The Superconducting Superconductor-Insulator-Superconductor (SIS) receivers have become the preferred method for (sub)millimeter-wave radio astronomical observations due to the extremely low receiver noise temperature. The coupling noise of the local oscillator (LO) system is also a part of the receiver noise. In many years of astronomical observations, it is found that the coupling noise of LO can not be completely ignored, which has a certain impact on the sensitivity of astronomical observations. The noise temperature of the superconducting SIS receiver was tested using two different kinds of signal generators as the primary signal source of LO system. It is found that the base noise output by the signal generator can be coupled into the receiver and deteriorate the receiver noise. By adding a narrow band filter to the input of the signal generator, the base noise of the signal generator can be filtered, and this part noise of the receiver can be eliminated, the overall noise of the receiver is reduced, and the sensitivity of the telescope is improved.