question about the Quality factor of an air coil

[samy555]

Hi
If I wound a 100nH air coil to place in a 100MHz oscillator, how can I measure its Q?
Q= XL/R, where R is the ohmic resistance of the coil

thanks

 

[jeffrey samuel]

Xl=2*pi*f*L

so Xl= 2*3.14*100M*100n
= 6.18*10
= 61.8(approx)

i think so

 

[samy555]

I said Q not XL

 

[jeffrey samuel]

You can express the resistance,
R, of the coil as
R=ρ×N×π×D×4/(π×d2×1000)=ρ×N×D/(250×d2)=ρ×g×h×D/(250×d2×d2),
where ρ is the wire resistivity in ohmmeters and the resistance is expressed in ohms.




You can express the resistance, R, of the coil as R=ρ×N×π×D×4/(π×d2×1000)=ρ×N×D/(250×d2)=ρ×g×h×D/(250×d2×d2), where ρ is the wire resistivity in ohmmeters and the resistance is expressed in ohms."]You can express the resistance, R, of the coil as R=ρ×N×π×D×4/(π×d2×1000)=ρ×N×D/(250×d2)=ρ×g×h×D/(250×d2×d2), where ρ is the wire resistivity in ohmmeters and the resistance is expressed in ohms.

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as you know the inductance of th coil do the reverse to cal the resistance of the coil

 

[mtwieg]

At RF frequencies the best method is to resonate the coil with a very high Q capacitor at the frequency of interest, and measure the Q of the resonance with pickup loops (usually with a network analyzer).

 

[zsolt1]

hi,
as i remember from some high school lessons , series resonance can be used to measure Q of a coil ( Lx ), using 2 electronic voltmeters ( VE1, VE2) , a signal generator , and a condensator (Cv) in series with the coil
**broken link removed**

you vary the frequency of the signal generator (or the variable condensator Cv), until you get to resonation . You know that you are at the resonation point when you see that VE2 shows highest voltage level U2 . At that point you measure the voltage U1 at the output of the generator with VE1, then Q is U2/U1

as far as i remember ..... google it to be shore

Ps : as you can see from Q = U2 / U1 ; you don't even have to know the resonation frequency f0 ( or any frequency) , meaning that every generator can be used , and also any variable condensator. You just need 2 electronic voltmeters (that can operate on higher frequencys ....)

 

[zorro]

You can express the resistance,
R, of the coil as
R=ρ×N×π×D×4/(π×d2×1000)=ρ×N×D/(250×d2)=ρ×g×h×D/(250×d2×d2),
where ρ is the wire resistivity in ohmmeters and the resistance is expressed in ohms.

It is not the d.c. resistance what is needed, but the resistance at high frequency, that is different because of skin effect and other factors.
You can have good insights looking for the principle of operation of the Q-meter (an old but nice instrument).
Regards

Z

 

[FvM]

At RF frequencies the best method is to resonate the coil with a very high Q capacitor at the frequency of interest, and measure the Q of the resonance with pickup loops (usually with a network analyzer).

Putting the DC coil resistance into Q= XL/R will most likely overrate Q by a considerable factor. At older days, people used a grid-dipper for a Q estimation. For an air-coil, a calculated Rac value aware of the skin effect can't be very wrong.

Network analyzer is always an option, provided you have one and know how to operate it.

 

[D.A.(Tony)Stewart]

What test equipment do you have avail? SA? Pulse gen? Sig gen? scope?
There are several methods to measure Q. U can expect Q of 50~100 with wire diam D and air gap 2D depending on proximity effects and helix shape.
What impedance is the coil at 100MHz? Series mode or parallel?

 

[mtwieg]

Putting the DC coil resistance into Q= XL/R will most likely overrate Q by a considerable factor. At older days, people used a grid-dipper for a Q estimation. For an air-coil, a calculated Rac value aware of the skin effect can't be very wrong.

Uh, right. And I never suggested using the DC resistance...? My method is 100% empirical. You resonate the coil with a known capacitance at the frequency of interest, and the Q of the observed resonance will be equal to the Q of the inductor (assuming that the Q of the inductor is much lower than the Q of the capacitor, which is usually the case). I've used this method to measure RF coils with Q>300.

Did you intend to quote someone else?

 

[FvM]

My comment wasn't strictly related to your post. Sorry for confusion.

 

[D.A.(Tony)Stewart]

You can look at phase noise spectral shape, and also inject broadband phase noise with coil nearby with 3nS Pulse derived from 100MHz dividing by 1024 say and using XOR one shot.

Then measure noise levels of proximity injected noise on 100MHz output. With this noise spike every 1MHz up to 300 you can measure 3dB suppression of LC oscillator sidebands of ingress.