The question was asked. How to make a frequency doubler. Several answers were offered. The advice of one member was one test is better than the advice of one thousand experts. I kind of like the empirical method myself so here we go. What does it take to make a doubler? A signal source. Check i have a dip meter so far so good. A tank circuit. I took an inductor and a capacitor from the parts bin. Check have a tank. A way to clip the input to less than 90 degrees pulses. I have a diode, resistor and a battery. Check can build a clipper. Bread boarded the circuit like this.
This simple circuit puts a 1.5 volt battery potential across a diode in series with a 10 K ohm resistor reverse biasing the diode. As I drive the circuit my input will have to exceed 1.5 volts and forward bias the diode. If I can the wave forms will look something like this.
This is actually from a transistorized doubler but the wave forms will be the same. As the diode switches on it will pass the peaks. I hope to get less than 90 degrees. My signal is adjusted by adding or removing loops on the dip meter coil. My test setup looks like this.
My son gave me the scope and I'm still learning how to use it. It does have a frequency reading at the bottom of the screen. I'll zoom in on it.
As you can see the frequency is just over doubled. If it were a serious circuit I would tweak it a little. I'll call it a proof of concept and set it aside for now.
The green waveform is not distorted as it seems. Every other peak 'shrinks' as power is drawn by the tank.