Dual coil for mpp

[green]

godigit, what do you see if you place the loop on the operating coil without the test coil?

I see the tx cycle .

I re-wired the coil inside to outside and redamped. the whole coil.
It came out just a hair faster than outside to inside. Performance seemed the same. Thats it for the predamped tests now I will try the calculated damping resistor on the coil see how close it is.
Final damp went from 568 ohms to 737.

My test inviroment here is very bad. On a coil tested else where and here Im loosing 2 to 3 inches in air tests. Holding my porbe in the air I get a .56Mv mains signal and something else there too.

Im going to disassymble the coil to recheck it with the calculated resistor and I will try some more SRF measurements then.
I thought I read that 2 Mhz was the min for SRF testing with a signal generator.
Even if the one I made doesnt work I have other plans for it. I set it up with a 4 comparitor chip I can run at least two I think and have a double pulsing 10,000pps front end.

I dont know why we are getting such different measurements we are not magicians we take pictures of the results.
Other than we are not actually comparing apples with apples .
what Rigol do you have my scope is the 70 MHz 2 channel.
There have been scope updates for some of the rigol scopes but i can find none for mine so I think its good.
Works fine for everything else.
Maybe your scope has different impediance than mine , Im just trying to narrow things down.

Why do you want a lower SRF?

I'm just trying to figure out why I get 6.7MHz SRF and you get 1.3MHz SRF for the small coil. Don't think AWG30 would be much different than AWG28. I have a RIGOL 1052E 50MHz scope, don't think our scopes could cause the difference. I twisted the wires to see what effect it would have measuring SRF, would have to make a lot of twists to get resonance near 1.3MHz. Maybe someone else has a guess at why our SRF measurements are a lot different. Your picture of the small coil looks like a flat spiral with no spacing?

Looked at your picture reply #72 again. Scope leads were connected to coil, not a loop. Tried SRF measurement with my leads connected to the small coil(probe x1)reads 1.35MHz about the same as you. Try the loop when you disassemble your coils.

[green]

Made the large coil.

[green]

Was wondering what the SRF for the small coil would read with the signal generator method vs PI excited method. Don' have a signal generator so tried with spice. PI excited method is closer for the 115uH coil(small coil), both methods the same for the other two coils(300uH).

[godigit1]

Just ran a couple quick tests.
The small coil is harder to excite using a live pi than the larger coils.
To get it to resonate in a way I can measure it, I have to have the probe at 1 times and distance the coil away frow the Live coil to reduce amplitude enough to measure it.
On the larger coils Im able to use the 10 x on the probe and tape directly to the live coil.

The small coil will not resonate at 10x probe setting, I start seeing the flyback pulse some ringing but it is damped to much to get a measurement.

I tried a 6" loop of 26 awg and Im getting only the TXcycle, Im not getting SRF at all. I tried 1x 10 times coil shorted and open.
I only get a inductive look at the Tx Cycle.

When pulsing with the coil attached to a live Pi and both the damping resistor and input resistor removed, I can measure just using the probe as a loop and see resonation .
But I get the same measurement as live pulsing.

Im wondering exactly how you made your loop.
Updates to windows 10 have left me unable to save pictures I keep trying they just dissapear even my phone changed and flipps all the pictures now.
I made my 6" wire loop out of 26 awg magnet wire with no twists and attached to pos and neg of the probe.


.But I really think its all a probe issue. I think the Rigol probes are of a much nigher impediance that the ones you are using now.
The weird thing is I get allot of coils hitting at 1.389Mhz Its too much a coincidence.
Thinking it has to be a probe Issue.

[godigit1]

Hi Green,
Also forgot to mention.
your scope is upgradable to 100Mhz with a software download.
I saw this on you tube but I have not seen it for my scope.
Didnt know if you had seen that or not.
Best

[green]

Hi godigit, some pictures of my measurements. PI Tx coil_8inch critical damped, size not critical but coil should be critical damped. Used about 2inch spacer(about a 2inch spacer seems to be best)between TX and test coil(roll of masking tape)piece of foam worked too. RIGOL probe x1, ground lead connected to probe making a loop. Probe not connected to anything. Scope pictures from probe loop.

I made a loop(one turn)of wire because the RIGOL probe didn't work the first time I tried the other day. Found my ground lead had opened(if you look at pictures you can see solder joint where I repaired the lead). RIGOL probe works as good as my other probes.

[godigit1]

Hi Green,

I set up pretty much exactly as you and Im able to get better SRF but the wave forms I get still look damped.
Im not sure whats up with that but both coils together look better.

Here is the test set up And shot of the damped 6.5"Id coil.



Small coil SRF is now 6.667Mhz


The full waveform


Large coil SRF is now 4.808Mhz


Coils combined SRF Is 4.545Mhz



When I originally tried this I was using my dual field coil I couldnt get SRF.
When I saw you were using a unshielded coil I switched to the same .
With the unshielded coil Im at least getting SRF.
Im not sure if thats fully the culprit as I do not get the same wave form as your getting.

[green]

Hi Green,

I set up pretty much exactly as you and Im able to get better SRF but the wave forms I get still look damped.
Im not sure whats up with that but both coils together look better.

Here is the test set up And shot of the damped 6.5"Id coil.
[ATTACH]47719[/ATTACH]
[ATTACH]47720[/ATTACH]

Small coil SRF is now 6.667Mhz

[ATTACH]47721[/ATTACH]
The full waveform
[ATTACH]47722[/ATTACH]

Large coil SRF is now 4.808Mhz
[ATTACH]47723[/ATTACH]

Coils combined SRF Is 4.545Mhz
[ATTACH]47724[/ATTACH]
[ATTACH]47725[/ATTACH]

When I originally tried this I was using my dual field coil I couldnt get SRF.
When I saw you were using a unshielded coil I switched to the same .
With the unshielded coil Im at least getting SRF.
Im not sure if thats fully the culprit as I do not get the same wave form as your getting.

Think my Tx coil is faster than yours. Mine decays in a little over 500ns, yours maybe 3us. We are getting the same answers now, the picture looks different. If you sampled the oscillation at 5us the waveform wouldn't be damped.

[godigit1]

Our coils are acting different.
On my inside coil I cant even measure at 5 Us the oscillations are so small and on a declining scale I have to get way out on the tail at 100ns to line up enough to even measure.
Have you hooked your coils up and damped yet?
Try the small coil first and damp as you normally would a mono coil. and then try the calculated value or visa versa.
That is where I am at ready to try the calculated value.

Im showing my finished coil at the op amp as fully sampleable at less than 5 us.
Current shot is from the coil fly back and op amp with the inner coil damped seperatly on a PI and wired inside to outside.
I picked up almost 2 us on the coil versus wireing outside to inside.


Am I looking at things wrong ? I see this coil as sampleable at 3 us on the bench.

[godigit1]

There is over a three time variation between live pulsing and the probe loop method.
I think we need to be double checked by a signal genny.

[green]

There is over a three time variation between live pulsing and the probe loop method.
I think we need to be double checked by a signal genny.

Reply #93 I did a comparison of SRF measurement with loop pickup PI excited and using signal generator. Both methods were close to calculated SRF with higher Cpar values. Signal generator method reads lower when Cpar was 5pf. The 1pf series capacitor effected the reading, changing it to .1pf in the simulation gives a SRF close to calculated. I had thought coupling between PI coil and test coil might effect SRF measurement. Does have a very small effect with spice. Spacing appears to have a very small effect with actual testing. Haven't seen anything that suggests signal generator method is better. Maybe someone has a different opinion and could explain why signal generator method might be better or something wrong with my simulation.

What do you mean by live pulsing?

[green]

On my inside coil I cant even measure at 5 Us the oscillations are so small and on a declining scale I have to get way out on the tail at 100ns to line up enough to even measure. https://www.geotech1.com/forums/attachment.phpattachmentid=47722&d=1570252346 (your small coil)

Try Y position so center screen is near full Tx decay(2.5 to 3us after Tx off)set Y scale to maybe 100mV/div and X scale to 50ns/div.

Don't have a MPP, been playing with spice. Will send coils for measurement comparison when I get done playing.

[dbanner]

I take live pulsing to mean taking the readings off a directly driven coil, whereas the probe loop method induces a current/voltage. Maybe there is some small but significant difference in the results between the two methods, the question is what can this be attributed to, and how can it be rationalised. At first glance, I would say that coupling, however small, can affect the results, Just as it would in a transformer.

[godigit1]

What I meant by live pulsing was that I am exciting the coil with another Operating PI coil the probe directly hooked to pos and neg of the coil.
Ive been taping the test coil directly to the bottom of a whites dual field.
After recent tests on a unshielded coil, Im thinking that the dual field is pullng the coils down due to it being shielded<,I think its partially loading the coils down Could this be correct?
As well as dbanner is suggesting about the probes effecting the reading .

The reason I was wanting to try the signal generator method was mainly to learn Im still in that imperical mode where I need to see touch and feel to learn.
Im not spice savvey or anywhere close to being able to design complicated circuts.
Plus Im thinking once a signal genny set up is made I can just clip any coil up.

Green,
I have not tried the scope set up you suggested yet , I will try again after I do one last damping test.
Hooked the small coil up to damp on the pi and double check my last resistor value before trying the calculated resistor form the whites pattent formula.
Blew my damping set up playing around too long.
If any one damps the inner coil seperatly you want to monitor your flyback or turn down your on time as the small coil can put out some voltage .

[green]

I take live pulsing to mean taking the readings off a directly driven coil, whereas the probe loop method induces a current/voltage. Maybe there is some small but significant difference in the results between the two methods, the question is what can this be attributed to, and how can it be rationalised.

When I first tried the loop method I thought coupling between the PI Tx coil and the test coil might be a problem. Very small effect with my tests. When coil is connected to signal generator circuit, a PI Tx circuit or probe connected across test coil while exciting with a PI Tx the circuit effects SRF. There would be a very large difference in SRF if the coil was connected to a PI Tx circuit without the series diode, not as much with the diode included. Think 6.7MHz is correct SRF.
SRF
6.7MHz____PI coil exciting small coil, loop pickup
3,87MHz___small coil connected to Tx with diode, loop pickup
2.82MHz___small coil connected to Tx with diode, x10 probe across coil
550kHz____small coil connected to Tx no diode, x10 probe across coil
1.3MHz____PI coil exciting small coil, x1 probe across small coil
3MHz_____ PI coil exciting small coil, x10 probe across small coil