| 摘要 |
In the prior art (fig.1) the choice of loop filter damping resistor and VCO Kv results in a compromise between random jitter, tuning range and stability. The present invention overcomes these limitations by providing a VCO with two independent control nodes connected to the filter. The present loop filter / VCO circuit comprises a damping resistor connected between a relatively high kv input node and a relatively low kv input node, and a loop filter capacitor coupled in series with the damping resistor. The relatively high Kv input node is at the junction between the damping resistor and the loop filter capacitor. This is a low-noise, stable node with a slow response to loop adjustment, and gives the PLL a wide tuning range without producing excess jitter. Signals from the two input nodes are combined in a transistor circuit (fig.3) which supplies the control input of a Voltage Controlled Oscillator. The invention reduces the effect of thermal noise generated in a resistor in the PLL loop filter. |
