Abstract: A system for transmitting digital data employs an angle modulated carrier signal having constant amplitude and a limited spectrum. The system comprises an offset quadrature phase shift key (QPSK) modulator with a "tracking filter" phaselocked loop at the output of the QPSK modulator. The phaselocked loop has a voltage controlled oscillator which produces a constant amplitude carrier signal at the output of the loop, and also has a low pass loop filter which reduces the spectrum of the phase modulated carrier signal. Additional spectrum shaping is obtained by connecting L-R-C circuits to the low pass loop filter to eliminate predetermined frequency components from the output of the loop.

Abstract: The clock signal required for demodulation of a received NRZ digital data stream is generated by detecting each transition across a reference axis made by the received NRZ digital data stream. This transition data is then utilized to change the voltage applied to a voltage controlled oscillator. The change in the voltage applied to the voltage controlled oscillator causes the frequency of the output from the voltage controlled oscillator to change. The change in the frequency of the output of the voltage controlled oscillator adjusts the period of the clock signal until the clock signal is synchronized with the transition data at which time it may be used to demodulate the recieved NRZ digital data.

Abstract: Two oscillators are utilized to convert digital data into an FSK modulated signal. One oscillator is at the mark frequency and the other is at the space frequency. FSK modulation is accomplished by switching between the two frequencies based on the state of the digital data. Phase and frequency shifting techniques are also provided to insure that a desired difference between the mark frequency and space frequency is maintained and to also insure that a proper phase relationship is maintained between the output from the two oscillators.

Abstract: A clock signal required for demodulation of a received NRZ digital data stream is generated by comparing transition data for the received NRZ digital data stream to a clock signal which has a period equal to the bit period of the NRZ digital data stream. The results of the comparison is utilized to shift the clock signal in such a manner that the clock signal will be synchronized with the received NRZ digital data stream so as to provide a clock signal which may be utilized to demodulate the received NRZ digital data.