Neutrodyn modtager

First neutrodyne radio receiver.jpg
The first neutrodyne radio receiver, presented by Louis Alan Hazeltine in a lecture on March 2, 1923 titled "Tuned radio frequency amplification with neutralization of capacity coupling" to the Radio Club of America at Columbia University, New York. Invented by Hazeltine at Stevens Institute, Neutrodyne receivers were widely sold until they were replaced by superheterodynes in the 1930s. The Neutrodyne circuit was a type of Tuned Radio Frequency (TRF) receiver. In a TRF receiver, the radio signal from the antenna is amplified at its original radio frequency by one or more vacuum tubes before it is rectified in a detector to extract the audio signal. However the triode tubes available at the time had high interelectrode capacitance between the plate and the grid which allowed signal from the output plate circuit to feed back into the grid, causing the tube to oscillate This produced interfering sounds, shrieks, whistles, and groans from the speaker. The Neutrodyne circuit solved this problem by adding a circuit which coupled some of the signal from plate to grid circuit with the opposite phase, canceling the feedback through the interelectrode capacitance. This was usually done with an additional winding on the interstage coupling transformers.

Hazeltine's prototype Neutrodyne is shown here, being examined by members of the Radio Club of America. It had five tubes: two stages of RF amplification, a detector, and two stages of audio amplification. The interstage coupling coils between the tube stages that provide the neutralizing signal are visible, mounted diagonally, between the tubes. The rectangular objects in foreground are "B" batteries providing the plate voltage, about 45 V, to the tubes. They are special types with multiple taps, allowing the plate voltage to be changed by cutting more or fewer cells into the circuit.