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PCBD-June2014

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58 The PCB Design Magazine • June 2014 lightning speed laminates ity. There are also multiple areas of impedance matching, and the impedance values can pur- posely change greatly from one area to another on the circuit. The power amplifier IC itself will often have input impedance of much less than 10 ohms, whereas the traces on the PCB will be 50 ohms. To get the energy to efficiently transi- tion from 50 ohms down to less than 10 ohms, it is critical to have clean signal energy input into the power amplifier IC. There are several different types of imped- ance. The most common type of controlled impedance on a PCB is referred to as character- istic impedance. Other types include input im- pedance, wave impedance, image impedance, and others. Many of the impedance issues are frequency-dependent, and this can be an issue for wideband power amplifiers. The impedance matching network that is necessary for the de- sign of a power amplifier can only tune the im- pedance transitions over a range of frequencies. Many times the actual power amplifier IC can be used over a wider range of frequencies (wide- band) than the tuning networks that are used to match the impedance for traces going into and out of the chip. The impedance value of a PCB is dependent on many variables. In order of their significant impact on impedance, these variables include: thickness of the substrate, dielectric constant, conductor width and copper thickness. High- frequency circuit materials are formulated to have a tight thickness control as well as dielec - tric constant control. The same can be said for the laminate's copper thickness. But remember that this thickness can vary on the final circuit because of differences in PCB fabrication tech- niques. PCBDESIgN John Coonrod is a market devel- opment engineer for rogers Corporation, advanced Circuit Materials Division. To reach Coonrod, click here. realtimewith.com CliCk To View Video Interview What capabilities should you look for when you're selecting a new fabricator? is a survey good enough, or should you require a full audit? rob Scott of the auditing company next level PCB explains the criteria that he assesses when auditing a PCB supplier. PCB Auditing at the Next Level by Real Time with... IPC APEX EXPO WHEN IS CoNTRoLLED IMPEDANCE IMPoRTANT? continues

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