the superregenerative 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 automatic gain control (agc).