1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.-!).4%.!.#%G0G0-%!352%-%.434/G0G0“%G0G0#!22)%$G0G0/54G0G0/.G0G06 amended at New Delhi, 1960;Geneva, 1964; Mar del Plata, 1968 and Malaga-Torremolinos, 1984)The CCITT,in view o
2、fRecommendation R.72 on the periodicity of maintenance measurements to be made on international voice-frequency telegraph (VFT) channels,consideringthat it should be clearly laid down what maintenance measurements are indispensable to ensure the correctoperation of VFT channels,unanimously declares
3、the view(1) that maintenance measurements and any necessary adjustments of amplitude-modulated VFT channelsshould be made in the following order:a) the power supply voltages;h) the value of the frequency transmitted to line by the channel;c) the output level of each send filter in condition Z and in
4、 condition A;d) the output level of each send filter after the control current has been interrupted;e) the output level of each receive filter in condition Z;f) the degree of distortion with sequences of significant intervals each having a duration of one or two unitintervals. (It would be advisable
5、 for this measurement to be made at normal, maximum and minimumlevels. All the modifications of level should be made after the receive filter.) The measurement andadjustments may be first carried out on local and then on line, or on line only, so as to minimize the degreeof distortion;g) the receivi
6、ng relay if any (if the results obtained at point f) should make this desirable);h) the threshold of the receiver;i) the degree of distortion, in accordance with the method described in Recommendation R.5 and bearing inmind (1) and (2) of Recommendation R.74;(2) that maintenance measurements and any
7、 necessary adjustments of frequency-modulated VFT channelsshould be made in the following order:a) the power supply voltages;h) the values of the frequencies transmitted to line by the channel;c) the frequency emitted after the control current has been interrupted;d) the output levels of each send f
8、ilter for the characteristic frequencies A and Z;e) the output levels of each receive filter for the characteristic frequencies A and Z, if possible;f) the frequency drift, if the channel is used for this measurement (see below);g) the degree of distortion with sequences of significant intervals eac
9、h having a duration of one or two unitintervals; the measurement and adjustment should be first carried out on local and then on line, or on lineonly, so as to minimize the degree of distortion;2 Fascicle VII.1 - Rec. R.73h) the receiving relay, if any;i) the threshold of the receiver (at blocking);
10、j) the degree of distortion, in accordance with the method described in Recommendation R.5 and bearing inmind (1) and (2) of Recommendation R.74.The measurement referred to in f) above must be carried out to check, where necessary, whether there is anyfrequency drift on the VFT bearer circuit by mea
11、suring the pilot frequency when the system is operated with one;otherwise, Administrations should agree to measure a characteristic frequency at the output of the line for a mutuallydetermined channel. The result of this measurement will be compared with the result of the measurement made whenthis f
12、requency is sent; the difference will show any drift due to transmission on the VFT bearer circuit;(3) that, unless otherwise specified, the measurements should be effected at the nominal modulation rate of thechannel (50, 100 or 200 bauds). However, if a 100-baud channel is operated with a rate of
13、50 bauds, in accordance withRecommendation R.35 bis, the measurements should be effected at the rate of 50 bauds and adjustments made if thelimits mentioned for 50 bauds in Recommendation R.57 are no longer respected;(4) that the results of the measurements made on the international channels should
14、be exchanged directly bytelegraph or telephone between the measuring stations, at the request of one of these stations;(5) that, since maintenance work is a cause of interference on circuits in service, maintenance measurementsshould be made outside busy hours as far as possible;(6) that, when maintenance measurements are carried out on circuits in operation, every precaution should betaken in accordance with Recommendation R.76 to avoid disturbances.
