DEEPER PI DETECTION DEPTH

[Aziz]

Yeah, I probably missed your point. Maybe you are saying: do an FFT on the response signal and look at that for patterns? How do we sample enough points to do that, isn't it tough to get even a few, or not? What are you proposing?

-SB

Hi SB,

no I don't say do the FFT on the response signal. I'm just saying look at the AC response in the posting ( http://www.geotech1.com/forums/showp...&postcount=221 ).
I'm just saying to do more AC response spice simulations. It answers some fundamental (but interesting) questions.

So, what's the simple graph is telling us?

Put the following command line somewhere and change the transmitter inductance (don't remove the L=300µ command):

.step param L list 150µ 300µ 600µ

And look what happens.

Report all here, what you have found.

Cheers,
Aziz

[Qiaozhi]

One important point, when using SPICE's AC analysis, is that it's a "small-signal analysis". This is why you can set the voltage source parameters (AC amplitude and AC Phase) as 1 and 0, so that the Bode Plot shows the gain of the circuit directly. You need to be aware that the SPICE models are first linearized around the operating point before the simulation starts. In many cases there can be a huge difference between the large-signal and small signal results. It may not cause any problems in this instance, but it's something to bear in mind.

By the way, whatever independent source you use in an AC simulation, the frequency you define on that source (SINE, for example) is not used. Instead, the simulation uses the frequencies defined in the .AC statement.

[Aziz]

Ok guys,

I have put the simplest form of an AC analysis (TX -> Target -> RX). Regardless of small signal (1 V) or full signal (battery voltage), BTW this is totally irrelevant in the provided simple spice simulation, what can we obtain from it?

Is there a free lunch for us?


Comeon, feel free to comment it, what you see. We will get very valuable information from it.

Aziz

[Qiaozhi]

I have put the simplest form of an AC analysis (TX -> Target -> RX). Regardless of small signal (1 V) or full signal (battery voltage), BTW this is totally irrelevant in the provided simple spice simulation, what can we obtain from it?

The use of a 1V signal does not imply "small-signal". Since the models are linearized around the operating point, prior to the AC simulation starting, means that only a "small-signal" variation will yield correct results. Any non-linear effects that may occur in practice are removed by the simplified models. In other words, some circuits will give different results during an AC simulation for different operating points. The "trick" of setting the AC amplitude to unity, is simply used to display the gain of the circuit directly in the Bode Plot. In fact, 1V may be an excessively high input voltage for some circuits, but this does not affect the results of an AC analysis.

Just thought I need to make this clear.

[Sean_Goddard]

Re: The PI I am working on. I need to clarify a few points.

Dave Emery kindly helped me improve my design after I sent him the
schematics for his opinion. I mentioned Dave in my previous post so as to
give him the credit for this. I should have explained that this was the
only involvement Dave had and that we are not working together on any kind
of a commercial venture (I didn't proof read what I had typed and implied something which was NOT true).

Dave kindly helped me even though he was fully aware that I plan(ned) to sell my detectors in one way or another. One last thing, my opinion of Eric Foster's detectors is mine alone and not Dave's, it was meant in fun and in NO WAY was it a slight at Eric's incredible achievements over the years which FAR outweight anything I have done.

SORRY ERIC, no offence meant .

You can also note that against Daves advice I made further modifications to his suggested layout (which I have since lost as the only mods which existed were Dave's mark-up on MY design) and managed to burn up a whole batch of FETs. Depth is off the map, but the FETs just keep blowing after 20 hours or so. I've tried backing off various things which SHOULD, in theory, solve the probelm, but nothing seems to be helping, not even changing the FET for one with different parameters. I've NO IDEA what is causing the problem.

Dave has rightly (after reading my comments) refused to help further so I'm not sure I will be doing ANYTHING with this design.

Sorry if I gave people the impression that something was happening which was NOT! And an even BIGGER apology to Dave for implying that you were involved with a "project" when you were not.

[Sean_Goddard]

My intention in posting what I did was attempt to kick start what has, as far as I can see, become a stalled process by getting people to analyse what they were doing in more depth (pardon the pun). Using Dave's name to imply that something was imminent when in fact this was NOT the case. It was wrong of me and I admit full liability.

Dave has expressed his anger regarding this matter, and I have apologised for the misuse of his good name. Dave in NO WAY endorses any of my work (and nor should he) and contrary to what was implied has NO intention of working on ANY project (commercial or otherwise) with me now or in the future!

One a personal level I have made NO advances worthy of note regarding PI Development, but have made a few discoveries which I shall be keeping to myself.

In light of the offence I have cause, this will be the LAST post I shall me making on this forum!

Thanks for all the great stuff I've learned and keep up the good work folks!

[simonbaker]

Ok guys,

I have put the simplest form of an AC analysis (TX -> Target -> RX). Regardless of small signal (1 V) or full signal (battery voltage), BTW this is totally irrelevant in the provided simple spice simulation, what can we obtain from it?

Is there a free lunch for us?


Comeon, feel free to comment it, what you see. We will get very valuable information from it.

Aziz

Ok, I'll bite...

I guess my first impression is that the spectral shape is very sensitive to target time constant but not too sensitive to RX/TX coil inductance, so you can use that information to differentiate targets.

However, you have pretty radical differences in target resistance. Is that realistic?

For fun, I tried substituting actual metal conductivity resistances for the target resistor values. I have no idea if that is valid. So let me know if that is meaningless or not.

Below is a comparison of the RX response for those targets.

Does it mean anything, or just BS?

-SB

P.S. Gee, the phases look nice and differentiated above 100 kHz -- maybe some sort of phase detector would be useful... what does that remind me of...

[simonbaker]

My intention in posting what I did was attempt to kick start what has, as far as I can see, become a stalled process by getting people to analyse what they were doing in more depth (pardon the pun). Using Dave's name to imply that something was imminent when in fact this was NOT the case. It was wrong of me and I admit full liability.

Dave has expressed his anger regarding this matter, and I have apologised for the misuse of his good name. Dave in NO WAY endorses any of my work (and nor should he) and contrary to what was implied has NO intention of working on ANY project (commercial or otherwise) with me now or in the future!

One a personal level I have made NO advances worthy of note regarding PI Development, but have made a few discoveries which I shall be keeping to myself.

In light of the offence I have cause, this will be the LAST post I shall me making on this forum!

Thanks for all the great stuff I've learned and keep up the good work folks!

Hot FETs or hot people... which is tougher problem?...

Well, didn't seem like such a big offense, but not me to say I guess...anyway seems like you made it right...

Look forward to more hot discoveries you come up with in the future...

-SB

[NC-Dave]

My intention in posting what I did was attempt to kick start what has, as far as I can see, become a stalled process by getting people to analyse what they were doing in more depth (pardon the pun). Using Dave's name to imply that something was imminent when in fact this was NOT the case. It was wrong of me and I admit full liability.

Dave has expressed his anger regarding this matter, and I have apologised for the misuse of his good name. Dave in NO WAY endorses any of my work (and nor should he) and contrary to what was implied has NO intention of working on ANY project (commercial or otherwise) with me now or in the future!

One a personal level I have made NO advances worthy of note regarding PI Development, but have made a few discoveries which I shall be keeping to myself.

In light of the offence I have cause, this will be the LAST post I shall me making on this forum!

Thanks for all the great stuff I've learned and keep up the good work folks!

Sean, there is no need to quit the forum. Your post was dumb but you obviously did not intend to cause any problems. I enjoy reading all the stuff from people like yourself and Aziz as well as countless others on this forum. I would hate to have you simply fade away. To back this up I am going to ask the rest of the readers on the forum to all post a message requesting Sean to keep on posting. On a final note, Sean is a lot closer to a vastly improved beach machine than he realizes. His work is excellent. His only problem is that he tries to work too fast causing him to make a few mistakes. Keep up the good work, Dave. * * *

Dam it all, life is way too short and this is only a hobby.

[Qiaozhi]

Ok, I'll bite...

I guess my first impression is that the spectral shape is very sensitive to target time constant but not too sensitive to RX/TX coil inductance, so you can use that information to differentiate targets.

However, you have pretty radical differences in target resistance. Is that realistic?

For fun, I tried substituting actual metal conductivity resistances for the target resistor values. I have no idea if that is valid. So let me know if that is meaningless or not.

Below is a comparison of the RX response for those targets.

Does it mean anything, or just BS?

-SB

P.S. Gee, the phases look nice and differentiated above 100 kHz -- maybe some sort of phase detector would be useful... what does that remind me of...

Simon - In AZIZ's simulation he use targets with time constants of 1us, 10us, 100us and 1ms. These all seem very reasonable values to use, except perhaps the 1ms target.
However, in your version you have the following:
silver = 625ns
copper = 588ns
gold = 435ns
aluminium = 370ns
zinc = 167ns

These all seem to be far too short.
Remember that TC = L/R.

[Aziz]

Ok, I'll bite...

I guess my first impression is that the spectral shape is very sensitive to target time constant but not too sensitive to RX/TX coil inductance, so you can use that information to differentiate targets.

However, you have pretty radical differences in target resistance. Is that realistic?

For fun, I tried substituting actual metal conductivity resistances for the target resistor values. I have no idea if that is valid. So let me know if that is meaningless or not.

Below is a comparison of the RX response for those targets.

Does it mean anything, or just BS?

-SB

P.S. Gee, the phases look nice and differentiated above 100 kHz -- maybe some sort of phase detector would be useful... what does that remind me of...

Hi SB,

sorry, but you are onto wrong path (yet again). Don't go too fast. Just step by step. We want to reveal the basic fundamentals only. And we aren't doing any ground balance nor discrimination here.

What kind of information can we get from the response in the given configuration:
- in regards to the spectral energy of the response (the maximum possible response information)?
- in regards to the bandwidth of the response?
- in regards to target response of different time constants targets?
- in regards to selecting the TX pulse frequency?
(just to list some basic issues here)
...
(we can line up more questions here of course)

Don't think too complex. Just look at the Bode plot. You even do not need to look at the phase information.

Cheers,
Aziz

PS:
Thanks for clarifying this Qiaozhi.
Sean, don't go please!
Glad to see Dave here.

[simonbaker]

Simon - In AZIZ's simulation he use targets with time constants of 1us, 10us, 100us and 1ms. These all seem very reasonable values to use, except perhaps the 1ms target.
However, in your version you have the following:
silver = 625ns
copper = 588ns
gold = 435ns
aluminium = 370ns
zinc = 167ns

These all seem to be far too short.
Remember that TC = L/R.

Yes, I just took a wild stab, I modified one of Aziz's target resistors slightly and made the others proportional.

Why would metals with a conductivity range of 1:7 have orders of magnitude different time constants?

I need education on target modeling. How do you come up with those TCs?

-SB

[simonbaker]

Hi SB,

sorry, but you are onto wrong path (yet again). Don't go too fast. Just step by step. We want to reveal the basic fundamentals only. And we aren't doing any ground balance nor discrimination here.

What kind of information can we get from the response in the given configuration:
- in regards to the spectral energy of the response (the maximum possible response information)?
- in regards to the bandwidth of the response?
- in regards to target response of different time constants targets?
- in regards to selecting the TX pulse frequency?
(just to list some basic issues here)
...
(we can line up more questions here of course)

Don't think too complex. Just look at the Bode plot. You even do not need to look at the phase information.

Cheers,
Aziz

PS:
Thanks for clarifying this Qiaozhi.
Sean, don't go please!
Glad to see Dave here.

Aziz: why don't you just go ahead and say what's on your mind? We're all ears...

-SB

[Qiaozhi]

Yes, I just took a wild stab, I modified one of Aziz's target resistors slightly and made the others proportional.

Why would metals with a conductivity range of 1:7 have orders of magnitude different time constants?

I need education on target modeling. How do you come up with those TCs?

-SB

To find the time constant, simply divide L by R.

I can see what you were trying to do in your example, but you cannot replace R by the resistivity of the material. For one thing, resistivity is measured in ohm-meters, and for copper (for example) the resistivity is 1.68 x 10E-8, not 0.17. Also, the time constant of a target depends very much on its size, shape and orientation, and not just its resistivity.

All you can really do is use a set of targets with various time constants, as Aziz did in his simulation. Trying to define a target for a particular material is not possible, otherwise you could use this method to provide a PI with target discrimination, which you cannot.

[simonbaker]

To find the time constant, simply divide L by R.

I can see what you were trying to do in your example, but you cannot replace R by the resistivity of the material. For one thing, resistivity is measured in ohm-meters, and for copper (for example) the resistivity is 1.68 x 10E-8, not 0.17. Also, the time constant of a target depends very much on its size, shape and orientation, and not just its resistivity.

All you can really do is use a set of targets with various time constants, as Aziz did in his simulation. Trying to define a target for a particular material is not possible, otherwise you could use this method to provide a PI with target discrimination, which you cannot.

Ok. I wasn't trying to calculate the actual resistance, just make relative targets (1.68 x 10E-8 is ohms per something).

But if there is data that shows that range of "time constants" is representative, that's what we have to deal with.

However, how do we relate the time constant of a target to its shape and metal? We need some kind of relationship for Aziz's graphs to be useful.

-SB