Abstract: An underwater communication system provides reliable underwater transmission of digitally encoded communications as a series of consecutive acoustic frequency pulses (40) wherein the frequency of each pulse corresponds to one of two preselected frequency settings (F.sub.A,F.sub.B). A communication is transmitted as a series of frequency pulses (40) divided into a header section (44), a message section (45), and a parity bit (46). The header bits are of a first pulse width (T1), and the message and parity bits are of a second pulse width (T2). Received communications are converted into an electrical receive signal by a hydrophone (76), which is processed by a sonar control (61) and provided to a CPU (62) which decodes the communication for display on a display unit (60).

Abstract: An electrically tuned super-regenerative receiver (10) comprises a feedback type oscillator having a signal output and a signal input, phase shift components (30) connected in a feedback loop for coupling the signal output to the signal input to cause oscillations, and a quench oscillator (35) coupled to the signal input for switching the oscillator between an oscillating and a non-oscillating condition. A variable capacitance device (40), such as a varactor diode, is connected between the signal output and a reference potential (21), and a microprocessor (105) provides a tuning voltage (42) which is applied at the signal output for varying the current in the varactor diode, thereby varying the oscillator center operating frequency. The oscillator center operating frequency is detected by a spectrum analyzer (112) and compared to a desired center operating frequency by a controller (118).