DEEPER PI DETECTION DEPTH

[simonbaker]

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...

Wouldn't bandwidth of MDs like TGSL be essentially the synchronous detector bandwidth, which is only a few Hz (depending on integration time), maybe for TGSL 100 Hz or so?

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...

Would you expand on that some more? I'm not clear on the concepts you are describing.

Regards,

-SB

[Carl-NC]

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?

I don't know of a single VLF that achieves 1A peak current, maybe 100mA max. Also, in a PI, the current turn-off time is not the same as the voltage decay time. Typically, the current shuts off in 1-2 us. Most PI's peak at 2-3 amps.

But the real question is, what defines "better"? Deeper?

[simonbaker]

I don't know of a single VLF that achieves 1A peak current, maybe 100mA max. Also, in a PI, the current turn-off time is not the same as the voltage decay time. Typically, the current shuts off in 1-2 us. Most PI's peak at 2-3 amps.

But the real question is, what defines "better"? Deeper?

Even though the current turns off in 1-2 uS, what does the di/dt turn-off look like? Is that what stimulates the target?

Also, isn't the voltage decay time the confounding characteristic that makes detecting small targets difficult (in a basic PI design)?

Regards,

-SB

[Davor]

I don't know of a single VLF that achieves 1A peak current, maybe 100mA max.

... times the tank Q of say ... 10 ... there you have it. Most of the current is recycled in resonant tanks. In case of resonance the only current you can measure accounts for losses.

[simonbaker]

... times the tank Q of say ... 10 ... there you have it. Most of the current is recycled in resonant tanks. In case of resonance the only current you can measure accounts for losses.

I'm interested in that potential for hi-Q Tx tanks, but so far I don't know if it is really a good idea (Tx voltage easily affected by ground). I hope to try some experiments (I think others are going to also, maybe you?).

Apparently Carl doesn't know of an MD that actually uses such a high-Q tank/oscillator configuration.

Regards,

-SB

[Carl-NC]

Even though the current turns off in 1-2 uS, what does the di/dt turn-off look like? Is that what stimulates the target?

Let's say 2 amps in 2us, reasonably linear. So the "1,000,000 A/s" slew rate is actually reasonable.

Also, isn't the voltage decay time the confounding characteristic that makes detecting small targets difficult (in a basic PI design)?

The voltage decay is something that just gets in the way. Use an IB coil to get rid of it, and you can then see smaller targets.

[Tinkerer]

I'm interested in that potential for hi-Q Tx tanks, but so far I don't know if it is really a good idea (Tx voltage easily affected by ground). I hope to try some experiments (I think others are going to also, maybe you?).

Apparently Carl doesn't know of an MD that actually uses such a high-Q tank/oscillator configuration.

Regards,

-SB

For a deep searching detector we want at least 1A/us of moment. But 1A alone does not say all. We also need to say how many turns in the coil and what the diameter of the coil is.

1A/us at 10 turns of magnetic moment in a coil of one square meter area gives good depth for a target of about 20mm diameter.

1A/us at 20 turns still gives good depth for a 10mm diameter target.

Then we want to take the first sample of the target response when the response is at it's peak.

And then we want to do that at a frequency or repetition rate of 5000Hz, to have many samples to stack.

Davor, you keep saying that the RX front end is not good. Can you come up with a better RX front end?

Tinkerer

[Carl-NC]

... times the tank Q of say ... 10 ... there you have it. Most of the current is recycled in resonant tanks. In case of resonance the only current you can measure accounts for losses.

There is no need to measure the current, it's easily calculated. Resonance or not.

[moodz]

...as Davor has pointed out previously a lot of these problems have been addressed in other fields such as Rf engineering. Its just that the application has changed. The problem of maintaining a high Q ( ok let's say optimal ) which ensures maximum power transfer was solved years ago in the induction heating field. By using a PLL to lock the current phase to the voltage phase of a tank circuit the Q will be maintained. The actual Q will vary though due to variable losses from the tank coil to the load however maximum current can be maintained at all times in the coil ..... Easily several amps.

The answer to Aziz question is the VLF. (power density vs bandwidth not to mention receiver bandwidth advantage and synchronous continuous sampling advantage)

Look at the circuit here ....

http://uzzors2k.4hv.org/projectfiles...Ts_4046_IH.GIF

Moods . Free lunch dinner and all your base are mine.

[Davor]

Even though the current turns off in 1-2 uS, what does the di/dt turn-off look like? Is that what stimulates the target?

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.

Now about the previous questions...

Wouldn't bandwidth of MDs like TGSL be essentially the synchronous detector bandwidth, which is only a few Hz (depending on integration time), maybe for TGSL 100 Hz or so?

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.
With PI you also have a kind of integration during a sample pulse, but in absence of proper weighting function it is more of a window than an integrator. A proper weighting function would be some kind of fade-in, only I’m not yet sure of what kind. IMHO with PI you should sample first, apply the weighting function, and integrate/amplify later.

Would you expand on that some more? I'm not clear on the concepts you are describing.

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.

[Tinkerer]

There is no need to measure the current, it's easily calculated. Resonance or not.

Still, the question remains: Is 100mA the power consumption or is this the peak coil current.
100mA with 100 turns at 20kHz, brings us back to 1A/us didt.

Tinkerer

[Aziz]

Hi guys,

thanks for the hot contributions. Everybody is welcome (there is no need to be a "guru").

A clarification to my posting:
The values are chosen somehow arbitrary or reasonable of course. Don't confuse it with power consumption. We assume a comparable configuration of course. And don't focus too much to the irrelevant details. Just focus to the given facts.

Well the term "better" should be interpreted as much gain to the operator as possible:
- could be more depth (well, the most important item I think)
- could be more operating duration (power consumption)
- could be more signal integrity (less noise)
and so on.

Moodz gave you the answer. But I am missing the discussion "why?".

Cheers,
Aziz

PS: Where are the other gurus?

[Davor]

Davor, you keep saying that the RX front end is not good. Can you come up with a better RX front end?

I'm contemplating on it. There are some other things on my mind, like finishing a disertation, so you can only imagine a mess that goes around in my head.
Anyway, from what I've seen so far, there is no need for opamp on frontend. It makes things worse. Instead you need something with hard grip on ground like chopper or Tayloe, or something. Say 1MHz mixer. Next, you must make the most out of the time in which you may detect a signal. To do so you'll need a proper weighting function, in this case some kind of fade-in. Point is that however Tx coil is not exactly emitting power after Tx transistors quench off, the noise power continues to drop for some time, hence, however the target signal is decaying, so is the noise, and you may expect a near constant S/N for some more time. Applying a weighting function gain would make for a much longer duration of signal with good S/N. Hence much longer integration.
Next, discrimination. Zero crossing detection is much easier task with properly weighted signal, so there you have it.

IMHO an opamp at frontend is a mistake. You are not fighting thermal noise there, but a hard slamming switching transistor. A properly balanced analogue switch instead of an opamp with optimum impedance match would be a good start.

About Tx, I just might have a clue on how to make for a nice pi pulse that behaves, and presents itself as low impedance. More to come...

[Carl-NC]

Still, the question remains: Is 100mA the power consumption or is this the peak coil current.

Peak coil current.

100mA with 100 turns at 20kHz, brings us back to 1A/us didt.

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.

[Aziz]

BTW,

we have to talk about the (TX) coils reactance X(L) later too:
X(L) = w*L = 2*pi*f*L
And the impedance Z(L) as well.
Z(L) = sqrt(X(L)*X(L) + R(L)*R(L))

We will see later, why the flyback period in a PI generates a high voltage pulse. The fact U = -L*dI/dt isn't the real proof. But we aren't so far yet.

Aziz