Signal original form to a waveform expressed as a current ("I") or a voltage ("V"), or an electromagnetic waveform, for example, an optical signal or radio transmission. Once expressed as an electronic signal, the signal is available for further processing by electrical devices such as electronic amplifiers and electronic filters, and can be transmitted to a remote location by electronic transmitters and received using electronic receivers. In Electrical engineering programs, a class and field of study known as "signals and systems" (S and S) is often seen as the "cut class" for EE careers, and is dreaded by some students as
The Thing (listening device) consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna; it had no power supply or active electronic components. The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter. This is currently referred in NSA parlance as "illuminating" a passive device. Sound waves (from voices inside the ambassador's office) passed through the thin wood case, striking the membrane and causing it to vibrate. The movement of the membrane varied the capacitance "seen" by the antenna, which in turn modulated the radio waves