Neutrodyne

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.