Ideas & Solutions
Impedance and multilayer PCBs
Developers and designers of high-frequency circuitry know that they must calculate the impedance of printed conductors and allow for it in drafting CAD layouts. What is less well known is that in many standard digital circuits the impedance of printed signal conductors and the power supply increasingly play a role that is functionally decisive. The reason for this lies in the shorter switching times (signal rising edge) of modern modules. Although not always essential to practical use, the clock-pulse rates of signal circuits now approach the 100 MHz range and that of chips the GHz range. The switching times of associated chips have fallen to 1 or 2 nanoseconds or even less. The end result is high-speed circuitry requiring allowance to be made for physical conditions that could previously be disregarded.
Elementary design rules must be observed to ensure reliable future module function. This includes overall power planes offering wideband decoupling via capacitive properties. Strict routing strategies avoiding slot antennae (which can cause high-frequency noise interference emission from the planes) and ensuring optimal signal feedback must also be applied. The circuitry design and choice of components must guarantee signal transmission in an homogenous electrical field using impedance-controlled circuitry. A pre-condition in fulfilling this requirement is the application of these technical specifications to the PCB, which hence has a functionally decisive role as carrier of the module. There is no reason to believe the PCB might be unable to meet the specifications set.
If you have interest to find out more about this topic, we suggest you to read pages 27-31 of  "Ambitionierte Multilayersysteme für Highspeed-Baugruppen" of a leading specialist in this field, Mr. Arnold Wiemers.
Please feel free to ask our engineers if you need additional information on this topic.