Transmitter limit improvement factor is one of the most important technological indexes for delivery test of CINRAD and it comprises transmitter input and output limit improvement factors. Based on the testing methods offered by Plessey Siemens Company, a spectrum analyzer is used to measure the signal power spectrum density distribution, and then the signal to noise ratio (S/R) is obtained. However, during the delivery test and patrol examination, the interfering spikes at different frequency are often found on the graph of signal power spectrum density distribution and they may affect measurement indexes of transmitter limit improvement factor and transmitter phase stability and so on. This paper analyzes test signal circuit and examines causes for interfering spikes and repairing methods and hence provides useful field maintenance experience for radar technologists.