Neutrodyne
Encyclopedia
The Neutrodyne was a particular type of Tuned Radio Frequency
(TRF) radio receiver, in which the instability-causing inter-electrode capacitance
of the triode
RF tubes is cancelled out or "neutralized". In most designs, a small extra winding on each of the RF amplifier
s' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim capacitor
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 Hazeltine
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 RCA
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 America
.
By the 1930s, advances in vacuum tube manufacturing had yielded the tetrode
and, soon after, the pentode
, both of which had reduced control grid to plate (Miller) capacitance. These advances finally made Edwin Armstrong
'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.
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).
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. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
(TRF) radio receiver, in which the instability-causing inter-electrode capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
of the triode
Triode
A 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. The triode vacuum tube was the first electronic amplification device...
RF tubes is cancelled out or "neutralized". In most designs, a small extra winding on each of the RF amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
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 Hazeltine
Louis Alan Hazeltine
Louis Alan Hazeltine was an engineer and physicist, the inventor of the Neutrodyne circuit, and the Hazeltine-Fremodyne Superregenerative circuit. He was the founder of the Hazeltine Corporation....
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 RCA
RCA
RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...
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 America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
.
By the 1930s, advances in vacuum tube manufacturing had yielded the tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
and, soon after, the pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
, both of which had reduced control grid to plate (Miller) capacitance. These advances finally made Edwin Armstrong
Edwin Armstrong
Edwin Howard Armstrong was an American electrical engineer and inventor. Armstrong was the inventor of modern frequency modulation radio....
'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., 1500 kHz) 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 ReceiverCrystal radio receiverthumb|Boy listening to a modern crystal radioA crystal radio receiver, also called a crystal set or cat's whisker receiver, is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long wire antenna...
- Low IF ReceiverLow IF receiverIn 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 ReceiverSuperheterodyne receiverIn electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
- Tuned Radio Frequency ReceiverTuned radio frequency receiverA 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. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned...
External links
- Louis Alan Hazeltine "Scanning the Past" the Proceedings of the IEEE Vol. 81, No. 4, April 1993
- Neutralisation of Philco Neutrodyne-Plus Receivers
- The Neutrodyne circuit