外差法是指机器自己产生一个时钟频率，这个频率比接收到的无线电波频率要高一个固定的值。例如，中国的超外差式收音机要比天线接受到的频率高465KHZ，然后用这个时钟频率减去收到的无线电波频率，这样就得到了一个固定的465KHZ差频。当调节可变电容时，天线接收到的频率和机器的时钟频率会同步变化，以保证差频稳定不变，这样收音机的性能会更稳定。由于是调幅波，所以收到的声音频率取决于电波的幅度，而不是载波频率，所以可以把载波频率固定住，声音会更稳定。外差法不适用于模拟式的调频收音机。

Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden, in which new frequencies are created by combining or mixing two frequencies. Heterodyning is used to shift one frequency range into another, new one, and is also involved in the processes of modulation and demodulation. The two frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer. In the most common application, two signals at frequencies f₁ and f₂ are mixed, creating two new signals, one at the sum f₁ + f₂ of the two frequencies, and the other at the difference f₁ − f₂. These new frequencies are called heterodynes. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer. Heterodynes are related to the phenomenon of "beats" in acoustics.