The Armstrong Oscillator is used to produce a sine-wave output of constant amplitude and of fairly constant frequency within the RF range. It is named after the electrical engineer Edwin Armstrong, its inventor. It is generally used as a local oscillator in receivers, as a source in signal generators, and as a radio-frequency oscillator in the medium- and high-frequency range.
The Armstrong Oscillator is identified by the feedback link positioned as a secondary winding on the tuning coil. The feedback mechanism is a coil that couples signal from the output of the amplifier back to the LC resonant circuit with the proper phase for positive feedback. The identifying characteristics of the Armstrong oscillator are that (1) it uses an LC tuned circuit to establish the frequency of oscillation, (2) feedback is accomplished by mutual inductive coupling between the tickler coil and the LC tuned circuit, and (3) it uses a class C amplifier with self-bias. Its frequency is fairly stable, and the output amplitude is relatively constant.