1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 / TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU30%#) amended Geneva, 1976 and Melbourne, 1988)The requirements for the characteristics of an instrument capable of assessing the impulsive noise performanceof telephone-type circuits are described below and
2、must be adhered to in order to ensure compatibility of results obtainedby equipments standardized by the CCITT and produced by different manufacturers.1 Principle of operationThe instrument will record the number of times that the instantaneous voltage of the input signal exceeds apredetermined thre
3、shold during the period of measurement. The maximum rate at which the instrument can recordimpulses exceeding the threshold is 8 2 counts per second. The threshold level is calibrated in terms of the r.m.s.value of a sinusoidal input signal (dBm) whose peak value is just sufficient to cause the inst
4、rument to operate thecounting mechanism.2 Definition2.1 dead timeFor the purpose of this specification the dead time is defined as the time after which the counter is ready toregister another pulse following the start of the preceding pulse.3 Specification clauses3.1 Input impedance (frequency range
5、 300 Hz to 4 kHz) Balanced, earth free Input longitudinal interference loss . 46 dB3.1.1 Terminating impedance (other impedances optional). 600 ohms Return loss . 30 dB3.1.2 High impedance. approx. 20 kohms Bridging loss across 300 ohms . 0.15 dB3.2 Input balanceWith a pulse which is 60-dB higher th
6、an the threshold setting applied between the midpoint of the sourceimpedance and the earth terminal of the instrument the counter shall not operate.3.3 Operate level rangeThe minimum operate level range to which the instrument responds shall be from 0 to 50 dBm (i.e. 0 to 50 dB with respect to 1.1 V
7、, which is the peak voltage of a sine wave having a power of 1 mW in 600 ohms). Thethreshold shall be adjustable in 3 dB steps ( 0.5 dB) and the thresholds for positive and negative polarities of inputpulse shall not differ by more than 0.5 dB.3.4 Dead timeWhatever values of dead time are included i
8、n a particular instrument, a value of 125 25 ms shall beprovided in all cases._1)The text of this Recommendation has been established under the responsibility of Study Groups IV and XVII. Further elaborationof this Recommendation shall be the joint responsibility of these Study Groups.2 Fascicle IV.
9、4 - Rec. O.713.5 Attenuation/frequency characteristics3.5.1 Flat bandwidthResponse within the range 1 dB from 275 to 3250 Hz: 3 dB point 1 dB at 200 Hz; below 200 Hz, the attenuation shall rise at about 18 dB per octave; at 100 Hz, minimum attenuation17 dB; above 3250 Hz, the rise in attenuation sha
10、ll be compatible with the sensitivity requirement indicated in 3.7 below.3.5.2 Optional bandwidthsBy means of additional filters the equipment may provide other optional bandwidths.In any case it should be designed so that external filters can be added.One of the filters shall have the following cha
11、racteristics:Flat within 1 dB from 750 Hz to 2300 Hz: 3 dB points at 600 Hz and 3000 Hz; below 600 Hz and above 3000 Hz the response shall fall off at about 18 dB per octave.For measurements of impulsive noise in the 75 bit/s return channel, a filter with the following characteristicshas been used:
12、3 dB points at 300 Hz and 500 Hz; below 300 Hz and above 500 Hz the response shall fall off at about 18 dB per octave.For measurements of impulsive noise with a 1020 Hz test signal (see Recommendation O.6) applied to thecircuit under test, a notch filter at 1020 Hz shall be provided as an option. Th
13、is filter shall have the characteristics givenin Table 1/O.71.TABLE 1/O.71Characteristics of the notch filter3.6 CalibrationWith the instrument switched to the flat condition, a continuous sinusoidal 1000 Hz signal applied to the inputat a voltage equivalent to 0 dBm in 600 ohms, and with the operat
14、e level control set to 0 dBm the instrument shall beadjusted by means of a calibration control to register 8 2 counts per second. When the input signal is reduced in levelto 1 dBm the instrument shall not count.When the input level is reduced to any value within the operate level range, the operate
15、level setting at whichthe instrument just fails to count shall not differ from the actual input level by more than 1 dB.Frequency(Hz)Attenuation(dB)1700 1330 1180 50.0Note It should be noted that measurement results may differ ifmeasurements are performed with and without test tone.Fascicle IV.4 - R
16、ec. O.71 33.7 SensitivityWith the instrument calibrated in accordance with 3.6 in the flat condition and the operate level set to 0 dBm,rectangular pulses of either polarity of 50 milliseconds duration having a peak amplitude of 1.21 V with an intervalbetween pulses in excess of the dead time shall
17、be applied to the instrument and cause the counter to operate at thecorrect rate. When the width of these pulses is gradually reduced, the counter shall count at the correct rate when thepulses have a duration of 50 microseconds but shall not count when the pulses are 20 microseconds.3.8 Display of
18、measurement results3.8.1 Impulsive noise counterEach event to be counted shall be recorded as one unit on a counter. The counter shall be able to register atleast 999 events.3.8.2 Relative duration of impulsive noise events (optional)To allow an easier estimation of data transmission errors which ma
19、y result from impulsive noise, theinstrument shall provide means to calculate and indicate the relative duration of the impulsive noise events. This quantityis the ratio of the time that the input signal exceeds a designated threshold to the total measurement time. Results shall beindicated in a ran
20、ge of 1 101to 1 108.3.8.3 Seconds containing impulsive noise events (optional)As a further option, the instrument shall provide means to calculate and indicate the percentage of secondscontaining one or more occurrences of impulsive noise. Results shall be indicated in a range 0 to 100% with one dig
21、itafter the decimal point.3.9 TimerA built-in timer capable of switching off the instrument after a predetermined time shall be provided. Thistimer shall be adjustable from 5 to 60 minutes in steps of 1 minute.Significant testing intervals will be 5, 15, 30 and 60 minutes.4 Operating environmentThe electrical performance requirements shall be met when operating at the climatic conditions as specified inRecommendation O.3, 2.1.
