Question on Q of toroid coil

Dave a écrit :

Q is the ratio of circulating energy to losses. Q = L w/Rs

With Rs the equivalent series resistance. Rs accounts for core loss, and copper loss and have a minimum value at low frequency, so your Q will vanish at low freq.

Also Rs varies quite a lot with frequency, so its almost impossible to tell the HF value from the LF one.

Q is XL / R, so for a higher frequency you will get a better Q.

BUT: core losses and skin effect are represented by R and they will rise somewhat, too. Above the self resonance frequency, parasitic C will dominate and X will dwindle, so Q will not rise forever.

Anyway, your LCR meter is probably OK.

regards, Gerhard

of

meter

does

(Nice meters those Extechs'). The 14uH will be correct but the reported Q value is worthless (unless you're working at 1kHz) and would still be as worthless if tested at 10kHz

"Fred Bartoli"

Rather big non sequitur there.

....... Phil

Huh. A Q meter. This interests me. Any ideas on where I could find info on building one of those? (I have a lot more time than money, and it does sound interesting...)

Thanks for the info.

Dave

They turn up on eBay from time to time. They sell for less than the parts.

Phil Allison a écrit :

Phil I know you can do better. Maybe my wording wasn't the best but you also know I'm not english native and I know you know the answer, so...

Anyway, when Lw tends to zero and Rs has to be > Rcopper (which happens to be finite) Q obviously tends to 0. No non sequitur at all.

Since when the ratio of a known quantity (Lw) to an unknown one (Esr) gives a known value?

"Fred Bartoli"

I'd like to be, under the sea

In an octopus's garden, with you ........

Fishing up such a putrid red herring from the depths of the ocean could impress only those congenitally out of their depth.

Only aging hacks play to that audience.

You can do much better.

....... Phil

You can measure the Q of the coil by driving it at the frequency of interest and measuring the phase angle between input and output.

Q = arctan phase angle

Cheers

PeteS

"PeteS"

Aliens brains have a thicker consistency ....

But not as thick as some.

...... Phil

That should have read:

Q = arctan (90 - phase angle) = arccotan phase angle.

Cheers

PeteS

Phil Allison a écrit :

Don't know, but I know places where cheese tastes as good as caulk.

This one is a nice design but needs an external signal generator.

Aah. Yes, I found that last night thanks to Google. Also two others, one of which I have the pay for. Am looking at options. But thank you very much for the referrence. I'm saving that as a favorite, thinking I'm going back there.

Take it easy...

Dave

Now, *this* is really interesting. But how would you accurately measure the two phase angles? Please share whatever information you have on using this method of determining Q.

Thanks,

Dave

Hi Dave, You can use an oscilloscope and a signal generator to get a pretty good idea of Q. Do you have an oscilloscope and signal generator? If so, I'll post the method I have used. Mike

Hello Mike,

I have an Oscope, but no RF signal source. Am now planning on correcting this second-place deficiency however, so please feel free to post your information. I'll save it, and use it as soon as I can. Knowledge is never wasted.

I am planning on building a Q meter, probably the Poor Man's Q-Meter from Silicon Chip Online at

For this project I also need a signal source, so it is on the agenda.

Thanks much,

Dave

Oh come on, do you really need the underlying physics and electronics of standard Vector Network Analysers (VNA) explained to you?

Hey Mike,

I have the Oscope, and am planning on getting a signal generator. Icurrrently have a source of RF, but only at one frequency (for whatever crystal I plug into it.) Please, speak! Knowledge is never wasted, and I would like to hear what you have to say...

Thanks,

Dave