Test & Screening
the left side of the board, while high- and low-frequency digital components are placed on the right side; the system clock is in the middle and connectors are located at the board’s edge. In this way, analog and digital components are totally separated and traces can easily be routed separately. Signals transmitted underneath analog and digital components must also be clean and well segregated. An internal plane layer underneath the trace layers may be bifurcated so that power and ground layers are split for proper signal transmission (Figure 3). This technique reduces EMI effects and suppresses noise. Ideally, the analog section must be totally isolated in terms of placement, routing and plane separation. Analog traces should run only underneath their analog reference power or ground plane. Conversely, digital traces should run under the digital section with respective power and ground planes. Impedance is thus kept constant, and there is a good return signal path.
Using Radius Bends and Blind/Buried Vias
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[ 46 ] COTS Journal December 2005
When routing RF circuits, it is important to avoid the extreme bends at 45° angles that many digital designers rely on to conserve routing space. These angles hinder proper propagation to high-frequency performance, causing impedance mismatches. Using arcs instead of sharp angles eliminates this problem. A high-frequency effect created by this conventional PCB layout practice is return loss reflection, which cancels an incoming signal due to out-of-phase reflections. This problem can be resolved by using mitered corners or a radius bend to change the trace’s direction. Blind vias, as the name implies, go from an external layer— such as the component or solder side of the board—to one of the internal planes, whereas buried vias go from one internal layer to another internal layer. Blind and buried vias rather than through-hole vias are more effective for minimizing EMI/RFI and maintaining high signal integrity, especially in high-speed boards. For instance, in the through-hole approach, a signal may go from the top layer to layer three, but if the hole goes all the way to a bottom layer, such as layer 12, then a stub is created. This stub is the portion of the through-hole via not used for signal transmission. In effect, the stub serves as an antenna that creates added noise. Blind vias are used to transmit signals between layers and provide exact termination at the exact layer. For example, a blind via can go from layer one to layer three and terminate at layer three, even if that PCB has 12 or more layers. Nexlogic Technologies San Jose, CA. (408) 436-8150. [www.nexlogic.com].