Duplex Radio-Telephone (Revisited)

Main What is super-regeneration? W. B. Lewis Back to projects

The super-regenerative receiver (SRR) was invented by Edwin Armstrong in 1922. Popular for its very high gain and remarkable simplicity, in those days it was the only practical way to amplify VHF signals. The simplest SRR uses one active device - a single transistor or valve, yet can detect and amplify microvolt signals to headphone volume.

The SRR is just an oscillator coupled to an antenna. Oscillation is quenched (stopped) and allowed to build up again repeatedly. Supersonic quench rates between 30 and 100 KHz are typical. Oscillation build-up starts from signal and/or noise. Amplitude growth is exponential and may continue until it limits, in which case the circuit is said to operate in logarithmic mode, or may be quenched before limiting, in which case it operates in linear mode. Peak amplitude reached in linear mode is proportional to received signal strength. Time taken to reach limiting varies with the log of signal strength.

The SRR is deaf whilst quenched and external signals are swamped by the local oscillation once build-up has started, so there is actually only a short interval, known as the sensitive phase, during which the received signal is sampled.

If the quench waveform is rectangular, the SRR operates in the so-called step-controlled state; if sawtooth or sinusoidal, in the slope-controlled state. Quench waveform shape affects characteristics such as bandwidth.

Wideband FM can be demodulated by slope detection; but the SRR is best suited to AM. Audio may be extracted by various means, such as inserting a transformer in series with the power supply. Logarithmic mode introduces distortion; however, this is tolerable if modulation depth is not excessive.

An SRR may be separately or self-quenched. Self-quenched circuits are inherently logarithmic. A parallel RC network in series with a terminal of the active device causes a relaxation oscillation. Amplitude limiting of the HF oscillation drives the active device non-linear, rectification occurs and the capacitor charges. The bias point shifts and HF oscillation is quenched. The capacitor discharges, bias is restored and the cycle repeats. Self-interrupted oscillation, intentional or not, is also known as squegging.

Much work was done during World War II, on applications such as IFF (Identify Friend or Foe) transponders, to tame the SRR, which had a reputation for needing constant adjustment. This was not a problem for amateur operators; but reliable and repeatable designs were required for mass production. "Super-Regenerative Receivers" by J. R. Whitehead (Cambridge University Press, 1950) was a by-product of this wartime work.

Unless isolated from the antenna by an RF amplifier stage, a super-regenerative oscillator (SRO) is not only sensitive to hand-capacity effects around the antenna; but also radiates interference. When SRRs were most popular, no doubt many operators experienced quench locking!