1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 / TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU30%#)&)#!4)/.3G0G0&/2G0G0-%!352).G0G0%15)0-%.4G0G0(ZG0G02%&%2%.#%G0G04%34G0G0&2%15%.#9)45G134G0G0RecommendationG0G0/ (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation O.6 was published in Fascicle VI.4
2、 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recommend
3、ation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, incl
4、uding photocopying and microfilm, without permission in writing from the ITU.Fascicle IV.1 - Rec. 0.6 1Recommendation O.6Fascicle IV.1 - Rec. 0.61020 Hz REFERENCE TEST FREQUENCY(Melbourne, 1988)1 IntroductionThe intent of this Recommendation is to specify a single nominal reference frequency of 1020
5、 Hz in order toprovide guidance to manufacturers and Administrations in the design and operation of new equipment and systems. ThisRecommendation is not intended to have an effect on existing equipment or systems except where modifications arerequired to allow for interworking. For instance, an olde
6、r analogue exchange would need to be provided with newreference frequency capability if circuits were provided between it and digital exchanges.2 Test frequencies on circuits routed over PCM systemsThe selection of a suitable test frequency is a major consideration when testing circuits routed over
7、PCMsystems. An error in level measurement can arise on circuits routed over PCM systems if the test frequency is a sub-multiple of the PCM sampling rate. This error can be nearly as great as 0.15 dB at 800 Hz and 0.20 dB at 1000 Hzwith a sampling rate of 8000 Hz employing 8-bit coding. In addition,
8、errors in other parameters, such as total distortion,may be even more significant.Therefore, it is recommended that the use of a reference test frequency that is a sub-multiple of the PCMsampling rate should be avoided. Studies within CCITT reveal that some Administrations have employed nominalrefer
9、ence test frequencies offset from 800 Hz or 1000 Hz by varying amount but within the ranges 804-860 Hz or1004-1020 Hz. These studies have confirmed that where interworking is not required, no significant problems inmaintenance have been encountered by Administrations and existing test procedures and
10、 equipment may continue to beused.In the case of interworking and for new equipment and systems, the Administrations expressed a strongpreference for the selection of a reference test frequency of 1020 Hz.3 Considerations for new measuring equipment specificationsThe following should be considered f
11、or new measuring equipment specifications in the Series ORecommendations:i) A reference test frequency of 1020 Hz is recommended for test frequency generating circuits orinstruments that provide reference test frequencies. The specified frequency tolerance should be +2 to 7 Hz1).ii) The nominal leve
12、l of the reference test frequency when used on in-service equipment should not begreater that 10 dBm0 0.1 dB.iii) Measuring circuits or instruments which utilize the reference test frequencies should provide, if possible,for measurements of any frequencies within the nominal range of 1000 to 1025 Hz
13、By agreement between the Administrations concerned, in the absence of the required sending or measuringapparatus, the use of a measuring frequency in the range of 800 to 860 Hz is admissible. Other considerations about thedeployment and use of reference test frequencies are given in Recommendation
14、M.20 1.References1 CCITT Recommendation Maintenance philosophy for analogue, digital and mixed networks, Volume IV,Recommendation M.20._1)The negative tolerance of 7 Hz is intended to allow the use of digitally generated test signals that are generated by asufficiently high number of samples to achieve the measurement accuracy specified in certain Series O Recommendations(e.g. Recommendation O.133).
