Neutrodyne

Lore & Background

The Neutrodyne receiver emerged from the need to stabilize early TRF receivers, which suffered from oscillation due to high interelectrode capacitance in triode vacuum tubes. This capacitance created unintended feedback between the plate and grid circuits, producing audible whistles or howling in speakers. Louis Hazeltine developed a method to cancel this capacitive coupling by introducing an equal and opposite feedback path, a technique he patented beginning in 1919 and described in a 1923 paper. In most designs, a small extra winding on each RF amplifier's tuned anode coil generated an antiphase signal, adjustable by variable trim capacitors, to cancel the stray signal coupled via plate-to-grid capacitance.

Practical adjustment methods that made neutralization usable in multi-stage receivers were developed in 1922 by Harold A. Wheeler, working in Hazeltine's laboratory at Stevens Institute of Technology. The Neutrodyne circuit was also developed partly to avoid the regenerative receiver patents held by Edwin Howard Armstrong. Commercial production began in the early 1920s, and by the mid-1920s, between twelve and fourteen independent manufacturers were producing licensed Neutrodyne receivers under Hazeltine's patents, allowing companies outside RCA to compete.

Reader's Guide

The Neutrodyne receiver was significant as a practical solution to the instability that plagued early TRF receivers, enabling higher gain without parasitic oscillation. Its neutralization technique allowed independent manufacturers to produce competitive receivers during a period when key regenerative and superheterodyne patents were controlled by RCA. The circuit remained popular in radio receivers until the 1930s, when advances in vacuum tube manufacturing yielded the tetrode, which had reduced control grid to plate (Miller) capacitance. These advances made neutralization unnecessary for TRF receivers and also made Edwin Armstrong's superheterodyne design practical for domestic use. Consequently, the TRF circuit, including the Neutrodyne, became obsolete in radio receivers and was superseded by the superheterodyne design. However, the neutralization technique itself continued to be used in other applications, such as suppressing parasitic oscillation in RF power amplifiers in radio transmitters.

Did You Know?

• Harold A. Wheeler developed practical adjustment methods for neutralization in 1922 at Stevens Institute of Technology.
• A Neutrodyne receiver was used on US President Warren Harding's yacht, the Mayflower.
• The neutralization technique is still used in RF power amplifiers to suppress parasitic oscillation.