11.通信系統(tǒng)中信源是消息的發(fā)源地，其作用是通過傳感器把消息轉(zhuǎn)換為(      )

A.語音信號                                                      B.原始電信號

C.IP分組                                                         D.已調(diào)信號

12.通信系統(tǒng)中發(fā)送設(shè)備的功能是將信道匹配于(      )

A.交換設(shè)備                                                      B.信源

C.傳輸設(shè)備                                                      D.信號

13.幅度調(diào)制中，調(diào)制的實質(zhì)是進行(      )

A.頻譜搬移                                                      B.幅度縮小

C.幅度放大                                                      D.頻率擴大

14.對于模擬信號，信號帶寬與信道帶寬的關(guān)系是(      )

A.前者大于后者                                               B.兩者相等

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

15.夜晚，在天波傳播中電離層反射高頻電磁波主要是在(      )

A.A層                                                             B.D層

C.E層                                                             D.F層