4th Mar 2019, 12:34 am | #81 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
|
Re: Q-meter questions
Quote:
|
|
4th Mar 2019, 1:46 am | #82 | |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Q-meter questions
Quote:
https://www.digikey.co.uk/en/article...loscope-probes I think the Q of the gimmick cap is important and it has to be fairly high to minimise dielectric losses . In the cheap Ukrainian active probe article and the DIY FET-amplifier article, you will see the tips of the active probe or FET amplifier have to be carefully made and designed: http://www.crystal-radio.eu/fetamp/enfetamp.htm 0.3pF input copper plate capacitance would have to be estimated using empirical equations based on the dielectric properties. I am not sure any capacitance meter can measure this. |
|
4th Mar 2019, 8:48 am | #83 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
|
Re: Q-meter questions
Scope probes aren't *quite* as bad as they're being painted. A lot of the drop in impedance at higher frequency is capacitive reactance. This will pull the tuning of any tank it is probing, only the resistive component presented will damp the Q. The resistive component also falls, so the effect is still significant.
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
4th Mar 2019, 11:02 am | #84 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,766
|
Re: Q-meter questions
For what it's worth, I've attached another constructional article on a 'Q' Meter. Namely, the 'Cymar' which appeared in Radio & Electronics World in January 1984. Despite the design being 35 years old, it doesn't require any exotic obsolete components.
I always thought that R&EW was published by AMBIT - later Cirkit, who sold components and kits back in the day when people still liked to build things rather than to buy them. However, according to the americaradiohistory site, it states that R&EW was a successor to Radio Constructor, which ceased publication in 1981. ( R&EW ran from 1981 - '89, and though there is a section for it on the ARH site next to Radio Constructor, there doesn't appear to be any scanned issues as yet). Hope that might be of interest.
__________________
David. BVWS Member. G-QRP Club member 1339. |
4th Mar 2019, 6:42 pm | #85 |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
Re: Q-meter questions
There do seem to be a few similarities between some of the homebrew designs. The Cymar design uses a test source with a very high source impedance which we know isn't a good idea.
It also compounds this by using a common source JFET buffer amplifier ahead of the detector. This is definitely not a good idea as the input (Rp shunt) resistance of the common source JFET in that circuit will drop alarmingly even by 2MHz. Rp could be as low as 50k ohm in parallel with 10pF by 2MHz and it could be less than 5k ohm in parallel with 8pF by 20MHz. That's even worse than a x10 scope probe. So this design should definitely be avoided unless the aim is to make something that allows crude comparative measurements of Q. In other words, this design is a toy at best.
__________________
Regards, Jeremy G0HZU |
4th Mar 2019, 8:26 pm | #86 |
Nonode
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,580
|
Re: Q-meter questions
Thank you David for the details on the 'Cymar' Q-meter. On the ARH website, a number of issues of R&EW have been added recently: https://www.americanradiohistory.com...nics-World.htm
Regards, Symon. |
4th Mar 2019, 9:58 pm | #87 | |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,766
|
Re: Q-meter questions
Quote:
What a wonderful resource.
__________________
David. BVWS Member. G-QRP Club member 1339. |
|
4th Mar 2019, 11:53 pm | #88 |
Octode
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,654
|
Re: Q-meter questions
There's someone advertising a Wayne Kerr B801 admittance bridge on the vmars email list. These are the details of the post:
The following Wayne Kerr Admittance Bridge set of units is going to the Tip; unless somebody is curious about it. Vintage:- is about 50 !! years old. VHF Admittance Bridge; type B801. [I think this is based on Ratio Transformers with switched selection of 'Taps'.] [allows 'Y' parameter measurements used with control unit below.] Bridge Source; type S1618. [5MHz to 100MHz.] Control Unit; type C.U. 681. [Provides bias volt and current setting for active devices.] Transistor Adaptor; type Q801A. Standards; type Q761 [components in polished wood box]. Anyone who wants to follow this up and is not on the vmars list - please PM me. Richard |
5th Mar 2019, 2:20 am | #89 | |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
Re: Q-meter questions
Quote:
When this is transformed and plotted as Rp and Cp it gives the equivalent parallel (damping) resistance and the parallel capacitance vs frequency. I think I've posted up this type of plot before but it might be useful to see it again on this thread. It shows how quickly the Rp value drops as the frequency increases. The plot is just for a model but a typical scope probe will look very similar. I've seen some x10 probes where the equivalent model is 90R in series with 11pF but usually it's around 50 or 60R in series with 12pF and this basic model holds true across maybe 1MHz to 100MHz or so. So the plot below should be quite useful I think.
__________________
Regards, Jeremy G0HZU |
|
5th Mar 2019, 8:16 am | #90 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
|
Re: Q-meter questions
I use somewhat better probes than the usual cheapies, and I avoid the switched x1/x10 jobbies, their loading is still a problem for any RF work.
Sealectro did some rather useful resistor tips for SMC cables in 500 anf 5k Ohm flavours, these are a lot more useful at moderate RF. Tek do a'passive' probe which is also a resistor ahead of normal 50 Ohm coax. The price isn't good for what it is! Otherwise I break out the HP 500MHz active probes. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
5th Mar 2019, 7:33 pm | #91 | ||
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Q-meter questions
Quote:
Exactly, they are one-touch lethal. I enjoyed my excursion into them for a while, but I had an ahah-moment one day and got rid. I will never encourage/endorse a build with one now. Same goes for the newer type with a high-powered inverter on board. They're potentially even worse, as silent (unless you can hear the supersonic component at 25KHz or so). Quote:
Having said that, I have looked all around for decent research and I found it. The take-away is that Tesla secondaries tend to have a Q between 200 (statistical mode) and 280. I've heard some claim Q's of 400 but this may be exceptional. This is from the IEEE's transactions on plasma science. Source: 'Optimising the secondary coil of a Tesla transformer to improve spectral purity 42(i) pp143.148 Authors: Raven, RM; Smith, IR; Novac, BM. Published under the creative commons license, so extensive citation is ok with acknowledgement. The paper addresses Tesla coil usage in non-recreational areas of usage including EHT generation for physics (especially plasma) research environments and so on. If the complex impedance of the coil is measured by a vector network analyser (VNA)...measuring this impedance at resonance and then finding the frequencies at which the inductive reactance and the capacitative reactance are equal in magnitude to eachother enable half-power points to be found for the fundamental and also higher mode resonances, denoted by fn. This response can be measured, since the lower (fl) and upper (fu) frequencies, corresponding to a halving of the input power, can be determined.This is nice. I don't have a VNA at the moment, although forum member MerlinMaxwell kindly sent me a mini-one. I just haven't got round to getting onto a Windows environment yet. Data published by this research group shows, as I say, the commonest value of Q to be 200. It peaks for secondary coils self-resonant at 630KHz, where it may reach 280, and tapers markedly for coils resonant at 800KHz or higher. What do you make of all that?
__________________
Al |
||
5th Mar 2019, 8:43 pm | #92 | |||
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Q-meter questions
Quote:
I calculated Q using Javatc 3D software. I have tried three different combinations of secondary coils and two primary configurations with different primary surge impedance, they all have predicted values of Q around 180-200. I have never been able to measure the actual Q. http://www.classictesla.com/java/javatc/javatc.html But I will be able to do once I receive the new toy Siglent SDS1202X 200 MHz scope tomorrow. I have receieved my Poor Man's active probe from Ukraine today. It is harder to measure Q with tesla coils because any human body or walls near the coil will affect the results. The same is true with the measurement of resonance frequency that I have to model the spark loading with a wire during physical measurement and software modelling with Javatc. I think Q is not as critical as coupling coefficient k in most cases becuase you cannot change it once you wind the coils. I make my tesla coil having adjustable coupling coefficient k (very fine adjustment of secondary coil using screw mounted riser platform). Quote:
Quote:
I think it is healthy to feel scare when one is handling high voltage because I develop the respect for it. I have never got zapped once since I started designing and building tesla coils. Last edited by regenfreak; 5th Mar 2019 at 9:00 pm. |
|||
5th Mar 2019, 10:21 pm | #93 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Q-meter questions
To be able to understand what are the limitations of the oscilloscope probe and coaxial cable, I will really have to grind through this Feng Shui chart and stomach the abstract concepts; the Ying Yang of L network matching with different topologies and frequency response:
http://highfreqelec.summittechmedia....E0306_Rhea.pdf The "Q of load" is discussed in part 2. : http://www.highfreqelec.summittechme...Rhea-part2.pdf Well more on my reading lists. It is difficult stuff. Figure 5 and 6 are very interesting! Last edited by regenfreak; 5th Mar 2019 at 10:42 pm. |
5th Mar 2019, 11:11 pm | #94 |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
Re: Q-meter questions
As I said earlier, I have no experience with Tesla coils. However, if you just want to know the Q of the Tesla resonator when it is stood vertically on a base then I'd have thought you could use the E and H field method. But this is just a guess.
Ideally, you would want to use a swept system like a (scalar) network analyser or a spectrum analyser with a tracking gen. But you could also use something cheaper like the Analog Discovery 2. The idea is that you transmit via the port 1 of the analyser with the H field probe and you receive with the E field probe on port 2. Move each probe closer and closer to the coil until you get a peak reading on the swept display and try and keep the probes far away enough that you get a steady reading but the bandwidth doesn't widen (through remote loading of the Tesla coil). Then just measure the 3dB bandwidth of the peak on the analyser and you can work out the Q. I use this method a lot for LC resonators or helical resonators. The H field probe couples a tiny amount of energy into the coil and the energy then sloshes back and forth inside the resonator between E and H fields. The E field probe can then pick up the E field part of all this. As I said earlier, I wrote some software that runs on my little netbook and this controls the VNA and it measures the Q in real time using active markers on the VNA. So once connected up, it can measure Q in real time. It will also measure the loaded Q if you bring anything near the coil that loads it. eg another smaller (primary?) coil. This method does away with source impedance error and also the detector error because the coupling to the system is remote and very light. But you do need a sensitive analyser as the signals returning will be in uV. With a coil as big as a Tesla coil the E and H field probes could probably be a couple of feet away and still get a solid measurement. But this might not be the goal you are seeking. I'm just guessing that you want to know the Q of the Tesla coil when it is acting as a resonator?
__________________
Regards, Jeremy G0HZU |
6th Mar 2019, 9:53 am | #95 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
|
Re: Q-meter questions
Jeremy , did you read the method in my post no. 91 up there , from a published research group /IEEE (not recreational Tesla coil use , for physics research applications)
Cheers
__________________
Al |
6th Mar 2019, 10:46 am | #96 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,902
|
Re: Q-meter questions
Whatever method you use, the coupling into and out of the resonator under test has to be VERY light. Trad Q meters need a tiny series impedance to inject signal and a very high resistance on their metering side. These are two ways of achieving light coupling electrically. Jeremy's field probes have to be used a good distance from the resonator for light coupling.
The aim is that the losses of the input and output couples have to be much smaller than the losses of a tank of the highest Q the instrument is to be capable of measuring with reasonable accuracy. If the couple losses are known, indicated Q figures can be corrected to an extent, but this multiplies errors just at the end of the scale where things get interesting. Murphy! David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
6th Mar 2019, 6:15 pm | #97 | ||
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
Re: Q-meter questions
Quote:
Quote:
But the Tesla coil is a parallel resonant system. A revised method would be to look at the right side of the smith chart. In theory at least you could find the points where the +/- imaginary value matches the parallel resistance Rp (at resonance). This would give the 3dB BW. But I don't think a regular VNA can do this because the impedances are so high. So I think I've missed what the research group means here.
__________________
Regards, Jeremy G0HZU |
||
6th Mar 2019, 11:09 pm | #98 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
|
Re: Q-meter questions
Thanks Guys. I will stick to the ring-down method for measuring Q in tesla coils. It is the simplest method and requires just a scope and a function generator. The other Q measurement technique is to use frequency sweep 3db method using an "antenna" dangled above the tesla coil and connected directly to the scope. I will leave the VNA and Smith charts to the experts.
I will also try to use very weak inductive or capacitive coupling method using the cheap active probe to measure Q. The active probe uses dual-gate Mosfet (like a tetrode) with a gate capacitance of about 2pF. As a hobbyist, I don't have deep pockets like you pro guys here. The cheap Ukrainian active probe from ebay has very small capacitive loading on the circuit (0.75pF) and high input impedance at the typical resonance frequency of tesla coils (100kHz to 450kHz). With frequency less 1MHz, the poor man's active probe is a very effective RF probe. This video (Mr Carlson) demonstrates the value of cheap DIY active probe (different design but similar idea): https://www.youtube.com/watch?v=NlbC-cMIBTY This guy did his university thesis on the evaluation of DIY active probe: https://cdn.instructables.com/ORIG/F...ZPIPJTFO82.pdf The passive 1:10 probe tends to load down the Q and detune the tank circuit with its large capacitance and low impedance at higher frequency. The passive 1:100 probe has too much attenuation and the amplifier noise or vertical sensitivity of the scope will become a big issue. The Q for the primary coil of a tesla coil is usually extremely high (never needed to be predicted or measured) because it is made of very thick copper pipes in order to minimise the skin effect. My large tesla coil has about 800-1000 amps current through the primary coil. The general rule is that Q for the secondary coil should high but there is no advantage to make it very high because once the sparks hit as the ground objects, the Q will be loaded down anyway. To improve the performance of a tesla coil with a given input power, it is better to increase the surge impedance of the secondary coil by increasing the secondary inductance in order to reduce the massive current through the primary coils. The higher the primary current, the more losses through the spark gaps. Also it helps to use large tank capacitance to move away from the resonance rise of the neon sign transformers and use the biggest topload to create the fattest and brightest sparks. This describes two Q measurement techniques for tesla coil: http://www.teslathon.de/stefan/tc/tech4.htm PS I always deliberately detune the tesla coil so that the resonance frequency coincides with the condition during the longest spark ground strike. The spark acts like stray capacitance and lowers the resonance frequency significantly. Last edited by regenfreak; 6th Mar 2019 at 11:27 pm. |
7th Mar 2019, 2:47 am | #99 | |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
Re: Q-meter questions
Quote:
The cheapo Ukranian probe is going to be a better bet I think.
__________________
Regards, Jeremy G0HZU |
|
7th Mar 2019, 2:57 am | #100 | |
Dekatron
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
|
Re: Q-meter questions
Quote:
Presumably your new scope will have an FFT mode and if you select it you should be able to configure it to see quite small signals just like a spectrum analyser. You should be able to get down into the uV region like this although you would be best to select the 'flat top' window function if you want the best amplitude response behaviour as you sweep a signal with a sig gen and watch it on the scope.
__________________
Regards, Jeremy G0HZU |
|