Regenerative Circuit - Super-regenerative Receiver

Super-regenerative Receiver

The super-regenerative receiver uses a second lower frequency oscillation (within the same stage or by using a second oscillator stage) to provide single-device circuit gains of around one million. This second oscillation periodically interrupts or "quenches" the main RF oscillation. Ultrasonic quench rates between 30 and 100 kHz are typical. After each quenching, RF oscillation grows exponentially, starting from the tiny energy picked-up by the antenna plus circuit noise. The amplitude reached at the end of the quench cycle (linear mode) or the time taken to reach limiting amplitude (log mode) depends on the strength of the received signal from which exponential growth started. A low-pass filter in the audio amplifier filters the quench and RF frequencies from the output, leaving the AM modulation. This provides a crude but very effective AGC (Automatic Gain Control).

Types of Signals for Regenerative vs. Super-Regenerative (super-regen or superregen) Detectors. Super-Regenerative Detectors work well for wide-band signals such as FM, where it performs "slope detection". Regenerative Detectors work well for narrow-band signals, especially for CW and SSB which need a heterodyne oscillator or BFO. A super-regenerative detector does not have a usable heterodyne oscillator - even though the super-regen always self-oscillates, so CW (Morse Code)and SSB (Single side band) signals can't be received properly.

Super-regeneration is most valuable above 27 MHz, and for signals where broad tuning is desirable. The super-regen uses far fewer components for nearly the same sensitivity as more complex designs. It is easily possible to build super-regen receivers which operate at microwatt power levels, in the 30 to 6,000 MHz range. These are ideal for remote-sensing applications or where long battery life is important. For many years, super regenerative circuits have been used for commercial products such as garage-door openers, radar detectors, microwatt RF data links, and very low cost walkie-talkies.

Because the super-regenerative detectors tend to receive the strongest signal and ignore other signals in the nearby spectrum, the super-regen works best with bands that are relatively free of interfering signals. Due to Nyquist's theorem its quenching frequency must be at least twice the signal bandwidth. But quenching with overtones acts further as a heterodyne receiver mixing additional unneeded signals from those bands into the working frequency. Thus the overall bandwidth of super-regenerator cannot be less than 4 times that of the quench frequency, assuming the quenching oscillator produces an ideal sinewave.