| 摘要 |
A phase-locked loop having improved off-frequency detection. A variable frequency output signal is to be precisely locked to the frequency and phase of an alternating input signal. This is accomplished by alternately utilizing two feedback loops. A reference signal, having a frequency approximately equal to the frequency of the input signal is input to an initialization feedback loop in which it is mixed with the output signal. The initialization loop produces a feedback signal for controlling a voltage controlled oscillator which generates the output signal. An off-frequency detector detects the frequency difference between the output signal and the reference signal. When this frequency difference decreases below a predetermined level, the off-frequency detector disables the initialization feedback loop and enables a primary feedback loop. In the primary feedback loop the phase of the output signal is compared to the phase of the input signal, and the difference signal controls the voltage controlled oscillator in such a manner that the phase of the output signal tracks the phase of the input signal. With the scheme, the initialization loop, which has a wide bandwidth, is used to control the voltage controlled oscillator over a wide range of frequencies of the output signal (i.e., the initialization loop has a broad capture range). When the frequency of the output signal has been brought within the capture range of the primary loop, operation is switched to the primary loop. The primary loop has a narrower bandwidth and therefore provides better noise rejection than the initialization loop.
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