| 摘要 |
A crystal oscillator comprises a cristal 1 and a crystal oscillator circuit 3 for providing an output signal 5. In order to adjust the output frequency, for example during the testing stage of production, a bank of load capacitors C1 to C6 are provided. Each capacitor C1 to C6 may be switched into or out of the circuit by a respective switch 9 to 19. The switches are controlled by control lines D0 to D5, which are set by a 6 bit register 21, known as the trim register. The value in the trim register is set according to the frequency of the output signal. To counteract any swing in the output signal, a bank of current switches T1 to T4 are provided, which enable resistors R1 to R4 to be switched into or out of the circuit. Each resistor R1 to R4 is switched by a respective one of the control lines D2 to D5. Thus, adjusting the load capacitance to obtain a desired frequency causes the oscillator core current to be changed automatically, thereby maintaining a constant swing in the output signal. |
