| 摘要 |
<p>A cellular type high capacity mobile communication system requires a large number of channels for use on a selective basis within each of several small cells. In the system described, switching from one channel to another to avoid interference within the new cell is accomplished as the mobile communicator moves from one cell to another. Both the base station within each cell and the mobile units require the means for generating the necessary frequencies and for pulling in the frequency of the transmitter or receiver of the base station and the mobile unit rapidly and stabilizing it at the new frequency. In this frequency synthesizer, phase-locked loop instability resulting from over-leaping intermediate channel frequencies is overcome by detecting, ahead of the frequency and phase comparator, the absence of an output from the frequency countdown circuitry of the feedback portion of the phase-locked loop. The signal that is representative of the absence of the normal feedback signal or its complete lack is applied to a transistor circuit that biases the voltage-controlled oscillator to the midpoint of its range and that has such a low output impedance that it "swamps out" the relatively high output impedance of the feedback path after the comparator. As a stable feedback signal is restored so that a detectable frequency countdown signal is obtained, the transistorized circuit is disconnected. The mode of operation of this transistorized circuit is somewhat analogous to circuits commonly called crowbar circuits.</p> |
