| 摘要 |
PURPOSE:To eliminate the need for a band pass filter with a strict standard or operation of a DC rejection high pass filter by detecting a carrier signal after the carrier signal is demodulated and band limit is applied. CONSTITUTION:The carrier signal converted into a digital value by an A/D converting circuit 4 is demodulated by a demodulation circuit 7 and an unnecessary out-band frequency is rejected by a roll-off filter 9. The carrier signal from the roll-off filter 9 is detected by a carrier detection circuit 6 and transmitted as a start signal for a MODEM. Since a DC generated by the A/D converting circuit 4 is cut off by the roll-off filter 9, no effect is given to the carrier detection circuit 6. A reception signal modulated from a line enterring a transformer 1 is demodulated by a demodulation circuit 7, and then an unnecessary frequency is rejected by the roll-off filter 9, and amplified to a proper level by an automatic gain adjusting amplifier circuit 10 and transmitted. |
