DEEPER PI DETECTION DEPTH

[simonbaker]

Hi Carl,

I am only learning how to do simulations. I am sure that much better simulations could be done.

So what I am doing, is to make a simulation that gives me results as close as possible to the results I have observed with real circuits.
As you said before, I do things backwards, normally one would make a simulation first and then build the circuit. However, I built many circuits to try to understand what is happening and now I do the simulations to see if the theory fits the results.

Below is another simulation.
With the same TX coil current I get twice the RX signal.
I kept getting this surprising result with a real circuit and it did not fit the theory.
Eventually I found a scientific article for a geological application that talked about the same effect. Unfortunately I have not been able to find the link to the article again to post it here,
but anyway, the simulation gives the same result as the real component circuit.

The simulation below has the same coil current, the same targets and the same trace colors.


Tinkerer

Hi Tinkerer:

You are doing yeomans work, it is always appreciated.

Can you attach your LTSpice file each time you post an output graph so we can better understand all the factors involved and how you model your system?

Best regards,

-SB

[Aziz]

Anyone heard of JOHNSON noise? It pertains to resistors (specifically carbon) so, USE METAL FILM IN THE FRONT END!!!!!! Formula is SQROOT (value of resistor) * 4nV/Hz

SOoooo...Can you see where this is going??? In order to keep your SYSTEM noise low, requires a little more thought than just throwing amplifiers together using arbitrary values to get gain levels.

Aziz is there as is Moodz..

I'm NOT deliberately trying to be evasive, I'm trying to help people to learn more about electronics by encouraging them to think a little outside the box!

Well, this (the Johnson noise) isn't an issue. The low noise design is well undestood and indeed very very trivial.

What you trying to offer isn't novel. So tell us more please! But I presume, it's already known.

Aziz

[Sean_Goddard]

Anyone heard of JOHNSON noise? It pertains to resistors (specifically carbon) so, USE METAL FILM IN THE FRONT END!!!!!! Formula is SQROOT (value of resistor) * 4nV/Hz

SOoooo...Can you see where this is going??? In order to keep your SYSTEM noise low, requires a little more thought than just throwing amplifiers together using arbitrary values to get gain levels.

Aziz is there as is Moodz..

I'm NOT deliberately trying to be evasive, I'm trying to help people to learn more about electronics by encouraging them to think a little outside the box!

[Aziz]

There are simple/advanced rules for avoiding or coping with (EMI) noise:
(besides the PCB rules, avoiding ground loops, decoupling, low-noise designs, etc.)

<turning on the novice mode>

1. Avoiding: Avoid it, i.e., don't measure it if possible (shield your coil and cables).

<turning on the advanced mode>

2. Cancelling: Make the noise common mode to cancel it in a differential stage. Or use an anti-interference coil (figure-8 coil, or use a "reference" coil).

3. Band limitting: Don't measure out of interest bandwidth. You don't need it.

<turning on the expert mode>

4. Synchronizing: If the noise source isn't a gaussian nature (gaussian noise can't be modelled and predicted), then synchronize your measurement to the noise source to avoid the modulation of your measurement (the noise modulates your measurement). The noise source typically comes from the mains power source, radio stations carrier frequency, switched power supplies, CPU (program code dependent), other switching devices, etc.
Well, if the noise source is a gaussian nature, sh1t happens. You can't predict it, when it comes.

<turning on the Einstein/genius mode>
(to cope particularly with the evil gaussian noise)

5. Predicting: It's beyond the scope of most users now (even experts):
Use the out of interest band to cancel/predict the noise in the region of interest band. Some noise sources are wide band gaussian nature (like the wide band random pulses), which intersects with the band of interest region. The noise in the out of band correlates with the region of interest band.
Oh man!, you need a lot of cpu processing & brain power for this.

<turning off the brain demolution mode>

The list isn't complete. Just to give you an idea.


Aziz

[Sean_Goddard]

NOW you're on the right track Moodz!!!

[Tinkerer]

Tinkerer,

With the short TC target this shows exactly the opposite of what I expect! The ramp TX causes the 'on' eddies to have a max negative value at turn-off, yet the flyback eddies are lower than the normal TX, where the 'on' eddies have largely died out. However, the long TC responses look more like I expect.

- Carl

Hi Carl,

I am only learning how to do simulations. I am sure that much better simulations could be done.

So what I am doing, is to make a simulation that gives me results as close as possible to the results I have observed with real circuits.
As you said before, I do things backwards, normally one would make a simulation first and then build the circuit. However, I built many circuits to try to understand what is happening and now I do the simulations to see if the theory fits the results.

Below is another simulation.
With the same TX coil current I get twice the RX signal.
I kept getting this surprising result with a real circuit and it did not fit the theory.
Eventually I found a scientific article for a geological application that talked about the same effect. Unfortunately I have not been able to find the link to the article again to post it here,
but anyway, the simulation gives the same result as the real component circuit.

The simulation below has the same coil current, the same targets and the same trace colors.


Tinkerer

[Aziz]

( single ended high gain amplifier stages being an obvious problem )

Moodz is rising an interesting fact: Problems with single ended designs.
(I am making an advertisement for differential design now. Moodz know the benefits.)

Fortunately, PI designs fit perfectly into the fully balanced differential front-end amplifier applications. This means, differential input and differential (balanced) output.
You will have the differential integrator after this stage either. So you can subtract out a lot of common noise up to the integrator stage (included).
Oh yes, and if you use a differential coil, a lot better it is then.

Aziz

[Carl-NC]

Tinkerer,

With the short TC target this shows exactly the opposite of what I expect! The ramp TX causes the 'on' eddies to have a max negative value at turn-off, yet the flyback eddies are lower than the normal TX, where the 'on' eddies have largely died out. However, the long TC responses look more like I expect.

- Carl

[moodz]

*** Deleted at request of poster - sensitive information. ***

What's missing is fundamental design .... If you are touching the metalwork and getting a signal well no wonder it picks up noise from the CPU and probably everything else in the vicinity. PCB layouts are usually not the cause of that problem but some more fundamental error in the circuit itself ( single ended high gain amplifier stages being an obvious problem )

[Tinkerer]

It is true that the eddies from an unsettled turn-on will partially cancel the turn-off eddies, but I also doubt it is very significant. It would be interesting to test for this, by using a series resistor to flat-top the current on a wide TX pulse width, then removing the series R and reducing TX pulse width until the peak coil current at turn-off is the same as the flat-top current.

I tried some simulations with a Flattop and a Sawtooth TX ramp. The results correspond to real circuit responses, but are not yet accurate enough to answer the question of the cancelling of the TX ON eddy currents.

Attached are the simulations of a flattop TX and a Sawtooth TX of about the same coil current.
The coil current is the green trace.
As we can see, the target responses are quite different in amplitude.

The influence on the long TC target is different to the short TC target.

When we look at the target response, during the ON ramp, we can see how the eddy currents in the short TC target (pink trace) reaches a flattop at the end of the Sawtooth TX ramp. We have lots of eddy currents in the target.

With the Flattop TX ramp we see how the short TC target (pink trace) quickly reaches the peak and then decays to near zero at the time of switch OFF. The eddy currents are practically gone.

When we look at the signal amplitude after switch OFF, we see that the short TC target amplitude is significantly higher (184.36mA) with the Sawtooth TX ramp, than the Flattop TX ramp (147.50mA)

Observing the red trace, of a longer TC target, we can see the differences during the ON ramp as well as the difference in amplitude after switch OFF.

The effect on the different TC targets is different.

This simulation shows a lot, but it does not yet answer the question of the influence of the cancelling of the ON eddy currents, because there are still too many other factors that are different between the 2 simulations, nevertheless, the simulation can give some understanding how every little difference in the setup, will produce a difference in the results.
PI's are so simple yet so complex.

Tinkerer

[Sean_Goddard]

No, No expert mate, just pragmatic about what I THOUGHT I knew, which turned out to be JACK S@*T.

I thought "It should be REALLY easy to design a good deep PI" so after AMY different builds, ALL with **** poor performance, I decided to run some sims of existing machines to see what got. Pretty similar results as all the machine had pretty similar performance.

I looked at what I had and made a few changes, a few did sod all, BUT most did. the opposite to what I expected. SO much for theory!

SO I want back and did the OPPOSITE of what I thought would work and.....I started to get results.

Moving from simulation (Matlab, Simulink) to breadboard proved I was on the right track, so I do more of what I "wasn't doing" and even better results appeared. Then I over did it and it stopped working altogether.

Backing off a little got me to an "edge" where the machine was extreme in depth, but sometimes broke into oscillation, backing off 25% made it stable but lost a little depth. Refining what I was left with will be the machine I go with.

[golfnut]

You could try an LTSpice simulation to see if you could demonstrate a significant advantage to locating the cap near the coil.

Again I wonder if one advantage of putting the cap on the PCB is to filter out some cable noise.

There could be some advantage to putting a high-Q TX coil-cap tank in the search head to minimize the driving current in the cable -- that is, if you ever wanted to play with high-Q TX tanks, which may have some disadvantages such as ground sensitivity.

Sean your clearly a real expert.
Steve

[Sean_Goddard]

Actually it's BOTH!!

Going back to what I said, if the PCB's are designed properly then there should be much less of a problem. If this weren't the case, why do most of the units I've built from on here sound off when you tough parts of exposed metalwork and WHY even when using a genuine search head, do most of these machines have poor performance?

No criticism of the hard work people do, but SOMETHING MUST BE MISSING in the final analysis. That's a fact!

[Aziz]

*LOL*

No, no!

It's the coil, that picks up the noise.
*LOL*

Aziz

PS: µC noise isn't much an issue. Can be handled well.

[moodz]

*** Deleted at request of poster - sensitive information. ***

LOL. ... I would have to disagree ..... Of course digital circuits generate hash. ... The problem is that the analogue ccts pick this up ...oh I see what the problem is ... It's the analogue ccts . So really it's the analogue ccts not the CPU that is the problem.

Moodz