This paper dealt with a general the Qrem of frequency doubler. By summarizing the frequency doubler theorems, a new frequency doubter theorem can be derived. "In time invarient system, if the input waveform shows the half-wave symmetric characteristic and if the even function is the mapping from input into output, then the frequency of output is at least 2-times of input frequency." By the theorem, this paper propose a new wide band square wave frequency doublerusing negative resistance circuit.e circuit.