Measuring resonant frequency and bandwidth for a 2mH coil

[MishuRF]

Hello all,
I have one question related with measuring ferrite coils, unknown values(in the area of 1mH to 4 mH). I need the resonant frequency (arround 125kHz) with les than 1KHz accuracy. There is an ideea of creating a field with a signal generator(sweep freq @ 125KHz) and a known coil resonating at this frequency , than receiving with an other coil and a multimeter the signal strenght from this coil and making a calibration of the results(calibrating the system exactly like S21 VNA calibration). Than introduce the unknown coil between the both known coils and from this new results extract the datas for this new coil.
This measurement is very easy to be done also with VNA, but I need a much simple solution and cheap.First method requires signal gen,a good multimeter and some software,which is not the cheapest way.
Ideas?
Thank you in advance.
Mihail

 

[Skogsgurra]

If you have an oscilloscope, you can measure voltage across coil when connected in series with a capacitor.

Circuit: switch-coil-capacitor-other end of switch.

Open switch and charge capacitor. Remove power supply and close switch. The circuit will resonate with decreasing amplitude. Read oscillating frequency and damping and use text-book formula to calculate undamped resonance frequency and bandwidth.

Really cheap USB based DSOs are available from many sources. Google is your friend.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

Good advice by Gunnar. Measuring the frequency of the decaying transient response will give the same frequency as you would have gotten by doing a frequency sweep and looking for the peak response frequency ... . but will be a lot easier.

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[electricpete]

By the way, you mentioned bandwidth – that can also be determined from the transient response.

Assume a transient response of the form: I0*exp(-sigma*t)*sin(wd*t)
look at pos-peak-to-pos-peak time equal 2PI/wd, decay ratio= R
exp(-sigma*2*Pi/wd) = R
-sigma*2*Pi/wd = ln (R) = logdec
sigma = -logdec*wd / (2Pi).

Damping factor is:
zeta = (sigma)/sqrt(sigma^2+wd^2)
= -logdec*wd / [(2Pi) *sqrt( (logdec*wd / (2Pi))^2+wd^2)]
= -logdec*wd / [wd * sqrt(logdec^2 + (2PI)^2)]
= -logdec / sqrt(logdec^2 + (2PI)^2)

For lightly damped systems logdec^2 << 2PI^2
Zeta ~ -logdec / (2Pi)

Halfpower Bandwdith:
This is the distance from where the magnitude is 70.7% of peak below resonance to where the magnitude is 70.7% of peak above resonance. It is given by:
BW = 2*sigma = 2*logdec*wd / (2*Pi)


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Eng-tips forums: The best place on the web for engineering discussions.

 

[Skogsgurra]

Thanks Pete. I was too lazy to look it up.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MishuRF]

Hello Gunnar and Electricpete,
Thank you for input, very good idea using transient response than frequency responce. I will try with the system I have today and prezented in first post, and then with the ideea from you and compare results. The most important will be accuracy of measurement!
Thank you again !
Mishu

 

[kontiki99]

Before sophisticated electronic test equipment came along, engineers used to use reference books that listed bridge circuits assembled using known and unknown components similar to wheatstone bridges.

I have don't have first hand experience with those techniques, but I've have a pocket reference:

Maybe surf for:

Wein Bridge
Schering Bridge
Hay Bridge
Owen Bridge
Maxwell Bridge
Resonance Bridge

I think this is how WWII vintage General Radio test equipment worked.