ITU-T O 41-1994 Psophometer for Use on Telephone-Type Circuits - Specifications for Measuring Equipment - Equipment for the Measurement of Analogue Parameters (Study Group 4) 17 pp.pdf

1、ITU-T RECMN*0.43 94 4862593 Ob00377 868 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU 0.41 (1 0/94) SPECIFICATIONS FOR MEASURING EQUIPMENT EQUIPMENT FOR THE MEASUREMENT OF ANALOGUE PARAMETERS PSOPHOMETER FOR USE ON TELEPHONE-TYPE CIRCUITS ITU-T Recommend

2、ation 0.41 (Previously “CCITT Recommendation”) FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them

3、with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WT?SC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval o

4、f Recommendations by the Members of the IT-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). ITU-T Recommendation 0.41 was revised by ITU-T Study Group 4 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 15th of October 1994. N

5、OTE In this Recommendation, the expression ?Administration? is used for conciseness to indicate both a telecommunication administration and a recognized operating agency: O ITU 1995 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic

6、 or mechanical, including photocopying and microfilm, without permission in writing from the ITU. CONTENTS 1 Introduction 2 General . 3 Specific requirements . 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 Input impedance 3.1.1 Terminating mode . 3.1.2 Bridging mode 3.1.3 Complex input impedances . Longi

7、tudinal losses . Calibration accuracy at 800 Hz . Relative gain versus frequency (frequency weighting) 3.5.1 Optional frequency characteristic . Detector circuit characteristics 3.6.1 Detector circuitry tests 3.6.2 Turnover Detector and display dynamics (measurement averaging time) . 3.7.1 Instrumen

8、tation with continuous signal monitoring . 3.7.2 Instrumentation with non-continuous signal monitoring . Measuring range . 3.7.3 Damped response Linearity Operating environment . Output indicator 3.10.1 Immunity to electromagnetic fields Annex A . Comparison of ITU-T and North American weightings .

9、Annex B . Level and noise measurements at interfaces with complex impedances . Level measurements at interfaces with real impedance . General considerations . Level measurements at interfaces with complex impedances . B.2.1 B.2.2 B . 1 B.2 Level meters with complex input impedances Psophometers with

10、 complex input impedances B.3 Conclusion References . Page 1 1 1 1 1 2 2 2 2 2 2 4 4 5 5 5 5 6 6 6 6 6 7 7 10 10 11 11 12 12 13 Recommendation 0.41 (10/94) 1 ABSTRACT Defines the basic requirements for an instrument to measure noise and other interfering signals on telephone circuits. KEYWORDS Compl

11、ex impedances, measurement, noise measurement, psophometer, tester, weighting filter. 11 Recommendation 0.41 (10/94) Recommendation 0.41 PSOPHOMETER FOR USE ON TELEPHONE-TYPE CIRCUITS (Published 1972; revised 1984, 1988,1992, 1995) 1 Introduction This Recommendation provides basic requirements for p

12、sophometers to be used for the measurement of noise and other interfering signals on international telephone circuits and circuit sections. 2 General To accomplish the measurements as stated above, a psophometer should have the following significant characteristics: a) The relative sensitivity of th

13、e instrument, at various frequencies, should be as specified by the psophometric weighting characteristics. (See Note.) The reference point for the sensitivity of the instrument should be O dBm (one milliwatt) at 800 Hz. The r.m.s. (root mean square) value of the weighted noise signal should be dete

14、cted and displayed. (See Note.) The dynamics of the detector and display device should meet requirements given in clause 3. The overall accuracy of the instrument when being used in its normal range and environmental conditions should be * 1 .O dB or better. Specific tests for accuracy of various as

15、pects of the instrument are given in clause 3. NOTE - The characteristics of the weighting filter combined with an r.m.s. detector can be described by the formula (Pp means psophometric power): b) c) d) e) In this formula, W(f) is the weighting filter of the psophometer with the limiting frequencies

16、 F1 = 16.66 Hz and F2 = 6 kHz as given in Table 1. b) sinusoidal signais; c) any periodic signal having a peak-to-r.m.s. ratio of 8 dB or less. 4 Recommendation 0.41 (10/94) ITU-T RECMN*O-41 94 M 48b2.591 Ob00385 934 TABLE 2/0.41 Characteristics of the optional flat filter with an equivalent noise b

17、andwidth of 3.1 kHz (bandwidth of a telephone channel) Frequency (Hz) 3400 Attenuation Increasing 24 dB/octave (Note 1) Approximately 3 dB (Note 2) I k 0.25 dB OdB I $: 0.25 dB Approximately 3 dB (Note 2) Increasing 24 dB/octave (Note 1) NOTES 1 24 dBfoctave up to an attenuation of at least 50 dB. 2

18、 3.1 kHz k 155 Hz. Below 300 Hz and above 3400 Hz the attenuation shall increase at a slope not less than The exact cut-off frequency shall be chosen to achieve an equivalent noise bandwidth of 3.6.1 Detector circuitry tests The following test is recommended to assure that the detector circuitry is

19、functioning as prescribed. a) Apply pulses of an 1800 Hz sine wave at a pulse rate of 80 Hz, with 20 percent of the cycle at full amplitude and 80 percent of the cycle 8.4 dB below full amplitude. The indicated r.m.s. value should be 5.0 2 0.5 dB lower than the level of the ungated full amplitude si

20、ne wave. Alternatively, psophometers manufactured to previous design specifications (see Annex A) shall meet the following test: Successively apply two sinusoidal signals of different frequencies, which are not harmonically related and which provide the same output level on the output indicator. The

21、n apply both these signals at the same levels simultaneously. The increase on the output indicator should be 3 dB t: 0.25 dB above the reading for the single frequency input. This condition should be fulfilled using different pairs of frequencies at different levels. b) 3.6.2 Turnover Apply a rectan

22、gular wave form with a 20 percent duty cycle and a repetition rate of 600 pulses per second to the input of the instrument, and note the noise reading. Invert the input leads, the two readings shall agree within 1 dB. This test should be performed at several levels over the specified operating range

23、 of the set. 3.7 Detector and display dynamics (measurement averaging time) The response time for the detector and indicating means shall meet one or both of the following requirements: 3.7.1 Instrumentation with continuous signai monitoring The application of an 800 Hz sinusoidal signal with a dura

24、tion of 150 to 250 ms should produce an output indication which is the same as that produced by the application of a continuous 800 Hz signal of the same amplitude. Applied signals of shorter duration should produce lower readings on the output indicator. Recommendation 0.41 (10/94) 5 ITU-T RECMN*0-

25、43 74 W 4862573 Ob00386 870 H When performing this test the reading error shall be less than rt 0.2 dB. 3.7.2 Instrumentation with non-continuous signal monitoring With the application of bursts of 800 Hz tone to the input of the psophometer, gated at a duty cycle of 50 percent, with half the cycle

26、at full amplitude and the other half down 8.4 dB from full amplitude, the output device shall indicate a variation as shown in Table 3. The levels should be chosen to avoid autoranging points. TABLE 3/0.41 Variation of the output indication with the application of specified bursts of 800 Hz at the i

27、nput of the psophometer Peak-to-Peak Gating frequency indicator variation It is permissible to adjust the total input power with a 1 dB vernier control to a point where the display does not change so as to pass the less than 1 dB requirement. 3.7.3 Damped response (Under study.) 3.8 Linearity The fo

28、llowing test is recommended to assure that excessive error is not caused by overload in the presence of signals which have a large peak-to-r.m.s. ratio. Apply a signal at a frequency of approximately 1000 Hz in 5 ms pulses separated by 20ms at an r.m.s. level corresponding to the highest value withi

29、n any selected range of the instrument. When the level is decreased over a range of 10 dB the psophometer reading shall be proportional to the applied level decrease with a tolerance off 0.5 dB, for all ranges of the instrument. 3.9 Output indicator If an analogue meter is used, the spacing of the m

30、eter markings shall be one dB or less over the normally used portion of the meter scale. If a digital display is used, the noise reading shall be displayed to the nearest 0.1 dB. The result shall be rounded rather than truncated. The update rate for a digital display shall be at least once per secon

31、d. Optionally, instruments using digital displays may provide additional display characteristics to expand the application of the instrument. Such additional display characteristics shall be defined by the manufacturer to assist the user in interpreting the results. 3.10 Operating environment The el

32、ectrical performance requirements shall be met when operating at the climatic conditions as specified in 2.V0.3 5. 6 Recommendation 0.41 (10/94) ITU-T RECMN+0.4L 74 4862593 Ob00387 707 3.10.1 Immunity to electromagnetic fields The unit should not be affected by the presence of electromagnetic fields

33、 (50 Hz). The test for this immunity is given below: a) With the instrument in the weighted measurement mode, an electromagnetic field strength of 16 A/m at 50 Hz shall cause an output indication of less than -85 dBm. b) With the instrument in an unweighted measurement mode (optional, see 3.5.1), an

34、 electromagnetic field strength of 0.8 Nm at 50 Hz shall cause an output indication of less than -85 dBm. Annex A Comparison of ITU-T and North American weightings (This annex forms an integral part of this Recommendation) Telephone circuit noise impairment is normally measured with “C-message” weig

35、hting within the North American domestic telephone networks i, 2. The frequency response of this weighting differs somewhat from the IT-T psophometric weighting specified in this Recommendation. As a consequence, the relationship between measurements made with the North American noise meter and the

36、ITU-T psophometer is dependent on the frequency spectrum of the noise being measured. In addition, it should be noted that measurements made with the North American noise meter are expressed in dBrn (decibels referred to -90 dBm or decibels above a reference power of 10-12 watts). For example, if on

37、e milliwatt of white noise in the 300 to 3400 Hz band is applied to both an ITU-T psophometer and a North American noise meter, the following readings are obtained: ITU-T psophometer (195 1 weighting) -2.5 dBm North American noise meter (C-message weighting) 88.0 dBrn Recognizing that the relationsh

38、ip of the output fadings of the differently weighted instruments will change for other noise spectra, the following rounded conversion formula is proposed for practical comparison purposes: Psophometer reading (in dBm) = C-message noise meter reading -90 (in dBm) This conversion includes the effect

39、of the difference between the reference frequencies (800 Hz for psophometric weighting and 1000 Hz for C-message weighting) used in the two types of noise meters. The C-message weighting coefficients and accuracy limits at various frequencies are given in Table A. 1. A comparison between psophometri

40、c and C-message weighting is shown on Figure A. 1. Another weighting frequently used for measuring telephone circuit noise impairment within the North American domestic telephone networks is referred to as “3 kHz Flat” weighting i. This weighting is intended for the investigation of the presence of

41、low-frequency noise (power induction, etc.) on the circuit under test. It is characterized as a 3 kHz low-pass weighting of Butterworth shape attenuating above 3 kHz at 12 dE3 per octave. The specification for this weighting is given in Table A.2. Recommendation 0.41 (10/94) 7 ITU-T RECMN*O-LtL 94 W

42、 4862571 0600388 643 TABLE A.UO.41 C-message weighting coefficients and accuracy limits Frequency (Hz) 60 1 O0 200 300 400 500 600 700 800 900 1000 1200 1300 1500 1800 2000 2500 2800 3000 3300 3500 4Ooo 4500 5000 Relative weight (dB) -55.7 42.5 -25.1 -16.3 -11.2 - 7.7 - 5.0 - 2.8 - 1.3 - 0.3 0.0 - 0

43、.4 - 0.7 - 1.2 - 1.3 - 1.1 - 1.1 - 2.0 - 3.0 - 5.1 - 7.1 -14.6 -22.3 -28.7 Tolerance (k dB) 2 2 2 2 1 1 1 1 1 1 0.0 (reference) 1 1 1 1 1. 1 1 1 2 2 3 3 3 NOTE - The attenuation +all continue to increase above 5000 Hz at a ratc of not less than 12 dB per Octave until it reaches a value of -60 dB. 8

44、Recommendation 0.41 (10/94) ITU-T RECMN*0.43 94 = 4862593 Ob00389 58T Relative loss (dB) Tolerance (a) dB O O O O 0.8 3.0 12.3a) f2.5 f 1.7 f0.5 f0.2 f 1.0 f 1.8 F3.0 - 100 200 300 400 500 700 1000 2000 3000400050006000Hz Frequency T404750441dC C-message weighting - - - Pcophometric weimting FIGURE

45、A. 1/0.41 Comparison between psophanetric and C-message weighting TABLE A.YO.41 3 kHz flat weighting characteristic I Frequency (Hz) I 30 I 60 I 400 I lo00 I 2000 I 3000 I 6000 I a) The loss shall continue to increase above 6000 Hz at a rate octave until it reaches a value of 60 dB. The loss at high

46、er frequencies shall be at least 60 dB. Recommendation 0.41 (10/94) 9 ITU-T RECMN*Om4L 94 m 4862573 Ob00390 2TL Annex B Level and noise measurements at interfaces with complex impedances (This annex forms an integral part of this Recommendation) 3 B.l Level measurements at interfaces with real imped

47、ance - General considerations This annex provides background information concerning measurements at interfaces with complex impedances. It should be taken into account when calibrating level measuring equipment - especially psophometers - for measurements at such interfaces. Figure B. 1 shows a simp

48、le circuit consisting of a voltage source S with the source impedance RSource which is connected to a load RLoad. The source produces a Voltage Vs, the voltage across the load is VL FIGURE B.UO.41 The source may represent an unknown signal to be measured and the load may be the input impedance of a

49、level meter. In spite of the fact that level meters mostly measure - voltages, they may be calibrated for voltage level measurements in dB or power level measurements in dBm. In case of power level calibration, the reading follows the formula: Resultat = 10 log (Nx / NRef) dBm (B-1) In this equation NX is the unknown power level to be measured and NRef is the reference power level of 1 mW if the level meter is calibrated in dBm. If the input impedance of the level meter (RLoad in Figure B.l) is 600 Q, the voltage V required to produce a power of 1 mW is V = d1 - lW3 W - 600