DEEPER PI DETECTION DEPTH

[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

[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 )

[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

[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

[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

[Sean_Goddard]

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

[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]

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]

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

[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

[simonbaker]

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

- I think it is a good point to distinguish EMI noise problems from circuit noise problems. Circuit noise problems follow the usual rules about focusing on the front-end components, making them as low-noise as possible.

- Certainly a good point about "synchronizing" to any predictable noise signals -- subtract them out, integrate them out, map them to a constant voltage (SD), etc.

The noise in the out of band correlates with the region of interest band.

- I don't think that is correct for gaussian noise. Maybe some other kind of noise.

- Band limiting is always a good idea, just have to watch out your filter doesn't distort your signal, or at least you know how to compensate.

- As for differential mode, that's great to knock down common mode noise such as the cable acting like an antenna. However, can it really help with noise picked up by the coil? There must be noise that looks just like the target signal, so you can't have it both ways -- block the noise, you block the target.

Not knowing anything, my impression of PI detectors is that the real challenge is calibration. It seems to boil down to picking off exact voltage points at exact time delays. Some kind of normalizing circuitry/algorithm would seem to be desirable to enable looking for smaller changes.

Cheers,

-SB

[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

SB,

here is the simulation of the Sawtooth TX and the TEM TX side by side. Both simulations use about the same TX peak current, but the TEM simulation recycles the current and uses only a fraction of the power that the Sawtooth TX uses.

Also attached are the LTSpice files of each.

As you can see on the .asc files, the difference in the schematic is minimal, but the TEM gives twice the signal amplitude and uses a fraction of the power.

The blue trace is the Flyback voltage.
The green trace is the coil current.
The red and pink traces are 2 targets with different TC.

Tinkerer

[Carl-NC]

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

Johnson noise applies to ALL types of resistors. Bulk carbon resistors have additional noise on top of that, carbon film are a little better, and metal film the best. But even metal film can't escape 4kTR.

- Carl

[Carl-NC]

Below is another simulation.
With the same TX coil current I get twice the RX signal.

Hmmm, looks to me like twice the TX current as well. Post #93 shows ~450mA single-ended, but #97 shows +/-450mA.

In any case, I am working on a circuit to measure this phenomenon. I'll post results next week.

- Carl

[Tinkerer]

Hmmm, looks to me like twice the TX current as well. Post #93 shows ~450mA single-ended, but #97 shows +/-450mA.

In any case, I am working on a circuit to measure this phenomenon. I'll post results next week.

- Carl

It is twice the same current, charge and discharge of the same capacitor.

With LTSpice, TEM.asc, I use the inductor L7 to measure the current. I click Control L7 to get the average current, it is about 8mA. For the Sawtooth TX it is about 26mA.

The slope, or rate of change is slower, so one would expect a lesser magnetic moment, but the target response shows that the longer slope produces more eddy currents.

There are several differences in the circuits. TEM allows for a much higher pulse repetition rate and results in a heavier coil to get low power losses.

It also gives very good FE discrimination.

Tinkerer