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In the earliest generations of printed wiring boards the etched circuits were little more than neatly laid out replacements for the tangles of wire that connected all the pins of the components (Do you remember vacuum tubes?) in "hard-wired" electronics. Some of us can remember owning the early 9-volt battery-operated "transistor radios" with their little printed wiring boards and generally scratchy reception of AM radio stations. In the latest "radios," many of which we know as "cellular phones" or "base stations," printed wiring is no longer simply a way to provide voltage bias or carry current from one place to another. In many cases the circuitry actually includes or is a part of active functional elements including amplifiers, power dividers, couplers, filters and antennas. This photo (left) is of a small part of a typical microwave PWB with printed and etched active components. At RF frequencies these active (and passive) components interact with the substrate material on which they are fabricated in ways that are determined by the frequency at which they operate and the requirements of their design. For instance:

-- Circuit lines can become unintentional "antennas" for ambient signals and noise, especially if their length is close to the wavelength of a harmonic of the primary frequency;

-- Closely spaced transmission lines may need to be "shielded" from one another to avoid coupling, or "cross-talk";

-- High-speed digital data transmission requires that wavefronts (consisting of an infinite number of harmonics of the primary frequency) remain coherent -- in other words the whole square wave signal gets where it's going at one time;

-- Boards have capacitance and inductance that can distort signals both in-plane (in transmission lines) and as they go from one layer to another through plated vias;

-- Circuit element size is inversely proportional to the square root of the relative dielectric constant of the PWB material;

-- Etched antennas may be subject to Passive Intermodulation Distortion;

-- PWB's have to deliver controlled Impedance (and thereby avoid problems with VSWR -- about which we will disclose additional information later);

-- And much, much more

If there are any of the above that you don't deal with every day (but your customers do), then this article is for you. It is intended to introduce anyone who makes, buys or sells PWB's, but isn't a microwave design engineer, to the basics of RF/microwave signals and how those basics impact the laminate materials choices that are made in frequency dependent circuitry. With this information at hand you will not only better understand why your customers (or your engineers) are selecting the materials they do, but you will hopefully be better able to partner with them during their design process, when you can still impact the produceability of the product you will later have to manufacture.

Downloads:
•Application Note: Everything You Wanted To Know About High Frequency Laminates