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PCBD-Dec2014

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December 2014 • The PCB Design Magazine 61 MAkING A CONNECTION WITH CONDUCTOR DISCONTINUITIES continues John Coonrod is a market development engineer for rogers Corporation, advanced Circuit Materials Division. To read past columns, or to reach Coonrod, click here. lightning speed laminates 1b. A frequency-S21 curve for a microstrip edge- coupled bandpass filter is shown in Figure 2. Even though there are multiple conductor discontinuities for the filter shown in Figure 1b, it can be seen that energy still propagates through this structure, given the energy is at the right frequency. In the case of the edge- coupled filter performance shown in Figure 2, a good example would have energy propagate through the filter at 2 GHz. . The blue curve in Figure 2 shows S21 performance, and there is a band of frequencies, from about 1.9 to 2.1 GHz, at which much of the energy is passed through the filter. If an application was gen - erating a lot of energy at 3 GHz (the far right frequency in Figure 2), the energy at that fre- quency is pretty much shut off with about 65 dB of loss. Summary Contrary to many of the concepts related to traditional PCBs, conductor discontinuities at microwave frequencies may not cause a defect in a circuit, and they could actually be the rea- son the circuit is performing properly. Addition- ally, there are many other design issues to con- sider when designing microwave PCB circuitry, and the rules regarding typical PCBs no longer apply. PCBDESIGN Figure 2: Frequency vs. s21 performance of a microstrip edge-coupled bandpass filter.

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