Parasitic Oscillations

Author: J.B. Hoag

Spurious oscillations, whose frequency is neither that of the fundamental nor harmonics thereof, often occur when a transmitter is first built. They are called parasitic oscillations. They extract energy from the desired operating frequency, throwing it away in non-useful form, and also interfering with transmission on other channels. Hence they are undesirable and must be removed. They are due to the excitation of various circuits at the natural resonant frequencies of these units. The elimination of parasitics consists of introducing properly located resistances, chokes, or bypassing condensers, so as to prevent the offending element from oscillating at its own frequency. The location and elimination of parasitics is an art in itself, much of which can only be acquired from actual experience. One can detect the existence of parasitic oscillations by running a receiver through wide frequency ranges on the sides of the carrier frequency. These oscillations are often of considerable strength, whereas harmonics are usually quite weak in comparison with the fundamental frequency. Furthermore, parasitics generally have poor stability. Often, when a transmitter cannot be tuned or adjusted properly by the customary procedures, the trouble will be found to be due to the existence of parasitic oscillations.