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30 The PCB Magazine • July 2016 ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1) Perturbed Resonant Cavities with Electric Field Oriented In-Plane of Dielectric: 5. Rectangular cavity and open resonator (ɛ r and tan δ) 6. Split post dielectric resonator - SPDR (ɛ r and tan δ) Aperture-Coupled Stripline with Electric Field Oriented Normal to Plane of Dielectric: 7. Bereskin resonator (ɛ r and tan δ) Descriptions of Measurement Methods Extraction of ɛ r from Impedance Measurements of Microstrips The objective of this method is to calculate normal ɛ r values from time-domain measure- ments [4]. This method does not directly yield tan δ and is a fixed value without frequency de- pendence. The principle of this technique is to back-calculate ɛ r from impedance values mea- sured on a time-domain reflectometer (TDR). The advantage of this method is that it can utilize any microstrip transmission line or even impedance coupons on a circuit board. The dis- advantages are that the values are limited by the pulse-width of the TDR and the back-calcu- lation of ɛ r requires a lot of assumptions. For this study, samples were prepared by etching multiple micro strip transmission lines. These micro strip transmission lines were bro- ken up into three line widths and two lengths. This produced six microstrip transmission lines of varying widths and lengths for each circuit board material sample. The microstrip lengths were 130 mm and 230 mm while the widths ranged from 210 μm up to 400 μm depending on the material. The three line widths were cho- sen for each sample based on the theoretical ɛ r and corresponding 50 Ohm line width. The de- sire was for the narrow line to have impedance greater than 50 Ohms and the wide line to have impedance less than 50 Ohms with the other line falling squarely in the middle. An example of two equal line width microstrips of two dif- ferent lengths can be seen in Figure 1. Once the microstrips were prepared, the physical dimensions of each were needed. Fig- ure 2 shows the measurements required for the impedance extraction method. Dielectric thick- Figure 1: Two example microstrip transmission lines for impedance extraction. Figure 2: Microstrip transmission line cross section [5] .