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Author Topic: Dally, Shark & Ruslan workbench  (Read 71543 times)

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Itsu,

I found another video about pulse amplifier.
There is slightly more info about tuning.
Here is a transcript. May be it helps.

Vasik


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Thanks Vasik,


i had seen that video before, thats where i got the info to set the pulse duty cycle on the FG to around 73% to get the correct drain signals, but the translated text helps greatly.

Itsu
   

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I do still have my doubts if this last test really shows the standing wave resonance and not again the LC resonance.
It's good to have doubts.

You should perform a self-educational experiment by taking a looooong piece of 50Ω coax and setting it up for a TDR measurement. Verify that the classic pulse reflection is visible when the coax is shorted and opened at the far end.
Next, connect a parallel combination of a ferrite torroidal coil and a good cap instead of the open and short termination at the far end. Verify that the TDR experiment still works. The LC resonant frequency of the coil&cap does not matter as long as it is different from that round trip reflection in the coax.

While you still measure the pulse reflections from the far end of the coax in the TDR manner, add another stimulating frequency* that will resonate the lumped LC circuit connected at the end of the coax.  Can you discern the round trip reflection in the coax from the response of the LC circuit at its end?
You can do all that using your FG and scope.  Use my FG programming technique to make it output nanosecond pulses (I made my 1st video about it ...if you remember).

* That can take a form of two different frequency sine waves added together, ...or a rectangular pulse train of specific duty cycle and pulse repetition frequency.
   

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Setup like this can be used to see coil's resonances.
There is a link to a video and short summary about this circuit in PDF file.


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It's not as complicated as it may seem...

Hi Poynt,

well, i uderstood from Stalker his video / text (rk_rev2.pdf on page 58/59 and on) that he determined the wave resonance by using the pulse duty cycle, like being done in TDR measurements.

But it turns out its not an accurate way to do so according to a comment from Vasik41 in his post #239 above.

So i still am trying to find a way to confirm the LC resonance and wave resonance points are identical or very close (as per calculator) before going to the next step which is to rewind it as a Grenade coil and do the same measurements there.

Itsu

If I understand correctly, then you are pulsing the coil at its resonant LC frequency, but using a 3.7% duty cycle? What then would we be looking for in terms of determining the wave resonance? Are the +250V ~50ns wide pulses not just inductive kickback pulses?

Is the ultimate goal to match the LC resonance with the wave resonance? Sounds like that might be a lofty goal.

If I was looking to determine the wave resonance of that coil/TL, I would start by applying a single pulse at one end and see if there is a reflection back (assuming the scope probe is measuring at the signal insertion point). If there is a reflection, then determining the timing between the initial pulse to the reflected pulse should allow you to determine the wave resonance. The other end's termination should be open or shorted, but should not matter which for this simple test (only difference is one reflection will be inverted, and the other non-inverted).

   

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Poynt,

Quote
If I understand correctly, then you are pulsing the coil at its resonant LC frequency, but using a 3.7% duty cycle? What then would we be looking for in terms of determining the wave resonance? Are the +250V ~50ns wide pulses not just inductive kickback pulses?

yes, these are kickback pulses.  At 3.7% duty cycle they suppose to react a certain way due to the attached coil, see new info (pdf/video) given by Vasik41 a few posts above (post #250).
From that i understand that the pulser used that way is primarily used to show with neon's and fluorescent lamp that at a certain frequency there are standing waves (what we are looking for).

Quote
Is the ultimate goal to match the LC resonance with the wave resonance? Sounds like that might be a lofty goal.

Lofty as in impossible?

Quote
If I was looking to determine the wave resonance of that coil/TL, I would start by applying
a single pulse at one end and see if there is a reflection back (assuming the scope probe is
measuring at the signal insertion point).
If there is a reflection, then determining the timing between the initial pulse to the
reflected pulse should allow you to determine the wave resonance.
The other end's termination should be open or shorted, but should not matter which for this
simple test (only difference is one reflection will be inverted, and the other non-inverted).

That is a good approach for a coax or 2 line wire, but as we are dealing with a single wire coil its not that easy (lofty even), see post #217 where we were discussing this (see link there too).

I tried it yesterday but no reflection was seen using the surrounding environment as return.

Regards Itsu
   

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It's not as complicated as it may seem...
Lofty as in impossible?
Not impossible, but possibly a great challenge.

Quote
That is a good approach for a coax or 2 line wire, but as we are dealing with a single wire coil its not that easy (lofty even), see post #217 where we were discussing this (see link there too).

I tried it yesterday but no reflection was seen using the surrounding environment as return.

Regards Itsu

What did you use as a return termination for your scope/TDR?

I would suggest trying earth ground, and if no joy there, try using a metal sheet under the coil. I realize this may alter the resonance frequency, but it still could provide a valuable result.

One other thing that may be killing the reflection is the connection to the TDR or pulse generator after the pulse has been sent. If the connection could be broken (high impedance) then the reflection may be measurable. One possible way of achieving this would be to use a 3-state driver (doesn't have to be big current or voltage) and design it so the high impedance state is selected right after the pulse is sent.
   

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It's good to have doubts.

You should perform a self-educational experiment by taking a looooong piece of 50Ω coax and setting it up for a TDR measurement. Verify that the classic pulse reflection is visible when the coax is shorted and opened at the far end.
Next, connect a parallel combination of a ferrite torroidal coil and a good cap instead of the open and short termination at the far end. Verify that the TDR experiment still works. The LC resonant frequency of the coil&cap does not matter as long as it is different from that round trip reflection in the coax.

While you still measure the pulse reflections from the far end of the coax in the TDR manner, add another stimulating frequency* that will resonate the lumped LC circuit connected at the end of the coax.  Can you discern the round trip reflection in the coax from the response of the LC circuit at its end?
You can do all that using your FG and scope.  Use my FG programming technique to make it output nanosecond pulses (I made my 1st video about it ...if you remember).

* That can take a form of two different frequency sine waves added together, ...or a rectangular pulse train of specific duty cycle and pulse repetition frequency.


That might be a good idea to get a better idea on what i could expect via such a controlled combined resonanse setup.

Let me see what i can do,   Itsu
   

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Setup like this can be used to see coil's resonances.
There is a link to a video and short summary about this circuit in PDF file.

Again good info on the subject Vasik,   i will see if i can make it to good use.

Itsu
   

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Not impossible, but possibly a great challenge.

What did you use as a return termination for your scope/TDR?

I would suggest trying earth ground, and if no joy there, try using a metal sheet under the coil. I realize this may alter the resonance frequency, but it still could provide a valuable result.

One other thing that may be killing the reflection is the connection to the TDR or pulse generator after the pulse has been sent. If the connection could be broken (high impedance) then the reflection may be measurable. One possible way of achieving this would be to use a 3-state driver (doesn't have to be big current or voltage) and design it so the high impedance state is selected right after the pulse is sent.


Thanks Poynt,

i used "the surrounding environment" or mostly capacitive coupling i guess like suggested in the link on Post #217.
In there it was also suggested to use a metal sheet etc., but due to the negative responses from there i did not try that.

I can take a shot at it once more to see for my self i guess.

Itsu 
   

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Itsu,

Z for your coil is 2.7Kohm (sqr(143uH/19pF))
I think you need add 2.7K resistor in series with pulse source to match impedance and see reflections.

Vasik


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Here is a transcript. May be it helps.
Did you make a transcript of this video, too ?
https://youtu.be/v6FrGTF731o
   

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Did you make a transcript of this video, too ?
https://youtu.be/v6FrGTF731o

Only summary, not complete transcript, it is in the same file


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The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
   

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Itsu,

Z for your coil is 2.7Kohm (sqr(143uH/19pF))
I think you need add 2.7K resistor in series with pulse source to match impedance and see reflections.

Vasik

Thanks, i can try that when using a strip of copper tape underneath the long coil to act as a return.

But are you sure that 2.7K is Ok?   How do you calculate?

Itsu
   

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Thanks, i can try that when using a strip of copper tape underneath the long coil to act as a return.

Itsu

Itsu,

Strip of copper tape underneath will significantly change capacitance.

Vasik


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yes i know, but now i have no reflection at all (perhaps with your 2.7K it does, please see my above doubts about 2.7K) so i will give it a try with the copper tape too.

Itsu
   

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Itsu,

I calculate it as sqrt(143uH/19pF) = sqrt(L/C) = Z.

sqrt(143 10e-6 / 19 10e-12) = sqrt(143/19) 10e3 = 2.74K

Vasik

Edit: sqrt() = square root
« Last Edit: 2021-04-23, 17:50:26 by Vasik041 »


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Ok, indeed 2.7K it is  O0

Will try to set up the pulsing across ground this weekend.

Itsu
   

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I did some very rough tests using a copper strip underneath the coil as return for the TDR pulse, see picture.

When shorting the far end of the coil to this copper strip, there is a very tiny negative going pulse visible on the scope about 200ns away from the initial pulse.

This 200ns points to a length of 59.8m (no velocity factor yet) which is the length of the coil wire plus the 1m copper strip length.

The coil suppose to be then 58.8m and when applying a velocity factor of 66% (guestimated) it brings the length of the coil to 38.8m which is very close to the 37.5m it really is.

Will do some better tests and video tonight.

Itsu
   

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This 200ns points to a length of 59.8m (no velocity factor yet) which is the length of the coil wire plus the 1m copper strip length.
I don't think you can add the length of the copper strip like that any more than you can add the length of the shield in a coaxial cable.
   

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I don't think you can add the length of the copper strip like that any more than you can add the length of the shield in a coaxial cable.

Hmmm,  when pulsing a shorted coax, the (negative) pulse travels along the shield back to the beginning, doesn't it, so we include (add) that length of the shield too  don't we?

itsu
   

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Hmmm,  when pulsing a shorted coax, the (negative) pulse travels along the shield back to the beginning, doesn't it, so we include (add) that length of the shield too  don't we?
No, we just double the time for the round trip.
If the pulse was moving forward using only the center conductor and backwards using only the shield, then the forward and backwards propagation delays would be different.
   

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It's not as complicated as it may seem...
Hmmm,  when pulsing a shorted coax, the (negative) pulse travels along the shield back to the beginning, doesn't it, so we include (add) that length of the shield too  don't we?

itsu
The pulse uses both the main conductor and the return on both trips. The total time and therefore length of the TL must be divided by two to determine the actual length of the TL. But I think you know all this ;)

I take it that you detect no reflection with the TL open-ended?
   

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You can expect the following current distributions in the center-driven dipole antenna (black). Notice, that current is always zero at the ends of the dipole, because it has nowhere to go.
Note, that when the current becomes zero at the midpoint, then the driving impedance approaches infinity ...and the VSWR peaks at that drive point ( S11, too ).

Dividing the length of the dipole by the period of the wavelength when VSWR peaks, will help you to calculate the velocity factor.


   

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I made a video of the tiny pulse / bump i see when shorting the far end of the coil.
No reflection seen with an open-ended coil.

So i think its not possible then to pulse a single coil.
Will continue to do some tests using the nanoVNA.

Video here: https://youtu.be/ouQc5z5RrmE

Itsu
   
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