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July 2016 • The PCB Magazine 37 ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1) thickness and permittivity in the materials be- ing measured. A configuration was chosen such that five resonances could be consistently and repeatedly measured. 26 GHz, 40 GHz, 49 GHz, 56 GHz, and 60 GHz were chosen. Data collec- tion and processing is done through commer- cial software that interfaces directly with the VNA. In both cavities, the sample is placed in the middle and evaluated in both the vertical and horizontal orientation. The open resonator is capable of precision for tan δ measurement of about 0.0001 since the Q of the resonator is 50,000–100,000. The disadvantage of this tech- nique is that it is quite tedious to perform and repeatability is limited by mechanical and am- bient environmental stability that needs to be maintained for the fixture. Figure 9 illustrates the overall resonant method concept. The higher quality factor (Qc) waveform is of the resonant cavity absent a cir- cuit board material sample and the lower qual- ity factor (Qs) waveform is of the same resonant cavity with a material under test present. The resonant frequency (fc) of the empty cavity is clearly shifted to a lower frequency (fs) and the quality factor (Qs) is clearly lower with a sample present. The resonant frequencies and band- widths of the unloaded cavity and the loaded cavity should be measured and quality factor calculated for the frequencies of interest [12] . Once mapped, the equations below can be used to calculate ɛ r and tan δ. These equations compare the differences in the resonant fre- quency and quality factor from the unloaded cavity to the cavity with a circuit board mate- rial present. where and Split Post Dielectric Resonator (SPDR) to Measure ɛ r and tan δ The split post dielectric resonator, as seen in Figure 10, utilizes two circular dielectric resona- tors to measure ɛ r and tan δ or a circuit board material. The method functions similarly to the previously described resonant cavities in that the unloaded quality factor (Qc) and resonant frequency (fc) of the resonator without a mate- rial sample is compared to the loaded cavities change in resonance quality factor (Qs) and shifted frequency (fs) with a material sample present. However, this method is different in that the Rayleigh-Ritz method is used to com- pute the resonant frequencies, the unloaded quality factors and all other parameters of the SPDR [13] . In this study, all calculations and mea- surements were accomplished with commer- cially available software and hardware. Figure 9: Illustration of resonant cavity method.