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42 DESIGN007 MAGAZINE I FEBRUARY 2023 cuit being built in volume production. If the electrical characterization is a project that will consider many different types of circuit mate- rials for qualification for future projects, it may be best to use the same fixtured test method for all materials. If the same test method is used and is well understood, the evaluation should be relatively impartial to the materials being tested. However, since different circuit materials have different processing needs for the circuit fabrication process, it may not be a good comparison to evaluate different circuit materials using the same circuit test method. ere are many test vehicle options available for circuit test methods. Some of the more common circuit test vehicles include ring resonators and transmission line circuits with different lengths; antenna patch radiators, 180-degree hybrids, edge-coupled filters, and delay lines are additional options. Ring resonators have been used for material characterization for many years and with good success. But over the past several years, and with more applications using millimeter-wave (mmWave) frequencies, obtaining accurate results when using ring resonator test vehicles has become problematic. Wavelength can be a simple way to think about these challenges, as related to lower and higher frequency. At mmWave frequencies, wavelengths are very small, and the propagating wave will be more sensitive to circuit anomalies, which can cause RF disturbances. ese small circuit anomalies may be normal for circuit fabrication and may not cause any difference in RF performance when operating at lower frequencies with lon- ger wavelengths. Many ring resonators are gap coupled; dif- ferences in the gap coupling can influence the ring resonator's center frequency, which impacts the Dk extraction accuracy. e RF differences due to the gap are usually related to etching and copper thickness variation. If the same ring resonator design is used for two cir- cuits built at different times and one circuit has thicker copper than the other, the circuit with the thicker copper will have more fields fring- ing in air than the circuit with thinner copper. Air has a very low Dk and having more fields in air will naturally lower the extraction of the Dk value. e lower Dk result will be related to the circuit fabrication process rather than the circuit material. e conductor trapezoi- dal shape and its normal variation from circuit to circuit can also impact the RF performance due to affecting the fields in the gap coupled areas. Again, at lower frequencies, these small differences due to coupling will be less signifi- cant, but at mmWave frequencies the differ- ences can be substantial. ere are also copper surface roughness effects to consider for Dk extraction. e cop- per surface in this case refers to the copper surface at the substrate-copper interface as the laminate is made. When the copper is rough, it will slow the wave propagation and alter the RF performance. e copper surface rough- ness is not perfect and there is normal varia- tion in roughness from batch to batch of the same copper foil. For a microstrip ring resona- tor, the copper roughness will not be the same for the signal plane as the ground plane, due to this normal variation in roughness. At lower frequencies, the differences in roughness are less significant than at mmWave frequencies. e copper roughness and its variation are other issues to consider for the test vehicle being used at higher frequencies. ere are many test methods and test vehicles that can be used to characterize high frequency circuit materials. Designers should contact the material supplier to inquire how the published Dk was obtained, as well as ask for test method recommendations that are appropriate to the RF design being considered. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns, click here.