《RF and Microwave Basics Impact PCB Design》.pdf

RF and Microwave Basics Impact PCB Design It is a given that printed circuit board designs are utilizing higher frequencies to meet performance demands. As data rates increase, the resulting bandwidth requirements are driving the upper limit of signal frequency to 1 GHz and beyond. And while this is a far shot from millimeter wave technology (30 GHz), it is indeed RF and low-end microwave. RF requires a design engineering approach that addresses the associated stronger electromagnetic field effects which naturally occur at these higher frequencies. These fields can induce signals in adjacent signal lines, or PCB traces, creating undesirable crosstalk (interference and overall noise), undermining system performance. Return loss (signal reflected back into the incident oncoming signal) is primarily caused by impedance mismatch and has much the same impact of added noise and interference to the primary signal. There are two effects of high return loss, both of which are bad news. First, signal reflection back towards the source adds noise to the system, making it more difficult for the receiver to distinguish noise from the signal. Second, any reflected signal is fundamentally a degradation of the signal itself since the “meaning” or shape of the inbound signal can be altered. While a digital system can be far more forgiving since it is only attempting to recognize a one or a zero (on or off), the use of harmonics for faster pulse rise times involves weaker signals at higher frequency. And, while we can implement forward error correction technology to fix some of these effects, the result is system degradation as capacity gets consumed in redundant transmissions. A much better answer is to understand and engineer the RF effects to help, not hurt, your signal management assignment. Overall recommended target values for return loss are minus 25 dB at the highest frequency of interest (usually the worse-case dat