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36 The PCB Magazine • July 2016 tan δ of a material under test by comparing the loaded (perturbed) and unloaded (unperturbed) resonant modes of a resonant cavity. The reso- nant frequency and quality factor will change with the loading of a resonant system with a dielectric material [8] . In order for the resonant cavity method to function, all circuit board samples must have all the copper cladding re- moved. With the resonant cavity, measuring the ɛ r and tan δ of a material is a quick and repeatable process. First, the resonant cavity must be con- nected to the VNA and the resonant frequencies and quality factors within the frequency band of interest must be mapped. Once this is accom- plished the circuit material under test must be placed in the resonator and the resonant fre- quencies and quality factors must be mapped again. Two different cavity resonant methods were implemented. The first was a rectangular wave- guide cavity resonator, shown in Figure 7, which was used to characterize the dielectric proper- ties of the circuit board materials up to 10 GHz. Specifically, the waveguide resonator is setup with only enough space to fit the circuit board material sample between the two halves of the resonant cavity. This is done to allow for the material under test to be inserted and removed without disturbing the cavity dimensions. The cavity is designed to have six resonant modes at frequencies of approximately 2.2 GHz, 3.4 GHz, 5.0 GHz, 6.8 GHz, 8.6 GHz and 10.4 GHz. Data collection and processing is done through automated commercial software that inter- faces directly with the VNA. It's important to note that the process was done twice for each sample, one with the sample in a vertical ori- entation and the other with the sample rotated 90 degrees in a horizontal orientation. This was done to determine if there are any differences in the in-plane permittivity and tan δ based on material orientation. The rectangular cavity has the advantage of being very simple and quick, but the precision of tan δ is limited to about 0.0005–0.001 since the resonator Q ranges between 2000–7000. The second resonator was an open cavity, shown in Figure 8, which im- plements two concave spherical re- flectors to create a concentric resonant cavity. This cavity was used to charac- terize the circuit board materials up to 40 GHz. The resonant mode frequen- cies are determined by the distance between the reflectors. Choosing the optimum cavity spacing requires some experimentation to minimize interfering modes across the range of ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1) Figure 7: Rectangular cavity resonator. Figure 8: Open resonator.