Beam deflection tube

Beam deflection tubes are vacuum tubes with an electron gun, a beam intensity control grid, a screen grid, sometimes a suppressor grid, and two electrostatic deflection electrodes on opposite sides of the electron beam, that can direct the rectangular beam to either of two anodes in the same plane. They can be used as two-quadrant, single-balanced mixers or (de)modulators with very linear qualities, their mode of operation similar to one-half of a Gilbert Cell, by applying an unbalanced signal f₁ to the control grid and a balanced signal f₂ to the deflection electrodes, then extracting the balanced mixing products f₁ − f₂ and f₁ + f₂ from the two anodes. Similar to a pentagrid converter, the cathode and the first two grids can be made into an oscillator. Two beam deflection tubes can be combined to form a double-balanced mixer.

They need extensive shielding against external magnetic fields. The ballistic transistors currently under development employ a similar principle.