PCB design is not easy!
It is useful at the start of a project to ask the question “What problem are we trying to solve?” When this is done with an electrical engineering project, the following is the result. All electronic systems generate voltage waveforms that contain information. This information may be analog, digital, or RF. Circuits take these voltage waveforms and perform some operation.
This operation may be to open a garage door; to play music on a speaker; to calculate the value of π to five thousand places or to send a picture to grandma over the Internet. This, then, defines the problem. It is to create voltage waveforms of sufficient quality to convey the intended meaning, transport those voltages to circuits utilizing them, and then use the waveforms to perform an operation, all the while making sure that other signals or noise do not corrupt the operation.
At the same time, the intended signals or voltage waveforms must not corrupt other signals. This corruption might take the form of cross-talk or EMI (electromagnetic interference).
It is worth noting that the usable signal in all cases is a voltage waveform. This is true whether the signal is RF, microwave, analog, or digital. It is also true no matter what type of driver is being used. It is true even if the signal source is described as operating in the “current mode.” As will be seen later, it is not possible to send “voltage” waveforms from one place to another. In order to deliver a voltage waveform, it is necessary to create and send energy in the form of an electromagnetic wave, either down a transmission line or through space. Understanding this and how electromagnetic waves behave is fundamental to successfully designing high-speed electronic devices.
Types of High-Speed PCBs
Over time, the world of high-speed electronics has been split into two general classes of PCBs: RF/microwave/analog and Digital. Characteristics of Two General Classes of High-Speed PCBs:
RF, MICROWAVE, ANALOG PCBs
- Low circuit complexity
- Precise matching of impedance
- Minimizing signal losses essential
- Small circuit element sizes
- Usually only 2 layers
- High feature accuracy need
- Low/uniform dielectric const
DIGITAL-BASED PCB
- Very high circuit complexity
- Tolerant of impedance mismatches
- Tolerant of lossy materials
- Small circuit element sizes desirable
- Many signals and power layers
- Moderate feature accuracy needed
- Dielectric constant secondary
The industry has developed design tools, materials, and manufacturing methods that are optimized for each major class of PCBs. The rate at which speeds of logic circuits have increased has moved many digital designs into the speed range normally considered to be RF or microwave. It is useful to compare these two classes to see how they differ.
Source: Right the first time
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