Q measurement methods

Author Topic: Q measurement methods (Read 1582 times)

Verpies	Re: Q measurement methods « Reply #50 on: 2026-05-01, 22:11:59 »
Group: Administrator Hero Member Posts: 4856	Did the low-capacitance low-voltage TVS diodes (like SP0402B-ULC) and low C_OSS MOSFET like the EPC2037 arrive, too ?

Itsu	Re: Q measurement methods « Reply #51 on: 2026-05-02, 08:51:22 »
Group: Moderator Hero Member Posts: 4848	Yes, they did, but they are unhumanly small especially for the elderly I am working on a small!! PCB in Kicad to be able to have the correct footprints to put those fleas on: Itsu

gyula

Re: Q measurement methods « Reply #52 on: 2026-05-02, 09:16:12 »

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Hi Itsu,

Thanks for the clarification on the ordered capacitors, I missed that you got it.

Unfortunately, the GJM1555C1H470FB01D type has no specs under 100 MHz, only for higher than that frequencies.

Gyula

Quote from: Itsu on 2026-05-01, 21:44:18

Gyula,

to avoid any confusion, the 6 parallel 47pF SMD capacitors i am using now instead of the variable air capacitor are the newly ordered ones.
But they are not the " Murata ERB series capacitors" verpies had recommended as they seem to be obsolete, and instead i was directed to this substitute: GJM1555C1H470FB01D which are also 47pF smd (very small 0402 type) capacitors with LOW ESR.
No indication on how low the ESR is however, but better than the variable air capacitor.

Itsu

Verpies

Re: Q measurement methods « Reply #53 on: 2026-05-02, 12:12:38 »

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Quote from: Itsu on 2026-05-02, 08:51:22

Yes, they did, but they are unhumanly small especially for the elderly

Of course. Just like with any capacitor construction - the smaller the surface area, the smaller the capacitance.
The permittivity of the material between these surfaces matters, too. Vacuum/Air is the smallest, FR4 is larger.

Quote from: Itsu on 2026-05-02, 08:51:22

I am working on a small!! PCB in Kicad to be able to have the correct footprints to put those fleas on:

The placement of components wrt to the LC tank connector matters, ...the inter-trace capacitance and trace lengths, matter too.

P.S.
I just thought that if you are going to manufacture a PCB, you might as well add a comparator/zero-crossing detector like the TLV3202 and some jelly-bean ICs to make a nifty Q-meter with a digital readout.
Once the ringdown waveform (green trace) is converted to a rectangular waveform (blue trace), the digital pulses can be simply counted by BCD counters like the 74HC4518 and displayed by latched 7-segment LED drivers like the NE589 or 74HC4511.
At 4% hysteresis, the pulse count equals the Q.

Q measurement methods

Ringdown - comparator output.png (19.92 kB, 800x480 - viewed 68 times.)

Itsu

Re: Q measurement methods « Reply #54 on: 2026-05-02, 20:43:21 »

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Quote from: Verpies on 2026-04-30, 23:16:38

I can't see the annotated single loop on that photo.
If I can't see it then it is too small.
Consider using a very thick wire for it. Like the pencil-thick speaker wire... or a slice of copper pipe. You will cross 300 then.

Instead of the 0.8mm diameter single loop secondary, i now used as this single loop secondary a piece of 5mm diameter copper tubing, see picture:

The signals using the voltage probes are like this:

Yellow is the output signal across the 284pF capacitor (6x 47pF smd parallel)
Blue is the input signal across the 50 Ohm input resistor (FG set to 5Vpp)
purple is the signal across the single loop secondary.

Fres was 2141kHz at 15.3Vpp, -3dB points (10.8V) are at 2144.1 and 2137.6 difference is 6.5, so Q is Fres / difference = 2141 / 6.5 = 329.3

Itsu

Itsu

Re: Q measurement methods « Reply #55 on: 2026-05-02, 20:57:25 »

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Quote from: Verpies on 2026-05-02, 12:12:38

Of course. Just like with any capacitor construction - the smaller the surface area, the smaller the capacitance.
The permittivity of the material between these surfaces matters, too. Vacuum/Air is the smallest, FR4 is larger.
The placement of components wrt to the LC tank connector matters, ...the inter-trace capacitance and trace lengths, matter too.

P.S.
I just thought that if you are going to manufacture a PCB, you might as well add a comparator/zero-crossing detector like the TLV3202 and some jelly-bean ICs to make a nifty Q-meter with a digital readout.
Once the ringdown waveform (green trace) is converted to a rectangular waveform (blue trace), the digital pulses can be simply counted by BCD counters like the 74HC4518 and displayed by latched 7-segment LED drivers like the NE589 or 74HC4511.
At 4% hysteresis, the pulse count equals the Q.

Well, such a nifty Q-meter with a digital readout would be a nice thing to have if one is in the habit of measuring the Q of coils often (which i do lately, but i can't remember doing it earlier).

But even then, it will make a nice project to work on, so let me see what jelly-bean parts i have available here.

You said: 'At 4% hysteresis, the pulse count equals the Q.", so does that mean that in my present configuration, the coil will produce a ringing signal consisting of 329 pulses?

Itsu

gyula

Re: Q measurement methods « Reply #56 on: 2026-05-02, 21:40:53 »

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Hi Itsu, very good Q result, the single turn copper tubing together with the shorter wiring and the direct soldering of one end of the tubing to the PCB increased the Q from 291 to 329.
The T200-2 core still have inherent reverse Q of up to 425 at this frequency but to utilize it a tuning cap of even less ESR would be needed than the one now. Of course, only in case if a Q > 400 is needed.

Gyula

Quote from: Itsu on 2026-05-02, 20:43:21

Instead of the 0.8mm diameter single loop secondary, i now used as this single loop secondary a piece of 5mm diameter copper tubing, see picture:

The signals using the voltage probes are like this:

Yellow is the output signal across the 284pF capacitor (6x 47pF smd parallel)
Blue is the input signal across the 50 Ohm input resistor (FG set to 5Vpp)
purple is the signal across the single loop secondary.

Fres was 2141kHz at 15.3Vpp, -3dB points (10.8V) are at 2144.1 and 2137.6 difference is 6.5, so Q is Fres / difference = 2141 / 6.5 = 329.3

Itsu

Verpies

Re: Q measurement methods « Reply #57 on: 2026-05-02, 22:42:11 »

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Quote from: Itsu on 2026-05-02, 20:57:25

You said: 'At 4% hysteresis, the pulse count equals the Q.", so does that mean that in my present configuration, the coil will produce a ringing signal consisting of 329 pulses?

Yes, down to 4% of the initial amplitude.

The only thing that can dampen that parallel LC ringdown is poor connections, thin wires and the active feedback circuit inside of your contactless current probe (or the ferrite of the probe itself but this ferrite should be very high Q because your probe is sensitive down to mA and has 70MHz bandwidth, AFAIR) .

Verpies	Re: Q measurement methods « Reply #58 on: 2026-05-02, 22:49:40 »
Group: Administrator Hero Member Posts: 4856	Quote from: Itsu on 2026-05-02, 20:57:25 But even then, it will make a nice project to work on, so let me see what jelly-bean parts i have available here. It's a rewarding project that doesn't cost much. Good for times when you find yourself idle, e.g.: waiting for the center post of that exotic core.

Verpies	Re: Q measurement methods « Reply #59 on: 2026-05-02, 22:51:46 »
Group: Administrator Hero Member Posts: 4856	Quote from: Itsu on 2026-05-02, 20:43:21 That's more like it. I can see the secondary now. Do you get higher Q with the secondary connected to the ground plane ...or without that connection ? Quote from: Itsu on 2026-05-02, 20:43:21 Fres was 2141kHz at 15.3Vpp, -3dB points (10.8V) are at 2144.1 and 2137.6 difference is 6.5, so Q is Fres / difference = 2141 / 6.5 = 329.3 You crossed 300 !

Itsu

Re: Q measurement methods « Reply #60 on: 2026-05-03, 11:06:05 »

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Quote from: Verpies on 2026-05-02, 22:51:46

That's more like it. I can see the secondary now.
Do you get higher Q with the secondary connected to the ground plane ...or without that connection ?
You crossed 300 !

I did not measure the Q without the secondary connected to the ground plane.

It was quite hard to solder that 5mm copper tubing to the ground plane so i would rather not try to remove it again.

Itsu

Itsu

Re: Q measurement methods « Reply #61 on: 2026-05-03, 13:56:03 »

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I went back to verpies his ringdown circuit here and left everything the same except for the parallel capacitor of the DUT.
Instead of the High ESR ceramic cap i now used the 6x 47pF parallel low ESR caps (284pF).

I expected that the Q will increase from the initially measured 19 to the around 300 which i reached with Gyula his method.

This is the result:

The time between pulses was measured to be 464ns, so with 300 pulses we would have a ringdown time of 139.2us which is about the time between the 2 vertical cursors.
So now we can see how that looks like and i wonder if the pulses still could be measured by using a comparator.

Itsu

Verpies

Re: Q measurement methods « Reply #62 on: 2026-05-03, 14:39:26 »

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Quote from: Itsu on 2026-05-03, 13:56:03

So now we can see how that looks like and i wonder if the pulses still could be measured by using a comparator.

Zoom around the right cursor and see.

Quote from: Itsu on 2026-05-03, 13:56:03

I expected that the Q will increase from the initially measured 19 to the around 300 which i reached with Gyula his method.

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.

Verpies	Re: Q measurement methods « Reply #63 on: 2026-05-03, 14:53:22 »
Group: Administrator Hero Member Posts: 4856	Quote from: Itsu on 2026-05-03, 11:06:05 I did not measure the Q without the secondary connected to the ground plane. Too bad. Are you familiar with the performance of symmetric vs. asymmetric cables for e.g.: microphones ? « Last Edit: 2026-05-03, 15:11:41 by Itsu »

Itsu

Re: Q measurement methods « Reply #64 on: 2026-05-03, 15:08:55 »

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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.

Zoomed in around the right cursor 139.2us:

The horizontal A cursor is on the middle line, the B cursor at the top peak pointing at 3.4ma amplitude which is about 4% of the starting amplitude of 84mA.

Pulses look OK to me to be measured by a comparator.

OK about the pulse length, so start with a low duty cycle.

Itsu	Re: Q measurement methods « Reply #65 on: 2026-05-03, 15:13:13 »
Group: Moderator Hero Member Posts: 4848	Quote from: Verpies on 2026-05-03, 14:53:22 Too bad. Are you familiar with the performance of symmetric vs. asymmetric cables for e.g.: microphones ? Sorry, edited your post instead of answering No not really, is there a relationship with the secondary being grounded versus floating?

Verpies

Re: Q measurement methods « Reply #66 on: 2026-05-03, 15:24:04 »

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Quote from: Itsu on 2026-05-03, 15:13:13

Sorry, edited your post instead of answering

I know, it can sometimes happen accidentally. Just pay attention to the yellow window outline - its presence means that you are editing someone else's post.

Quote from: Itsu on 2026-05-03, 15:13:13

No not really, is there a relationship with the secondary being grounded versus floating?

Yes, just like with symmetric vs. asymmetric microphone cables.
It has to do with flapping the ground plane wrt Earth ground.
Because of its large surface area, the ground plane is one plate of a large capacitor.
Ideal capacitors don't dissipate energy but ESR, radiation and lossy dielectrics, do.

Magluvin	Re: Q measurement methods « Reply #67 on: 2026-05-04, 15:44:00 »
Group: Mad Scientist Hero Member Posts: 1241	i agree with verpies. that ground plain probably introduces capacitance issues, considering the small value of intended caps involved. i see the spacer between the large core and the copper pcb, but again, how ever small the capacitance there is between the 2 could alter calculations product. mags

Itsu	Re: Q measurement methods « Reply #68 on: 2026-05-04, 16:05:47 »
Group: Moderator Hero Member Posts: 4848	OK, but as i followed this design of the HP4342A, which has the single turn firmly attached to ground, i also solidly grounded it: Itsu

Verpies

Re: Q measurement methods « Reply #69 on: 2026-05-04, 16:56:07 »

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Quote from: Itsu on 2026-05-04, 16:05:47

OK, but as i followed this design of the HP4342A, which has the single turn firmly attached to ground, i also solidly grounded it:

This schematic already has one error, namely the 0.001Ω vs 0.01Ω impedance miscalculation.
It is conceivable that the grounding is erroneously or ambiguously marked on that schematic, too ...and in reality is as depicted on the attached schematic.

Also, note that an autotransformer would have be sufficient for the impedance transformation if galvanic isolation between the primary and secondary winding was not planned.

Q measurement methods

Coil Q measurement HP4342A.png (21.1 kB, 695x264 - viewed 44 times.)

Q measurement methods

Autotransformer Solution.png (19.84 kB, 678x242 - viewed 44 times.)

gyula

Re: Q measurement methods « Reply #70 on: 2026-05-04, 21:04:00 »

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Hi Itsu and all,

In the meantime I have found further useful information on the issues mentioned.

The 0.001 Ohm versus the 0.01 Ohm umbiguity came from my reverse calculation, I did not think the source impedance labeled as Z₀ could be much less than 50 Ohm or even much less than 25 Ohm,
the latter came logically from the 50:1 turns ratio that insures an impedance transformation of 2500 from the supposed 25 Ohm but not to 1/1000, only to 1/100.

So it is not 25 Ohm but around in the range from 0.5 Ohm to 1.5 Ohm, this now comes from

first: the schematic diagram found in the online user manual of HP4342A, see the 1st attached picture.

and second: it comes from a very thorough series of measurements on the HP4342A Q meter described in this paper:
http://www.ve2azx.net/technical/HP4342A_Q%20Meter_Tests1.pdf This paper needs careful reading because it is full of data and includes measurements also on replicated 50:1 transformers.

So the impedance converter circuit seems to be an emitter follower having an output impedance in the range from 0.5 Ohm to 1.5 Ohm impedance within 20 kHz and 70 MHz,
this drives the primary coil of the 50:1 transformer which steps it down to around 1 milliOhm.

Yes, Math wise the 1 Ohm divided by 2500 gives 0.4 milliOhm, of course but as it turns out from VE2AZX measurements, this small value is little higher in the practical implementation (mainly due the estimated K=0.8 coupling factor between the primary and the secondary of the 50:1 transformer) and gradually increases in the function of frequency.

Taken from his text, it is 1.8 milliOhms at 100 kHz, 14 milliOhms at 10 MHz, 130 milliOhms at 50 Mhz and 228 milliOhms at 70 MHz.

I have also found a youtube video which shows how the 50:1 transformer is built, see it here: https://www.youtube.com/watch?v=tqjJgGce05Y and from video time around 8 minutes he starts disassembling the structure which directly encloses the toroid core.

I attached a screenshot of the 50:1 turns ratio transformer.

Probably there remains some further questions that are not clear yet. For instance in the tuning capacitor assembly circuit there is a 75 Ohm resistor shown across the primary coil of the transformer: most likely it trimms, compensates the impedance favorably on the primary side.

Gyula

Tuning cap circuit of HP4342A Q meter.jpg

Q measurement methods

Tuning cap circuit of HP4342A Q meter.jpg (132.18 kB, 1405x655 - viewed 41 times.)

Q measurement methods

HP4342A 50 to 1 transformer picture.jpg (161.73 kB, 942x828 - viewed 433 times.)

Verpies	Re: Q measurement methods « Reply #71 on: 2026-05-04, 21:13:02 »
Group: Administrator Hero Member Posts: 4856	I like the 1 turn secondary that spans the ENTIRE circumference of the toroidal core. I was going to suggest something like this but decided not to so as to avoid nitpicking too much.

Itsu	Re: Q measurement methods « Reply #72 on: 2026-05-04, 21:30:09 »
Group: Moderator Hero Member Posts: 4848	Gyula, thanks for this, it shows how an amazing instrument this HP4342A really is, nothing to compare with my setup. Itsu

gyula

Re: Q measurement methods « Reply #73 on: 2026-05-04, 21:54:07 »

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Itsu, no worry, your setup is able to measure Q values in the range of 300 in the 2-3 MHz range and this Q included the higher than desirable ESR of the tuning cap, And with a HP4342A you could measure a Q of around 410 or so the T200-2 core is capable of.

And with the inverter circuit (emitter follower) driving the primary coil the performance can be improved (in case the need arises, now it is not really needed).

Gyula

Magluvin

Re: Q measurement methods « Reply #74 on: 2026-05-04, 23:45:36 »

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Of the three schematics above one shows 2 separate types of ground for the left and the right of the total circuit. usually meaning they are not grounded together. where one side may be grounded to earth ground and the other to maybe an isolated chassis ground, or just negative reference, etc. so the top original circuit may not be incorrect.

for me the copper PCB under the large toroid could cause issues, again, the tiny SMD caps value needs to be THEE capacitance values for the test to give an accurate result as possible, and that copper plane may skew results by adding capacitance (small but maybe large enough to be a problem in reference to the SMD caps value) without realizing it.

mags