Recently I've been working on an AM transmitter for 40m. It occurred to me that FM should be much simpler to produce. I had ordered some varactor diodes recently and first tried to FM a crystal or ceramic resonator. I can't get much deviation and the audio did not sound good.
Then I remembered a post on Soldersmoke about a largely unknown but surprisingly stable oscillator called a Franklin oscillator. It has two transistors and has a very light load on the tuned circuit.
Rather than the variable supply transistor I fitted a 5V three terminal regulator. Coupled to the tuned circuit is a varactor diode with an electrolytic capacitor through which line audio is inserted. There's a trim pot to set the DC to the diode.
Initial results are quite promising.
More to be done...
