The Neutrodyne

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. ...

, invented in 1922 by Louis Hazeltine, was a particular type of tuned radio frequency (TRF) receiver, in which the instability-causing inter-electrode

capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...

of the

triode A triode is an electronic amplifier, amplifying vacuum tube (or ''thermionic valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated Electrical filament, filament or cathode, a control grid, grid ...

RF tubes is cancelled out or "neutralized"US Patent No. 1450080, Louis Alan Hazeltine
"Method and electric circuit arrangement for neutralizing capacity coupling"
filed August 7, 1919; granted March 27, 1923 to prevent parasitic oscillations which caused "squealing" or "howling" noises in the speakers of early radio sets. In most designs, a small extra winding on each of the RF

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...

s' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

s to cancel out the stray signal coupled to the grid via plate-to-grid capacitance. The Neutrodyne circuit was popular in radio receivers until the 1930s, when it was superseded by the superheterodyne receiver.

History

The circuit was developed about 1922 by Harold Wheeler who worked in Louis Hazeltine's laboratory at Stevens Institute of Technology, so Hazeltine is usually given the credit. The tuned radio frequency (TRF) receiver, one of the most popular

designs of the time, consisted of several tuned radio frequency (RF) amplifier stages, followed by a detector and several

audio amplifier An audio power amplifier (or power amp) electronic amplifier, amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup (music technology), pickup, to a level that is high enough for dr ...

stages. A major defect of the TRF receiver was that, due to the high interelectrode

of early

vacuum tube A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause and effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handle ...

within the RF amplifier stages gave them a tendency to

oscillate Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulu ...

, creating unwanted radio frequency alternating currents. These parasitic oscillations mixed with the

carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or freq ...

in the detector, creating

heterodyne A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called ''heterodyning'', which was invented by Canadian inventor-engineer Reginald Fessenden. Heterodyning is us ...

s (beat notes) in the audio frequency range, which were heard as annoying whistles and howls from the speaker. Hazeltine's innovation was to add a circuit to each radio frequency amplifier stage which fed back a small amount of energy from the plate (output) circuit to the grid (input) circuit with opposite phase to cancel ("neutralize") the

which was causing the oscillation. This effectively prevented the high-pitched squeals that had plagued early radio sets. A group of more than 20 companies known as the Independent Radio Manufacturers Association licensed the circuit from Hazeltine and manufactured "Neutrodyne" receivers throughout the 1920s. At the time, RCA held a virtual monopoly over commercial radio receiver production due to its ownership of the rights to the Armstrong regenerative and superheterodyne circuits. The Neutrodyne ended this control, allowing competition. Compared to the technically superior superheterodyne the Neutrodyne was cheaper to build. As basically a TRF receiver, it was also considered easier for non-technical owners to use than the early superhets. After manufacture each tuned amplifier stage had to be ''neutralized'', adjusted to cancel feedback; after this the set would not produce the parasitic oscillations which caused the objectionable noises. By 1927 some ten million of these receivers had been sold to consumers in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. By the 1930s, advances in vacuum tube manufacturing had yielded the

tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids, and a plate electrode, plate (called ''anode'' in Bri ...

, which had reduced control grid to plate ( Miller) capacitance. These advances made it possible to build TRF receivers that did not need neutralization, but also made Edwin Armstrong's superheterodyne design practical for domestic receivers. So the TRF circuit, including the Neutrodyne, became obsolete in radio receivers and was superseded by the superheterodyne design. The Neutrodyne neutralization technique continues to be used in other applications to suppress parasitic oscillation, such as in RF power amplifiers in

radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter (often abbreviated as XMTR or TX in technical documents) is an electronic device which produces radio waves with an antenna with the purpose of signal transmissio ...

References

External links

Louis Alan Hazeltine
''"Scanning the Past" the Proceedings of the IEEE Vol. 81, No. 4, April 1993''

{{Webarchive, url=https://web.archive.org/web/20060221021111/http://www.digitaldeliftp.com/LookAround/lookaround_techspot.html , date=2006-02-21 Radio electronics History of radio Receiver (radio)

History

See also

References

External links