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Introduction of Third-order Intermodulation Test
The third-order intermodulation distortion caused by two frequencies is a common problem in modern communications system. When two (or more) carrier signals in a system pass a passive device, such as antenna, cable, filter, and duplexer, due to imperfect selection of the materials, unreliable  mechanical contact, false soldering, surface oxidization, etc. non-linear factors will occur at the joint places of different materials, this is like a mixer diode. Third-order intermodulation distortion (2F1-F2 and 2F2-F1) is the maximum intermodulation product which is further induced by the two carrier signals (F1 and F2) and their second harmonic (2F1 and 2F2). The typical target for third-order intermodulation product (IM3) is that the IM3 value produced by the device which is applied with two carrier signals of+43dBm simultaneously should not more than 110 dBm, i.e. 153 dBc.
Third-order intermodulation distortion will degrade the performance of a communications system. The exceeding third-order intermodulation product will interfere with other receivers, finally leading to the receivers being unable to operate normally. In the past designers are more concerned the intermodulation test of active devices. However with the development of communications system and the enhancement of the system quality, more and more attentions are paid to the intermodulation problem of passive devices. 
The InformationTransmissionLinesQualityInspectTestCenter, M. I. I. (the Inspection and Test Center of No 23 Research Institute, CETC) has been engaged in the measurement of RF cables, cable assemblies, and various connectors for long time. In order to better meet the demand of the customers and the need for 3G mobile communications test, our Inspection and Test Center additionally purchases a Model SI-2000 Passive Intermodulation Distortion Analyzer) (see the Fig. below) on the basis of exsisting Model SI-900 Analyzer (Summitek Instruments, Inc. U.S) so as to carry out the third-order intermodulation test for 3G mobile communications. As the only laboratory that provides third-order intermodulation testing service in China we are going to warmly provide vast connector and cable assembly manufacturers with quality service.

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