Dual coil for mpp

[Qiaozhi]

Still trying to figure out what I'm doing wrong. Another test looking at MMP amplifier out TP3.
from godigit's reply #118 https://www.geotech1.com/forums/atta...9&d=1570936662

Don't see much if any effect at TP3 with small coil Rd open or 500R. Increasing external Rd causes overshoot to increase. My damped signal almost as good as godigit's except my takes about 10us, his 5us to come out of saturation. Any suggestions why or what I might try?

Measure the parameters of your coils, and use the equations in post #234 to calculate the correct values for the damping resistors.

Try running two simulations, one with only the external Rd in place, and the second with only the inner Rd. In both cases there is a lot of ringing. You need both resistors to quieten things down.

[Qiaozhi]

How low does Tx have to drop to for the MPP amplifier to come out of saturation?

It looks like (from simulation) that it's about 204mV with a gain of 30k.

[Carl-NC]

Here's my twopenn'orth contribution to this thread...

Nice analysis.

[godigit1]

Tank you so much Qiaozhi,
Ive been super sick but that made me feel much better.
Thanks for correcting the formulas Im still trying to learn proper math procedures.
Best Regards!!

[godigit1]

I agree your trace is better than mine. I can get mine to look like yours with lower damping resistance but it flattens out near 10us not 5us like yours. Only way I can get near 5us is to underdamp. My question, what am I doing wrong?

Hi green,
The way my coil is connected is, TX pos to the inside of the inner coil,
Damping resistor between pos and neg of the inner coil.
positive of the outer coil to neg of the inner coil.
neg of the outer coil to RX.

I found this arrangement faster than going from the outside of the coil in.

For damping I hitched up just the inner coil and damped it with a resistor set up to determine damping resistor value.
then put the coils together and final damp.
Our coil are super close you should get the same results.
My only thought is cable attachment to the coil. Maybe we have diffferent capacitance there.
Im using a belden 110 ohm digital audio cable.
Pleas try this method of damping the coil .

I will try the values Qiaozhi provided see if I can get better results.

[godigit1]

Hi Green,
Sorry for just getting to things I recieved your package saturday on time.

Hooked up the coil tonight as you have it wired now (inside to outside) first with a 449 ohm inner resistor then a 530 ohm and a 1 k ohm.
On my inductance meter with the inner damping resistor installed I read the coil at 290uh and 3.8 ohms resistance.

So with no futher adue I will show the scope shots,

This is the 1 k inner and it damped out to 479 on the final damping resistor set up.



With the 530 and 449 ohm inner resistors I damped to the 449 inner resistor and used the same final damping value for the 530 ohm.
It damped at 1.585 K. Both were within .2us of each other so ill show the best the 449 ohm.



So Im seeing things a bit faster than the 10 us you were getting, Im not sure if its the cable totally yet.
Not sure about the twisted pair included was this your test cable?

The coil is fully shielded as you sent it and looks to still be very fast. Im interested to see whats inside but want to know if there is anything specific you would like done before I open it up and seperate the coils for testing?

Im able to get a very nice stable threshold at the lower inner damping values but once and a while it gets chatty with the inner 1k resistor not too bad.
I was actually expecting and hoping to see a little more speed out of the 28 awg vs the 30 awg and it apppears to be so at this point.

Thinking a litz spiral dual field could be pretty sweet?

[green]

Hi Green,
Sorry for just getting to things I recieved your package saturday on time.

Hooked up the coil tonight as you have it wired now (inside to outside) first with a 449 ohm inner resistor then a 530 ohm and a 1 k ohm.
On my inductance meter with the inner damping resistor installed I read the coil at 290uh and 3.8 ohms resistance.

So with no futher adue I will show the scope shots,

This is the 1 k inner and it damped out to 479 on the final damping resistor set up.

[ATTACH]48324[/ATTACH]

With the 530 and 449 ohm inner resistors I damped to the 449 inner resistor and used the same final damping value for the 530 ohm.
It damped at 1.585 K. Both were within .2us of each other so ill show the best the 449 ohm.

[ATTACH]48325[/ATTACH]

So Im seeing things a bit faster than the 10 us you were getting, Im not sure if its the cable totally yet.
Not sure about the twisted pair included was this your test cable?

The coil is fully shielded as you sent it and looks to still be very fast. Im interested to see whats inside but want to know if there is anything specific you would like done before I open it up and seperate the coils for testing?

Im able to get a very nice stable threshold at the lower inner damping values but once and a while it gets chatty with the inner 1k resistor not too bad.
I was actually expecting and hoping to see a little more speed out of the 28 awg vs the 30 awg and it apppears to be so at this point.

Thinking a litz spiral dual field could be pretty sweet?

Hi godigit, thanks for the testing.
Your first picture is similar to what I got when trying to critical damp. Second picture a lot better. The twisted pair is what I used for lead. See what you get with the twisted pair. The coil is just taped together, open it, do what ever you can think to try. Can be put back together.

[godigit1]

Hi Green,
Sorry for dragging this out rough week behind me.

First I forgot to thank you in the last post for sending your coil. Its been interesting having a shielded coil assembly to test.
Im just starting to learn more about shielding. Thank you

So Ive been trying different methods of damping and inner coil damping resistor values and trying to find a way to view the results on the inner coil once fully assembled.

I made a statement at the beginning of the thread that it was easy to damp the dual field coil. My method of damping the inner coil seperatly works good but turns out there is a bit more to the dual field.
It comes down to how aggressively you damp the inner coil.

In post # 182 dbanner describes what the results end goal of what the damping should be not just a test method.

"A circuit with a value of resistor that causes it to be just on the edge of ringing is called critically damped. Either side of critically damped are described as underdamped (ringing happens) and overdamped (ringing is suppressed).
The critically damped response represents the circuit response that decays in the fastest possible time without going into oscillation."

This is what I consider a aggressive damping stratagy for the fastest speed a coil is capable of. Ive always damped my coils with this mind set as well.
When I originally damped my dual field inner coil I damped to the fast side and noticed ringing when a target was introduced. Less agressive damping of the inner coil seemed to fix it, I can now show it by the end of this post, I hope.

With that said I want to add that once the two coils are joined you can change the inner resistor around willie nillie and still damp the final coil at the op amp and have it look perfect. The inner coil is hidden and compensated by the outer coil. the higher the damping on the inner coil the lower on the outer and visa versa>>>
Ive poured back through the thread and think Ive maybe found a differance in our inner coil damping set up.

Dbanner describes the inner coil should be connected as per patent then the inner coil damped first.
Im not sure if you did it that way as well or not.
But the way I see it I read the patent as the coil should be damped by its own merets. So in my testing I have seperated the inner coil and damped it alone to get the Rd values Ive used.

Ive found a way to check the inner coil after assembly of both coils.
At least as best I can.

For this test I first seperated the coils,
Inner coil Rd values were determined connected to the MPP with a standard damping set up.
A criticle damping at 236 ohms and aggressive damping resistor value at 328 ohms was chosen.
I also threw in a 1k wild card inner resistor as it will prove to be important to the end results.

I will show pictures of the complete 1 k results and the final results for the 230 and 330 ohm resistors to keep the pictures down.
And I have to appologize my phone is fllipping the pics sideways again.

Here is the inner coil seperated and shots of the op amp at 230 ohms and 330 ohms.







Here are shots of the complete 1 k tests with the coil fully assembled.
I will use the raw coil flyback of both coils for the first part of the test.

1k inner resistor criticly damped



Fly back pulse doesnt look too bad.



But if you zoom in the first signs of trouble, the inner coil looks underdamped but not too horrble.



Here is the same set up aggressively damped notice the speed differance.



Now looking at the flyback both coils are loooking underdamped.

.

The exact tests were done on the 230 and 330 ohm inner Rd.

for the next half of the test I remove the large coil from series by disconnecting it from the RX only. Then attach the rx to the small coil and remove the MPP damping resistor.
This way we can get a look at what the inner coil is doing at the op amp.




Have to continue on another post too many attachments. Missed it by 2. sorry

[godigit1]

Here we go now>>







So by these three end shots I choose the 230 ohm inner damping value.

Here is the coil speed fnally damped with the 230 ohm inner resistor.


this is conservative and I can push it to 4.6 us

Any way you look at it this is a fast coil, with your shielding the speed hardly changed if at all really.
I checked shielding conductivity and its good 1 to 2 k depending where I probe.
I have first sample set at 5 us on the Mpp and the coil is operating stable very stable for my bad test inviroment.

And this brings me to my last picture.
I tested the twisted pair leads and not too much of a speed differance maybe 1us. But its a good example of cable shielding and how bad my test inviroment is.


After Srf testing I would like to send back your coil with a cable and resistors I used to see if you can duplicate the results. If thats ok I think it would be good to double check me and or the Mpps maybe a bum chip or something if were still not nuts on.

[6666]

For post #248 most of the images are Invalid Attachment specified

[godigit1]

Thanks 6666 what a mess.
Lost internet in the middle of my post last night must have been it.
Im just going to paste the whole post below dont know what else to do.
Sorry Everyone

[godigit1]

Hi Green,
Sorry for dragging this out rough week behind me.

First I forgot to thank you in the last post for sending your coil. Its been interesting having a shielded coil assembly to test.
Im just starting to learn more about shielding. Thank you

So Ive been trying different methods of damping and inner coil damping resistor values and trying to find a way to view the results on the inner coil once fully assembled.

I made a statement at the beginning of the thread that it was easy to damp the dual field coil. My method of damping the inner coil seperatly works good but turns out there is a bit more to the dual field.
It comes down to how aggressively you damp the inner coil.

In post # 182 dbanner describes what the results end goal of what the damping should be not just a test method.

"A circuit with a value of resistor that causes it to be just on the edge of ringing is called critically damped. Either side of critically damped are described as underdamped (ringing happens) and overdamped (ringing is suppressed).
The critically damped response represents the circuit response that decays in the fastest possible time without going into oscillation."

This is what I consider a aggressive damping stratagy for the fastest speed a coil is capable of. Ive always damped my coils with this mind set as well.
When I originally damped my dual field inner coil I damped to the fast side and noticed ringing when a target was introduced. Less agressive damping of the inner coil seemed to fix it, I can now show it by the end of this post, I hope.

With that said I want to add that once the two coils are joined you can change the inner resistor around willie nillie and still damp the final coil at the op amp and have it look perfect. The inner coil is hidden and compensated by the outer coil. the higher the damping on the inner coil the lower on the outer and visa versa>>>
Ive poured back through the thread and think Ive maybe found a differance in our inner coil damping set up.

Dbanner describes the inner coil should be connected as per patent then the inner coil damped first.
Im not sure if you did it that way as well or not.
But the way I see it I read the patent as the coil should be damped by its own merets. So in my testing I have seperated the inner coil and damped it alone to get the Rd values Ive used.

Ive found a way to check the inner coil after assembly of both coils.
At least as best I can.

For this test I first seperated the coils,
Inner coil Rd values were determined connected to the MPP with a standard damping set up.
A criticle damping at 236 ohms and aggressive damping resistor value at 328 ohms was chosen.
I also threw in a 1k wild card inner resistor as it will prove to be important to the end results.

I will show pictures of the complete 1 k results and the final results for the 230 and 330 ohm resistors to keep the pictures down.
And I have to appologize my phone is fllipping the pics sideways again.

Here is the inner coil seperated and shots of the op amp at 230 ohms and 330 ohms.



Here are shots of the complete 1 k tests with the coil fully assembled.
I will use the raw coil flyback of both coils for the first part of the test.

1k inner resistor criticly damped


Fly back pulse doesnt look too bad.


But if you zoom in the first signs of trouble, the inner coil looks underdamped but not too horrble.



Here is the same set up aggressively damped notice the speed differance.


Now looking at the flyback both coils are loooking underdamped.



The exact tests were done on the 230 and 330 ohm inner Rd.

for the next half of the test I remove the large coil from series by disconnecting it from the RX only. Then attach the rx to the small coil and remove the MPP damping resistor.
This way we can get a look at what the inner coil is doing at the op amp.



this post from here all the pictures are in post #249

[godigit1]

Another thing to mention and should be taken into consideration for anyone who wants to make a dual field coil.

When testing flyback voltages there is a differance between the coils. Both are 115 uh so would think that if everything was perfect the voltages would match.
right now its at a 2 to 1 ratio outside to inside.
Outside coil 300 volts
inside 176 volts

Things to consider , adjusting inductance of the inner coil or posssibly coil spacing.

When the size of these coils was chosen it was scaled from a 12 " dual field.
I thought per the patent they were planar coils but they are bundle. maybe the planar coils need tighter coupling.
Just saying.

[dbanner]

godigit1, your experiments with dual field setup have been quite invaluable. I believe your conclusion of damping each coil on its own merits makes perfect sense. However, it should be clear that using both coils simultaneously in a dual coil arrangement introduces some subtle factors which affect the overall calculations. It is obvious that there is some reflective coupling effects between the coils which impact, however small, on the final damping values for both inner and outer. It requires some tweaking to get the best possible response.

Perhaps the best comparitive analysis might only be had by opening up a Whites dual field search loop (to have electrical access to the inner coil for scope probing), and doing scope measurements and srf etc, etc. in order to have a baseline of data for comparison. It might even be the case that a self made dual field coil might perform better if wound planar instead of jumble.

[godigit1]

godigit1, your experiments with dual field setup have been quite invaluable. I believe your conclusion of damping each coil on its own merits makes perfect sense. However, it should be clear that using both coils simultaneously in a dual coil arrangement introduces some subtle factors which affect the overall calculations. It is obvious that there is some reflective coupling effects between the coils which impact, however small, on the final damping values for both inner and outer. It requires some tweaking to get the best possible response.

Perhaps the best comparitive analysis might only be had by opening up a Whites dual field search loop (to have electrical access to the inner coil for scope probing), and doing scope measurements and srf etc, etc. in order to have a baseline of data for comparison. It might even be the case that a self made dual field coil might perform better if wound planar instead of jumble.

I agree I would like to see and measure the inside of a dual field, but not just curious enough yet to part one of mine.
I will say that the planar coil is more sensative to small gold but its a smaller coil as well.

I looked back 4 years ago on my completed Mpp shots and the 12 inch is sampling in range of 7us on a MPP with no upgraded amps.
It damped at exactly 681 ohms , The coil is no slouch, at 7 us.
On a MPP with upgraded amps its reducing sampling speed by almost 2us.
I have to and will revisit the 12 inch on my bench Pi and see if it damps faster. also a way to validate the op amps.

I cannot find the thread you found with the 7.5 inch apart. What was wrong with the coil? why did Jose take it apart?
I have to admit my first reaction was to think thats not even a whites coil.
I belive it is but I was really suprised to see how to me it looks like sloppy assembly practices and I dont understand tying the inner coil bundle together with the leads.
What no tape am I missing something is there a reason for doing that maybe coil tuning? fat solder joints ect,

Looking at that I see room for improvement of the dual field design, Litz wire a bundle wound threaded like the coilteck coils do and no knots on the leads should speed it up I would think.

Damping the dual field by performance is not too hard but seeing the results of what you have done on the inner once coupled is.
The raw coil flyback is the only tool Ive been able to come up with to monitor this.

My findings are with a damping resistor set up,
That if the inner coil is damped as fast as possible when the coils are coupled it will ring, but when hit with a target can you see it pop in there on the scope.
If you damp the inner coil on the heavy side when you first get a nice Knee at the op amp, the final product is slightly slower.
The optimal Ive found is to hit the mid point. It looks the best on the scope and tests the best with the targets Ive used.
Oddly enough there is a 100 ohm range there?????? not like the regular monos ive tested. Thinking maybe beacuse of the low inductance when seperate???

Not to say Im taking every aspect of the coupling into effect but I can say with confidence that anyone who wants to build a dual field can damp this way and be happy with the performance.

I will reviste the 12" and redamp I can see my inexperiance from 4 years ago in my old shots.
May take a bit everything is in SRF mode right now .