本文針對現行有線電視系統用戶終端信號品質查驗工程中單一波道載波雜訊比參 數之量測，因待測網路系統雜訊與量測用頻譜分析儀內背景雜訊間彼此位準相近時，易造成 量測結果誤差過大甚至發生量測錯誤之困擾，提出討論與解決問題之方法，另外在單一波道 載波雜訊比量測架構中之前置放大器與可變衰減器這二項裝置並非任何量測情況均可加入適 用，針對正確而適當之使用時機與程序提出說明並探討不適用量測情況下但加入這二項裝置 時之不良影響。 本文提出量測 CNR 參數前必需先重複作不聯接試驗，再依雜訊位準差值作 判定後，或逕行量測 CNR 參數不必修正也不必開前置放大器，或需修正測得 CNR 參數讀值 ，或需先開前置放大再量測 CNR 參數。

　　　　　In this paper, we deal with the measurement of in-channel carrier-to-noise ratio (CNR) parameter for the signal quality inspection engineering at the subscriber's position of a cable television (CATV) system. When the noise levels of both the employed spectrum analyzer and the under-measuring CATV system are approximated, this situation will introduce significant errors in measuring the CNR parameter and even obtain erroneous results. To solve above problem, a novel measurement procedure is proposed. Moreover, the applicability and influence of pre-amplifier and adjustable attenuator in implementing a measurement architecture for the in-channel CNR parameter are also discussed. In our method, disconnect test is first accomplished ahead of measuring the CNR parameter. Thus, the difference of noise levels can be roughly estimated. Based upon this noise difference, the subsequent measurement procedure can be adequately determined, i.e., direct measurement, correcting the readings of CNR parameter after measurement or activating the pre-amplifier in a spectrum analyzer before measurement. Experimental results demonstrate that the proposed method can efficiently improve the accuracy in measuring the CNR parameter of a CATV system.