Oscillator circuits are one of the basic building blocks of amateur radio equipment. Oscillator circuits are not only used to generate the signals we transmit. They are also an integral part of receivers, such as the superheterodyne receiver.
You can think of an oscillator as an amplifier with a tuned circuit at the input. This tuned circuit might be an LC circuit or a crystal. The values of the components in the tuned circuit determine the output frequency of the oscillator. There are three types of oscillator circuits commonly used in Amateur Radio equipment – Colpitts, Hartley and Pierce. (E7H01) Colpitts and Hartley oscillator circuits are commonly used in VFOs. (E7H06)
For a circuit to oscillate, it must have positive feedback with a gain greater than 1. (E7H02) In a Hartley oscillator, positive feedback is supplied through a tapped coil.
(E7H03) In a Colpitts oscillator, positive feedback is supplied through a capacitive divider. (E7H04)
In a Pierce oscillator, positive feedback is supplied through a quartz crystal. (E7H05).
In addition to these basic oscillators, there are a couple of other oscillator types that you have to know about for the Extra Class test. A magnetron oscillator is a UHF or microwave oscillator consisting of a diode vacuum tube with a specially shaped anode, surrounded by an external magnet. (E7H07) A Gunn diode oscillator is an oscillator based on the negative resistance properties of properly-doped semiconductors. (E7H08)
I could never keep Colpitts and Hartley straight, so I use a mnemonic. “Colpitts” and “capacitor” both startwith “C” and the Colpitts oscillator uses capacitive feedback. “Hartley” starts with “H”, like the unit for inductance, the Henry, and the Hartley oscillator uses inductive feedback.
I guess you could connect Pierce with “piezoelectric” for crystals.
Walter’s bit has worked for me for decades ;)