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article influence of via stub length and antipad size on the insertion loss profile continues Figure 6: Different stub lengths. Impedance Control To assure good matching of the transmission lines to the measurement equipment, the single ended and differential impedance of the traces was measured on each test panel. For ease of testing, dedicated impedance coupons (Figure 7) were used in conjunction with handheld probing heads and a standard, volume manufacturing impedance test system. The impedance testing confirmed that both the differences between the panels and between the two produced work orders were minimal. The absolute values were slightly below nominal, with an average single ended impedance of 47.6 ohm and an average differential impedance of 97.2 ohms on layer 3. The detailed readings can be found in table 1. 48 The PCB Design Magazine • January 2014 Single-Ended Insertion Loss Testing Measurement of single ended and differential insertion loss of the transmission lines including the effect of the via stubs was performed on a 4-port vector network analyzer capable of going to 40 GHz. High-quality coaxial cables with 2.92 mm connectors with a frequency rating of 40 GHz were used. A minimum warm-up period of two hours was ensured prior to calibration of the vector network analyzer. For this purpose an electronic calibration module, connected directly to the end of the coaxial cables was used. After completion of the calibration, the cables connected directly to the compression mount connectors on the test boards, without any additional adapters needed.