| 摘要 |
MODULATORS Digital modulation is applied to modulation means 1 to produce a modulated output from a voltage controlled oscillator 2 in a phase locked loop during transmission of a random modulating input. A component of the input voltage on coupling capacitor 18 will be due to half the swing between logic voltage levels but, when transmission ceases and a continuous logic 1 or logic 0 state is applied, the state of charge of the capacitor 18 will change causing an unduly large frequency swing on re-transmission. This is prevented by a biassing circuit comprising resistors 19, 20 fed by tri-state buffer 12 which hold the input terminal of the capacitor at the average level between logic 0 and logic 1 when no modulation is applied.
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