Lightning Speed Laminates: The Blending of High-Speed Digital and High-Frequency RF


Reading time ( words)

When the terms high-speed and high-frequency are mentioned, people think they describe the same issue. But in reality, they can be two very different matters. The term high-speed generally refers to digital technology which transfers data at very high rates. But the term high-frequency is typically related to radio frequency (RF), which involves analog signals moving energy at high frequencies.

An easy way to think of the two technologies is that one is related to time domain and the other is frequency domain. In other words, high-speed digital (HSD) applications have concerns with time related items such as rise time of the pulse which defines the 0’s and 1’s of the digital information. The high frequency applications are interested in how an electromagnetic wave responds at a certain frequency or a range (band) of frequencies. Many times, charts which describe the different technologies will be related to time for HSD and frequency for RF high-frequency applications.

There is a fundamental relationship between HSD and RF technology. The pulses used for digital information transfer are generated by RF waveforms.  Basically, a sine wave can be thought of as a RF waveform, and when several sine waves, which are at different frequencies, are combined, they can form a square wave. The square wave is used for the pulse generation of the digital signals.  As a simple example, a digital pulse speed (clock speed) that is 2 GB/s is formed by RF signals at frequencies of approximately 1 GHz, 3 GHz, 5 GHz, 7 GHz, etc.  In the order of frequencies given they are: the fundamental frequency (1 GHz), the 3rd harmonic (3 GHz) of the fundamental frequency, the 5th harmonic (5 GHz) and the 7th harmonic (7 GHz) frequencies.

The frequencies given for the 2 GB/s example 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 definition 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 addressed. 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.

To read this entire article, which appeared in the October 2016 issue of The PCB Design Magazine, click here.

Share

Print


Suggested Items

Stitching Capacitor: Crosstalk Mitigation for Return Path Discontinuity

06/13/2019 | Chang Fei Yee, Keysight Technologies
When the return path is broken due to the switching of reference planes with different potential, e.g., from ground to power or vice versa after layer transition on PCB, the return current might detour and propagate on a longer path, which causes a rise in loop inductance. This might lead to the sharing of a common return path by different signals that pose a high risk of interference among the signals due to higher mutual inductance. This interference results in signal crosstalk. To mitigate the crosstalk due to return path discontinuity (RPD), stitching capacitors are mounted on the PCB to serve as a bridge between the two reference planes of interest on different PCB layers.

Freedom CAD Book Excerpt: The Printed Circuit Designer’s Guide to… Executing Complex PCBs

05/31/2019 | Scott Miller, FreedomCAD Services
The following is an excerpt from Chapter 5 of "The Printed Circuit Designer’s Guide to… Executing Complex PCBs," written by Scott Miller of Freedom CAD Services.

Technically Appropriate Material Choices are Key to Design Success

05/16/2019 | Nolan Johnson, I-Connect007
Materials are no longer a passive part of the design; they play an active role in the manufacturability, reliability, and speed of a PCB. I-Connect007’s Nolan Johnson and Mike Creeden, founder of San Diego PCB Design, discuss several key characteristics that designers should consider in their material selection process.



Copyright © 2019 I-Connect007. All rights reserved.