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PCBD-Feb2015

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44 The PCB Design Magazine • February 2015 In the past, one of OEM customers' main concerns when dealing with their PCB fabrica- tors was characteristic impedance. Many times, a PCB design is considered controlled imped- ance and the PCB fabricator is held to a speci- fication for impedance control. A design may be 50 ohm nominal impedance, and the PCB is tested to show that it has the correct impedance within some tolerance. The tolerance is some- times ± 10%, other times ± 5% or less. As digital rates continue to increase, there are more requests for fabricators to perform in- sertion loss and other types of electrical testing. Understanding the basics of digital signaling can help explain why these new electrical tests are desired for ensuring a quality PCB for high- speed digital applications. Digital pulses are made up of analog sine waves. In order to make a digital pulse, sine waves of different frequencies are added to- gether to form the digital pulse. Each of these sine waves is a high-frequency RF wave at a spe- cific frequency. As an example, when a 10 Gbps pulse is generated from a combination of the RF waves, the frequencies used will be approx- imately 5 GHz, 15 GHz, 25 GHz and 35 GHz added together. Actually there can be more, but this is a simple example of showing the adding of sine waves using the fundamental harmonic (5 GHz), 3 rd harmonic (15 GHz), 5 th harmonic (25 GHz) and the 7 th harmonic frequency at 35 GHz. Figure 1 shows how the digital pulse can be formed from adding these sine waves. LIgHTnIng SPEED LAMInATES by John Coonrod rOGers COrPOrATION Insertion Loss: A Bigger Concern in High-Speed Digital? feature coulmn

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