频率测量 单片机测频汇编程序，以89C51为例，0--255Hz ，T0口输入信号。-Frequency measurement The single-chip frequency measurement assembler to 89C51, for example, 0- 255Hz, the T0 port input signal.

在超外差式接收机中，如果经过混频后得到的中频信号比原始信号的频率低，那么此种混频方式叫做下变频 （Down Converter or DC）。-In the superheterodyne receiver, if after mixing an intermediate frequency signal obtained by frequency lower than the original signal, then this method is called downconversion