1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 / TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU30%#) amended at Melbourne, 1988)1 IntroductionThis specification gives basic clauses describing the essential features to be provided in test equipment using asinusoidal test signal for quantizing distortion
2、 measurements on PCM channels. It is important that the characteristics ofquantizing distortion measuring apparatus of this type are sufficiently specified to ensure that they are capable ofinterworking and that they will give results of sufficient accuracy. This specification is based on a general
3、statement ofthe method described as Method 2 in 9 of Recommendation G.712 1.2 Testing methodThe testing method consists of applying a sine-wave signal to the input port of a PCM channel and measuringthe ratio of the received signal to distortion power, using the proper noise weighting (see 3.3.4 bel
4、ow). The methodalso requires the use of a narrow-band rejection filter in the receiver equipment to block the sinusoidal test signal fromthe distortion measuring circuits so that the distortion power may be measured.3 Specifications3.1 Test signal frequenciesA test signal in either of two frequency
5、bands may be required depending on the test-signal rejection filterbeing used to make the measurement. The preferred test frequencies are either 820 Hz or 1020 Hz. However, otherfrequencies in the rejection band of the test-signal rejection filter (such as 804 Hz or 850 Hz) may be used.3.2 Character
6、istics of the signal source3.2.1 Signal level rangeAt least 45 to + 5 dBm0 for relative levels according to 11 of Recommendation G.232 2 with a settingaccuracy of 0.2 dB.3.2.2 Output impedance (frequency range 300 Hz to 4 kHz) Balanced, earth free (other impedances optional) 600 ohms Return loss . 3
7、0 dB Output signal balance. 40 dB3.2.3 Distortion and spurious modulation ratio 50 dB3.2.4 Frequency accuracy and stabilityThe accuracy and stability of the test signal frequency shall be appropriate to the frequency used and itsposition with respect to the rejection band of the filter concerned. Th
8、e accuracy and stability must in any case be suchthat the frequency is never a submultiple of the PCM sampling rate.3.3 Characteristics of the measuring instrument3.3.1 Measuring range and accuracy10 to 40 dB signal-to-distortion ratio with an accuracy of 1.0 dB.3.3.2 Input signal rangeAt least 55 t
9、o + 5 dBm0 for relative levels according to 11 of Recommendation G.232 2.2 Fascicle IV.4 - Rec. O.1323.3.3 Input impedance (frequency range 300 Hz to 4 kHz) Balanced, earth free (other impedances optional) 600 ohms Return loss . 30 dB Input longitudinal interference loss (below 4 kHz) . 46 dB Input
10、longitudinal interference loss (at 40 Hz) 60 dB3.3.4 Measuring filterThe value of the distortion signal shall be weighted by the standard CCITT noise weighting filter for telephony(see Recommendation O.41). Alternatively, C-message weighting may be used (see Annex A to Recommendation O.41).A calibra
11、tion correction factor may be necessary when C-message weighting is used. The manufacturing tolerances onthe characteristics of these filters may have to be less than is permitted in their respective specifications, in order toachieve the measuring accuracy in 3.3.1.3.3.5 Test-signal reject filterEi
12、ther of two test-signal rejection filters may be provided, with characteristics as given in Table 1/O.132.TABLE 1/O.132Test-signal reject filter characteristics3.3.6 Detector characteristicsAn r.m.s. or quasi-r.m.s. detector having sufficient accuracy to meet the accuracy objective must be used form
13、easuring the distortion signal.804 to 850 Hz test-signal reject filterFrequency Loss1000 Hz 1105 Hz 1360 Hz50 dB (rejection band)1180 Hz 1330 Hz 1700 Hz50 dB (rejection band) 3.0 dB 1.0 dB 0.5 dBFascicle IV.4 - Rec. O.132 33.3.7 Bandwidth correctionThe calibration of the measuring instrument shall i
14、nclude a correction factor of appropriate value to accountfor the loss in effective noise bandwidth due to the test-signal reject filter. The correction factor assumes a uniformdistribution of distortion power over the frequency range involved and is of the following form:Correction (dB) = 10 log10
15、Effective bandwidth of standard noise weightingEffective bandwidth of the measuring instrument4 Operating environmentThe electrical performance requirements shall be met when operating at the climatic conditions as specified inRecommendation O.3, 2.1.References1 CCITT Recommendation Performance characteristics of PCM channels between 4-wire interfaces at voicefrequencies, Vol. III, Rec. G.712.2 CCITT Recommendation 12-channel terminal equipment, Vol. III, Rec. G.232.
