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Hazeltine's prototype Neutrodyne receiver, presented at a March 2, 1923 meeting of the Radio Society of America at Columbia University. It had two stages of tuned radio frequency amplification. (closeup, bottom) The 3 interstage coupling coils, from which the neutralizing signal is taken, are visible.

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".[1][2] 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 amplifiers' tuned anode coils was used to generate a small antiphase signal, which could be adjusted by special variable trim capacitors to cancel out the stray signal coupled to the grid via plate-to-grid capacitance.

The circuit was developed around 1922 by Harold Wheeler who worked in Louis Hazeltine's lab at Stevens Institute of Technology, so Hazeltine is usually given the credit.[3] Hazeltine's innovation was to add a circuit to each amplifier stage which fed back a small amount of energy from the plate (output) circuit to the grid (input) circuit with opposite phase to "neutralize" the feedback through the tube's interelectrode capacitance which was causing the oscillation. This effectively prevented the high-pitched squeals that had plagued early radio sets. A group of more than 20 firms known as the Independent Radio Manufacturers Association licensed the circuit from Hazeltine and manufactured "Neutrodyne" receivers throughout the 1920s.[3] 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.[3] The Neutrodyne broke this stranglehold, allowing competition in the radio market. Compared to the technically superior superheterodyne the Neutrodyne was cheaper to build. Also, 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.

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

Tuned RF amplifier stage used in TRF receivers. Cgp, 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 oscillate, producing noises in the loudspeaker.
Original Neutrodyne circuit. CN 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.
Modified Neutrodyne circuit in which the feedback is taken from the secondary of T2 instead of the primary.

Neutralization technique[edit]

To neutralize a Neutrodyne receiver the procedure went something like this:

  1. 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.
  2. Turn the receiver off.
  3. Start with the final RF stage and work backwards towards the antenna.
    1. 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 by using a neutralizing adapter between the tube and its socket.
    2. Turn the receiver on. If you can hear the signal that the receiver is tuned to, that tube needs neutralizing.
    3. Adjust the neutralizing capacitor for that tube for minimum volume.
    4. Re-tune the receiver for maximum volume.
    5. Adjust the neutralizing capacitor for that tube for minimum volume.
    6. Turn the receiver off.
    7. If you disabled the filament by unsoldering it, resolder the lead.
    8. Repeat with each tube until all RF stages have been neutralized.
  4. 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 somewhat 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).

Neutrodyne receiver used on US President Warren Harding's yacht, the 'Mayflower'

See also[edit]


  1. ^ US Patent No. 1450080, Louis Alan Hazeltine, "Method and electric circuit arrangement for neutralizing capacity coupling"; filed August 7, 1919; granted March 27, 1923
  2. ^ Hazeltine, Louis A. (March 1923). "Tuned Radio Frequency Amplification With Neutralization of Capacity Coupling". Proc. of the Radio Club of America (New York: Radio Club of America) 2 (8): 7–12. Retrieved March 7, 2014. 
  3. ^ a b c Lee, Thomas H. (2004). The Design of CMOS Radio-Frequency Integrated Circuits, 2nd Ed.. UK: Cambridge University Press. pp. 15–16. ISBN 0521835399. 

External links[edit]