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PCBD-Oct2016

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62 The PCB Design Magazine • October 2016 (3 GHz) of the fundamental frequency, the 5 th harmonic (5 GHz) and the 7 th harmonic (7 GHz) frequencies. The frequencies given for the 2 GB/s exam- ple are generally not considered high frequency for RF concerns. However, when the frequency is high, many extra design considerations must be utilized in order to ensure good quality defi- nition of the digital waveform. This concern for HSD signal quality is the focus of a term known as signal integrity (SI). Many years ago when HSD speeds were lower, SI was primarily focused on time domain issues and there were many concerns to be ad- dressed. Now, as HSD speeds are higher, the SI focus is still on time domain which is even more difficult but now RF issues become much more important and the SI engineer has to deal with frequency related issues as well. The job of the SI engineer is getting much more complicated as HSD speeds are increasing. As a very general statement, RF issues are moderate up to 10 or 15 GHz. But from 15 to 30 GHz they become more difficult, and from 30 to 60 GHz they can be very difficult. Beyond 60 GHz, the RF issues are really tricky but these generalities also depend on the circuit configu- ration. Achieving good RF performance is more difficult at higher frequencies. RF performance is affected by the circuit design. Some circuit designs are more robust at higher frequencies than others. High-speed digital processing currently done at 10 GB/s is relatively well understood, however not trivial, nonetheless the drive to higher speeds gets more difficult due to the in- fluence of the high frequency RF components. As another example, a HSD application oper- ating at 28 GB/s has RF signals at 14 GHz, 42 GHz, 70 GHz and 98 GHz. The higher-frequen- cy harmonics are very influential on the clarity of the rise time of the digital pulse and in order to have a well-shaped digital pulse, the circuit response to the RF frequencies at 70 and 98 GHz is very important. The frequencies in this range are considered millimeter-wave RF technology and the variables for having good wave behav- ior at these frequencies are incredibility tough to deal with in PCB design. When designing a PCB at millimeter-wave frequencies, the wavelength relationship to PCB design features is critical. The wavelength at 70 GHz for a stripline PCB using material with a Dk of about 3 is approximately 0.100" (2.54 mm). Fractions of the wavelength can af- fect the circuit performance. When circuit fea- tures are about the size of ½ the wavelength, wave performance can be affected significantly, which will impact the definition of the digital pulse. However, even at a ¼ wavelength, there are influences and a safe number is to have cir- cuit features less than 1/8 wavelength to avoid the feature having an unwanted impact on wave performance. The physical size related to 1/8 wavelength at 70 GHz with the stripline example is 12.5 mils (0.32 mm). Basically anything larger than 1/8 wavelength can impact the wave perfor- mance at 70 GHz and just trying to get the energy from the connector into the circuit can be a problem. For a stripline circuit, the plated through-hole via which transitions the signal from the connector down into the circuit at the signal layer can have dimensions greater than 12.5 mils and may cause the energy to be distorted when it gets to the signal layer. Even more concerning is the 70 GHz issue is only the 5 th harmonic for the 28 GB/s signal and the 7 th harmonic is at 98 GHz. The 1/8 wavelength dimension for that frequency is 8.6 mils (0.22 mm). Fractions of a wavelength at millimeter- wave frequencies can impact circuit perfor- mance. However, material properties can influ- ence the RF performance of the circuit. Most circuit materials used in high-frequency PCBs have layers of woven glass reinforcement to help mechanically stabilize the material. Some THE BLENDING OF HIGH-SPEED DIGITAL AND HIGH-FREQUENCY RF " The job of the SI engineer is getting much more complicated as HSD speeds are increasing. "

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