Neutrodyne

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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 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 in the early 1920s by Harold Wheeler who worked in Alan Hazeltine's lab at Stevens Institute of Technology, so Hazeltine is usually given the credit.[1] Hazeltine's invention effectively neutralized 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.[1] 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.[1] 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.

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 resonant at the same frequency), 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.

Neutralization Technique

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).

See also

References

  1. ↑ 1.0 1.1 1.2 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

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