Recommendation K.24



METHOD FOR MEASURING RADIO FREQUENCY

INDUCED NOISE ON TELECOMMUNICATIONS PAIRS



1.	Purpose of this Recommendation



	This Recommendation is intended to standardize the method for measuring radio frequency induced 
noise that may cause degradation of equipment performance and transmission quality. Standardization of 
the method for measuring induced noise makes possible the international standardization of the quality of 
the telecommunication system.



2.	Scope



	This Recommendation considers measurement methods for radio frequency induced noise at any tele-
communication pair. Locations for measurement are both the cable entry point into a building and the 
interface point of a terminal equipment.



	The frequency range to be considered is 10 kHz to 30 MHz.



Note - Above 30 MHz, the technical problems of making measurements have not been solved and are 
therefore still under study.



3.	Circuits for measuring radio frequency induced noise voltage



	1)	Measured mode of induced noise voltage



	Both transverse and longitudinal voltages should be measured.



	2)	Measured condition of telecommunication line



	Measurements should be made with all telecommunications equipment 	disconnected at measuring 
end and with a measurement termination 	network.



	i)	Termination network for measurements



	Measurements should be made at both the cable entry point into the subscribers premises and at 
the terminal equipment point. In the measurement, a T network shown in Figure 1 should be used. 
The longitudinal conversion loss of the T network should be at least 10 dB higher than the value of 
the LCL for the cable type to be measured (e.g., 60 dB cable requires 70 dB measurement termina-
tion network).



Note - Values of Rx and Ry are under consideration. Administrations and RPOAs are requested to make 
measurements at both sets of values indicated in Figure1.  

	ii)	Reference earthing point



	Either of two reference earthing points may be used. In order of preference they are: 1) the screen 
of the cable or 2) the primary protection ground terminal, protective earth, or nearby grounded 
metal work. Since it affects the result, the reference earthing point used for a measurement should 
be stated.

Note -  For transverse measurements, a connection to a reference point may not be required, but care 
must be taken with the capacitance of the measurement equipment to ground. This may be done by using 
battery powered measuring equipment. An isolating transformer for mains powered equipment, or a balun 
termination network, must be used when measuring metallic transverse voltage.



	iii)	Termination Network to use at the central office



On inside house wire (such as the S/T interface line of ISDN) it is important to termi-
nate the far end of the cable. However, when measuring at the entry point of the local 
network into the customer's premises (such as the two-wire interface to NT1 of ISDN), 
it is not important to have a termination at the far end if the cable lengths exceeds 1 
km. Below 1 km, it may still be possible to make measurements without terminating 
the far end, depending on the frequency of the interfering signal and the make-up of 
the local network.



	3)	Detector type



The detector shall have fundamental characteristics as defined in Section 1 of CISPR spec-
ification for radio interference measuring apparatus and measurement method, CISPR 16, 
1987.



	4)	Bandwidth of measurement



The bandwidth of measurement shall have fundamental characteristics as defined in Section 
1 of CISPR specification for radio interference measuring apparatus and measurement 
method, CISPR 16, 1987.



Improvements in narrowing the bandwidth and the standardization of appropriate measuring 
equipment needs further study in cooperation with CISPR.



	5)	Electric field immunity of measurement equipment



The test equipment should have an overall immunity to electromagnetic fields in accordance 
with CISPR publication 16. Adequate accuracy should be provided for extending the use of 
the equipment to locations with field strengths above 3 V/M to 10 V/M.