The Superheterodyne Radio: No really, that's its name

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超外差收音机在调台时改变本地振荡频率，让感兴趣的信号落在固定中频。Watson 用差频解释为什么可以把精细滤波留给同一个频率范围。

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the difference between the two frequencies.

the filter more precise because it is set to a fixed frequency