| 摘要 |
A modulation circuit that is particularly suited for a communication modem and provides for each of the channels precise carrier mark and space frequencies generated in response to external signals from an FSK communication station. The modulation circuit includes first and second frequency controlling circuits arranged with an inductor to form a resonant circuit of a voltage variable oscillator. The modulation circuit further includes a feedback control network and a voltage-frequency control adjustment circuit. The feedback control network generates a loop error signal applied to both frequency controlling circuits. The voltage-frequency control adjustment circuit generates a voltage-frequency adjustment control signal and has a switching circuit responsive to the external signals. The voltage-frequency control adjustment circuit includes an inverting network and a bias network. The inverting network receives the loop error signal and provides, when in an operative state, an output signal that is phase shifted by a predetermined amount from the loop error signal. The bias network is connected between the output of the inverting network and the second frequency controlling circuit and controls the operative and non-operative state of the inverting network. The combined operation of the inverter network and the bias network generates carrier frequencies that are shifted in response to the external signals.
|
