At first, we need to reset the central frequency, span and reference level according to the tested signal. Then under the circumstance of without overload reduce the pad value gradually. If the SNR is less than 15dB, we need to reduce the RBW. The less the RBW, the lower SNR of the spectrum analyzer, as a result, the higher sensitivity. What we need to pay attention, measuring result of the spectrum analyzer is the sum of the input signal and internal noise, in order to get the accurate result, the SNR needs to be over 20dB.