| 摘要 |
A circuit for generating a common signal as a function of either of two reference signals comprises a pair of phase-locked loops which share an oscillator. Each phase-locked loop generates a control signal as a function of one of the reference signals. One or the other of the control signals is coupled to the oscillator which generates the common signal. A comparator compares the two control signals and generates a signal indicative of their difference. While the control signal generated by the first phase-locked loop is coupled to the oscillator, the difference signal is applied to the second phase-locked loop where it controls the generation of the second control signal to minimize the difference between the two control signals. While the control signal generated by the second phase-locked loop is coupled to the oscillator, the second phase-locked loop is nonresponsive to the difference signal. Thus when a switch of reference signals occurs even if they are of different phase and even if that phase is slowly time-varying, the circuit can lock to the new reference signal at a phase dictated by the phase difference between the two references at the instant of the switch without causing the oscillator output to undergo significant frequency or phase changes. |
