Design007 Magazine

Design007-Nov2019

Issue link: http://iconnect007.uberflip.com/i/1183414

Contents of this Issue

Navigation

Page 61 of 121

62 DESIGN007 MAGAZINE I NOVEMBER 2019 the fastest rise time. When dealing with 1-ns rise times, the emissions can easily exceed the FCC/CISPR Class B limits for an unterminated transmission line. At high frequencies, traces on a PCB act as a monopole or loop antennas. Unfortunately, the high-frequency components of the fundamental radiate more readily because their shorter wave - lengths are comparable to trace lengths (particu- larly stubs), which act as antennas. Consequent- ly, although the amplitude of the harmonic fre- quency components decreases as the frequency increase, the radiated frequency varies depend- ing on the antennas/traces characteristics. Computer-based products tend to radiate on the odd harmonics. High emissions are gener- ally detected at the third, fifth, and sometimes seventh harmonic of the fundamental clock frequency. If this also occurs where the AC im- pedance of the PDN is high, then the radiation is projected even farther. Being able to view a problem in the frequen- cy domain is a powerful tool that provides an- other perspective that often reveals structure to a problem that isn't obvious in the time do- main alone. Key Points • The frequency domain can provide valuable insight to understand and master many SI effects • In the time domain, the system is evaluat- ed according to the progression of its state with time; however, in the frequency domain, the system is analyzed according to its response for different frequencies • The system is changed from time to frequency to make it easy to understand the response • The Fourier series expansion of a square wave is made up of a sum of harmonics • Impedance is defined in both the time and frequency domains; however, it is far easier to understand and apply the concepts of AC impedance in the frequency domain • If the square wave has an even mark-to- space ratio, then the even harmonics cancel • The high-frequency content of a square wave is significantly affected by the rise time of the waveform; a fast rise time results in higher-frequency components • One needs to consider the maximum bandwidth of a signal, including harmon- ics, rather than assume the perfect square wave fundament frequency model • Power consumption in FPGAs has become a primary factor for FPGA selection • To reduce power consumption, IC manu- facturers have moved to lower core voltages and higher operating frequencies, which of course mean faster edge rates • A faster edge rate creates ringing in the unterminated transmission line; this also has a direct impact on radiated emissions • The high-frequency components of the fundamental radiate more readily because their shorter wavelengths are comparable to trace lengths (particularly stubs), which act as antennas Further Reading • B. Olney, "Beyond Design: When Do Traces Become Transmission Lines," The PCB Design Magazine, October 2017. • B. Olney, "Beyond Design: Signal Integrity, Part 1," The PCB Design Magazine, October 2014. • E. Bogatin, Signal and Power Integrity: Simplified, Prentice Hall, 2008. DESIGN007 Editor's Note: Figures 1 and 3 drawn by Barry Olney. Barry Olney is managing director of In-Circuit Design Pty Ltd. (iCD), Australia, a PCB design service bureau that specializes in board- level simulation. The company developed the iCD Design Integ- rity software incorporating the iCD Stackup, PDN, and CPW Planner. The software can be downloaded www.icd.com.au. To read past columns or contact Olney, click here.

Articles in this issue

Links on this page

Archives of this issue

view archives of Design007 Magazine - Design007-Nov2019