Bucking Coils

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gyula

Re: Bucking Coils « Reply #75 on: 2026-04-19, 11:09:55 »

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Hi Itsu,

Thanks for your efforts. Whether the Darlington config is appropiate here or not, I do not know the problem, basically it should be.

Perhaps it would be worth doing a test with the 2SC5200-O alone, remove the 2N2222 and use the 2SC as a single transistor emitter follower. The top bias resistor (between the base and the positive rail) would be the 220 kOhm trimpot, the bottom bias resistor
(between the base the negative rail) could be 100 kOhm, emitter resistor remains 10 Ohm. Start with 20 V supply voltage.

Gyula

Itsu

Re: Bucking Coils « Reply #76 on: 2026-04-19, 12:11:22 »

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Thanks Gyula, i will try that lateron to see if the darlington setup is the problem.

Meanwhile, i was looking for a way to use the nanoVNA to measure / calculate the Q and i found this website: https://coppermountaintech.com/determining-resonator-q-factor-from-return-loss-measurement-alone/

It shows how to do it and how to circumvent the low 50 Ohm impedance of the nanoVNA by loosely coupling (inductive or capacitive) the nanoVNA to the DUT.

I choose to use 10pF capacitors to couple the nanoVNA to the DUT, so my measurement circuit looks like this:

I did a calibrate (open, short, load, isolate and thru) by using at thru the 2 10pF caps in series connected (series measurement configuration) and lateron connect each of the 10pF caps to the DUT.

Using the return loss or S21 Log Mag (dB) Graph to plot the resonance trace and from that take Fres. and the both -3dB points to calculate the Q, see here:

It once again turns out to be a Q of 13.1

Itsu

gyula	Re: Bucking Coils « Reply #77 on: 2026-04-19, 12:37:05 »
Group: Tech Wizard Hero Member Posts: 1385	Okay Itsu, that is a good find. You could try to reduce the 10 pF coupling caps to even lower values to make the nanoVNA loading effect less and less, perhaps try to go down to say 3.3 pF, or even 1 pF. Of course the driving level and the sensitivity of the instrument sets a limit for using too low value coupling caps. Did you use the bucking coils or the new coil on the T200-2 core? Gyula

Itsu	Re: Bucking Coils « Reply #78 on: 2026-04-19, 13:16:16 »
Group: Elite Experimentalist Hero Member Posts: 4848	My lowest value capacitors are these 10pF ones, but i could try to put several in series, or build some out of parallel wires. I guess this measurement is similar as with the scope probe measurement, which also has a capacitance in the 8 to 3.9pF range with similar Q results (13). I was using the original bucking coils.

Itsu

Re: Bucking Coils « Reply #79 on: 2026-04-19, 15:49:27 »

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Gyula,

Still with the T200-2 core.

Running with the 2SC5200 alone, 220K trimmer between positive rail and base (set to 3.6K), 100K between base and ground.
20V input at 460mA, FG set to 5Vpp input, but the scope shows only 500mVpp so the base is loading down the FG signal, so we can not use it as real input reference i think.

Input measured across the 10.4 Ohm emitter resistor shows 2.01Vpp and the output across the 100pF capacitor is 28.4Vpp pointing to a Q = 14.1

Screenshot here:

Yellow output across 100pF capacitor
White input from FG
Blue input AC voltage across 10.4 Ohm emitter resistor.

Itsu

« Last Edit: 2026-04-19, 20:51:52 by Itsu »

gyula

Re: Bucking Coils « Reply #80 on: 2026-04-19, 20:18:13 »

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Thanks Itsu.

With the latest 2SC5200 test alone, please clarify where the probe of the white channel is placed? You wrote:
"FG set to 5Vpp input, but the scope shows only 500mVpp so the base is loading down the FG signal, so we can not use it as real input reference i think."

And your wrote this under the Screenshot:
"Blue input from FG
White input AC voltage across 10.4 Ohm emitter resistor."

IF the white trace shows the AC voltage (500 mVpp) across the 10.4 Ohm resistor, then why did you divide the cap voltage of
28.4 Vpp by 2.01 Vpp? (The latter is supposed to be the loaded FG input voltage from the 5 Vpp, right?)

So if we divide the cap voltage 28.4 Vpp by the 500 mVpp AC emitter voltage, we get Q = 28400 / 500 = 56.8

This is a better value already but came from using the T200-2 core.

In this test you had 460 mA, this current established a virtual r_E emitter resistance of 26mV/460mA= 0.056 Ohm.
IF you would use say 1 kOhm or 1.2 kOhm fix resistor now to replace the 220 k trimpot, the emitter current would become about 1 Amper, further reducing r_E.

Note that the Amidon T200-2 core is capable of producing Q > 300 or 350 when the number of turns is between 25 and 40 (i.e. L is between 8uH and 20 uH) in the frequency range between 3 MHz and 4 MHz.
See the graph in Page 15 here again https://alexsradioshop.de/wp-content/uploads/2023/01/AmidonAMI.pdf

So the emitter follower with the 460 mA emitter current ruins the expectable Q > 350 to nearly Q = 57 value IF the white trace indeed shows the AC emitter voltage of 500 mVpp.

Thanks,
Gyula

Itsu

Re: Bucking Coils « Reply #81 on: 2026-04-19, 20:54:27 »

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Hi Gyula,

i swapped the blue and white traces

sorry about that. I have corrected it now in my earlier post.

As can be seen in the screenshot, the FG (set to 5Vpp) signal (white trace) when connected to the 100nF cap gets distorted and pulled down for some reason, so i do not think it is the correct value (500mVpp) to use as the input.

The blue trace is the AC voltage across the 10.4 Ohm resistor and reads 2.01Vpp, so the real FG input signal should be somewhat higher according to you, so say 2.2Vpp, but somehow it is not.

If i install a 1K fixed resistor for the 220K trimmer, i indeed get an emitter current of 1A.

Without further changes, the output signal across the 100pF cap is now 25Vpp and the AC voltage across the 10.4 Ohm emitter resistor is 520mVpp this a Q = 47

gyula

Re: Bucking Coils « Reply #82 on: 2026-04-19, 21:26:45 »

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Thanks for the clarification.

Itsu, in the meantime I found this paper on experiments with coils and Q measurements https://hrsasa.asn.au/downloads/files/coilq.pdf

In Page 2, the Q measuring principle of the HP4342A Q meter is shown, I attached a screenshot.
It insures as low as 0.001 Ohm driving impedance to a coil plus a cap in series with it. The transformer primary has 50 turns, the secondary 1 turn, the primary is terminated
with an 50 Ohm resistor, this is driven by a signal generator having also 50 Ohm output impedance, meaning the transformer steps down from 25 Ohm to 0.001 Ohm
BUT
there should be a typo : the stepped down impedance should be 0.01 Ohm (and not 0.001) with the given turns ratio and 25 Ohm primary impedance.
Nevertheless, the 0.01 Ohm would already be the lowest to be attained so far. I do not understand why the paper says one milliOhm output impedance. Turns ratio 50, the square of this is 2500 and the 25 Ohm primary impedance appears as 25/2500= 0.01 Ohm.

IF you agree, this method would also be worth testing as the last one in this Q measuring journey which so far has yielded low Q values.

Of course, it is possible the step down transformer measuring method may still yield a Q of around 13 - 15 with the bucking coils.

If this would be the case, it would be good to check the Q of the T200-2 type core with this method, the Amidon data sheet indicates Q > 350 with turns between 25 to 40 (between 8uH - 20uH) in the 3-4 MHz frequency range as I wrote in my previous post.

I just noticed your additional post above, so the Q now is 47 with the T200-2 core. IT would be good to attain at least a Q of 300 for this core in a test circuit and the method shown in the paper I refer to may insure this. Basically this is a refined version of what Smudge
suggested earlier, using a coupling coil, now with a single turn secondary.

The FY8300 FG has a 24 Vpp max sine wave up to 5 MHz if I am correct, so after the 50 times voltage division there remains > 400 mVpp across the 1 turn output to feed the series LC circuit. If you agree with this, use the T520-2 core, easier to wind the 50 turns.

What do you think?

EDIT: the 24 Vpp output of the your FG will be halved when the 50 Ohm resistor across the primary coil of the matching transformer terminates its output but the remaining > 200 mVpp is still enough to see it on the scope.

Gyula

Bucking Coils

Screenshot Coil Q measurement.png (86.93 kB, 743x643 - viewed 94 times.)

Itsu

Re: Bucking Coils « Reply #83 on: 2026-04-20, 09:24:36 »

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Gyula,

that is an interesting paper with good details on how it works.

I will have to read it several times to fully grasp it, but the principle is clear.

One thing i noticed when comparing the data from my real circuit and the simulation circuit (or datasheets) is that when adding a 4K resistor across the 100pF capacitor (mimicking a scope probe), the Q is drastically lowered as could be expected.
I am not sure if the simulator is including this "load" when placing a probe, but i expect its not

Anyway, i am halfway to adding 27pF smd capacitors onto each turn (42 total

) of the 2 delay line coils on my ferrite toroid.
When done i can make the measurement to compare my Q data with and without these delay lines attached to see if Smudge was right.

It would be interesting to use this new Q measure method later on to see if it is doing what it supposes to do.

Thanks, Itsu

gyula

Re: Bucking Coils « Reply #84 on: 2026-04-20, 10:32:19 »

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Hi Itsu,

No any load is imposed on a circuit in any circuit simulator when you use any probe in it.

In the real circuit, when a probe is hooked up in parallel with the 100 pF in the present LC circuit, the probe's self capacitance is added in parallel to the 100 pF, and as you correctly expected, it is not like at all when you deliberately shunt the 100 pF with 4.7 kOhm resistor to mimic the capacitive reactance.

IT is always good to have several probes and when you are ready to test this 'new' Q measuring method, would you use them (you wrote about in post #36)?

Basically, all of their self capacitances should be 'absorbed' as a small capacitance added to the 100 pF but the probes' 10 MOhm internal resistance is surely transformed into the LC circuit as a certain loss.
However, the equivalent parallel resistance of a decent 100 pF capacitor may be as low as some MegaOhm due to frequency dependent dielectric losses, unfortunately.

As an alternative capacitor check, would you consider paralelling 4 SMD caps (27 pF each) to replace the 100 pF cap you have used so far? The extra 8 pF should cause only a few kHz change in resonant frequency.
This is in case if the T200-2 core would perform way under the Q=300-350 range, we can trust in the Amidon cores Q wise too.
(In the HP4342A Q meter they used a good quality variable capacitor in series with the coil to be measured and a built-in RF voltmeter measured the resonant voltage across the variable cap to know the Q of the coil.)

Thanks also for your kind efforts,

Gyula

Itsu

Re: Bucking Coils « Reply #85 on: 2026-04-20, 16:41:13 »

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This results post is with reference to my post #36 quoted here below where i made some baseline Q measurements without the delay lines attached:

Quote from: Itsu on 2026-04-13, 19:46:55

I used 3 probes to measure / calculate the Q of my "parallel LCR" (16.5uH, 100pF and 0.1 Ohm) as shown earlier, by using the "frequency / bandwidth" method.

Scope TDS-3054B:

probe P6139B (passive) 10x (10MOhm / 8pF) Q=13
probe P6202 (active) 10x (10Mohm / 2pF) Q=11

Scope MDO-3054:

probe TPP0500B (passive) 10x (10MOhm / 3.9pF) Q=11

Surprising outcome as the probe with the highest load (P6139B) gives the highest Q (13)

Gyula his bipolar transistor probe schematic was also tested on the above device, but in "series LCR" configuration (had to run the scope channel in AC coupling)

Bipolar transistor 2n2222 (active) 11.2V input, 3.5V DC across emitter resistor (220 Ohm), 55mVpp input from FG, 565mVpp across 100pF cap. Q=10.7

So i think this is enough to have a baseline Q (13) to see if when adding the delays it will increase.

Itsu

The results with the delay line attached (2x 21 turn coils with each turn a 27pF smd capacitor attached, see picture below) can be found here:

Scope TDS-3054B:

probe P6139B (passive) 10x (10MOhm / 8pF) was Q=13, now with delay line: Q=14.1
probe P6202 (active) 10x (10Mohm / 2pF) was Q=11, now with delay line: Q=13.4

Scope MDO-3054:

probe TPP0500B (passive) 10x (10MOhm / 3.9pF) was Q=11, now with delay line: Q=14.2

Lateron in post #76 i also used my nanoVNA to make an initial Q measurement without delay lines resulting in Q=13.1

The same nanoVNA measurement with delay lines results in Q=14.6

The initial test with Gyula his bipolar transistor probe schematic which resulted in Q=10.7 was also repeated, but this made little difference as Q now with delay lines was measured (Fres/bandwidth) to be 10.1.
We tried many different configurations and may be we can present some better results lateron.

All in all i think that the Q with delay lines attached does increase compared to the Q without them

Itsu

Smudge	Re: Bucking Coils « Reply #86 on: 2026-04-20, 18:59:32 »
Group: Moderator Hero Member Posts: 2434	Itsu, That is encouraging. Is it possible to connect the two coils not bucking and see whether the delay line then reduces the Q. And a measurement with the coils as primary and secondary of a transformer to see what the delay time is? That will all help with the math. Smudge

Itsu	Re: Bucking Coils « Reply #87 on: 2026-04-20, 19:40:23 »
Group: Elite Experimentalist Hero Member Posts: 4848	Smudge, I guess this would be no problem, I will see what I can do. Itsu

Itsu

Re: Bucking Coils « Reply #88 on: 2026-04-20, 20:37:55 »

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Well, there is a problem.

The problem i have is that the both delay lines are active now, so i cannot make a comparison between the 2 litz wire coils in aiding mode "without delay lines active" and "with delay lines active".
I don't think it is valid to do a Q measurement in the "delay lines active" situation and compare that with the bucking coils "without delay lines active" situation.
To remove or disable the delay lines i have to desolder the 42 smd capacitors return wire.

If still doing the measurement, this is the result:
Putting the 2 litz wire coils in aiding gives an inductance of 194uH (was 16.5 in bucking).
The parallel capacitor stays at 100pF, so the resonance frequency lowered from 3770kHz to 595kHz.
The low -3dB point becomes 486kHz and the high -3dB point becomes 716kHz which is a difference of 232kHz.
Q = 595kHz / 232kHz = 2.56.

So considerable lower, but is this a valid situation to compare? I can remove the SMD caps return wire from them if needed, should i do that?

Itsu

gyula	Re: Bucking Coils « Reply #89 on: 2026-04-20, 20:45:24 »
Group: Tech Wizard Hero Member Posts: 1385	Well, could you use a 8-9 pF capacitor instead of the 100 pF, this would bring the resonant frequency back to 3.8-3.9 MHz range. Gyula

Itsu	Re: Bucking Coils « Reply #90 on: 2026-04-20, 20:51:02 »
Group: Elite Experimentalist Hero Member Posts: 4848	But i still would not have a comparison between "with delay lines" and without delay lines" in the new aiding mode situation.

Smudge	Re: Bucking Coils « Reply #91 on: 2026-04-21, 07:29:44 »
Group: Moderator Hero Member Posts: 2434	Itsu, I see your problem. The data you have given me I can work with to further our understanding so please carry on with the original experiment to see whether inceasing the delay increases the Q. Smudge

Itsu

Re: Bucking Coils « Reply #92 on: 2026-04-21, 16:31:37 »

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Smudge,

By increasing the delay i have to change out the capacitors, now 42x 27pF smd caps, for say 100pF caps which i have also 42 available.

So if i need to remove the present 27pF caps, i can also do your earlier request of measuring the Q in aiding mode and the time delay between primary and secondary.

As i already have the Q when in aiding mode with the 27pF caps delay lines active i now need to do the time delay measurement between primary and secondary.

So i tried to measure the time delay between primary and secondary when still having the 27pF delay lines active, but its hard to do so as there is quite some distortion at the secondary when using a square wave / pulse signal.
Also the time delay is different at different frequencies, so i tried to measure the time delay at 1Mhz (10Vpp) with a sine wave signal with max. amplitude selected (some reasonable vertical traces to compare the time difference), i get a time delay between primary input and secondary output signals of the transformer of 1.84ns.

If there is another better way to do this measurement, please let me know.

If not, i can now remove the 27pF caps delay line and do the same delay time measurement as above and later on the aiding coils Q measurement to compare with the one made yesterday.

Finally then i can install the 100pF caps delay lines and do further Q and time delay measurements.

Itsu

Itsu

Re: Bucking Coils « Reply #93 on: 2026-04-22, 21:26:27 »

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No response from Smudge, so i guess the time delay measurement method was OK,

I removed the 27pF delay lines and did the time delay measurement once again which turns out that there now is a time delay between primary and secondary of 1.46ns (was 1.84ns with delay lines active).

Then i did the aiding coils Q measurement without the 27pF delay lines active with as results:

Putting the 2 litz wire coils in aiding gives an inductance of 191uH (was 194uH with delay lines active and 16.5 in bucking mode).
The parallel capacitor stays at 100pF, so now the resonance frequency becomes 960kHz (was 595kHz with 27pF delay lines active!!!).
The low -3dB point becomes 695kHz (was 486kHz with delay lines) and the high -3dB point becomes 1300kHz (was 716kHz with delay lines active) which is a difference of 605kHz.
Q = 960kHz / 605kHz = 1.58 (was Q = 595kHz / 232kHz = 2.56 with delay lines).

So the Q WITH delay lines active is higher (2.56) than without delay lines active (1.58).

Strange i think is the difference in resonance frequency WITH delay lines (595kHz) compared to WITHOUT delay lines (960kHz).

I will now install the 42 100pF smd caps delay lines and redo the Q measurement and time delay measurement......

Itsu

Verpies	Re: Bucking Coils « Reply #94 on: 2026-04-22, 23:05:50 »
Group: Administrator Hero Member Posts: 4856	Quote from: Itsu on 2026-04-22, 21:26:27 Strange i think is the difference in resonance frequency WITH delay lines (595kHz) compared to WITHOUT delay lines (960kHz). Yeah, the resonance frequency affects the Q calculation and other things.

Smudge

Re: Bucking Coils « Reply #95 on: 2026-04-23, 07:59:36 »

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Quote from: Itsu on 2026-04-22, 21:26:27

No response from Smudge, so i guess the time delay measurement method was OK,

Sorry, I have been busy trying to get a wheelchair adapted vehicle suitable for my needs and one I can afford. I assumed that a simple phase measurement between sine wave input and output voltages would suffice where the output is into a resistive load, not a reactive load, and at the frequency of interest. I admit I am out of touch with the complexities arising in bench measurements, it is years since I last used an oscilloscope. I am seriously looking at converting a corner of my room in the extra care facility where I now live into a small laboratory where I can react to these problems.

Smudge

Itsu

Re: Bucking Coils « Reply #96 on: 2026-04-23, 09:24:00 »

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Quote from: Verpies on 2026-04-22, 23:05:50

Yeah, the resonance frequency affects the Q calculation and other things.

Yes, the delay lines do influence the resonance frequency more when the 2 coils are in aiding mode (595kHz with versus 960kHz without) than when in bucking mode (3535kHz with versus 3642 without), of course the inductance thus resonance frequency is different, but still.

I can imagine that the different resonance frequencies have their influence on the Q.

Itsu

Re: Bucking Coils « Reply #97 on: 2026-04-23, 09:34:12 »

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Quote from: Smudge on 2026-04-23, 07:59:36

Sorry, I have been busy trying to get a wheelchair adapted vehicle suitable for my needs and one I can afford. I assumed that a simple phase measurement between sine wave input and output voltages would suffice where the output is into a resistive load, not a reactive load, and at the frequency of interest. I admit I am out of touch with the complexities arising in bench measurements, it is years since I last used an oscilloscope. I am seriously looking at converting a corner of my room in the extra care facility where I now live into a small laboratory where I can react to these problems.

Smudge

No problem Smudge, the situation on the bench (and not every bench will be the same) can throw things at you, you don't expect, therefor i try to be so specific as possible to transfer the data i see onto this thread.

Itsu

Itsu

Re: Bucking Coils « Reply #98 on: 2026-04-26, 16:57:40 »

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I installed the 100pF capacitors delay line and made the 3 measurements (bucking coils Q, aiding coils Q and the time delay) using the "Fres / bandwidth (-3dB)" method on the first 2.

I was using the MDO-3054 scope with TPP0500B probes (10MOhm / 3.9pF) with the following results:

Bucking coils:
Base Q 11
27pF DL Q 14.2
100pF DL Q 18.9

Aiding coils:
Base Q 1.58
27pF DL Q 2.56
100pF DL Q 4.3

Prim / sec time delay:
Base 1.46ns
27pF DL 1.84ns
100pF DL 322ns!!!

This last (100pF DL) time delay measurement seems wrong, but i tried it several times with the same outcome (using a 1MHz sine wave signal of 10Vpp at the primary all the time)
If i reverse the secondary probe leads i get an even longer delay of 796ns.

Itsu

Smudge

Re: Bucking Coils « Reply #99 on: 2026-05-02, 16:56:14 »

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Quote from: Itsu on 2026-04-26, 16:57:40

I installed the 100pF capacitors delay line and made the 3 measurements (bucking coils Q, aiding coils Q and the time delay) using the "Fres / bandwidth (-3dB)" method on the first 2.

I was using the MDO-3054 scope with TPP0500B probes (10MOhm / 3.9pF) with the following results:

Bucking coils:
Base Q 11
27pF DL Q 14.2
100pF DL Q 18.9

Aiding coils:
Base Q 1.58
27pF DL Q 2.56
100pF DL Q 4.3

Prim / sec time delay:
Base 1.46ns
27pF DL 1.84ns
100pF DL 322ns!!!

This last (100pF DL) time delay measurement seems wrong, but i tried it several times with the same outcome (using a 1MHz sine wave signal of 10Vpp at the primary all the time)
If i reverse the secondary probe leads i get an even longer delay of 796ns.

Itsu

Maybe phase is not the best way to get delay time in this instance, sorry about that. Any chance of having a fast leading edge pulse and seeing the delay from that?

I appreciiate all you are doing on Q measurements and hope that work improves results from this bench.

Smudge