移動通信系統中的收信機的主要作用是把天線接收下來的射頻載波信號首先進行低噪聲放大，然后經過變頻、中頻放大和解調后還原出原始信號，最后經中頻放大器放大輸出。
1、正確錯誤
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Amanflowntoatimezonedifferentby10hourswill________eightdaystoreadjusthispalmsweat.
—Whydidn’tyoubuyanewcar?
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Hetalkedalotaboutthingsandpersons________theyrememberedintheschool.
Theirplanearrived________SanFranciscothismorning.
Johnsonwas________unknownbeforerunningforthepresidency.
