Q measurement methods

Author Topic: Q measurement methods (Read 1468 times)

gyula	Re: Q measurement methods « Reply #100 on: 2026-05-18, 20:19:56 »
Group: Tech Wizard Hero Member Posts: 1385	Hi Itsu, Yes, it is strange and I suspect if we find the cause of the higher single loop output, the e2 / e1 voltage ratio will bring a Q value very close to the -3 dB Q measuring method. It has to be caused by the 50:1 transformer itself I believe, might be leakage? Comments are welcome from anyone. Gyula

Itsu

Re: Q measurement methods « Reply #101 on: 2026-05-19, 09:41:59 »

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Gyula,

i measured the leakage induction of the both primary and secondary to be (Frequency measured is 2143kHz ):

Normal inductance of the primary coil with the secondary coil OPEN is 55uH
Leakage inductance of the primary coil with the secondary coil SHORTED is 45uH

Normal inductance of the secondary single loop coil with primary coil OPEN is 136nH
Leakage inductance of the secondary single loop coil with primary coil SHORTED is 116nH

I will measure the primary / secondary voltages to see if they are at a 50:1 ratio (which they should) and if not, try to find a way to correct this.

Here is the present circuit i work with:

Itsu

« Last Edit: 2026-05-19, 16:15:13 by Itsu »

Itsu

Re: Q measurement methods « Reply #102 on: 2026-05-20, 15:15:18 »

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When using the above-mentioned normal and leakage inductance of a coil, we can calculate the coupling factor with this calculator: https://e-magnetica.pl/doku.php/calculator/magnetic_coupling_coefficient

This means for the 50:1 toroid primary (55uH versus 45uH) a coupling factor K of 0.42
For the secondary it means (136nH versus 116nH) a coupling factor K of 0.38

So roughly I think we can say we have a 50:1 toroid coupling factor of 0.4

Itsu

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Re: Q measurement methods « Reply #103 on: 2026-05-20, 16:44:19 »

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Quote from: Itsu on 2026-05-20, 15:15:18

So roughly I think we can say we have a 50:1 toroid coupling factor of 0.4

At the risk of being boring, I will repeat that a winding, which symmetrically spans the entire circumference of a toroidal core, maximizes flux confinement in this core and consequently minimizes flux leakage, mutual magnetic coupling to other coils/cores, EMI generation and susceptibility to external EMI.

A single-turn winding can also span the entire circumference of the core by using a cuppola-style winding.
A multi-turn winding can symmetrically span the entire circumference of the core by existing in even number of reversing layers as shown below (the terminals of this winding should depart and arrive at the same place).

To minimize the parasitic capacitances the turns and layers should be kept apart with as much air between them as possible, i.e. they should not touch (use tiny glass beads as spacers held by CA glue at the wire crossovers. These beads are used for jewelry, bracelets, tassels, earrings ....and are very cheap). Yes, this is tedious but the results are worth it.

https://i.pinimg.com/originals/ec/b0/41/ecb041a5b0c680d93f7eb512bc82006b.jpg

Q measurement methods

https://i.pinimg.com/originals/da/99/77/da99775ad67e3c051242921d09397d2d.jpg

Q measurement methods

Itsu	Re: Q measurement methods « Reply #104 on: 2026-05-20, 21:49:47 »
Group: Moderator Hero Member Posts: 4848	Nothing boring here, tedious yes, but not boring. I cannot find any info on this cuppola-style winding, is that the style of the toroid in your picture? Anyway, i was thinking of putting the toroid in a copper box in such a way it forms a single turn around the toroid, like in the HP Q meter, but i am not sure what is more tedious. Itsu

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Re: Q measurement methods « Reply #105 on: 2026-05-20, 21:56:51 »

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Quote from: Itsu on 2026-05-20, 21:49:47

I cannot find any info on this cuppola-style winding, is that the style of the toroid in your picture?

No, that's for multi-turn winding.

Quote from: Itsu on 2026-05-20, 21:49:47

Anyway, i was thinking of putting the toroid in a copper box in such a way it forms a single turn around the toroid, like in the HP Q meter, but i am not sure what is more tedious.

Yes, something like this.
You can use a big copper pipe cap...

https://cdn.mscdirect.com/global/images/ProductImages/3689185-24.jpg

Q measurement methods

...or a short segment of large ID copper pipe with a flat end plate soldered on one side with a center post that are capable of containing the toroidal core with the multi-turn winding, without touching them. Solder a conical concentrator section on the other side.
The conical section is the most important feature because it symmetrically brings the current to the middle where the copper center post is. The finished cuppola looks like a door knob.

You have made conical sections like that in the past:

Q measurement methods

Cuppola.jpg (519.82 kB, 2602x1225 - viewed 81 times.)

Itsu	Re: Q measurement methods « Reply #106 on: 2026-05-21, 12:36:01 »
Group: Moderator Hero Member Posts: 4848	Thanks for visualizing it, looks great. First I need to find a suitable smaller toroid, as the present 50:1 is on a T520-2 (big), and in this case smaller is better. itsu

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Re: Q measurement methods « Reply #107 on: 2026-05-21, 13:14:33 »

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Remember that if you do not confine all of the magnetic flux to the toroidal core by a symmetrically reversing a multi-turn winding then the flux that leaks out of the core will induce eddy currents in the bulk metal of the cuppola and waste energy.
This phenomenon is similar to the one occurring in this arrangement:

Where the circular turn around the perimeter of the core represents the uncancelled circumferential current of the multi-turn winding and the rectangularish turn represents the path of the induced eddy current in some distant bulk metal (without the break/gap).
The flux leaking out of this core is like that of a permanent ring magnet polarized axially as depicted below:

Q measurement methods

...and this axial flux changes in time under AC drive, inducing EMF and current in distant closed conductors ... and other coils.

This undesirable induction is easily observed in the following experiment, where two one-layer windings are placed on two separate parallel toroidal cores which are placed coaxially above one another:

This arrangement creates an unintended 1:1 transformer.
The 1:1 ratio surprises most people because one core evidently has more turns on it than the other.
This happens because the transformer ratio, in this case, is determined by the circumferential MMF that generates the axial leakage flux - not the usual toroidal MMF (toroidal ampturns).

Itsu

Re: Q measurement methods « Reply #108 on: 2026-05-21, 15:35:05 »

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Ok, so even the 50 turn coil need a special winding technic to minimize losses.

I am not sure if it is all worth the (tedious) efforts for building such a Q meter as knowing the Q of a Coil or LC circuit is nice, but no showstopper IMO.

Meanwhile, i was thinking of using a AM503B current probe amplifier (https://w140.com/tekwiki/wiki/AM503B) as pickup for the ringing signals on your Q ringing method.

It needs a 50 Ohm input (into a scope, but could be used in any 50 Ohm input i guess).

So i build this TLV3202 circuit mentioned on page 15 / fig. 8-5 of the TLV3202 datasheet

But it needs some AC volts sine wave to get so decent square wave out of it.

So i think it needs some buffer amplifier in front of it to boost the 20mVpp and less ringing signals coming from the AM503B.

Itsu

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Re: Q measurement methods « Reply #109 on: 2026-05-21, 16:19:44 »

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Quote from: Itsu on 2026-05-21, 15:35:05

I am not sure if it is all worth the (tedious) efforts for building such a Q meter as knowing the Q of a Coil or LC circuit is nice, but no showstopper IMO.

It's just fun to do and such efficient 1:50 impedance transformer has other uses than for Q measurement.

Quote from: Itsu on 2026-05-21, 15:35:05

Meanwhile, i was thinking of using a AM503B current probe amplifier (https://w140.com/tekwiki/wiki/AM503B) as pickup for the ringing signals on your Q ringing method.
It needs a 50 Ohm input (into a scope, but could be used in any 50 Ohm input i guess).

Yes, it can be used for that purpose.
You can also use a GMR or TMR sensor cannibalized from a 2007+ hard drive.

Quote from: Itsu on 2026-05-21, 15:35:05

So i build this TLV3202 circuit mentioned on page 15 / fig. 8-5 of the TLV3202 datasheet
But it needs some AC volts sine wave to get so decent square wave out of it.

Even if you delete the feedback resistor (that sets the hysteresis) and set the V_REF voltage at the (-) input to several mV ?
If the (+) input of the comparator is located very close to the output of the AM503B then you can skip all comparator input termination resistors and the feedback resistor.

Itsu	Re: Q measurement methods « Reply #110 on: 2026-05-21, 20:48:51 »
Group: Moderator Hero Member Posts: 4848	I removed the feedback 1M and input termination resistors (330K) and put 0V (at +0.3mV it stops working, no output) at the + terminal. Input is a minimum 50mVpp AC sine wave (less won't work anymore, no output). I used my FG for now to input the signal using this circuit (used a different symbol for the TVL3202): The output looks like this, very noisy and feedback on the input signal: Itsu

Verpies	Re: Q measurement methods « Reply #111 on: 2026-05-21, 22:01:39 »
Group: Administrator Hero Member Posts: 4856	Quote from: Itsu on 2026-05-21, 20:48:51 Swap (+) and (-) comparator inputs and put a ceramic capacitor between the (-) input and comparator's ground.

gyula

Re: Q measurement methods « Reply #112 on: 2026-05-22, 15:27:09 »

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Hi Itsu and all,

In the meantime I did some simulations in LTspice. I have found that the needed 50:1 transformation ratio comes out only as being around 22:1, using the measured primary inductance of 55 uH and single turn secondary of 136 nH on the T520-2 core.

If we use this data sheet https://alexsradioshop.de/wp-content/uploads/2023/01/AmidonAMI.pdf and we use the formulas included in page 1, we can reverse calculate the number of turns for the measured primary and secondary inductances
(using AL = 207 uH/100 turns from data sheet for that core).

So the calculated number of turns is 51.5 for the measured 55 uH primary coil (which sounds reasonably close for your 50 turns, see note below).

However, the calculated number of turns comes out as 2.5 for the 136 nH secondary coil. The formula predicts 20.7 nH inductance for a single turn on this core.

The simulations show a 50:1 ratio if I use 28.1 nH inductance for the secondary coil, see attached screenshot. The 28.1 nH is much closer to the predicted 20.7 nH than the
136 nH. With the 28.1 nH secondary the Q of the LC circuit comes out as very nearly 370 at 2166 kHz.

Checking your 40 turns on the T200-2 core for your measured 18.8 uH inductance by reverse calculation, it gives 39.5 turns, very close to 40. (AL for T200-2 is 120 uH/100 turns.)

For consideration only: known rule of thumb for a 1 cm long wire inductance (OD = 1 mm) is about 10 nH and the T520-2 core has a permeability of 10.

I attach a simulation of the transformer with 28.1 nH secondary coil driving the LC circuit at resonance. For the total loss resistance in the LC circuit I used 0.69 Ohm.

Note: The little higher inductance of 28.1 nH I had to use (versus the reverse calculated 20.7 nH) to get the 50:1 ratio may come from the 50 turn primary coil which gave the 55 uH
primary inductance as you measured, probably a little high. If I use 52 uH instead of 55 uH to reverse calculate the number of turns, I get 50.1 i.e. 50 turns you have.
Using 52 uH in the simulation and using the 20.7 nH inductance for the secondary coil, I got V3 / V2 = Q = 45 Vrms / 0.1218 Vrms = 369.4 i.e. the same Q result, just the voltage levels are lower.

Gyula

50 to 1 transformer drives series LC.png

Q measurement methods

50 to 1 transformer drives series LC.png (92.44 kB, 1912x1014 - viewed 8 times.)

Verpies	Re: Q measurement methods « Reply #113 on: 2026-05-22, 18:08:48 »
Group: Administrator Hero Member Posts: 4856	I'd like to notice that any secondary leakage inductance of L2 will be added to the DUT's inductance L3.

Itsu

Re: Q measurement methods « Reply #114 on: 2026-05-22, 21:40:03 »

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Quote from: Verpies on 2026-05-21, 22:01:39

Swap (+) and (-) comparator inputs and put a ceramic capacitor between the (-) input and comparator's ground.

(+) and (-) comparator inputs swapped and placed a ceramic capacitor between the (-) input and comparator's ground
I also changed the 220k trimmer pot to a 1M one:

Signals seems slightly better, but minimum input went up to 400mVpp sine wave, also the trimmer pot still needs to be very close to 0V on the - terminal.

Itsu

gyula

Re: Q measurement methods « Reply #115 on: 2026-05-22, 21:45:25 »

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Quote from: Verpies on 2026-05-22, 18:08:48

I'd like to notice that any secondary leakage inductance of L2 will be added to the DUT's inductance L3.

Yes. Itsu measured leakage inductances, though the value he got seems to be also too high because the normal inductance of the single turn secondary seems to be too high as per
the simulations and reverse calculations show. So some tens of nanoHenry leakage is involved maximum, and this should be considered of course, especially when DUT inductance
happens to be under say 50 nH. Obviously this was the case in the HP Q-meter circuit, calibration was done anyway.

Gyula

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Re: Q measurement methods « Reply #116 on: 2026-05-22, 22:21:48 »

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Quote from: Itsu on 2026-05-22, 21:40:03

I also changed the 220k trimmer pot to a 1M one:

It would've been better if you'd changed the 200k pot to a resistor connected to the +5V rail and connected a 10k (or lower) pot to that resistor and the other side to ground while its center leg goes to the (-) input of the comparator.

Is the yellow trace the signal from the FG ?

P.S.
For comparator testing, triangle or sawtooth waveforms are the best.

Itsu

Re: Q measurement methods « Reply #117 on: 2026-05-23, 09:08:38 »

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Quote from: Verpies on 2026-05-22, 22:21:48

It would've been better if you'd changed the 200k pot to a resistor connected to the +5V rail and connected a 10k (or lower) pot to that resistor and the other side to ground while its center leg goes to the (-) input of the comparator.

Is the yellow trace the signal from the FG ?

P.S.
For comparator testing, triangle or sawtooth waveforms are the best.

Ok, did that, fixed 220K resistor from +5V to 5K trimmer pot to ground with its center lead to the (-) terminal.
Need about 10mV on the (-) terminal for the same signals as in the above screenshot, still with a minimum of 400mVpp input from the FG.
With a triangle waveform i need a minimum of 600mVpp input.

Yes, yellow is across the 50 Ohm resistor.

Itsu

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Re: Q measurement methods « Reply #118 on: 2026-05-23, 20:20:09 »

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Quote from: Itsu on 2026-05-23, 09:08:38

Yes, yellow is across the 50 Ohm resistor.

The interference appearing at the INPUT is unusual and unacceptable !

Adjust your FG to add a DC offset to the input signal so it does not go below ground. AFAIR this comparator does not like when its inputs go below its ground pin potential because this activates the comparator's internal protection diodes and the current from these diodes can mess with the inputs. The comparator could be already damaged by these excursions below ground.
By the same token, try lowering the +5V power rail because your scope indicates 8.120V ringing at the output, which is above the V_{CC_max} and might also activate the comparator's internal protection diodes. A little more load at the output might also dampen this ringing.

Finally, try to decouple the +5V power rail and the scope probe's ground with various techniques.
e.g. use your recently acquired low-ESR wonder-caps between the comparator's supply pins and between the (-) V_REF pin and comparator's ground pin.
Perhaps add a choke and f.bead at the +5V rail (before the cap), too.
Watch for radiative EMI coupling to you scope probe's ground clip (try using the springy type and use an improvised grounded copper foil barrier or mini-shield box).

If this bears fruit, I will think of a discrete JFET solution for the (+) input amplification and decoupling.

Itsu	Re: Q measurement methods « Reply #119 on: 2026-05-24, 09:54:07 »
Group: Moderator Hero Member Posts: 4848	I did lower the supply voltage yesterday to about 2V and the output (and input) signals improved a lot. I will try your other suggestions too, thanks. Itsu

Itsu

Re: Q measurement methods « Reply #120 on: 2026-05-24, 20:37:41 »

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I used the following settings on the FG (rigol):

Frequency 2145kHz
Sine wave 200mVpp
+100mV DC offset

The supply voltage was set to 2V
The DC voltage on the (-) terminal was set to 50.7mV.

Signals look like this:

Yellow input across 50 Ohm, blue is output across 100K
So input is somewhat lower, but any lower and i lose the output.

Lost a comparator in the process (its output seems grounded).

Still need to improve on the noise filtering.

I have a strong 169MHz intermittent signal being picked up by the (yellow) scope probe producing signal distortion and coming from somewhere outside (license free ISM band)

Itsu

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Re: Q measurement methods « Reply #121 on: 2026-05-24, 23:41:57 »

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Quote from: Itsu on 2026-05-24, 20:37:41

I have a strong 169MHz intermittent signal being picked up by the (yellow) scope probe producing signal distortion and coming from somewhere outside (license free ISM band)

If it is 169.65MHz then it could be the old P2000 Dutch emergency services system.
Otherwise it could be the wireless M-Bus (EN 13757-4) and similar standards used for remote reading of utility meters (electricity, gas, water, heat) and other sensors.

Itsu	Re: Q measurement methods « Reply #122 on: 2026-05-25, 09:24:07 »
Group: Moderator Hero Member Posts: 4848	Yes, it is 169.65MHz, the (not old) P2000 Dutch emergency services system.

Verpies	Re: Q measurement methods « Reply #123 on: 2026-05-25, 19:52:57 »
Group: Administrator Hero Member Posts: 4856	What kind of JFETs do you have ? I am thinking of a low capacitance buffer/amplifier like this to drive the comparator: Q measurement methods

Itsu	Re: Q measurement methods « Reply #124 on: 2026-05-25, 20:50:33 »
Group: Moderator Hero Member Posts: 4848	I have some BF245C on hand