Why RF design is so different from high speed digital design
You would think that the interconnection design for RF applications and high speed digital applications would be very similar. After all, the upper frequency range of the programs is similar. Figure 1 shows the frequency of various RF and microwave scripts and the bandwidth corresponding to different high-speed digital serial communication lines.
For example, 10Gbase Ethernet, at 10 Gbps, has a frequency component in the 25 GHz operating system, where K and Ka band radars operate. Aren’t RF engineers’ problems and design solutions like high-speed digital designers, if the frequency band of interest is the same?
The answer that comes as a surprise is no. These applications, although they share the upper frequency band, require very different design rules and are the main reason why there is usually a disconnection when an RF engineer tries to design a high speed digital product and vice versa.
The difference immediately begins with how an RF engineer views the world compared to a high-speed digital engineer. The RF engineer lives in the frequency range. Their intuition is rooted in the frequency band. How they approach signals and interconnections is all about the frequency spectrum.
Advanced digital engineers, by contrast, live in a time frame. Digital performance is measured in a timely manner; signal in the time zone. Their ideas about symbols and how they interact with each other and the connections are rooted in a time series.
There are four important differences in processing rates for RF and high-speed digital applications that make the processing system the best for one world, almost the other.
Bandwidth of the signal
Signal bandwidth is the numerical value for important signal information content. Thus, in RF application, there are frequency carriers that have some variability. The bandwidth of the RF signal is narrow, perhaps just a few MHz, according to the application, although the carrier may be at 2.5 GHz, as in 802.11abg Wi-Fi.
In high-speed digital applications, the signal has the highest frequency factor, depending on the signal rise time, but the important frequency factors reach all the way down to DC.
Very resistive idea
Resistance is probably the most important measure of the interconnection of any application. Interconnection resistance properties are the basis of interconnection performance in all high frequency applications. But it’s one of the most confusing topics between an RF engineer and a high-speed digital engineer. This happens because everyone uses the same term, but they give different resistances.
The use of the term “resistance” is unclear. There are many different types of resistors and each has its own specific definition. When RF engineers refer to interconnection resistance, they usually mean “input resistance in the frequency band” of the interconnection. When digital designers are rushing to talk about integration, they often mean the “form factor” of integration, which, in the case of a uniform transmission line, is the “temporary position” that sees the signal as it rotates the transmission. Line.
For the RF design, they do not care about the resistance of the interconnection at a given frequency, within a narrow frequency range. They can design the input resistance of the intermediate connector by adjusting its length, shape and even adding connected, or adjacent, arches. Input resistance is only important at load rates. What happens at other times is usually unrelated.
The digital designer cares about the importance of the line from DC to very high frequency. The key term is the immediate block that the signal sees as it moves with each step throughout the interface. The important design guide is to keep this impedance stable across the board. This means controlled cross-section and impedance control at the end, usually adding a resistor to simulate the signal so that you do not see any change in the impedance immediately.
Mention the word stub around RF designer and you will get a happy smile. A stub is a very powerful design feature for matching the line impedance with what the designer wants.
Mention the word stub around a digital designer and you get a scary, wild look in their eyes. They want to do what they can to avoid stocks because these features set the default limit on the amount of data their channel supports.
As a final example, consider how capacitors are used in these two worlds. All types of products use production machines, sometimes with up to 10-500 singles in the product. However, the applications are sometimes different and their capacitor requirements are different. Figure 4 shows an example of an RF radio output with more than 50 capacitors.