2019-10-12 RF Oscillator Won’t Make A Sine Wave, Only Sawtooth

I wanted to make an RF Oscillator that puts out a sinewave at about 734 kHz, which uses two convenient values, 100 uH coil and 470 pF capacitor. I used the toroid core with 16 turns of 30 AWG wire, with a tap at 4 turns then later 8 turns. I built the circuit in following photo and experimented with it a lot, changing values for the various parts and every time it would not oscillate or it would put out a sawtooth wave. I was monitoring the waveform at the emitter with my oscilloscope. The supply voltage was the same 4.5 volts throughout the experiment.

I started with the coil of 16 turns with a tap at 4 turns. Later I changed to 16 turns tapped at 8 turns. I started with the 2N4401 for the transistor and then changed it to a 2N3904 to see if it would change anything – it didn’t. The waveform stayed a sawtooth, with only changes to the frequency and amplitude. I varied the base bias resistor, starting at 150k, then I connected a 100k in parallel for about 68k, then I replaced those with two 22k in series for 44k.

I started out with a 10 nF or .01 uF DC blocking capacitor between the top of the coil and the base. When I reduced it to 1 nF or .001 uF, the sawtooth frequency went up. I put a 100 to 10k variable resistor in series with this capacitor and it varied the amplitude and frequency of the sawtooth, or else the sawtooth fell to nothing. I did a few other things that I haven’t mentioned. The only tendency I saw was the sawtooth got a slight wiggle in it somewhat like a damped sinewave.

This was supposed to be a Hartley oscillator. I’ve come to the conclusion that this circuit makes a very reliable sawtooth generator.

Update Oct 12 – I moved the .01 uF from emitter to +4.5V to emitter to – across the 1k resistor. I moved the scope probe from the emitter to the collector. Now the scope shows a sine wave on the sawtooth wave. Then while I was messing with it the sawtooth stopped and I saw for the first time just a sine wave! Yes! But the sawtooth came back, it just reappeared. It still is an unstable circuit. At this point I had set the 100 to 10k variable resistor to 100, so I increased it to 270 ohms. The sine wave was more stable, and it didn’t want to go into the sawtooth mode. I disconnected and reconnected the +4.5V several times, and most but not all of the times it put out only the sine wave. So it is still not stable and wants to go into sawtooth oscillation even though to sine waves are at the collector.

Update Oct 13 – I experimented with the values a lot and I have gotten a better feel for what puts the circuit into the sawtooth oscillation mode. I’m currently using the following circuit. It seems to be more stable and less prone to make sawtooth waves. The .01 uF from the top of the coil to the base has a lot to do with the sawtooth. I put a larger capacitor in parallel and it brought the sawtooth frequency down. Apparently this capacitor is charging and discharging. The 1k in series with the .01 uF reduces the gain enough to stop the sawtooth waves.

The waveform for the following circuit is sine wave but the bottom is skewed because it’s being clipped. Another odd thing is the L and C values should give a frequency of 734 kHz. But I measure 638 kHz with the DMM. This is too low and far off what it should be. I don’t understand what is causing the big difference. Normally the circuit is off no more than ten kHz.u

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