Abstract: A measure of the noise performance of an FDM system can be made by making measurements when the system is carrying traffic. The technique consists of creating a quiet channel in the system, and measuring the noise level in this quiet channel. The tester operates by Teeing off the broad band incoming signal, automatically attenuating the broad band signal to a predetermined level, and then measuring the power level in a narrow bandwidth lying within a single channel. The measurement is performed using an automatic attenuator controlled by a feedback loop containing, band pass filters, superhetrodyne detectors, a comparator controlling a counter, which in turn controls the automatic attenuator.

Abstract: TNR (Traffic Noise Ratio) is defined as the ratio of the portion of the noise representing a multi-channel signal that occurs in a narrow bandwidth (including inherent, and intermodulation) to the noise in the same bandwidth, in the absence of a traffic signal in that bandwidth, under specified traffic conditions. The TNR of a communications link is measured by measuring the noise power level in a predetermined bandwidth in a quiet channel with reference to a signal representative of the power that would be in the quiet channel if it was loaded under peak traffic conditions. The noise power level is measured using a digitally stepped automatic attenuator controlled by a feedback loop which includes filters for isolating the noise in the quiet channel.