1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 / TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU30%#)&)#!4)/.3G0G0&/2G0G0-%!352).G0G0%15)0-%.4%15)0-%.4G0G04/G0G0-%!352%G0G0./.,).%!2$)34/24)/.G0G053).G0G04(%G0G0 G134/.%).4%2-/$5,!4)/.G0G0-%4(/$)45G134G0G0RecommendationG0G0 (Extract from the “LUEG0“OOK)NO
2、TES1 ITU-T Recommendation O.42 was published in Fascicle IV.4 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 copyrigh
3、t 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 1988, 1993All rights reserved. No part of this publication may be reproduced or utiliz
4、ed 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.42 1Recommendation O.42Fascicle IV.4 - Rec. O.42EQUIPMENT TO MEASURE NONLINEAR DISTORTIONUSING THE 4-TONE INTERMODULATION METHOD(Malaga-Tor
5、remolinos, 1984)1 IntroductionNonlinear distortion impairments on analogue circuits are normally evaluated be measuring the harmonicfrequency signals resulting from a sinusoidal test signal, or by measuring intermodulation frequency signals resultingfrom the interaction of a multitone test signal. S
6、tudies and experience have shown that the harmonic distortion methodmay severely underevaluate the amount of nonlinearity present on a circuit under certain circumstances. When multiplesources of nonlinearity are present on a circuit, harmonic products may tend to cancel each other, whereas theinter
7、modulation products generated by a complex data signal may not cancel and may significantly impair thetransmitted message. This effect has become increasingly important with the advent of higher bit rates and withmultilevel/multiphase encoded data signals.The following intermodulation method of test
8、ing for nonlinear distortion using a 4-tone test signal isrecommended in order to achieve improved accuracy. This method measures certain 2nd and 3rd order distortionproducts resulting from the intermodulation of the tones in the prescribed test signal. The frequencies of the four testsignal tones a
9、re selected to generate 2nd and 3rd order intermodulation products that occur in the passband of ananalogue circuit and are easily separated from the applied test signal and measured. Four tones are used in order toachieve a test signal whose amplitude distribution is approximately Gaussian.2 Princi
10、ple of operationIntermodulation distortion can be broadly defined as the modulation of the components of a complex wavewith each other, as a result of which new components are produced that have frequencies equal to the sums anddifferences of integral multiples of those of the components of the orig
11、inal complex wave. Normally the 2nd and 3rdorder intermodulation components are sufficient to evaluate the circuit nonlinearity.A test signal is used which consists of four equal-level tones. Two of the tones are nominally 6 Hz apartcentred at 860 Hz and the other two are nominally 16 Hz apart centr
12、ed at 1380 Hz. To evaluate 3rd order distortion, thetotal power due to the six 3rd order intermodulation products in a narrow band centred at 1.9 kHz is measured andexpressed in dB below the received signal. For 2nd order distortion, the power due to the four 2nd orderintermodulation products in a n
13、arrow band centred at 520 Hz and the power nominally due to the four 2nd orderintermodulation products in a narrow band centred at 2240 Hz are also measured. These two 2nd order distortionproduct powers are then averaged and the result expressed in dB below the received signal.Second order intermodu
14、lation distortion is defined as follows:Intermod2nd= 20 log10(V4T/V2nd) dBwhere:V4Tis the r.m.s. voltage of the 4-tone signal, andVVV22nd22=+() ( )522where:V5is the r.m.s. voltage in the frequency band centred at 520 Hz, andV22is the r.m.s. voltage in the frequency band centred at 2240 Hz.Third orde
15、r intermodulation distortion is defined as follows:Intermod3rd= 20 log10 (V4T/V19) dB2 Fascicle IV.4 - Rec. O.42where:V4Tis the r.m.s. voltage in the 4-tone signal, andV19is the r.m.s. voltage in the frequency band centred at 1900 Hz.Depending on the relative levels of the intermodulation distortion
16、 products and noise on the circuit, the levelof the signals measured in the receiver with the 4-tone test signal may be due in part or entirely to circuit noise. Todetermine the contribution of this noise, an additional measurement is made using a 2-tone signal consisting of the highpair or low pair
17、 of tones at the same power level as the 4-tone signal. The resulting signal-to-noise level readings areused to correct the observed distortion readings. The correction may be accomplished automatically in the test set or bythe operator.3 Specific requirementsThe following provides a minimum set of
18、requirements that should be met by an instrument used to measurenonlinear distortion using the “4-tone” intermodulation method.3.1 Transmitter3.1.1 Level accuracyThe r.m.s. signal output level error shall be less than 1 dB.3.1.2 Level rangeThe output level range shall be at least 0 to 40 dBm. Calibr
19、ated attenuator increments of 1 dB or smallershall be provided unless a level indicator is part of the test set, in which case a vernier control is acceptable.3.1.3 SpectrumThe transmitted signal shall consist of four equal-level tones. Two of the tones shall be 6 1 Hz apart centredat 860 1 Hz and t
20、wo of the tones shall be 16 1 Hz apart centred at 1380 1 Hz. The tones shall be of equal levelwithin 0.25 dB.3.1.4 Harmonic distortionAny harmonic of any of the four tones shall be at least 35 dB below the tone.3.1.5 Background interferenceAny noise, distortion or interference falling within the dis
21、tortion filter passbands as specified in 3.2.4, shallbe at least 80 dB below the signal.3.1.6 Probability density functionThe probability density function of the transmitted signal shall be approximately that of four independentsinusoidal oscillators even if the tones are synthesized from a single s
22、ource.3.1.7 Signal-to-noise check signalIt shall be possible to disable either the two tones centred at 1380 Hz or the two tones centred at 860 Hz andincrease the other two tones by 3 0.25 dB. This signal-to-noise check signal is used to determine the interference ofthe noise on the circuit under te
23、st to the measurement.3.2 Receiver3.2.1 AccuracyThe measurement error shall be less than 1 dB.3.2.2 Input level rangeThe receiver shall meet the accuracy and measurement range requirements for an input level range of 0 to 40 dBm.3.2.3 Measurement and display rangeThe test set shall be capable of mea
24、suring and displaying the ratio of the signal level to the 2nd and 3rd orderdistortion products over a range of 10 to 70 dB.Fascicle IV.4 - Rec. O.42 33.2.4 Filter specificationsThe six 3rd order products to be measured fall in the range 1877 to 1923 Hz, the lower four 2nd orderproducts in the range
25、 503 to 537 Hz and the four upper 2nd order products in the range 2223 to 2257 Hz. (This allowsfor frequency shift in the channel and transmit signal frequency drift.)Filters used to recover the products must be wide enough to measure the total power within the overallaccuracy requirement of 1 dB an
26、d must be narrow enough to reject out-of-band noise. The filter bandwidths may bechecked by adding a 3.5 kHz band-limited white noise signal at a level of 40 dBm to the input of the set in addition tothe 4-tone signal at 10 dBm. The 2nd and 3rd order intermodulation levels displayed must each be at
27、least 46 dBlower than the power of the 10 dBm tone signal.Additionally with the 4-tone signal at 10 dBm applied to the input of the set, a test sinusoidal signal at alevel of 25 dBm shall be added. The 3rd order distortion reading shall be at least 55 dB below the signal level for alltest frequencie
28、s below 1600 Hz and above 2200 Hz. The 2nd order distortion reading shall be at least 55 dB below thesignal level for all test frequencies below 220 Hz, between 820 and 1940 Hz, and above 2540 Hz. At 180 Hz and lowerfrequencies, the rejection must be at least 25 dB greater than the above requirement
29、.3.2.5 DetectorsThe test signal and intermodulation distortion levels shall be measured with an average or an r.m.s. detector.3.2.6 Crosstalk with associated transmitterThe receiver shall meet overall accuracy requirements when its associated transmitter (if provided) is set toits highest output lev
30、el and terminated in 600 ohms, and a second transmitter, set 40 dB below this level, is used as asignal source for intermodulation measurement.3.2.7 Self-check capabilityA self-contained means should be provided to ensure that the receiver is calibrated within 1 dB for 2nd and3rd order distortion me
31、asurements.3.2.8 Improper received signal levelAn indication shall be provided for received test signals that are not within the input level range of 0 to 40 dBm.3.2.9 Signal-to-noise check signal indicatorAn indication shall be provided to indicate the presence or absence of the signal-to-noise che
32、ck signal.3.2.10 Correction for signal-to-noiseGenerally the correct signal-to-intermodulation distortion ratio is greater than the observed distortion readingdue to the presence of circuit noise. The operating instructions shall include a suitable correction curve or correctiontable, unless the tes
33、t set automatically makes the correction in the observed reading after the signal-to-noise checktransmission.3.2.11 Spurious tone monitorA means should be provided to determine if a spurious tone or noise equal to or greater than the test tone isbeing received. Frequencies closer than 100 Hz about 8
34、60 Hz and 1380 Hz are excluded from this requirement.3.3 Input and output impedancesAll given impedances are for a balanced (earth free) connection.3.3.1 Terminating mode (transmit or receive)When used in a terminating mode, the input/output impedance shall be 600 ohms with a return loss of 30 dB fr
35、om 300 to 4000 Hz.3.3.2 Bridging mode (receive)When used in a bridging mode, the tapping loss across 300 shall be 0.15 dB from 300 to 4000 Hz.3.4 Longitudinal lossesThe transmitter/receiver inputs and outputs should meet the following requirements. Measurements should bemade in accordance with Recom
36、mendation O.121.4 Fascicle IV.4 - Rec. O.423.4.1 Longitudinal conversion lossThe longitudinal conversion loss should be 46 dB between 300 to 4000 Hz.3.4.2 Input longitudinal interference lossThe input longitudinal interference loss should be 110 dB at 50 Hz. This requirement decreases 20 dB perdecad
37、e to 5000 Hz. The impressed longitudinal voltage shall not exceed 42 volts r.m.s.3.5 Output indicators3.5.1 AnalogueIf an analogue meter is used, the spacing of the meter markings shall be 1 dB or less over the normally usedportion of the meter scale.3.5.2 DigitalIf a digital indicator is used, the
38、result shall be displayed to the nearest 1 dB. The result shall be roundedrather than truncated. The instrument shall indicate within 1 dB of the final reading within 10 seconds after applicationof a test signal. After this initial period, the display shall be updated at least once every 5 seconds o
39、n the basis ofcontinuing measurements of both the received 4-tone level and the intermodulation products. An update period of twoor three seconds is recommended.3.6 Operating environmentThe electrical performance requirements shall be met when operating at the climatic conditions as specifiedin Recommendation O.3, 2.1.
