| 摘要 |
1460513 Measuring noise in a multi-channel FDM link POST OFFICE 16 May 1975 [30 May 1974] 24013/74 Heading G1U The noise performance of a multi-channel frequency division multiplex communications link is assessed by measuring the noise present in a quiet channel whilst the other channels are carrying traffic, this being effected by tapping the link 10a, Fig. 2, so as to extract a small fraction of the total signal over all the channels carried by the link, standardizing the resulting signal by passing it through an automatic gain and attenuator stage 12, 13, 14, where it is brought to such a level that the average power in the signal equals a pre-set reference level (either -29dBm for 1800 channel links or -32dBm for 960 channel links) and then measuring 16 the noise present in the quiet channel of this standardized signal. The result may be indicated or printed 51, and a logic circuit 21 can cause a plurality of links 10a, 10b,..., to be examined automatically successively. The automatic attenuator 13, Fig. 2, is shown in more detail in Fig. 3 and operates by comparing 26 the signal emerging from a switched attenuator 22 after detection 25 with a reference level 27, and adjusting the attenuator by a counter 29 until equality is attained. A similar arrangement is also employed to measure the noise in the quiet channel. The standardized wide band signal is passed via a second switched attenuator 31, Fig. 4, the signal present in the quiet channel is extracted by an appropriate filter 33a, 33b,..., the signal is demodulated 34, 52, 53, 37, to extract the noise per se, and then the attenuator 31 is adjusted by a counter 48 until the noise signal - which is detected 41 - is equal to a pre-set level 45, whereupon the count in the counter 48 is a measure of the ratio of the noise in the quiet channel to the total power being passed by the link. |
