ITU-T O 81-1988 Group-delay measuring equipment for telephone-type circuits《电话型电路的群时延测量设备》.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 / TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITUSERIES O: SPECIFICATIONS OF MEASURINGEQUIPMENTEquipment for the measurement of analogue parametersROUPG13DELAYG0MEASURINGG0EQUIPMENTG0FORTELEPHONEG13TYPEG0CIRCUITSITU-T Recommendation O.81Extract of “LUEG0“OO

2、KG0 &ASCICLEG0)6 G0 G9NOTES1 ITU-T Recommendation O.81 was published in Fascicle IV.4 of the Blue Book. This file is an extract from theBlueBook. 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 Bo

3、ok version and copyright conditions remain unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1984, 1988, 1993, 1997All rights reserved. No part of this publ

4、ication may be reproduced or utilized in any form or by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle IV.4 Rec. O.81 1Recommendation O.81Fascicle IV.4 Rec. O.81GROUP-DELAY MEASURING EQUIPMENTFOR TELEPHONE-TYPE CIRCUITS(G

5、eneva, 1972)The characteristics for a group-delay measuring set for telephone-type circuits which are described below mustbe adhered to in order to ensure compatibility between equipments standardized by the CCITT and produced by differentmanufacturers.1 Measuring principleIn the case of group-delay

6、 distortion measurements over a line (straightaway measurements), a signal for phasedemodulation is required on the receiving side whose frequency corresponds exactly to the modulation (split) frequencyon the transmitting side and whose phase does not change during the measurement. With the proposed

7、 measuringprinciple, this frequency is generated in a split-frequency oscillator in the receiver whose frequency is controlled withthe aid of a reference carrier having a fixed frequency of 1.8 kHz. The reference carrier is amplitude modulated with thesame modulation frequency as the measuring carri

8、er and is transmitted over the path to be measured in periodicalalternation with the measuring carrier. During the changeover from measuring carrier to reference carrier no phase oramplitude surge must occur in the sending signal. For the sake of identification the reference carrier is furthermoream

9、plitude modulated with an identifying signal.If the path to be measured has different group delay and/or attenuation for the measuring carrier and thereference carrier, a phase and/or amplitude surge appears at the output of the path to be measured at the carrierchangeover point within the receiver.

10、 This phase or amplitude surge is evaluated by the receiver of the measuring set.Thus, the receiver is provided with a phase measuring device for the purpose of group-delay measurements. Thismeasuring device includes the above-mentioned frequency controlled split-frequency oscillator whose phase isa

11、utomatically adjusted to the mean value derived from the phases of the split-frequencies transmitted with the measuringand the reference carriers. The split frequency voltage fed to the phase meter is taken from the output of an amplitudedemodulator which can simultaneously be used for measuring amp

12、litude variations. In order to recognize the actualmeasuring frequency on the receiving side particularly during sweep measurements a frequency discriminator maybe provided.If the frequency of the measuring carrier differs from the frequency of the reference carrier during themeasurement and if the

13、path to be measured has different group-delay and attenuation values for the two frequencies, asquare-wave signal appears at the outputs of the phase meter, the amplitude demodulator and the frequency discriminatorin the receiver, whose amplitudes are proportional to the respective measuring results

14、 referred to the frequency of thereference carrier and whose frequency corresponds to the carrier changeover frequency on the transmitting side. Thesethree square-wave signals are subsequently evaluated with the aid of controlled rectifiers and allow indications, togetherwith the correct signs, of d

15、ifferences in group-delay distortion, attenuation and measuring frequency between measuringand reference carrier frequencies.2 Technical details2.1 TransmitterThe modulation split frequency shall be 41.66 Hz (= 1000 Hz/24). With the aid of this signal the reference andmeasuring carriers are amplitud

16、e modulated to a modulation depth of 40%. Both sidebands are transmitted. Themodulation distortion factor shall be smaller than 1%. The changeover from measuring carrier to reference carrier iscarried out within a switching time of 100 microseconds. The changeover frequency is rigidly tied to the mo

17、dulationfrequency by binary frequency division and is 4.166 Hz (41.66 Hz/10). The carrier changeover occurs at the minimum ofthe modulation envelope. Deviations of 0.2 milliseconds are admissible. The carrier frequency which is nottransmitted in each case has to be suppressed by at least 60 dB refer

18、red to the sending signal.2 Fascicle IV.4 Rec. O.81The identifying signal which is required for identifying the reference carrier is also rigidly tied to themodulation (split) frequency. The assigned frequency 166.6 Hz is derived by multiplying the modulation (split)frequency by four or by dividing

19、1 kHz by six. The rectangular-shaped identifying signal derived from 1 kHz throughfrequency division can be used for direct modulation after having passed through an RC lowpass filter with a timeconstant of T = 0.43 milliseconds since a pure sinusoidal form is not required in this case. The modulati

20、on depth is 20%.The identifying signal is only transmitted during the last 24 milliseconds of the reference carrier sending time. The shapeof the different signals on the transmitting side shown as a function of time and their respective forms can be seen fromFigure 1/O.81.Figure 1/O.81, p.2.2 Recei

21、ver2.2.1 Group-delay measurements (see Figure 2/O.81)The signal coming from the path to be measured is demodulated and the modulation frequency of 41.66 Hz soobtained is filtered out by a bandpass filter. This modulation voltage is rectangularly phase modulated, the frequency ofthe phase modulation

22、being equivalent to the changeover frequency, 4.166 Hz. The phase deviation is proportional to thegroup-delay difference between the measuring carrier and the reference carrier. The phase demodulation is carried out ina phase meter whose second input is fed, for example, by a 1 kHz oscillator via a

23、frequency divider 24/1. This oscillatorforms a closed-phase control loop involving the phase meter and a lowpass filter which suppresses the changeoverfrequency. Thus, the modulation frequency generated in the receiver corresponds exactly to the modulation frequencycoming from the transmitter.At the

24、 output of the phase meter a 4.166-Hz square-wave voltage is obtained, whose amplitude is proportionalto the measuring result. In order to enable a correct evaluation of this signal, a controlled rectification is required. Thecontrol voltage is derived from the modulation (split) frequency which is

25、generated in the receiver by frequency division(10/1). The correct phase position with regard to the transmitting signal is enforced with the aid of the identifying signal166.6 Hz. The controlled rectifier is connected both to an indicating instrument and to the direct current output.Fascicle IV.4 R

26、ec. O.81 3Figure 2/O.812.2.2 Amplitude measurementsIf the amplitude measurement is to be referred also to the reference carrier, the signal at the output of theamplitude demodulator (4.166-Hz square-wave proportional to a) can be subsequently evaluated as already describedin the case of the group-de

27、lay measurements. Furthermore, it is possible to indicate the respective absolute carrieramplitude.2.2.3 Frequency measurementsFor sweep measurements it is necessary to generate in the receiver a voltage which is proportional to themeasuring frequency. This can be achieved with the aid of a frequenc

28、y discriminator which, in turn, supplies its outputvoltage to a controlled rectifier. The indicated measuring result is the frequency difference between the measuring carrierand the reference carrier. Optionally, only the measuring carrier frequency may be indicated.2.2.4 Blanking of transient disto

29、rtionDue to the carrier changeover it may happen that transient distortions occur in the path to be measured as wellas in the receiver. These interfering signals can effectively be blanked out by means of gate circuits. The gates willrelease the ensuing measuring devices only during those periods wh

30、ich are indicated in Figure 1/O.81.3 GeneralThe transmitter output and the receiver input must be earth free and balanced. It must be possible to apply amaximum direct current of approximately 100 mA to the connected measuring instruments for the purpose of loopholding.4 Fascicle IV.4 Rec. O.814 Spe

31、cifications for a group-delay measuring set for telephone-type circuits4.1 General4.1.1 Accuracy of group-delay measurements (see also 4.2.1 below):200 Hz to 400 Hz 100 microseconds400 Hz to 600 Hz 30 microseconds600 Hz to 1 kHz 10 microseconds1 kHz to 20 kHz 5 microsecondsofmeasuring range3%1)Outsi

32、de a temperature range of +15 C to +35 C the stated accuracy may be affected by variations of themodulation frequency, causing a measuring error of 4% instead of 3% (see 4.1.4 below).The additional error due to amplitude variations shall not exceed: variations up to 10 dB . 5 microseconds variations

33、 up to 20 dB . 10 microseconds variations up to 30 dB . 20 microseconds4.1.2 Measuring frequency 200 Hz to 20 kHz4.1.2.1 Measuring frequency accuracy: in temperature range +15 C to +35 C 1% of actual frequency reading 10 Hz in temperature range +5 C to +50 C 2% of actual frequency reading 10 Hz4.1.3

34、 Reference frequency 1.8 kHz(plus a vernier adjustment to avoid coincident interfering tones).There should be an option to include two additional reference frequencies to increase accuracy at the edges ofthe band.4.1.3.1 Reference frequency accuracy: in temperature range +15 C to +35 C 1% in tempera

35、ture range +5 C to +50 C 3%4.1.4 Modulation frequency (1 kHz : 24): 2) in temperature range +15 C to +35 C 41.66 Hz 0.5% in temperature range +5 C to +50 C 41.66 Hz 1% 4.1.4.1 Modulation depth 2). m = 0.4 0.054.1.4.2 Modulation distortion factor 2), 3). 1%4.1.5 Identifying frequency (1 kHz : 6) deri

36、ved from modulation frequency 2). 166.6 Hz4.1.5.1 Modulation depth 2). m = 0.2 0.054.1.5.2 Sending time of identifying signal 2)24 milliseconds terminating with the end ofthe sending time of the reference frequency4.1.5.3 The commencement of the identifying signal shall cause a decrease in the ampli

37、tude of the carrier (as shown inFigure 1/O.81)._1)The measurement range is taken to be the indicated value at full-scale deflection on the range in use.2)Requirements that have to be met on grounds of compatibility between equipments made by different manufacturers.3)The modulation distortion factor

38、 is taken to be:r.m.s. value of unwanted sidebandsr.m.s. value of wanted sidebands 100%Fascicle IV.4 Rec. O.81 54.1.6 Changeover frequency (1 kHz : 240) derived from modulation frequency 4)4.166 Hz4.1.6.1 Carrier changeover time 4). Less than 100 microseconds4.1.6.2 Deviation between carrier changeo

39、ver point and envelope minimum 4) 0.2 milliseconds4.1.7 Range of environmental conditions 5)4.1.7.1 Power supply voltage variation . +10 to 15%4.1.7.2 Temperature range +5 C to +50 C4.1.7.3 Relative humidity range 45% to 75%4.1.8 Additional requirements4.1.8.1 Speaker arrangements . Optional4.1.8.2

40、Internal check. Internal checking circuits shall be provided to verify the proper operation of thegroup-delay/frequency and attenuation/frequency distortion measurement functions, using appropriate outputs from thesender.4.2 Sender4.2.1 Error introduced by the sender in the overall accuracy of the g

41、roup-delay measurement (as indicated in 4.1.1above) shall not exceed 4): 200 Hz to 400 Hz 10 microseconds 400 Hz to 600 Hz 3 microseconds 600 Hz to 20 kHz 1 microsecond4.2.2 Range of send levels (average carrier power) (the maximum send level may berestricted as an option) . 40 dBm to +10 dBm4.2.2.1

42、 Send level accuracy . 0.5 dBat the reference frequency 0.3 dB4.2.3 Output impedance (frequency range 200 Hz to 20 kHz): balanced, earth free 600 ohms4.2.3.1 Return loss . 40 dB4.2.3.2 Signal balance ratio 46 dB4.2.4 Harmonic distortion of send signal 1% (40 dB)4.2.5 Spurious distortion of send sign

43、al 0.1% (60 dB)4.2.6 Frequency sweep rate Adjustable from 10 Hz/sec to 100 Hz/sec.At least four sweep rates shall be provided4.2.7 Preventing possible response of dial tone receivers Optional4.2.8 Provision for loop holding . Yes4.2.9 Arrangements shall be included in the sender so that when require

44、d, prior to measurement, the test andreference carrier frequencies can be measured to a resolution of 1 Hz. This may be achieved by providing suitableoutputs at the sender for use with an external frequency counter._4)Requirements that have to be met on grounds of compatibility between equipments ma

45、de by different manufacturers.5)These values are provisional and require further study.6 Fascicle IV.4 Rec. O.814.3 Receiver4.3.1 Input level range . 40 dBm to +10 dBm4.3.1.1 Dynamic range of receiver . 30 dB4.3.2 Input impedance (frequency range 200 Hz to 20 kHz): balanced, earth free. 600 ohms4.3.

46、2.1 Return loss . 40 dB4.3.2.2 Signal balance ratio 46 dB4.3.3 Range for measuring group-delayfrequency distortion0 100 200 500012510to microsecondsto milliseconds ,4.3.3.1 Accuracy of group-delay measurements in accordance with 4.1.1 and 4.2.1 above.4.3.4 Measuring range for attenuation/frequency d

47、istortion measure-ments 0 2, 5, 10, 20, 50 dB 6)4.3.4.1 Accuracy (+5 C to +50 C) . 0.1 dB 3% of measuring range4.3.5 Measuring range for input level measurements at the reference frequency . +10 dBm to 20 dBm4.3.5.1 Accuracy (+15 C to +35 C) 0.25 dB(+5 C to +50 C) 1 dB4.3.6 D.c. outputs shall be pro

48、vided to drive an X-Y recorder.4.3.7 Measuring ranges for frequency measurements 200 4200 20Hz to kHzHz to kHz4.3.7.1 Accuracy of frequency indications 2% 10 Hz4.3.8 Provision for loop holding Yes4.3.9 Noise immunity4.3.9.1 There shall be an option to include a lowpass filter to reduce the effect of

49、 interfering frequencies above4000 Hz, for example, metering pulses.The group-delay/frequency distortion of the filter shall not exceed 5 microseconds at 2600 Hz and 30 micro-seconds at 2800 Hz relative to the group delay at 1000 Hz. The attenuation/frequency distortion shall not exceed 0.1 dBat 2600 Hz and 0.2 dB at 2800 Hz relative to the attenuation at 1000 H