The
Neutrodyne was a particular type of
Tuned Radio FrequencyA tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. A 3 stage TRF receiver includes a RF stage, a detector stage and an audio stage...
(TRF) radio receiver, in which the instability-causing inter-electrode
capacitanceIn electromagnetism and electronics, capacitance is the ability of a body to hold an electrical charge.Capacitance is also a measure of the amount of electric charge stored for a given electric potential. A common form of charge storage device is a parallel-plate capacitor...
of the
triodeA triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode...
RF tubes is cancelled out or "neutralized". In most designs, a small extra winding on each of the RF
amplifierGenerally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of...
s' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim
capacitorA capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force...
s to cancel out the stray signal coupled to the grid via plate-to-grid capacitance.
The circuit was developed in the early 1920s by
Alan HazeltineLouis Alan Hazeltine was an engineer and physicist and the inventor of the Neutrodyne circuit, and the Hazeltine-Fremodyne Superregenerative circuit...
and made by a group of more than 20 firms that were licensed to produce Hazeltine-Neutrodyne receivers known as the
Independent Radio Manufacturers.
The
Neutrodyne was a particular type of
Tuned Radio FrequencyA tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. A 3 stage TRF receiver includes a RF stage, a detector stage and an audio stage...
(TRF) radio receiver, in which the instability-causing inter-electrode
capacitanceIn electromagnetism and electronics, capacitance is the ability of a body to hold an electrical charge.Capacitance is also a measure of the amount of electric charge stored for a given electric potential. A common form of charge storage device is a parallel-plate capacitor...
of the
triodeA triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode...
RF tubes is cancelled out or "neutralized". In most designs, a small extra winding on each of the RF
amplifierGenerally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The relationship of the input to the output of an amplifier—usually expressed as a function of the input frequency—is called the transfer function of the amplifier, and the magnitude of...
s' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim
capacitorA capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a voltage potential difference exists between the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force...
s to cancel out the stray signal coupled to the grid via plate-to-grid capacitance.
The circuit was developed in the early 1920s by
Alan HazeltineLouis Alan Hazeltine was an engineer and physicist and the inventor of the Neutrodyne circuit, and the Hazeltine-Fremodyne Superregenerative circuit...
and made by a group of more than 20 firms that were licensed to produce Hazeltine-Neutrodyne receivers known as the
Independent Radio Manufacturers. Hazeltine's invention effectively neutralized the high-pitched squeals that had plagued early radio sets. The design also neutralized the stranglehold that
RCARCA Corporation, founded as Radio Corporation of America, was an electronics company in existence from 1919 to 1986. Currently, the RCA trademark is owned by the French conglomerate Thomson SA through RCA Trademark Management S.A., a company owned by Thomson...
then held on the commercial radio industry. Compared to the technically superior Superheterodyne the Neutrodyne was cheaper to build and operate, and much easier for non-technical owners to use.
To properly set up a Neutrodyne receiver, not only did the circuitry need to be aligned for peak performance, (that is, getting all its tuned circuits operating "in step"), it also had to be
neutralized. However, this procedure only needed to be done once (albeit by a serviceman) and thereafter the radio could be tuned by anyone without special skill, a unique feature at the time. The neutrodyne was the first commercial receiver suited to use by the general public. By 1927 some ten million of these receivers had been sold to consumers in
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
.
By the 1930s, advances in vacuum tube manufacturing finally made
Edwin ArmstrongEdwin Howard Armstrong was an American electrical engineer and inventor. Armstrong was the inventor of frequency modulation radio, regeneration, and the superheterodyne....
's Superheterodyne design practical for domestic receivers. Ironically, these same advances made it also possible to build superior TRF receivers that did not need neutralization, but since an even more superior superheterodyne could be made for about the same cost, the TRF technique fell into disuse.
Neutralization Technique
To neutralize a Neutrodyne receiver the procedure went something like this:
- Turn the receiver on. Tune the receiver to a strong station near the high end of the dial (e.g., 1500KHz) or use a modulated signal generator set to that frequency and tune the receiver to it.
- Turn the receiver off.
- Start with the final RF stage and work backwards towards the antenna.
- Disable the tube filament (so the tube is not conducting) either by unsoldering one lead and taping the wire to prevent it from shorting to anything or (preferred) using a neutralizing adapter between the tube and its socket.
- Turn the receiver on. If you can hear the signal that the receiver is tuned to, that tube needs neutralizing.
- Adjust the neutralizing capacitor for that tube for minimum volume.
- Retune the receiver for maximum volume.
- Adjust the neutralizing capacitor for that tube for minimum volume.
- Turn the receiver off.
- If you disabled the filament by unsoldering it, resolder the lead.
- Repeat with each tube until all RF stages have been neutralized.
- The receiver should be realigned after being neutralized and if it still squeals the neutralization procedure repeated.
It is important to neutralize using the actual tube that will be in that socket as grid-plate capacitance varies some from tube to tube. Also once neutralized the tubes should not be exchanged between sockets. Often replacing a defective tube with a new one required neutralizing the receiver again (therefore tube replacement usually required a serviceman).
See also
- Crystal Radio Receiver
A crystal radio receiver — also called a crystal set — is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source except the power received from radio waves by a long outdoor wire antenna.-Overview:...
- Low IF Receiver
In a low-IF receiver, the RF signal is mixed down to a non-zero low or moderate intermediate frequency, typically a few megahertz. Low-IF receiver topologies have many of the desirable properties of zero-IF architectures, but avoid the DC offset and 1/f noise problems.The use of a non-zero IF...
- Regenerative Radio Receiver
- Superheterodyne Receiver
In electronics, the superheterodyne receiver is a receiver which uses the principle of frequency mixing or heterodyning to convert the received signal to a lower "intermediate" frequency, that can be more conveniently processed than the original radio carrier frequency...
- Tuned Radio Frequency Receiver
A tuned radio frequency receiver is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. A 3 stage TRF receiver includes a RF stage, a detector stage and an audio stage...
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