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Transmission/Reflection Measurement and Near-Field Mapping Techniques for Passive Intermodulation Characterisation of Printed Lines
Two-port reflection/transmission measurements and near-field mapping of PIM products in printed transmission lines have been applied to characterisation of PIM performance of PCB laminates. The two-port measurement setups based upon two different test fixtures with contactless and soldered cable launchers have exhibited noticeably different absolute level of PIM products in the same laminate material. Nonetheless the measurement results remain consistent within each setup, and the same qualitative behaviour of PIM products is observed in both setups. Therefore, for reliable characterisation of PIM performance of PCB materials, it is necessary to carry out comparative measurements of the printed lines using the same test setup. It was observed in the two-port measurements that even minor dissimilarity of the printed line specimens can cause significant variations of the PIM level and thus affect measurement uncertainty. This suggests that the statistical approach should be used to determine representative PIM characteristics of each laminate material with reference to the specific layout of printed conductors. In this respect the contactless launching has proved to be more advantageous technique for the production testing of PCB materials.
Near-field probing of PIM product distributions on printed lines has been used to gain insight into the mechanisms of PIM generation in printed lines. The presented mappings of PIM products are consistent with the two-port measurements and clearly show the cumulative growth of PIM level with the line length. The results of near-field probing have explicitly demonstrated that mismatch of the printed lines inflicts variations in the PIM level along the microstrip lines.
Schlüsselwörter: RF-30, TLG-30, PIM, passive Intermodulation, soldered microstrip launchers, contactless microstrip launchers
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