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. Th ...

, invented in 1922 by Louis Hazeltine, was a particular type of

tuned radio frequency A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector ( demodulator) circuit to extract the audio signal and usually ...

(TRF) receiver, in which the instability-causing inter-electrode

capacitance Capacitance is the capability of a material object or device 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 ...

of the

triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 19 ...

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 oscillation Parasitic oscillation is an undesirable electronic oscillation (cyclic variation in output voltage or current) in an electronic or digital device. It is often caused by feedback in an amplifying device. The problem occurs notably in RF, audio, and ...

s 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 may increase the power significantly, or its main effect may be to boost the v ...

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

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

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 A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carri ...

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 A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

and several

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

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

of early

vacuum tube A vacuum tube, electron tube, 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. The type kn ...

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 handled ...

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 pendulum ...

, creating unwanted radio frequency alternating currents. These

s mixed with the

carrier wave In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has a ...

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 Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

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 The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...

held a virtual monopoly over commercial radio receiver production due to its ownership of the rights to the Armstrong regenerative and

circuits. The Neutrodyne ended this control, allowing competition. Compared to the technically superior

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 A tuned amplifier is an electronic amplifier which includes bandpass filtering components within the amplifier circuitry. They are widely used in a variety of wireless applications. Schemes There are several tuning schemes in use, * Staggere ...

stage had to be ''neutralized'', adjusted to cancel feedback; after this the set would not produce the

s 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 and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

. 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 (called ''anode'' in British English). Th ...

, which had reduced control grid to plate (

Miller A miller is a person who operates a Gristmill, mill, a machine to grind a grain (for example corn or wheat) to make flour. Mill (grinding), Milling is among the oldest of human occupations. "Miller", "Milne" and other variants are common surname ...

) capacitance. These advances made it possible to build TRF receivers that did not need neutralization, but also made

Edwin Armstrong Edwin Howard Armstrong (December 18, 1890 – February 1, 1954) was an American electrical engineer and inventor, who developed FM (frequency modulation) radio and the superheterodyne receiver system. He held 42 patents and received numerous awa ...

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

, such as in RF power amplifiers in

radio transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...

s. {{multiple image , align = center , direction = horizontal , header = , image1 = Tuned RF amplifier stage.png , caption1 = Tuned RF amplifier stage used in TRF receivers. ''C''_gp, the internal capacitance between grid and plate in the tube ( Miller capacitance), creates a positive feedback path from output to input which can cause the stage to

, producing noises in the loudspeaker. , width1 = 224 , image2 = Neutrodyne circuit.png , caption2 = Original Neutrodyne circuit. ''C''_N applies a second feedback signal to the grid which is 180° out of phase with the first, canceling ("neutralizing") it, to prevent oscillations. The signal is taken from an opposite phase winding (''b'') on the interstage coupling transformer ''T2''. , width2 = 222 , image3 = Neutrodyne circuit - modified.png , caption3 = Modified Neutrodyne circuit in which the feedback is taken from the secondary of ''T2'' instead of the primary. , width3 = 234 , footer =

References

External links

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

Radio electronics History of radio Receiver (radio)

History

See also

References

External links