GB 16787-1997 30MHz to 1GHz sound and television signal cable distribution system radiation measurement methods and limits
Some standard content:
GB167871997
This standard is formulated based on the radiation content in the International Electrotechnical Commission standard IEC728-1:1986 "Cable distribution system for sound and television signals at 30MHz~1GHz", and is consistent with the corresponding content of the international standard in terms of technical content. This standard is also a supplement and improvement to the measurement method and limit value part of GB/T6510--1996 "Cable distribution system for sound and television signals at 30MHz~1GHz" on the radiation of the system. The formulation of this standard is conducive to the improvement of the quality of Chinese cable television and the production of cable distribution system products.
The appendix to this standard is the appendix to the standard.
This standard is proposed and technically coordinated by the National Radio Interference Standardization Technical Committee. Drafting units of this standard: Standardization Planning Institute of the Ministry of Radio, Film and Television, Sichuan Provincial Radio and Television Bureau, Yubo Sanling Radio Factory The main drafters of this standard: Wang Ximing, Luo Zhuwei, Yu Binhui. 341
1 Scope
National Standard of the People's Republic of China
30MHz～1GHz Sound and Television Signals
Cable Distribution System Radiation Measurement Methods and Limits Measurements and limits of radiation of cabled distribution systems primarily intended for sound and television signals operating between 30 MHz and 1 GHz GB 16787—1997
This standard specifies the radiation measurement methods and limits of 30MHz1GHz sound and television signal cable distribution system (hereinafter referred to as system).
This standard is applicable to the evaluation of system radiation during system design and acceptance. 2 System Radiation Measurement Method
This method measures the system radiation at a certain frequency. The signal of this frequency is fed to the input port of the system by a power signal generator, and its level is as specified in 2.2.2 of this standard. The radiation along the cable path is mainly measured outside the building. In order to represent the frequency band used for distribution of the measured system, the measurement should be carried out at enough frequency points. The radiation of the system can also be measured by the method of Appendix A (Standard Appendix). 2.1 Required equipment
Half-wave dipole antenna.
Low noise broadband preamplifier.
Bandpass filter.
Spectrum analyzer.
Power signal generator.
2.2 Measurement steps
2.2.1 The system should be operated under normal conditions and the power signal generator should be connected to its input port. 2.2.2 Before making measurements, adjust the power signal generator to the pilot frequency and adjust the level to the level of the nearest image carrier. For cable systems that do not use pilots or carriers outside the standard frequency band, do not select the local or local received space signal frequency. 2.2.3 Perform a patrol test along the cable path. The equipment is loaded on a car. The instrument connection diagram is shown in Figure 1. Switch S is in position 1.
Approved by the State Administration of Technical Supervision on May 28, 1997 342
Implemented on June 1, 1998
Half-wave dipole antenna
GB16787—1997
Filter
Power signal
Generator
Amplifier
Figure 1 Schematic diagram of the equipment connection for measuring system radiation Cheek harmonic
Analyzer
2.2.4 Note the locations where all cable radiation presents the maximum value. At each maximum value location, measure the leakage signal level using the following steps:
Place the antenna at a suitable distance from the point to be tested and adjust the dipole length to the test frequency. Rotate and/or move the antenna vertically and horizontally to achieve the maximum level, then measure the distance d between the center of the antenna and the system cable to confirm that the measured signal is emitted by the system (for this purpose, the power signal generator can be modulated with a fixed single tone). The leakage signal level is determined by comparison with a calibrated reference level (switch S is set to 2), and the corresponding radiated power is calculated using the following formula for a half-wave dipole antenna: P = U + K + 20 lg(d/7)
Equivalent radiated power, dBpW.
Where. P—·
U——leakage signal level, dBμV.
Coefficient of a half-wave dipole receiving antenna, dB. K-
d——distance from the center of the antenna to the cable, m. Comparison of the calculated power level with the limit values given in Chapter 3 of this standard 3 Limit values for system radiation
Due to the great differences in site conditions, the apparent radiation level will vary greatly. Therefore, several measurements should be made at each location and the median value should be taken as the radiation level. This level should not exceed 20dBpW. 343
A1 Introduction
GB16787-1997
Appendix A
(Standard Appendix)
Finnish Method
This appendix describes a measurement method for measuring radiation from a cable distribution system or part of a system, including indoor installations. In addition, an auxiliary method based on the substitution principle is described for measuring the radiated power from a single leakage point. A2 Working Principle
A cable distribution system covers a wide area, and its shielding effectiveness may vary from part to part. In order to obtain its complete radiation characteristics, it is necessary to measure all its areas. It can help to find out all the local strong leakage points caused by shielding effectiveness defects. The field strength at any leakage radiation point is the result of the synthesis of several leakage points. It has random phase and amplitude, although only one source is dominant near the main defect. In a cell without a main defect, the statistical characteristics of the field strength are similar to the Rayleigh distribution, and in a large area, the statistical characteristics are a combination of the Rayleigh and log-normal distributions. The method is given to measure the probability of exceeding a preset limit. The method requires a carrier signal receiving device that can be identified on the system and a device suitable for data acquisition in the vehicle, and can calibrate the threshold level of the entire measurement system. The lower value of the threshold level is used as a zone limit, and the higher value is used as a criterion for repairing defects. A3 Measurement equipment
A3.1 General arrangement
The measurement equipment has a measuring receiver, a recorder, a data processing unit and an output device for obtaining test results and a car for loading the equipment. A 1/4 wavelength rod-shaped vertical antenna is installed on the roof of the vehicle. The most suitable position for the antenna is the center of the roof. Apart from this, no other antenna is installed on the roof of the vehicle.
The reason for using a vertical antenna is that it simulates the typical arrangement of the most likely to be affected in radio stations, such as land mobile radio stations. The vertical antenna is non-directional in the horizontal plane and the field to be measured does not have any preferred polarization. According to practical experience, branch networks are the main radiation sources, especially in unit houses containing a significant number of vertical cables. The data processing unit shall include at least two adjustable level comparators by means of which the probability of exceeding two preset threshold levels in the field strength distribution can be measured.
A3.2 System Excitation
The radiation of any carrier signal may be measured on a cable distribution system, but in order that the measurement is not confused with spatial signals, the measured signal should be identified as being radiated by the system. Broadband interference sources (ignition noise) should also be identified. In general, defects causing excessive radiation in a system will result in similar overall effects on all frequencies, although the field strength at a location may vary with frequency, and in some circumstances this variation may be quite severe, perhaps 20 dB. In order to use commercially available receiving equipment, signals in the positive band will be suitable, and their frequencies should be selected away from FM broadcast signals transmitted by the system (not on their original open-circuit broadcast frequency) or broadcast signals used locally. As far as possible, 500 kHz spacing should be left on both sides of the selected frequency. The original program signals transmitted by the system can be identified by their content, but as a special test signal, an FM signal modulated with a readily recognizable audio frequency (e.g. 1 kHz) should be used. Add the test signal to the front end of the system at a level equal to that of the normal operation of the image signal (the signal provided to the system should not cause unacceptable interference to the system). When calibrating the comparator of the data processing unit, any difference in the voltage should be considered. In a system with dual TV and FM output ports, measurements in the II band may give different results than those in the TV band, because the user's connecting cable may be the main radiation source.
A3.3 Measuring receiver
GB16787-1997
Any receiver that is suitable for the frequency band used and has a signal level output with an appropriate dynamic range can be used for measurement. To ensure the overall sensitivity of the measurement system, the noise caused by the receiver itself and the interference caused by the measurement vehicle itself or other vehicles cannot cause a probability of exceeding the lower threshold level of 1%.
Taking into account the overflow and working voltage conditions that occur in actual applications, the error of the signal level output combined with the drift of the receiver frequency drift and the drift of the comparator level should not exceed the error of the RF threshold level ± 3dB. In order to identify the measurement signal and identify possible interference, the receiver should be equipped with a loudspeaker.
In order for the measurement system to meet the rapid fluctuations of the field strength at normal drift rates, the signal output level of the receiver should follow the envelope of the measurement signal, which requires the measurement time constant to be less than 50ms×100MHz/f. A3.4 Data processing unit
The operation of the data processing unit is controlled by a pick-up connected to the vehicle trip meter sensor. Thus, approximately one pulse is obtained per meter. The microprocessor controlled by these pulses will read the output state of the comparator and calculate the cumulative probability of exceeding the threshold and the number of recorded samples
. In order to facilitate the determination of individual defects, an alarm will be issued when the higher threshold level is exceeded. Figure A1 shows the block diagram of the measuring equipment.
Sound alarm
Comparator
Measurement receiver
Signal level output:
A3.5 Calibration
40dB pW
Comparator
20dB pw
Microprocessor
Acid meter
Sensor
Figure A1 Block diagram of measuring equipment
Cut-and-go switch
The operating level of the comparator is adjusted to the limit value equivalent to the radiation, for batches, a vertical half-wave dipole is located 3m away from the receiving antenna and at the same height as it, and is alternately fed with:
Power equivalent to the limit value, and the difference between the highest distributed signal level in the system and the test signal level fed into the system is deducted from this value; Power equivalent to the higher threshold level (for example, 20dB higher than the lower threshold level). Before measurement, check the direction of the measuring vehicle, which does not affect the level adjustment of the comparator by more than ±3dB. For calibration, a sufficiently large area without obstacles should be found so that reflections due to the operating conditions do not cause errors in the measurement. NOTE: From the measurement distance selected for the calibration requirements, there is a corresponding relationship between the limit values given in terms of absolute transmitted power and the actual measured parameters, such as: special points in the field strength distribution.
A4 Measurement
The measuring vehicle is checked along the streets within the area of the cable distribution network. Streets wider than 6 m are measured on both sides. If there are trunk cables in the area, the measurements should be made before they are connected to the network. The measurement route should be planned before the actual measurement. The number of measurement points should be at least 100. Branch networks in individual buildings or housing areas should be measured along the general traffic routes. As measurement results, the probability of exceeding the threshold level points in each local area studied and in the entire area of the network should be given. In addition, the leakage points where the local radiation exceeds the higher limit level should be reported. If the probability of exceeding the lower threshold level for the entire network or part of the network is less than 10%, the limit value is considered to be met. The individual leakage points will be measured separately using the method defined in A5 below.
A5 Front-end with a single local leakage point
GB 16787-1997
To find out whether the system meets the limits specified for radiation, a special measurement method needs to be specified for measuring the front end or an exposed leakage point.
A5.1 Measurement method
The measurement uses the substitution method. The measurement arrangement is shown in Figure A2. When measuring, the frequency allocated in the system at the highest field strength that has been shown to produce radiation should be used. If it is not possible to measure this field strength, a signal of any frequency distributed in the system can be selected for measurement, but the difference in level compared with the allocated signal with the maximum level must be considered. A5.2 Radiation measurement
The measurement is carried out at a distance d from the measured point (the assumed radiation source). In principle, the measurement distance does not affect the measurement result. However, the recommended standard measurement distance is 3, 10, 30, 100 or 300m, and it should be carried out at a point with as few obstacles as possible in the direction of the measured point. The indication of the measuring receiver should be maximized as much as possible by rotating the direction of the receiving antenna and adjusting the half-wavelength of the antenna. A5.3 Measurement of replacement power
The half-wave dipole antenna is placed near the measuring point. The position of the test receiver is the same as the distance specified in A5.2. The half-wave dipole is fed with a frequency as close as possible to the radiation to be measured in A3.2. The minimum possible frequency difference is determined by the receiver selectivity. According to A5.2, the indication of the measuring receiver should be as large as possible. After that, the level of the standard signal generator is adjusted so that the measuring receiver obtains the same reading as obtained in A5.2. This level is the radiated power of the measured point, a)
1--System # to be tested
2--Antenna and selective measuring receiver with sufficient sensitivity, which measures the signal's packet power (the power at the synchronization pulse of the television signal); 3-Standard generator and half-wave dipole antenna: 4-Standard generator, whose output level is set to make the system operate at a normal level. Figure A2 Connection of equipment1 Measurement methodbzxZ.net
The measurement uses the substitution method. The measurement arrangement is shown in Figure A2. When measuring, the frequency allocated in the system at the highest field strength that has been shown to produce radiation should be used. If it is not possible to measure this field strength, a signal of any frequency allocated in the system can be selected for measurement, but the difference in level compared with the allocated signal with the maximum level must be taken into account. A5.2 Radiated measurement
The measurement is made at a distance d from the measured point (the assumed radiation source). In principle, the measurement distance does not affect the measurement result. However, the recommended standard measurement distance is 3, 10, 30, 100 or 300m, and it should be made at a point with as few obstacles as possible in the direction of the measured point. The indication of the measuring receiver should be maximized as much as possible by rotating the direction of the receiving antenna and adjusting the half-wavelength of the antenna. A5.3 Measurement of substitution power
The half-wave dipole antenna is placed near the measurement point, and the test receiver is located at the same distance requirement as specified in A5.2. The half-wave dipole is fed with a frequency as close as possible to the radiation to be measured in A3.2. The minimum possible frequency difference is determined by the receiver selectivity. According to A5.2, the indication of the measuring receiver should be as large as possible. After that, adjust the level of the standard signal generator so that the measuring receiver obtains the same reading as obtained in A5.2. This level is the radiated power of the measured point, a)
1--System to be measured #
2--Antenna and a selective measuring receiver with sufficient sensitivity, which measures the signal's packet power (the power at the synchronization pulse of the television signal); 3-Standard generator and half-wave dipole antenna: 4-Standard generator, whose output level is set so that the system operates at a normal level. Figure A2 Connection of equipment1 Measurement method
The measurement uses the substitution method. The measurement arrangement is shown in Figure A2. When measuring, the frequency allocated in the system at the highest field strength that has been shown to produce radiation should be used. If it is not possible to measure this field strength, a signal of any frequency allocated in the system can be selected for measurement, but the difference in level compared with the allocated signal with the maximum level must be taken into account. A5.2 Radiated measurement
The measurement is made at a distance d from the measured point (the assumed radiation source). In principle, the measurement distance does not affect the measurement result. However, the recommended standard measurement distance is 3, 10, 30, 100 or 300m, and it should be made at a point with as few obstacles as possible in the direction of the measured point. The indication of the measuring receiver should be maximized as much as possible by rotating the direction of the receiving antenna and adjusting the half-wavelength of the antenna. A5.3 Measurement of substitution power
The half-wave dipole antenna is placed near the measurement point, and the test receiver is located at the same distance requirement as specified in A5.2. The half-wave dipole is fed with a frequency as close as possible to the radiation to be measured in A3.2. The minimum possible frequency difference is determined by the receiver selectivity. According to A5.2, the indication of the measuring receiver should be as large as possible. After that, adjust the level of the standard signal generator so that the measuring receiver obtains the same reading as obtained in A5.2. This level is the radiated power of the measured point, a)
1--System to be measured #
2--Antenna and a selective measuring receiver with sufficient sensitivity, which measures the signal's packet power (the power at the synchronization pulse of the television signal); 3-Standard generator and half-wave dipole antenna: 4-Standard generator, whose output level is set so that the system operates at a normal level. Figure A2 Connection of equipment
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