1、Rec. ITU-R SM.1446 1 RECOMMENDATION ITU-R SM. 1446 DEFNITION AND MEASUREMENT OF NTERMODULATION PRODUCTS N TRANSMITTER USWG FREQUENCY, PHASE, OR COMPLEX MODULATION TECHNIQUES (Question ITU-R 21 Ul) (2000) The ITU Radiocommunication Assembly, considering that intermodulation (IM) products are part of
2、the unwanted emissions (RR No. S1.146); that IM products are generated either in the radio transmission system itself and/or by interaction between a) b) different radiating elements at the same radio site; cl Appendix S3 of the RR and Recommendation ITU-R SM.329; d) e) f) IM products in an anomalou
3、s and uncontrolled way, and these will all add together at receivers; g) that the IM products due to amplitude-modulated radio transmitter are considered in Recommen- dation ITU-R SM.326: that the limits of spurious emissions cover only single- or multichannel IM products, and are prescribed by that
4、 the limits of out-of-band emissions cover only single- or multichannel IM products, and are under study; that no limits are defined for inter-transmitter IM between different systems; that there is a rapid increase of shared radio sites, and that each site can potentially radiate passive and active
5、 h) techniques , that there is a need to define methods of measurements of IM products, particularly for digital modulation noting that Report ITU-R SM.2021 contains general principles on generation of IM products and the relevant a) mitigation techniques to minimize IM, recommends 1 definitions and
6、 the relevant measurement techniques for each type of IM given in Annex 1 should be used. that when considering the types of mechanisms which generate IM products in the transmission system the 2 Rec. ITU-R SM.1446 ANNEX 1 IM products in the transmitter TABLE OF CONTENTS Page 2 1 Definitions of the
7、different IM types in the transmitter 1.1 Type 1 - Single-channel IM 3 1.2 Type 2 - Multichannel IM 4 1.3 Type 3 - Inter-transmitter IM 5 Type 4 - IM due to active antennas Type 5 - IM due to passive circuits 1.4 5 1.5 6 1.6 Transmitter IM attenuation 7 2 Radiocommunication services considerations 7
8、 3 Measurement techniques Generic measurement methods for single-channel IM measurement (Type 1) . 3.1.1 Analogue modulation 3.1.2 Digital modulations . Generic measurement methods for multicarrier IM measurement (Type 2) . 3.2.1 Descriptions of measurement methods 3.2.2 Comparison of the methods .
9、Generic measurement methods for inter-transmitter IM measurement (Type 3) 3.3.1 Principle 3.3.2 Measurement set-up . 3.1 3.2 3.3 7 7 8 8 9 9 10 11 11 12 Appendix 1 of Annex 1 - Examples of IM performance . 13 1 Definitions of the different IM types in the transmitter IM products are generated at non
10、-linearities in the transmitter output amplifier, e.g. at semiconductors, klystrons, etc., and in passive devices like combiners, circulators, connectors, etc. IM products at the frequency fl are generated by two or more unwanted signals at the frequencies fi, f2, . at non-linearities in the output
11、of a transmitter. The relation betweenfiM andfl,f2, . can be expressed very general: flM= I m1f1 + m2f2 + . I with mV= O, fl, f2, . The order of the IM product is given by n = ml + m2 + This means that the frequency for 2nd order IM products, ZM2 with n = 2, ml = m2 = 1 results inhM = I fi I f2 I an
12、d of the 3rd order ZM3 (n = 3, ml = 2, m2 = 1) inhM = I 2f1 I f2 I or in f-0? Therefore, it is important to maintain the linearity of passive circuits such as waveguides, cables and connectors. Rec. ITU-R SM.1446 7 1.6 Transmitter IM attenuation The transmitter IM attenuation is a measure of the cap
13、ability of a transmitter to inhibit the generation of signals in its non-linear elements caused by the presence of the carrier. This definition is indicated in Fig. 8 with fi as the carrier frequency having the output power PlV1) and f2 as the interfering signal having the power P2. FIGURE 8 Definit
14、ions of transmitter LM attenuation and reverse LM factor P2(f2) c- The transmitter IM attenuation, AIM, is then defined by: ZA43 25 W. Repeat steps 4 through 6. The lower of the two readings obtained in steps 6 and 8 is the IM attenuation. - Feeder Branching RF TxRx receiver filter network Transmitt
15、er - APPENDIX 1 TO ANNEX 1 Examples of IM performance In general, the IM performance depends on a huge variety of different factors, e.g. like modulation and access schemes, hardware components and cost for equipment. The following examples are aimed only to illustrate which ranges of IM suppression
16、s are achievable in real systems. They are not intended to define any limits for IM products. 1 Fixed service IM limits in current European standards for microwave radio-relay links are listed only for information (see Fig. il). They only make reference to the inter-transmitter IM problem (Type 3),
17、i.e. there are no specific values available for IM attenuation in European standards. FIGURE 11 Typical block diagram for fixed service A B C D ! Antenna Conformance to the fixed link IM limit is to be declared by the equipment manufacturer during installation. The maximum permitted IM level is quot
18、ed referenced to point B in the receive chain. All fixed link installations are to meet the given criteria when wanted power levels at point C equal 28 &m. 14 2 Rec. ITU-R SM.1446 Mobile service Many mobile standards do not contain any specific values for IM attenuation. The values for IM emissions
19、are included in the limits for spurious emissions. Some examples of available IM attenuation are listed in Table 2. The values required range from 40-70 dl3. The interfering signal is usually assumed as CW with power 30-50 dl3 below the wanted signal. TABLE 2 Examples of IM attenuation in mobile rad
20、io IM attenuation Service I GSM900 Base transceiver station and mobile private branch exchange (PBX) 70 dB GSM1800 Between mobile stations 50 dB TETRA Base station, only one channel more channels several transmitters Mobile station 40 dB 70 dB 60 dB 60 dB, measured at the antenna connector PMR For a
21、ll components Several base stations at the same site 40 dB 70 dB 3 Satellite service The IM products in satellite services are generated by multicarrier operation through common amplifiers in the RF transmit subsystem OM Type 2). Examples for the performance requirements of INTELSAT earth stations a
22、re given in Table 3. TABLE 3 Examples of the performance requirements for IM products transmitted from INTELSAT earth stations Uplink transponder 4/6 GHz: Hemispheric and zone beams Global beams and C-spot 4/6 GHz: Global, hemispheric and zone beams 11/14 GHz: spot e.i.r.p. density limits at 10“ ele
23、vation angle and beam edge Any combination of INTELSAT carrier types except those formed exclusively by interaction among SCPC carrier: 21 dB(W/4 kHz) 24 dB(W/4 kHz) Formed exclusively by interaction among pre-assigned SCPC carrier: 31.5 dBW 48.1 - 20 loglo N for 2 7 Any combination of INTELSAT carr
24、ier types: 10 dB(W/4 kHz) 16 dB(W/4 kHz) for INTELSAT V-VI for INTELSAT VII-VIII Rec. ITU-R SM.1446 15 Frequency Absolute power (dBm) (dB) Examples for typical values of IM attenuation for land earth stationskoast earth station (LESKES) and attenuation of IM products due to multiple carriers for Inm
25、arsat-2 and Inmarsat-3 satellites are depicted in Table 4. fi f2 2fl -f2 2f2 -fi 3fl -f2 55 25 15 -5 -20 - - 10 30 45 TABLE 4 Examples of the IM attenuation from Inmarsat earth and space stations System component I I IM attenuation I Inmarsat LES/CES 3rd order IM products: 30 dB below the e.i.r.p. o
26、f each of two test carriers each having an amplitude of e.i.r.p.mux -3 dB where e.i.r.p.mux is the maximum e.i.r.p. Inmarsat-2 space station Forward direction transponder Return direction transponder Due to multiple carriers: 13.5 dB 22.0 dB Inmarsat-3 space station Forward direction transponder Ret
27、urn direction transponder Due to multiple carriers: 14.0 dB 21.5 dB 4 Broadcasting services For broadcasting transmitters installed at the same site, the reverse IM factor is often used to characterize the inter-transmitter IM, AHM (Type 3). For illustration, the following example is picked up: the
28、wanted FM sound broadcasting transmitter operates at fi = 98 MHz, having power 300 W, and is interfered by a signal of 300 mW generated by a second transmitter. The frequencyfz is varied between 87.5 MHz and 108 MHz (Band 11) with a frequency separation of If1 -f2 I 300 kHz. The results are given in Table 5. TABLE 5 Examples of the reverse IM factor at FM transmitter sites Typical values of the reverse IM factor for FM transmitters are 10 dl3 or larger. The use of isolators may increase the protection.
