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OCTOBER 2021 I DESIGN007 MAGAZINE 75 Split Cylinder Resonator Test per IPC-TM-650 is test method is a cylinder resonator and as the name implies, it is split and can be opened and closed. Aer the laminate is made and all copper is etched off, the material to be tested (MUT) is put between the split cylin- der and it is closed. e resonator will have several different resonant peaks for the user to choose to evaluate the Dk and Df. ese differ- ent resonant peaks are at different frequencies. is test method will evaluate the X-Y plane of the material and not the Z-axis. Due to this difference and the fact this test method can also operate at the same frequency as the clamped stripline test method (Z-axis method), a comparison of data when evaluating the same material in these two tests will give infor- mation regarding the Dk anisotropy of the MUT. Additionally, if the material is aniso- tropic, it should be expected to obtain a dif- ferent Dk value in the split cylinder test when evaluating the same material with the clamped stripline test. Microstrip Ring Resonator Test Method is method is a circuit test method, and the ring resonator circuit pattern is the test vehicle which is built on the material to be evaluated. e ring resonator typically has open-ended 50-ohm transmission lines (feedlines), which bring the RF energy to both sides of a ring cir- cuit pattern (ring pattern looks like a very thin donut). e gaps between the two feedlines and the ring structure are critical and varia- tions in the gap areas can cause the Dk extrac- tion to be inaccurate. Also, if the ring circuit structure is plated with thicker copper in the PCB process, as compared to another ring resonator built on the exact same material but having thinner copper plating, the gap area will have more fields in air for the thick copper- plated circuit and the resonant peak will shi to be different than the ring resonator with the thinner copper plating thickness. Because of the difference in copper plating, the extracted Dk value will be different and is not correct when trying to evaluate the Dk of the material only. e gap and the copper plating thickness variation is a normal circuit variable; a circuit test will include this, but most material tests do not. Copper plating thickness has a natural variation in the PCB fabrication process and that thickness difference can cause inaccurate Dk results when using a ring resonator. Assum- ing the engineer is aware of the copper thick- ness issue and accounts for it in the extraction process, the correct Dk can be found. Addi- tionally, this test method is affected by copper surface roughness, whereas the previous two test methods are not impacted by roughness effects. e ring resonator evaluates the Z-axis of the material. Understanding the differences between test methods can be very important for a design engineer and especially when the engineer is comparing Dk and Df values on data sheets. If a comparison of data sheet Dk values is being done, the test method which generated the Dk values needs to be understood. Ideally, it would be best to compare Dk data using the same test method and at the same frequencies, but that is not feasible sometimes. However, if different test methods are used for Dk values on data sheets being compared, and both data sheets are reporting the Z-axis Dk at approxi- mately the same frequency, it should be a good comparison. When comparing datasheets or investigat- ing material properties for a new design, it is important to consult the material manufac- turer to understand how the critical data on the datasheet was obtained. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here.

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