Grid-leak detector

A grid leak detector is an electronic circuit that both demodulates an amplitude-modulated alternating current and amplifies the recovered modulating voltage. It works by using a vacuum tube's nonlinear conduction between the cathode and control grid, along with the tube's amplification factor. Lee De Forest invented the circuit around 1912, and it served as the demodulator in the first vacuum tube radio receivers until the 1930s.

History

Early triode tubes used as detectors often had no resistor in the grid circuit. Sewall Cabot may have been the first to use a resistance there, in 1906. He noted that a pencil mark could discharge the grid condenser, and that touching the grid terminal would restart a detector that had stopped. In 1915, Edwin H. Armstrong described using a resistance of several hundred thousand ohms across the grid condenser to discharge it.

The grid leak detector peaked in the 1920s, when battery-operated, multiple-dial tuned radio frequency receivers with low amplification factor triodes and directly heated cathodes were common. Examples include the Zenith Models 11, 12, and 14. After screen-grid tubes became available in 1927, most manufacturers switched to plate detectors and later to diode detectors. The grid leak detector remained popular for many years among amateur radio operators and shortwave listeners building their own receivers.

Operation

The stage performs two functions. For detection, the control grid and cathode act as a diode. At small radio frequency carrier amplitudes, square-law detection occurs due to the nonlinear curvature of the grid current versus grid voltage characteristic. At larger carrier amplitudes, detection becomes linear because of unilateral conduction from cathode to grid.

For amplification, the varying direct current voltage on the grid controls the plate current. The recovered modulating signal's voltage is increased in the plate circuit, so the grid leak detector produces greater audio frequency output than a diode detector at small input signal levels. The plate current includes the radio frequency component of the received signal, which is used in regenerative receiver designs.

In operation, the control grid and cathode function as a diode while the grid voltage also controls the electron stream from cathode to plate.