Q measurement methods

Author Topic: Q measurement methods (Read 1554 times)

Verpies	Re: Q measurement methods « Reply #75 on: 2026-05-05, 07:18:51 »
Group: Administrator Hero Member Posts: 4856	Quote from: gyula on 2026-05-04, 21:04:00 I think I see feed-through caps next to the toroid.

gyula

Re: Q measurement methods « Reply #76 on: 2026-05-05, 09:29:39 »

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Mags:

from the HP4342A manual it turns out the main internal circuits are fed from a PS which gives +25V and -25V DC with respect to the common ground.
using split DC supplies for RF and HF circuits is common, helps unwanted interactions between circuit stages.

regarding the copper PCB area under the large toroid: I see what you mean but consider the HP toroid core enclosure revealed in the teardown video,
they used teflon insulation spacers directly under and above the core and the dielectric constant of teflon is 2, i.e. twice that of the air, meaning
the stray capacitance added to the measuring circuit was quasi doubled, yet they used it. The spacer Itsu applied between the large core and the PCB surface
should contribute much less unwanted coupling capacitance I think.
see pages 25 and 26 for replicated transformers and measurements in this paper: http://www.ve2azx.net/technical/HP4342A_Q%20Meter_Tests1.pdf
he used 65 mil plastic spacers at bottom and top

Verpies: Yes, the feed-through cap is included in the schematic too, labelled as C4, 56 pF, see the attached schema in Reply 70, previous page.

Gyula

Itsu

Re: Q measurement methods « Reply #77 on: 2026-05-11, 11:52:51 »

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Quote from: Verpies on 2026-05-03, 14:39:26

Zoom around the right cursor and see.
Without the upgraded diodes and low C_OSS MOSFET I don't expect it to reach 300.
When you upgrade them - do not pulse them for a long time. Terminate the high pulse before the drain current gets up to 2A. Keep the PRF low too, e.g. 10Hz.

OK, i have the recommended parts put together on a PCB and the schematic looks like this:

The first result with a 5V DC 1kHz 10% duty cycle input pulse looks like this:

At the top the pulse with ring down, and at the bottom the zoomed in part of the start of the ring down.

So i need to count the ring down pulses to acquire the Q

Itsu

Verpies

Re: Q measurement methods « Reply #78 on: 2026-05-11, 15:01:29 »

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Quote from: Itsu on 2026-05-11, 11:52:51

So i need to count the ring down pulses to acquire the Q

Yes. If you want to do it automatically, the first step is to convert the ringing to digital pulses with a fast comparatpr like the TLV3202.

Also the exciting pulse can be automatically determined by a second comparator that turns off the MOSFET as soon as it drain voltage or current reaches max value.

Itsu	Re: Q measurement methods « Reply #79 on: 2026-05-11, 15:24:52 »
Group: Moderator Hero Member Posts: 4848	Right, so i need to have a current transformer of some sort where now the current probe is to pick up the ringing signal to be processed by the TLV3202. I have some LTS15-NP (perhaps to sturdy) which i could try for this.

Verpies

Re: Q measurement methods « Reply #80 on: 2026-05-11, 17:25:24 »

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Quote from: Itsu on 2026-05-11, 15:24:52

Right, so i need to have a current transformer of some sort where now the current probe is to pick up the ringing signal to be processed by the TLV3202.

Yes, so you could wind as many non-touching turns with a very thin wire as possible on a toroidal core and thread the parallel LC tank's wire inside it to see if your scope's voltage probe can pick up the ringing current.

Itsu	Re: Q measurement methods « Reply #81 on: 2026-05-11, 20:02:37 »
Group: Moderator Hero Member Posts: 4848	Well, the problem with that is that the toroidal core (i tried several small and medium cores) hugely impact the number of rings thus the Q of the parallel LC tank circuit (like a ferrite bead would to avoid oscillations).

Verpies

Re: Q measurement methods « Reply #82 on: 2026-05-11, 21:04:51 »

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There are high-loss ferrite materials such as the ones used in ferrite beads and there are low-loss ferrite materials such as the core used in your current probe. Smudge should chime in here...

If you don't have a low loss core then we can think of more exotic ways of measuring the current. This method does not have to be very linear because the goal is only to count the pulses so you can use such things as Rogowski or Hall sensors or a TMR or GMR read heads (w/ preamplifier) from a broken hard-drive (circa 2006 or later).

There are also non-magnetic Ohmic methods of measuring µV at two different points of the copper wire that connects the p-LC tank.

Finally, there are also capacitive methods that sense the voltage across the LC tank's capacitor, but they worsen the low <1pF isolation capacitances that the TVS diodes provide when the MOSFET turns off.

Itsu	Re: Q measurement methods « Reply #83 on: 2026-05-11, 21:48:51 »
Group: Moderator Hero Member Posts: 4848	Ok, i had an defective P6302 current probe which i toke the ferrite core off which sits in the clamp part, so i think it should be the correct low loss core: I used this core the first, and it gave me the reduced ringing, then some other cores seems worse.

Verpies	Re: Q measurement methods « Reply #84 on: 2026-05-11, 23:37:34 »
Group: Administrator Hero Member Posts: 4856	Quote from: Itsu on 2026-05-11, 21:48:51 I used this core the first, and it gave me the reduced ringing, then some other cores seems worse. I did not have this core in mind. I had the one in mind that the DUT wire is threaded through.

Itsu	Re: Q measurement methods « Reply #85 on: 2026-05-12, 09:17:19 »
Group: Moderator Hero Member Posts: 4848	Yes, i was meaning to say that i used this core to thread the DUT wire through (next to the current probe).

Verpies	Re: Q measurement methods « Reply #86 on: 2026-05-13, 01:41:04 »
Group: Administrator Hero Member Posts: 4856	How does the resistance of this non-inductive current sensor compare with the DC resistance of your LC tank (ESR of the cap + DCR of the coil) ?

Itsu

Re: Q measurement methods « Reply #87 on: 2026-05-13, 18:21:46 »

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To measure the DC resistance of the capacitor and coil i connected them in series LC configuration so that at resonance they should cancel out their reactances.

I put my nanoVNA in "series measurement" to measure this expected low impedance.

I set the sweep frequencies as close to the resonance point as possible.

This is the result:

It seems to me that the total ESR and DCR resistances at resonance is 0.084 Ohm (84 mOhm) as can be seen in the red circle.

I checked the DCR of the coil to be 0.040 Ohm using an Ohm meter so that would make the ESR of the capacitor in the 0.044 Ohm range.

Itsu

gyula

Re: Q measurement methods « Reply #88 on: 2026-05-14, 15:28:29 »

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Hi Itsu and all,

I still think that the best for estimating the Q for the coil wound onto the T200-2 toroid core is to consider its data sheet info, see here:
https://www.overunityresearch.com/index.php?topic=4949.msg119037#msg119037 It says a Q of 425 at and around 2 MHz for a 20 uH coil on this core from #18 (1mm) wire.

The 20 uH has X_L = 251 Ohm reactance at 2 MHz, so the measured coil in the data sheet had a total loss resistance of X_L / Q i.e. about 251 / 425 = 0.59 Ohm.
Here we have to assume the tuning capacitor in their test (if they resonated their coil for the measurements) should have had a Q = > 10,000 or so.

Your coil has about 19.3 uH, the inductive reactance is 262 Ohm at 2157.35 kHz. So the Q of your coil, assuming the same total loss of 0.59 Ohm should be 262 / 0.59 = 444 maximum, assuming the tuning capacitors have a Q = > 10,000 this involves an ESR of 0.0262 Ohm.

If we use the DC resistance of your coil as 0.04 Ohm, the coil Q appears to be 262 / 0.04 = 6550. This does not seem correct, too high.
Data sheet would have included such a high Q. We need to figure out why you got such a low loss resistance.

Comments are welcome.

Thanks,
Gyula

Itsu

Re: Q measurement methods « Reply #89 on: 2026-05-14, 21:53:59 »

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Gyula,

i have put the T200-2 core / coil to the test by using the nanoVNA, so JUST the core with coil.

Sweeping from 100kHz to 4MHz and looking at:

S11 phase (i understand that for a coil to test we need to check it at its +90 degree point),
S11 series inductance,
S11 series resistance and reactance.

The results are here:

So at 90 degree phase (which is at 429kHz) noted by the red marker 1 we have an induction of 18.483uH and a series R of 1.367 Ohm, see in the green circle on the right.
The reactance of this 18.483uH at 2MHz is 253 Ohm as can be seen by the green circle bottom middle and the line pointing to the data.

So there are some differences with the earlier measured resistances both by the Ohm meter and the nanoVNA when using the LC circuit (coil with capacitor).

Next i will try to measure in this same way the used parallel capacitor (284pF).

Itsu

gyula	Re: Q measurement methods « Reply #90 on: 2026-05-14, 22:11:28 »
Group: Tech Wizard Hero Member Posts: 1385	Okay Itsu, thanks, will comment tomorrow.

Itsu	Re: Q measurement methods « Reply #91 on: 2026-05-15, 09:34:11 »
Group: Moderator Hero Member Posts: 4848	The problem i have is with the resistance (1.367 Ohm). This is just a 2 meter or so 1mm diameter magnet wire, which should not have such a high resistance. My Fluke Ohmmeter shows 0.1 Ohm with short (5cm) leads, so it must be in that range. Itsu

Itsu	Re: Q measurement methods « Reply #92 on: 2026-05-15, 10:28:06 »
Group: Moderator Hero Member Posts: 4848	I zoomed in on the lower graph (series R) and it shows that the resistance starts low as expected, but then quickly rises to 2.8 or so Ohms around 1MHz and stays there. I will run some further tests. Itsu

gyula	Re: Q measurement methods « Reply #93 on: 2026-05-15, 12:05:54 »
Group: Tech Wizard Hero Member Posts: 1385	Hi Itsu, The resistance is one issue, the other one is reactance: if we accept the inductance of 18.483 uH or around that value, this is close to the 19.3 uH the L meter showed at 100 kHz, then its inductive reactance at 2 MHz should be around 232 Ohm (instead of the 253 Ohm the nanoVNA shows). Gyula

Itsu

Re: Q measurement methods « Reply #94 on: 2026-05-16, 11:42:14 »

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Gyula,

I carefully recalibrated the nanoVNA and made the following measurements (shunt measurement):

Frequency 2MHz

Coil
S11 series L 18.8uH
S11 series X 238 Ohm

Capacitor
S11 series C 287pF
S11 series X -275 Ohm

The DC resistance (DCR) of the coil needs to be measured using the Shunt-thru measurement, but gives nonsensible results (3.3 Ohm).
But the Agilent U1733C LCR meter shows at 100Hz an R of 40mOhm, which agrees with the DC resistance specification of an AWG 18 (1mm diameter) of 21 Ohm/km, so (40 turns of 5cm = 2m) 2m = 42mOhm.

The ESR of the capacitor was measured using the method showed here: https://www.youtube.com/watch?v=eytQh_ucDxw and turns out to be 49.4mOhm (-54.1dB using the table / graph mentioned in the video description).

Itsu

« Last Edit: 2026-05-16, 15:23:11 by Itsu »

gyula

Re: Q measurement methods « Reply #95 on: 2026-05-16, 16:15:44 »

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Hi Itsu,

These results sound much better I think, though there is still one thing to discuss.
I think we have to consider the T200-2 total core loss which can be reverse calculated from the Amidon data sheet as about 0.59 Ohm, see my reply #88 above.

This 0.59 Ohm by Amidon measurements should already include the DC resistance too and because you use the same #18 wire, we can say with good certainty the total loss of your coil is 0.59 Ohm at 2 MHz.

If we add to this the ESR of your capacitors, we get 0.59 + 0.0494 = 0.6394 Ohm. So the Q of this series LC (18.8 uH, 287 pF) circuit at 2.1667 MHz where they resonate, would be 255.939 Ohm / 0.6394 Ohm = 400.2

This is very close to the data sheet Q value. (The 255.939 Ohm comes from either the X_L or X_C reactance at the resonant frequency 2.1667 MHz).

Thanks for your kind efforts and hopefully we can find a measuring setup with which this Q of around or above 400 can be measured for this series LC circuit, the very low, 49.4 mOhm ESR of your capacitors makes this possible too.

Gyula

Itsu

Re: Q measurement methods « Reply #96 on: 2026-05-16, 17:03:56 »

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Gyula,

OK, so besides the DCR of the coil of 42 mOhm, we also have the core losses totaling 590 mOhm.

I removed the ground plane underneath the 50:1 coil, and thus isolated the single secondary loop from it.

I connected the T200-2 core/coil directly to the 287pF capacitor in series and to this single secondary.

Resonance is at 2143.1kHz (3Vpp input from FG) and Vpp across this single loop is 135mVpp with a Vpp across the 287pF capacitor of 20.1Vpp.

Taking this for Q measurement i got 20.1Vpp / 0.135Vpp = 148.8

IF i use the -3dB bandwidth method, i get a Q of 2143.1 / 6.5 = 329.7

But this is with the voltage probes attached, which might have some influence on the measurement.

Itsu

gyula

Re: Q measurement methods « Reply #97 on: 2026-05-16, 17:56:35 »

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Well, the voltage probe adds its own 5 pF or so self capacitance to the 287 pF capacitors, lowering resonant frequency from 2.1667 MHz to 2.1431 MHz (this comes with about 293.35 pF) and also adds its input resistance in parallel with the 287 pF, which is certainly less than 10 MegaOhm, and a certain loss is also introduced by the 1 turn secondary coil feeding the series LC circuit.

These two losses (introduced) could explain, more or less, the Q of around 330 instead of the around 400 value with the -3 dB measurement method

Why the ratio of the secondary voltage and the capacitor voltage brings a much lower Q of 148.8 value (i.e. the HP Q-meter measuring principle) remains to be solved. We will figure it out.

Gyula

gyula

Re: Q measurement methods « Reply #98 on: 2026-05-17, 21:53:08 »

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Hi Itsu and all,

I noticed an interesting lossless transformator model among the LTspice Example files and decided to use it for simulating this setup.
The received Q was 349.2 which is close enough to your measured 329.7 value. I used the same component values, loss resistances that you measured with the nanoVNA.
I included the probe impedance across the 287 pF tuning capacitor but to get the 20.1 Vpp across it I had to use a 680 kOhm as the probe's resistor.
This reduces the simulated unloaded Q of 398 (no probe) to 349.2 mentioned above. With no any probe resistor the e2 voltage across the C1 capacitor is 22.9 Vpp (8.14 Vrms).

What still remains to be figured out is why this e1 and e2 voltage measuring method (which now works in the simulation) does not work in your setup?
Because the -3 dB Q measuring method gave to you Q=329.7 but the e1 and e2 voltage ratios gave Q=148.8 in the very same setup.
Comments are welcome.

I blown up the voltage scale in the scope display vertically so that the e1 voltage 57.7 mVpp should be seen correctly, this is why the e2 voltage 20.15 Vpp goes out of the display vertically. (Otherwise the e1 voltage would be a flat line.)

Gyula

Series LC simulation with ideal transformer.png

Q measurement methods

Series LC simulation with ideal transformer.png (91.97 kB, 1914x1012 - viewed 26 times.)

Itsu	Re: Q measurement methods « Reply #99 on: 2026-05-18, 19:39:34 »
Group: Moderator Hero Member Posts: 4848	Gyula, nice find this lossless transformer model, now we perhaps can find where the difference between theoretical / simulated Q values and my real life Q value comes from. It seems strange to me that my single loop output (e1 voltage) is higher than your simulated one while the output voltage (e2) is the same. Itsu