| 摘要 |
PURPOSE:To preserve a frequency stabilizing degree of the signal obtained by frequency conversion accurately and stably by controlling a frequency of a reference oscillating device in the phase synchronizing loop which generates a local oscillating signal of the frequency converting part. CONSTITUTION:The signal is a four-phase DPSK (QPSK) signal whose frequency is converted, namely, a signal immediately before demodulation and the signal, in which a frequency stabilizing degree is required in the severest way, is inputted to the first frequency divider 31 and divided. Further, a direct current voltage obtained at an output edge 38a of a control part 38 is added to a terminal of a variable capacity diode 39 connected to a reference oscillating device 16 of the first frequency converting part 11. At the circuit, the relation of a frequency of the final signal (QPSK signal) obtained by frequency conversion and a frequency of an oscillating signal of the reference oscillating device 16 of the phase synchronizing loop is always monitored. When by the internal factor (temperature drift, etc.) or the external factor (external disturbance), frequency fluctuation exists at the signal for demodulation, the frequency of the reference oscillating device 16 is controlled and the frequency fluctuation can be suppressed. |
