Overview

Spurious emissions (a.k.a. "spurs," output energy that bears no apparent harmonic relationship to the input signal) and inter-modulation products (a.k.a. "inter-mod," or "IMOD," output energy functionally related to either the input signal, or some harmonic ^[1] thereof) are as ubiquitous in the RF world as water, and are therefore favorite topics of discussion among most RF design engineers. As mentioned in the text, any time you send a pulse of energy (mechanical or electrical) into anything that resonates, including an invisible "parasitic" circuit composed of stray capacitance and inductance, that resonator will extract the frequency component(s) it is tuned to, from the near infinite number of components that make up that pulse, and will begin to oscillate at that frequency. In other words, when our tuned resonator is hit by any sufficiently strong pulse of energy, the portion of energy at its resonant frequency (or any multiple thereof) drives the "device" into oscillation much the same way striking a tuning fork imparts enough energy into it to drive it into oscillation. This effectively explains one of the possible mechanisms involved in the generation of "random" spurious emissions from a circuit, but is by no means the only mechanism.

Another possible source of spurious noise is the active devices used in the system itself, particularly when these devices are operated in a decidedly nonlinear mode (e.g., Class "C" amplifiers during on/off transitions). During this operation they function as mixers, effectively combining all the signals injected into them...