题目内容

调频和调相，错误的说法是

A. 调频时最大相偏仅与调制信号的振幅有关系
B. 调频时最大相偏不仅与调制信号的振幅有关，而且与调制信号频率有关
C. 调相时最大频偏不仅与调制信号的振幅有关，而且与调制信号频率有关
D. 调频与调相的有效通频带公式相同

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设中心频率为1MHz、频偏为50Hz的已调频波，调整为调频收音机所需的75kHz频偏的已调频波，中心频率为100MHz。下列步骤中不正确的是

A. 经15次倍频后，再经20次倍频，然后与310MHz混频后取差频；最后10次倍频
B. 经15次倍频后，与16MHz混频后取差频；然后100次倍频
C. 经1500次倍频后，与1600MHz混频后取差频
D. 经20次倍频后，与19MHz混频后取差频；然后75次倍频，与25MHz混频后取和频；

对于调频：若调制信号频率为400Hz，振幅为2.4V，调制指数为60。当调制信号频率减小为250Hz，同时振幅上升为3.2V时，调制指数变为______

A. 120
B. 96
C. 256
D. 128

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