Interesting to know, that in the high frequency region, all targets respond with the same level of response. Whereas in the low frequency region, low TC targets will be missed due to low response.

So if you want to detect the low TC targets, you need some higher bandwidth (covering the high frequency region). Higher bandwidth is achieved with fast amplifiers and fast coils (see bbsailor's article).

Aziz

Another possible conclusion is that a "pulse" is not the ideal TX signal, because a lot of the energy is spread in the lower frequencies. Perhaps we need a "high frequency pulse" (whatever that is) that concentrates all the energy up in the higher frequencies. Well, that is probably some kind of sync function that moodz and others were toying with. Or heck -- just make a continuous wave hi-freq MD. Oops -- I forgot. The phase difference seems to disappear at higher frequencies, making discrimination harder. But wait... maybe a tuned RX coil would help (probably not much). Well, maybe we have a trade-off to work on.

We'll see more interesting designs I'm sure.

-SB

[Tinkerer]

Another possible conclusion is that a "pulse" is not the ideal TX signal, because a lot of the energy is spread in the lower frequencies. Perhaps we need a "high frequency pulse" (whatever that is) that concentrates all the energy up in the higher frequencies. Well, that is probably some kind of sync function that moodz and others were toying with. Or heck -- just make a continuous wave hi-freq MD. Oops -- I forgot. The phase difference seems to disappear at higher frequencies, making discrimination harder. But wait... maybe a tuned RX coil would help (probably not much). Well, maybe we have a trade-off to work on.

We'll see more interesting designs I'm sure.

-SB

Logic deductions get you there. It is the TEM method. A continuous wave TX, with 2 different wide spectrum bands of high power stimulating frequencies.
Just scroll up a bit to see the wave forms.

Tinkerer

[Aziz]

...
Oops -- I forgot. The phase difference seems to disappear at higher frequencies, making discrimination harder.
...
-SB

Hi SB,

I see, you are still after the discrimination and get worried about the less phase difference at higher frequencies.

But you are just looking at the resistive response cased phase response. Note, that the spice simulation didn't take reactive response effects into account.

Don't be sad, there are possibilities for your required phase difference.


In an induction balanced (IB) coil configuration, magnetic materials do change the coil coupling coefficient between TX and RX and the RX coil will see more signal from the TX coil. (Phase info).

The coil coupling coefficient change can be used further to increase the mutual inductance between TX and RX by placing a load in the RX coil (damping R). (more Phase info).

Well, (de-)magnetising and/or magnetic viscous effects do add more phase info for your convenience too.

And the skin effect:
Ferromagnetic metals cause a high skin effect (less penetration), which will reduce the targets TC dominantly. But the magnetising effects will be superimposed with the TC.

Cheers,
Aziz

[simonbaker]

Hi SB,

I see, you are still after the discrimination and get worried about the less phase difference at higher frequencies.

But you are just looking at the resistive response cased phase response. Note, that the spice simulation didn't take reactive response effects into account.

Don't be sad, there are possibilities for your required phase difference.


In an induction balanced (IB) coil configuration, magnetic materials do change the coil coupling coefficient between TX and RX and the RX coil will see more signal from the TX coil. (Phase info).

The coil coupling coefficient change can be used further to increase the mutual inductance between TX and RX by placing a load in the RX coil (damping R). (more Phase info).

Well, (de-)magnetising and/or magnetic viscous effects do add more phase info for your convenience too.

And the skin effect:
Ferromagnetic metals cause a high skin effect (less penetration), which will reduce the targets TC dominantly. But the magnetising effects will be superimposed with the TC.

Cheers,
Aziz

I'm not on your wavelength as to what you are saying; maybe a good sim of balanced coils would help show it.

My reasoning is that if at high frequencies these targets' responses all converge to approximately the same phase shift, then it will be harder to discriminate between them because all our MDs can detect is their response. Perhaps what you are saying is that if we look at them as part of a total dynamic system (which they are in our sims), we can vary some other part of the system to enhance the phase differences of some signal measured somewhere. That's what I was hoping, but I'm not seeing exactly how to do that. Especially because the targets are so weakly coupled to everything else, it seems all we can do is measure their responses more or less.

Regards,

-SB

[Davor]

IMHO there are several opportunities in optimising Rx input for better impedance matching, and true balanced operation. Point is that in radio technology there is something called "link balance" that says (in many words of complicated lingo) that you can get only a certain optimum dynamic range, and that remains the same or becomes worse even if you overpimp your Tx. The lower margin is limited by Rx design (system noise, detection limits) and natural noise. However the noise bandwidth here is much larger than with the VLF IB solutions, it is apparent that most of the problems are induced by TX side, and we are waaaaay far away from noise margins here.
My simple deduction would be to go for nicer Rx with proper balancing and careful impedance matching. Once Rx is working its best the Tx will not have to be as powerful as it is currently a standard, and it will be much closer to ideal because of less problematic components with less parasitics, and less awful transients. With less self-induced problems the dynamic margins will widen and hey presto! - a better PI.

[Aziz]

Hi all,

it's getting almost quiet here. It's time to throw another brain food to keep the ball rolling.

Let's look, whether there is a "free lunch" for us.

Following conditions: VLF vs. PI
VLF: 20 kHz operating frequency, 1 A peak TX coil current
PI: 1 kHz pulse frequency, 10 A peak, 10µs damping decay time

Let's look at the maximum TX coil current change (dI/dt):
VLF: max (dI/dt) = max(1 A*sin(wt)/dt) = 1 A*w*1 = 1 A*2*pi*20kHz = 125,664 A/s
PI: max (dI/dt) = max(10 A/10µs) = 1,000,000 A/s

Which detector is effectively better (delivers more) and why?

Cheers,
Aziz

[Davor]

So far I learned that whenever you think that you just found a free buffet there must be something wrong in your reasoning.

In case of PI vs VLF it is time vs frequency, and noise bandwidth is in play. While BW in case of WLF is ~15Hz (e.g. tens of milliseconds), and energy is integrated over larger periods, with PI it is the other way around, you have tens of kHz bandwidth, strong pulses AND ... a dead period.

Both concepts suffer from sub-optimal front-ends and efficiency problems. VLF's inherent trouble is ground reference for mixers, and balanced solutions are only half way better due to hidden PWM clocking problem. Phase information is extracted by synchronous detection, which is fine, but again the reference is floating a bit. Quadrature approach is generally avoided, and it could lead to more sublime solutions, such as Costas filtering etc. Indication, e.g. human interface, is usually something crude, same as with PI.

PI on the other hand is flawed by "hit me baby one more time" syndrome and a pursuit of a misleading single coil design, thus losing valuable microseconds of treasure signal. There is a phenomenon equivalent to phase shift in VLF, and it is zero crossing. Using VLF analogies, PI detection is mainly concentrated on partial detection of I component, while Q (zero crossing) is avoided. Detection is achieved by sampling/integrating, and - what do you know - floating reference problem again. To compensate for poor detection, a crude approach of more-juice-better-detection is employed. Nicer detection should lead to less power, which should lead to even better detection yet again ... to some limits, but much longer battery life. Given a choice of battery weight - I'd rather go with smaller.

Both concepts are in pursuit of targets with the same physical properties.

In short, both systems have venues yet to discover, and many small SNAFU-s to iron.