PI: Effects of pulse shape and transmit time.

[Tinkerer]

The attached symbol:CMF20120

[Teleno]

Tinkerer: Have you taken into account both the parasitic capacitance and resistance of the coil? I believe green had left them out.

[Qiaozhi]

Tinkerer: Have you taken into account both the parasitic capacitance and resistance of the coil? I believe green had left them out.

The LTSPICE inductor model allows these values to be passed as parameters. On the left of Green's schematic he shows these hidden values.
You can actually make these values visible by using <CTRL> with the right-hand mouse button to bring up a different dialog box. Then double-click in the right-hand column (under V) to change the visibility.

[Qiaozhi]

The attached symbol:CMF20120

Thanks, but there's now another problem.
It cannot find the "Cree_Z-FET_Models_Rev0p3.lib".

[Tinkerer]

Thanks, but there's now another problem.
It cannot find the "Cree_Z-FET_Models_Rev0p3.lib".

Here is the file.

[Tinkerer]

Tinkerer: Have you taken into account both the parasitic capacitance and resistance of the coil? I believe green had left them out.

Yes, the parasitic capacitance is very important. For this sim I used 200pF (rather on the low side) to include the coil inter-wire capacitance and the twisted pair cable and mosfet/diode capacitance. You can see the effect of this capacitance clearly in the red current trace, which goes down, then up and down again. Since the di/dt of the current is what kicks the target, this explains well how important the parasitic capacitance is.

The green voltage trace does not show anything of that.

While we are looking at the parasitic capacitance, we might try and find ways to reduce it, specially the coil to shield capacitance.

Green has been doing good work on that. Maybe, if we all put some effort into it, we find some good solutions?

Here is one idea:

A good way to reduce capacitance is to put the capacitances in series.

More ideas?

[Teleno]

Here is one idea:

A good way to reduce capacitance is to put the capacitances in series.

A Tx D-coil connected in series would have half the capacitance of each half D coil, but the total inductance will be larger than the sum.

It can also work during flyback for the components. If instead of one series diode you use two, their joint capacitance during flyback (reverse polarized) is halved.

The diode's capacitance gets higher as reverse voltage gets smaller. This technique has more effect as the coil voltage approaches zero, improving stability at the time of sampling.

[green]

While we have the sim at hand, we could look at the target response.

We see the TX eddy currents in the targets, to the time of TX switch Off and then the new eddy currents generated by the switch OFF di/dt.

v(n011) is the coil voltage decay

I(L2), ..L3, L4, L5, L6 are the responses of targets with TC's of 5us, 10us, 100us, 500us

We see how the short TC target eddy currents raise to a certain level during TX, but then start decaying again.
The long TC eddy currents keep increasing right to the time of switch OFF, because their TC is longer that the TX time.

We see that the switch Off must first destroy the negative eddy currents from the TX before it can build the positive eddy currents from the switch OFF.

Now, just one more hint:

When we use an IB coil, we can sample the TX eddy currents just before the TX switch OFF, invert the result and add it to the sample after switch OFF, obtaining an enhanced peak target response.

Of course, if we really want some more target information, there are many many more ways to extract this information from these target response curves.

If I plot the target during coil turn on with LTspice I get a similar response as you did in reply #44. If I plot the on response with a DD coil on the bench I get something different. Low tau targets have a higher response at turn off, high tau targets have a higher response at turn on. The turn on response is highest at the start, then decays instead of increasing. Any idea why? Been thinking about looking at the target during coil on and off times.

[Tinkerer]

If I plot the target during coil turn on with LTspice I get a similar response as you did in reply #44. If I plot the on response with a DD coil on the bench I get something different. Low tau targets have a higher response at turn off, high tau targets have a higher response at turn on. The turn on response is highest at the start, then decays instead of increasing. Any idea why? Been thinking about looking at the target during coil on and off times.

There are several significant to consider:

1) For the ON time, the TX current pulse shape is very important.
To get similar results, the pulse shape needs to be the same.

2) The simulation does not take into account the Earth's magnetic field.
Even in air tests, the Earth's magnetic field is always present. When we switch TX ON, the coil field changes the field vectors of the surrounding Earth field. Near the coil this generates a relative strong target response which then again decays.

3) The induction balance of a DD coil is not perfect.
This caused the switching noise and coil transients to be relatively more visible than the target response.

4) The ferrite core.
There is a very vast choice of ferrite cores available. A core might be optimized for a 50Hz frequency or 5MHz frequency.

Running out of time, will continue...........

[Qiaozhi]

2) The simulation does not take into account the Earth's magnetic field.
Even in air tests, the Earth's magnetic field is always present. When we switch TX ON, the coil field changes the field vectors of the surrounding Earth field. Near the coil this generates a relative strong target response which then again decays.

I think you'll find that the Earth's magnetic field has no effect in an air test, where the coil is stationary and the target is in motion. This is because the EF does not change relative to the coil, but remains fixed. Any eddy currents that are generated in the target, due its relative motion within the EF, will be so weak as to be undetectable.

[green]

I think you'll find that the Earth's magnetic field has no effect in an air test, where the coil is stationary and the target is in motion. This is because the EF does not change relative to the coil, but remains fixed. Any eddy currents that are generated in the target, due its relative motion within the EF, will be so weak as to be undetectable.

Don't know if it matters. All of my tests are static. Both the coil and target are stationary. The circuit is DC coupled, no capacitor coupling.

[Tinkerer]

I think you'll find that the Earth's magnetic field has no effect in an air test, where the coil is stationary and the target is in motion. This is because the EF does not change relative to the coil, but remains fixed. Any eddy currents that are generated in the target, due its relative motion within the EF, will be so weak as to be undetectable.

Moving a coil through the Earth's magnetic field generates a current in the coil. This is something very different from what I am talking about.

Let's see If I can explain what happens when the coil and the target are static:

We all including the coil and the target are surrounded and penetrated by the constant (nearly) magnetic field of the Earth. The field intensity is about 0.5 Gauss. No movement, no change, no eddy currents.

Now, let's assume we switch this field OFF. At the moment of switching OFF, eddy currents will be generated within everything that is permeated by the Earth's field.

OK, we can not switch the Earth's field OFF, but, within the sphere of the magnetic field that we generate with our coil, we change the field vectors of the Earth's field. Depending on the strength of our dipole coil field, which can be up to several Gauss, the Earth's field vectors will be totally displaced, deflected or enhanced.

When we switch the coil OFF, the Earth's field lines will return to their original alignment. In doing so, the Earth's field lines cut or move across the coil. We know that a coil moving across magnetic field lines generates current in the coil. It is the same when magnetic field lines move across a coil. The speed at which this happens, determines the amplitude of the current generated.

[Carl-NC]

Superposition applies to magnetics, and when analyzed this way the Earth field has no effect.

Looking at it from examples, if the coil generates +1 Gauss which is positively aligned with the 0.5g Earth field, then the net field is 1.5g. At turn-off the field returns to 0.5g, which is a transient of -1g.

If the coil generates +1 Gauss which is negatively aligned with the 0.5g Earth field, then the net field is -0.5g. At turn-off the field returns to 0.5g, which is a transient of +1g.

No matter how the TX field aligns with the Earth field, the transient field is always 1g.

[green]

There are several significant to consider:

1) For the ON time, the TX current pulse shape is very important.
To get similar results, the pulse shape needs to be the same.

2) The simulation does not take into account the Earth's magnetic field.
Even in air tests, the Earth's magnetic field is always present. When we switch TX ON, the coil field changes the field vectors of the surrounding Earth field. Near the coil this generates a relative strong target response which then again decays.

3) The induction balance of a DD coil is not perfect.
This caused the switching noise and coil transients to be relatively more visible than the target response.

4) The ferrite core.
There is a very vast choice of ferrite cores available. A core might be optimized for a 50Hz frequency or 5MHz frequency.

Running out of time, will continue...........

1) Any idea what the transmit pulse would have to look like to cause the on target signal to be opposite what we both get with the spice simulations, Or is something else causing it?
2) If there is an effect, doesn't balancing the coil for a zero signal cancel it out?(Missed Carl's reply while I was replying)
3)The switching transients are less than 3 usec long. The rest of the no target trace is a lot less than the target traces.
4)I have a couple different ferrites. Both have a similar response as some clay from the yard, maybe the same.

[Tinkerer]

Superposition applies to magnetics, and when analyzed this way the Earth field has no effect.

Looking at it from examples, if the coil generates +1 Gauss which is positively aligned with the 0.5g Earth field, then the net field is 1.5g. At turn-off the field returns to 0.5g, which is a transient of -1g.

If the coil generates +1 Gauss which is negatively aligned with the 0.5g Earth field, then the net field is -0.5g. At turn-off the field returns to 0.5g, which is a transient of +1g.

No matter how the TX field aligns with the Earth field, the transient field is always 1g.

In most places on earth, the EF is about parallel to the surface, while the coil field is perpendicular.
One way to check would be with a free floating compass needle that could align in the coil field.