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The Blending of High-Speed Digital and High-Frequency RF

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 3^rd harmonic (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 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.