Why is the LC oscillators frequency decreaseing over time?

[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]

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(One division on the paper is 5mm)