Waveforms + Measurements

[Tinkerer]

This is basically an induction balanced DD coil configuration.

Present a crown cork in the vertical position to the coil and observe the points in time:
20us, 80us, 120us, 140us.

Present the crown cork in the flat position and observe the same points.

Keep the target at a distance that does not saturate the preamp.

Repeat the sequence above with your 1$ coin.

Have a lot of fun.

Tinkerer

[WM6]

Very crude but here are the results of using induction balance using two differential coils.


The flyback decay is improved over the monocoil.


The coils as positioned for "null"

Crude really for us mono lowers. Can you post picture of signal response with some targets?

[moodz]

A quick test of Induction Balance.

Very crude but here are the results of using induction balance using two differential coils.

It was very easy to position the coils for a null. The large 18 inch coil drives the input to the diff amp. The smaller coil ( also a diff coil ) is the TX. The "centre tap" of each diff coil was connected to analogue ground ... which in my configuration is +12 volts ( battery pos ).

The amp gain was 100 as in monocoil results.

This configuration is significantly more sensitive than the monocoil .... however it is now also more sensitive to ferrites as well as conductive targets ... the monocoil configuration is not very sensitive to ferrites.

The flyback decay is improved over the monocoil.

moodz


Green trace = output of diff amp ... gain = 100 ... not even overloading !!
White trace is MOSFET gate drive.

The coils as positioned for "null"

[moodz]

But the ground should have a significant effect. The high mineral ground can change the inductance of the coil and hence the delay time. Of course, it's impedance too.

Aziz

True ... I will have to do more tests ... on real ground. I have 3 Kg of pure iron oxide and it shows no effect at all, also ferrites etc no effect. Only conductive targets show a response.

regards,

moodz

[Aziz]

adding a sheild has no effect as the balanced flyback +/- 500 volts cancels out and there is no ground effect as there is no capacitive coupling to ground.

The ultra short pulse is a result of the ruthruff configuration ... delayed signal and undelayed signal is fed to diff amp ... see CL Ruthruff description of balun. The presence of target in "core" changes permeability which changes delay.

moodz

But the ground should have a significant effect. The high mineral ground can change the inductance of the coil and hence the delay time. Of course, it's impedance too.

Aziz

[moodz]

adding a sheild has no effect as the balanced flyback +/- 500 volts cancels out and there is no ground effect as there is no capacitive coupling to ground.

The ultra short pulse is a result of the ruthruff configuration ... delayed signal and undelayed signal is fed to diff amp ... see CL Ruthruff description of balun. The presence of target in "core" changes permeability which changes delay.

moodz

[Aziz]

Aziz ... I agree with your analysis however is only valid for low frequency. The use of twisted pair ( bifilar ) in the differential coil actually behaves as a type of transmission line transformer. ( see my other post in theory section ... Ruthroff voltage balun )
The PI pulse is broadband and the results so far are not explained by analysis using lumped constants ... there is a whole new field opening here for detectors coils based on different configurations of coiled transmission lines where a broadband transformer is formed with the target / ground as the core / coupled element.
Some of my latest experimental results confirm this ... we have to move away from the simple inductive narrowband "transformer" analysis.

moodz.

Hi moodz,

I suspect, that trying to process this ultra-short period wouldn't work in the field. You will have much higher ground effects.

Did you try it with a shielded coil? Does it behave the same?
Aziz

[moodz]

No , this is not correct.

The imbalance is caused due to parasitic capacitance from to the TX mosfet. There are two parasitic capacitances, which will be seen by the active coil half:
Drain-Gate and Drain-Source path. The other coil half will see a different capacitive load however.

To make the capacitive load nearly symmetric, a symmetric TX Driver stage would be good. Even the other half is not active.

A center-tapped coil will have equal inductance coil halfs. The mutual inductances are of same amount.
Ltot = Lh1 + Lh2 + 2*k*sqrt(Lh1*Lh2),
Lh=Lh1=Lh2, k=1 (coupling coefficient)
-> Ltot = 4*Lh

Aziz

Aziz ... I agree with your analysis however is only valid for low frequency. The use of twisted pair ( bifilar ) in the differential coil actually behaves as a type of transmission line transformer. ( see my other post in theory section ... Ruthroff voltage balun )
The PI pulse is broadband and the results so far are not explained by analysis using lumped constants ... there is a whole new field opening here for detectors coils based on different configurations of coiled transmission lines where a broadband transformer is formed with the target / ground as the core / coupled element.
Some of my latest experimental results confirm this ... we have to move away from the simple inductive narrowband "transformer" analysis.

moodz.

[Aziz]

The reason for the imbalance is because the inductances on each side of the differential coil are not equal. If you center tapped the coil (at ½ the total turns) then you have a turns ratio of 2:1 and an inductance ratio of 4:1 for the two sections of the autotransformer. If your desire is a balanced flyback signal on each leg of the differential coil, the center-tap should occur at .707 of the total turns giving an inductance ratio of 2:1.

No , this is not correct.

The imbalance is caused due to parasitic capacitance from to the TX mosfet. There are two parasitic capacitances, which will be seen by the active coil half:
Drain-Gate and Drain-Source path. The other coil half will see a different capacitive load however.

To make the capacitive load nearly symmetric, a symmetric TX Driver stage would be good. Even the other half is not active.

A center-tapped coil will have equal inductance coil halfs. The mutual inductances are of same amount.
Ltot = Lh1 + Lh2 + 2*k*sqrt(Lh1*Lh2),
Lh=Lh1=Lh2, k=1 (coupling coefficient)
-> Ltot = 4*Lh

Aziz

[KingJL]

Hi KingJL,

it is not clear to me how to explain this number 707?

Please more explanation!

Lets say that total inductance is 600uh... You want 300uh on each side of the tap(1/2 the total inductance). Turns ratio is square root of inductance ratio. Square root of .5 is .707. Try it... you'll like it. Another way to look at it is that total windings is 1.414 time the primary winding. The primary winding is driven by the TX MOSFET.

I made the same mistake and did not realize what I had done until I really delved into why I had such an imbalance on the differential legs. You have to center-tap the inductance not the turns. It is the inductance that generates the flyback pulse with the collapse of the electromagnetic field. It is the flyback and subsequent decay that you want balanced to the input of the diff amp!

Regards,
JLK

[Tinkerer]

The reason for the imbalance is because the inductances on each side of the differential coil are not equal. If you center tapped the coil (at ½ the total turns) then you have a turns ratio of 2:1 and an inductance ratio of 4:1 for the two sections of the autotransformer. If your desire is a balanced flyback signal on each leg of the differential coil, the center-tap should occur at .707 of the total turns giving an inductance ratio of 2:1.

This is an interesting observation.
Could we go back to the coil building and redesign it?
I want to try the differential coil with my differential front end, since many are asking for a mono coil system.

What are your suggestions for an experimental differential coil winding and TX circuit?

All the best

Tinkerer

[WM6]

, the center-tap should occur at .707 of the total turns giving an inductance ratio of 2:1.

Hi KingJL,

it is not clear to me how to explain this number 707?

Please more explanation!

[KingJL]

The inputs to the differential amplifier are fed off either side of the differential coil which will + and - with respect to the centre tap analogue ground.
To investigate this I disconnected the negative input to the amp and connected it analogue ground. The amplifier was now only amplifying the signal at the + input side of the coil. I could see that the "huge" signal I am getting for the target is right at the base of the decay which is practically vertical voltage drop. The diff amp will try to amplify the difference at the inputs ... somewhat obvious. If the decay on one input is slight slower than the other there will be a relatively huge voltage differential this is what is being amplified ... a slight difference in the + and - decays caused by the target.
How is this .... ??? ..... the collapsing magnetic field of the TX coil will cause an equal and opposite flyback in the two sides of the diff coil. However the collapsing magnetic field of a target will be opposed to the main field ... so it will induce a very small decrease of field in the one coil and a very small increase in field in the other coil thus producing an ever so slightly different rate of decay in the coils and a shift in balance which the diff amp detects.
Because the waveforms at each input are falling at a huge dV/dt at this measure point it only takes a few nano seconds of shift to generate 10s of mv of difference between the two diff signals thus relatively small targets produce relatively huge ouputs. By varying the damping resistor in the way I had it connected I was effectively balancing the two coils which even a small coin would unbalance to produce a target response. If the two coils are each damped independantly the decays of each side may be balanced by varying one side to produce a balanced condition. This is what I inadvertantly achieved with my variable damper which had an offset resistance.

The reason for the imbalance is because the inductances on each side of the differential coil are not equal. If you center tapped the coil (at ½ the total turns) then you have a turns ratio of 2:1 and an inductance ratio of 4:1 for the two sections of the autotransformer. If your desire is a balanced flyback signal on each leg of the differential coil, the center-tap should occur at .707 of the total turns giving an inductance ratio of 2:1.

[Tinkerer]

...how does a relatively small target such as a 2 cm coin change this shape by 2 volts amplitude variation ????

Any target changes the Flyback wave form in its total life time. It is just a matter of finding ways to look at it, and read the information that these changes in wave form can tell us.

When we clip the wave form and distort the shape with the diodes and opamp saturation, it makes it a bit more difficult to see the pertinent information about the target, but some of it is still there.

I think that your differential "mono coil" and preamp is a great idea and it is worthwhile to spend time on it to see how far ahead it can take PI development.

Tinkerer

[moodz]

My guess is that the squiggle at the bottom of the Flyback is the reverse recovery of the diodes. If you try diodes of different recovery type, fast and hard or soft and slow, you will see a difference in the signal shape.

Tinkerer

...how does a relatively small target such as a 2 cm coin change this shape by 2 volts amplitude variation ????