Q multiplier
A regenerative circuit that sharpens receiver selectivity via positive feedback.
A Q multiplier is an electronic circuit added to a radio receiver to boost both selectivity and sensitivity. It works as a regenerative amplifier that introduces controlled positive feedback inside the receiver. This feedback narrows the receiver’s bandwidth, making it behave as if the Q factor of its tuned circuits had been raised. During the vacuum tube era, Q multipliers were common in shortwave receivers, either built in at the factory or added later as an accessory. To get the best sensitivity and rejection of interference, the user had to adjust the Q multiplier to a point just before it started oscillating.
The same circuit could also be set to act as a notch filter, which helped reduce interference from signals close to the desired frequency. In some receiver designs, the Q multiplier could be adjusted into oscillation to serve as a beat frequency oscillator, allowing reception of single sideband or Morse code. However, when used this way, it no longer improved selectivity.
The principle of regeneration in radio receivers was developed by Edwin Armstrong, who patented a regenerative receiver in 1914. At least one console-model broadcast superheterodyne receiver from 1926 used positive feedback to improve selectivity. Q multipliers were especially common in shortwave general-coverage and communications receivers during the 1950s. Their popularity declined after crystal and ceramic intermediate frequency filters became available.
- Inventor of principle
- Edwin Armstrong
- Year of regenerative receiver patent
- 1914
- Earliest known use in superheterodyne
- 1926
- Peak popularity period
- 1950s
- Common application
- Shortwave general-coverage and communications receivers
Lore & Background
The principle of regeneration applied to radio receivers was developed by Edwin Armstrong, who patented a regenerative receiver in 1914. At least one console-model broadcast superheterodyne receiver used positive feedback to improve selectivity in a 1926 design. Q multipliers were common on shortwave general-coverage and communications receivers of the 1950s, appearing as either factory installations or add-on devices.
In use, the Q multiplier had to be adjusted to a point just short of oscillation to provide maximum sensitivity and rejection of interfering signals. It could also be adjusted to act as a notch filter, useful for reducing the interfering effect of signals on frequencies near the desired signal. In some receiver designs, the Q multiplier was made to also serve as a beat frequency oscillator by adjusting it to oscillate, enabling reception of single sideband or Morse radiotelegraphy, though in that case the circuit no longer provided improved selectivity.
With the advent of crystal and ceramic intermediate frequency filters, the Q multiplier was no longer popular.
Reader's Guide
The Q multiplier was a notable accessory in vacuum-tube-era shortwave receivers, offering a practical means to enhance selectivity and sensitivity without requiring expensive or bulky components. Its ability to function as either a bandwidth-narrowing device or a notch filter gave operators flexibility in managing interference. The circuit's regenerative nature required careful adjustment to avoid oscillation, but when set correctly it provided a significant improvement in rejecting adjacent signals. Some receiver designs integrated the Q multiplier with a beat frequency oscillator, allowing it to serve dual purposes for different modes of reception, though this sacrificed its selectivity-enhancing role. The decline of the Q multiplier came with the introduction of crystal and ceramic intermediate frequency filters, which offered more stable and maintenance-free selectivity. Despite its obsolescence, the Q multiplier represents an important step in the evolution of receiver design, demonstrating how controlled positive feedback could be harnessed to improve performance in an era before modern filter technology.
Did You Know?
- A Q multiplier is a regenerative amplifier adjusted to provide positive feedback within a receiver.
- It could be adjusted to act as a notch filter to reduce interference from nearby signals.
- In some designs, the Q multiplier could serve as a beat frequency oscillator for single sideband or Morse reception.
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