Clock regeneration system for modem - uses colpitts oscillator, having positive and negative feedback, synchronised with output from pulse train generator

摘要

The clock regeneration circuit comprises a pulse shaping circuit (40) containing NAND (48) and NOR (50) gates. The output is a pulse train at the band rate, and a time constant (52,54) is chosen to provide narrow pulses. The pulses are injected via a potentiometer (56) to determine the amplitude of the sync. pulse to be applied to a Colpitts oscillator (44). Capacitors (60,62) form a passband filter to improve S/N ratio and reduce instability of the input pulse. The oscillator has additional positive feedback provided by capacitors (64,66) while a resistor (68) and capacitor (70) improve frequency and amplitude stability. The system is capable of regenerating the clock oscillator either at the bit rate, or at the band, even if no synchronisation pulse is present during extended clock periods with ones or zeros uninterrupted.