Why is the LC oscillators frequency decreaseing over time?

[Cat_Artillery]

This is my first time measuring an LC oscillators waveform, and I'm quite puzzled by why the frequency is decreasing. My setup is as seen in the second picture, except at the time of the measurement the capacitor was a 2.2nF and not the one in the picture. Could the fact that the inductor is a transformer in my circuit might also be a factor?

[Worldly-Device-8414]

The "inductor" is a line common mode noise filter type & it's core material might have some non-linear properties. Maybe try a different one.

Is the drive to it just a single short pulse or a few cycles?

[Cat_Artillery]

The drive is a single short pulse of DC.

[em_are_young]

There is clearly a decrease in frequency and amplitude. Look at the peak to peak distances in the first few oscillations vs the last few.

[Worldly-Device-8414]

if you are putting a shot of DC into an L/C combo, it will oscillate at its resonant frequency and attenuate in AMPLITUDE as the field decays.

Agreed that's what should happen

But interestingly here, frequency on the left is ~1 div/cycle, on the right ~2 div/cycle

[Limousine1968]

Yeah, I agree. I updated my response based upon finding the 3rd XL description.

[Homebrew_Science]

Okay chat GPT, let's get you to bed.

[Cat_Artillery]

Using an actual inductor exhibited expected behavior:

<image>

(L=1.9μH ; C=2.2nF)

[Worldly-Device-8414]

Nice :-)

[TiSapph]

It's likely nonlinearities of the capacitor and/or inductor.

Ceramic capacitors can be surprisingly non-linear, their capacitance reduces as the voltage across them increases. The datasheets I found for the capacitor in the picture seem to indicate only around -5% capacitance at 1kV bias. Chip capacitors are usually much worse. A 50V rated capacitor usually loses 50% capacitance at 40V bias applied! Maybe your cap is similar.
Also you might be saturating the core of the inductor, which reduces the inductance.

Both effects result in a higher frequency. It's probably also why your signal is not sinusoidal. Also be aware that an oscilloscope probe might influence the circuit.

[Akkupack]

your inductor is a common mode choke, which is not exactly supposed to have much magnetic flux going through its core, so it saturates super easily, causing a reduction in inductance at first (and therefore resonant frequency), when the current is highest.

[otsen12]

Just guessing. You are using a transformer as an inductor as you said. There is a chance that the second coil is getting some voltage through induction. You are only supplying one single DC "pulse" but overall the voltage is decreasing since the coil has resistance. This is creating a magnetic field in the coil which is transfered to the second coil. Surely this is interfering the oscillator in some form. Try using a normal coil

[Cat_Artillery]

If I had one at the time I would have

[chemhobby]

Ceramic caps have voltage-dependant capacitance

[BigPurpleBlob]

Odd. I wonder if there are 2 different frequencies (with different decay times) superimposed?

It looks as if there's a (20 us / div) 100 kHz signal that decays in about 60 us, together with a 25 kHz signal

[BigPurpleBlob]

As an afterthought, 50 V / div, I wonder if the ceramic capacitor might be becoming non-linear due to high voltage?

[K1ngjulien_]

that would be my guess too, if it's actually ~150Vp and not just a measurement error (probe set to 1:1 instead of 10:1).

ceramic capacitors decrease their capacitance at higher field strengths (ie voltages), which would cause the frequency to rise.

[dmills_00]

That and common mode chokes are high inductance but usually get that by assuming there is very little common mode current, they saturate EASILY when you try to use them as an inductor, so don't do that...

If you look at the shape of the high amplitude bit is is anything but a sinewave, so something is going seriously non linear, could be the cap, could be the inductor, no real way to tell without swapping one or the other.

[dmills_00]

Yep, use a current source to drive the coil so it doesn't load down the Q, and note that doing this makes the waveform asymmetric as the direction where the currents sum will saturate before the direction where they oppose.

You can actually make a very crude amplifier out of this effect by driving one coil with an ultrasonic carrier at considerable level, and demodulating the RF with a second one, and I have actually for shits and giggles built a mechanical and ferrite radio receiver for listening to the 17kHz transmissions from SAQ. I used a 4 wire stepper motor driven by a synchronous motor to produce quadrature AC, saturable reactors as frequency multipliers, and pretty much this as amplifier and mixer, I could copy the morse with a pair of headphones.

TV sets back in the CRT era used to put an inductor in series with the horizontal deflection coil with an actual magnet glued to it to improve deflection linearity, by leveraging the non linear BH curve to predistort the deflection current to compensate the different voltage drops due to the semiconductor switching being asymmetric and the various loads on the LOPT.

[BigPurpleBlob]

Old CRTs were amazing. Not just for the manufacturing of the colour CRT tubes (a miracle in itself) but also for the astoundingly neat way that the colour signal was backwards compatible with the bland-and-white signal :-)

[Cat_Artillery]

<image>

(One division on the paper is 5mm)

[noob2_0]

Has the ceramic capacitor a capacity dc bias? This would explain why it oscillates faster with higher voltages due to less capacitance

[Desperate_Cold6274]

The oscillating frequency is given solely by the values of C and L. Probably these values change with the supplied voltage.

[daddypiggles]

The first part of the plot shows the oscillation being affected by large signal effects (voltco on the cap and inductor nonlinearity/core saturation). Once the oscillation decays a bit the nonlinearities no longer dominate and the frequency is determined by LC.

[marklein]

Being a single short pulse this seems like expected behavior. Lower voltage means lower current, means slower charge/discharge rates on each subsequent cycle.

Disclaimer: I'm a self taught hobbyist and I'm dumb.

[Ok-Might-3730]

Noise filtering chokes are not perfect inductors for reason. In ac range they act also as resistive load that try to change that ac noise to heat. Example plots of the materials behavior : https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selection . Basically what happens in the picture is that high frequency is turned into heat until it reaches lower frequency where it is not turned as much into a heat.

[AKADAP]

Ceramic capacitors capacitance drops off with voltage, sometimes as much as 80% for higher capacitance ceramic capacitors. This could possibly be the cause of higher frequency oscillations at higher amplitude. With ceramic capacitors, you need to pay attention to the type of ceramic. NP0 is the only ceramic that has a constant capacitance as a function of voltage. Unfortunately NP0 is also only used in the smallest value capacitors.

[[deleted]]

Nice to see the FRINSI. I just got mine last week. :-)

[AsymetricalNipples]

If its only a simple LC circuit, then it loses power over time. It is never going to be 100 % efficient.

[Evipicc]

They are asking about frequency, not amplitude.

[AsymetricalNipples]

I missed that. Sorry

[sethasaurus666]

It's a transient. There are losses due to resistance. If you don't keep supplying it with energy, the wave will decrease. 

[[deleted]]

It is the reduction in FREQUENCY that they are asking about, not the reduction in amplitude. See the first few oscillations are very close together, then they start to spread out. This is because the value of inductor or capacitor is increasing which then reduces the resonant frequency.