The excitation source is the provider of the radio frequency excitation modulation pulse signals of the launch subsystem and the radio frequency test modulation pulse signals of the BITE subsystem for the fully coherent Doppler weather radar. If the power of the output RF modulation pulse signals output by the excitation source drops obviously, or the amplitude and width of the sequential control pulse change, or the frequency and power of the first LO (Local Oscillator) and the second LO signals entering the excitation source change, the transmitter power will drop greatly, so to induce the breakdowns such as unusual echo signals. The breakdown causes of the CINAD/CC weather radar excitation source are analyzed, as an example, by means of the breakdown case of the weather radar excitation source occurred on 27 October 2010 at the Aqsu radar station, Xinjiang. The key point waveforms under normal and abnormal conditions are analyzed and compared, and some suggestions for fault diagnosis and troubleshooting are given.