Finding tank circuit's resonance

[vijitbrisk]

Let's say I have a capacitor and inductor (used in a tank circuit for induction heating application). It operates at a PLL using CD4046. However, to calibrate the starting frequency I must know the tank's resonant frequency. Using either a micro-controller or any other logic, what would be the best way to identify the tank's resonance frequency by sweeping from a high frequency in such a way that I get a number displayed as the resonance of the circuit.

 

[KlausST]

Hi,

you say you have a capacitor and indutor. --> so f(r) = 1/ (2 * Pi * sqrt(L * C))

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But it seems you don´t have the values...

for sure you may swep frequency and find the max. or min current.

Maybe it is faster and maybe mor precise to check the phase.
If it is leading (I referenced to U), then it is capacitive. If lagging it is inductive.
Since the phase changes extremely fast at resonance frequency it is a method to find the frequency very precisely.

Try to run a simulation.

Klaus

 

[vijitbrisk]

Hi @KlausST
Thanks for replying.

Do you know any IC, which can be used to implement sensing the output current feedback while sweeping the frequency from a high to low and stop at the resonant frequency.

Vijit.

 

[chuckey]

Your tank circuit must operate at some nominal frequency either with no object in it and with a object in it. So find the first frequency is easy, so set your sweep frequency ,say 20% lower. Then put in the largest object with the most lossy material into the coil and see what frequency the circuit resonates to, higher?, if so set your sweep frequency to be +20% higher. So now you have constrained your sweep to a more manageable figure, say 15 -> 20 MHZ. Now you are in the realm of the possible.
Frank

 

[KlausST]

Hi,

Do you know any IC, which can be used to implement sensing the output current feedback while sweeping the frequency from a high to low and stop at the resonant frequency.

You did not give a single specification.
But to choose an IC one need a lot of specifications.
Supply voltage, frequency range, signal voltage and current, precision....

Klaus

 

[Warpspeed]

An easier way to do this is to make the tank circuit self oscillate at very low power, and then just measure the frequency directly.

The simplest way to do that is probably with a negative resistance Lambda oscillator circuit using P channel and N channel JFETs.
A circuit like that will oscillate from sub audio to VHF frequency with just about any parallel combination of L and C.

http://users.tpg.com.au/users/ldbutler/NegResDipMeter.htm

 

[BradtheRad]

An opamp can start oscillating automatically, at the resonant frequency of a series LC.



This circuit can be finicky to get started. You'll want to try different values for the potentiometer, supply voltage, ratio of L:C. Etc.

The 40 ohm resistor represents some amount of DC resistance in the inductor. The greater it is, the more difficult it is to make the circuit oscillate.

The op amp input must not be exposed to a voltage outside the supply rails. Therefore the potentiometer adjustment should be watched, because resonant action can cause volt levels to soar.

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Almost forgot to add:

Of course the circuit will only work, if the op amp is fast enough to operate at the LC resonant frequency. If it is not, there are other oscillators which operate from one or two transistors, however they are not necessarily any easier to get going.

 

[Warpspeed]

There are a million different possible oscillator circuits, but the problems of phase margin and gain, and sufficient circuit Q mean that oscillation can sometimes be difficult to start up with radical LC component values.

The beauty of the Lambda circuit is its extreme bandwidth and tolerance to vastly different impedances.

Its very simple with only two components, and with a digital frequency counter it is ideal for finding the resonance point of just about any LC circuit you are ever likely to encounter.

 

[kcyang]

The best way to identify the resonant is to measure the current through the inductor. The inductor current is maximum at resonant. So you can sweep the frequency until the inductor current reaches the peak. You can do that either measuring the inductor voltage or place a shunt resistor in series with the inductor for current measurement. You can learn more about resonant and maximum inductor current with these links.

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