11.通信系统中信源是消息的发源地，其作用是通过传感器把消息转换为(      )

A.语音信号                                                      B.原始电信号

C.IP分组                                                         D.已调信号

12.通信系统中发送设备的功能是将信道匹配于(      )

A.交换设备                                                      B.信源

C.传输设备                                                      D.信号

13.幅度调制中，调制的实质是进行(      )

A.频谱搬移                                                      B.幅度缩小

C.幅度放大                                                      D.频率扩大

14.对于模拟信号，信号带宽与信道带宽的关系是(      )

A.前者大于后者                                               B.两者相等

C.前者小于后者                                               D.前者大于等于后者

15.夜晚，在天波传播中电离层反射高频电磁波主要是在(      )

A.A层                                                             B.D层

C.E层                                                             D.F层