Every system design begins with a block diagram. The interconnections between blocks illustrate the functional flow of signals within the product.
If your block diagram shows the signaling rate, total bandwidth, protocol, and signaling amplitude that each connection requires, you can use it to help partition the connections in your system into groups of compatible signals. For example, consider a 3.3-V TTL bus and a 250-mV LVDS serial link. If you route them too close together, the big bus hammers the smaller signal with crosstalk. These signal types are naturally incompatible because of their wildly differing amplitudes.
Differing tolerances for noise also cause incompatibility. In the case of analog video, even though the peak-to-peak range of the signal is fairly large (about 1 V), analog-video signals require extraordinary protection from crosstalk because the human eye can discern incredibly small amounts of signal interference.
A good partitioning groups together signals of similar amplitudes, bandwidths, and protocols. Within each partition, you can then determine the trace-to-trace separation required to obtain acceptable crosstalk performance. Between segments, you should identify the amount of additional crosstalk attenuation required to keep your loud, high-powered partitions from interfering with your itty-bitty quiet ones.
To solidify your noise-control ideas, create a noise-partitioning diagram. It should correspond roughly to the way you plan to lay out your system. The diagram should identify all your partitions, showing the number and type of signals flowing through each interconnection. The color coding in Figure 1 shows what type of signals go where.
You can isolate the partitions in many ways. If you need only mild amounts of crosstalk attenuation—for example, 60 dB—a modest amount of horizontal spacing works perfectly well, assuming the circuit overlays a solid reference plane. For greater amounts of isolation, you might use vertical spacing (for example, opposite sides of a solid reference plane), cuts in the power and ground planes, separate pc boards, or, ultimately, separate metal boxes. An EMC engineer can help you assess the degree of isolation that each method obtains.
In many noise problems, common-mode currents circulating on the ground connections create as much or more noise than anything else. A complete noise-partitioning diagram, therefore, must show the signal paths and the common-mode ground connections between partitions. In addition to the intentional ground connections, your diagram must also include all unintentional ground pathways. Unintentional pathways arise due to parasitic capacitance. Even when you intend to separate two ground regions, the irreducible capacitance between them still creates a strong, low-impedance connection. For example, a digital ground plane in close proximity to a metal chassis easily shares enough parasitic capacitance with the chassis to create a less-than-1Ω connection at 1 GHz.
Don't forget to include your power planes in the partition exercise. Power planes carry noise that easily couples into any signals routed next to the power plane. Power planes also share parasitic capacitance with other objects, creating unintentional pathways that affect the overall picture.
A good noise-partition diagram combines all of your noise-control information, helping you understand, and forestall, many classes of noise, interference, and EMC difficulties.
W Michael King (www.systemsemc.com) is a systems design advisor. He is the author of EMCT: Electromagnetic Compatibility Tutorial (ISBN 0-7381-3340-X), available through Elliott Laboratories and the IEEE Standards Information Network.