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A modulated spurious signal is a carrier signal that emanates from an information processing device and can be intercepted. It is a type of compromised signal referred to in Telecommunications Electronics Material Protected from Emanating Spurious Transmissions (TEMPEST), which refers to the investigation and testing of compromising signals. These signals are generated from the modulation of carrier signals and the electromagnetic field that can be created around them.
Modulated spurious signals are primarily created when a carrier signal is modulated. The carrier can be in the form of a parasitic oscillation generated within information processing equipment and the modulation can be in the form of angle modulation or amplitude modulation. The introduction of modulated data signals creates a spurious signal to modulate, and the result is emitted into the air or merged into the external conductors of the underlying equipment.