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Subject: Re: RF / Microwave Printed Circuit

Date: 07/17/00 at 4:01 PM
Posted by: John W. Childers
E-mail: jwchilders@goldengategraphics.com
Message Posted:

In Reply to: RF / Microwave Printed Circuit posted by Parito Lee on 07/17/00 at 5:06 AM:

This is a great topic for the Printed Circuit Design forum.

"What is RF / Microwave Printed Circuit?"

One thing to keep in mind is that RF and Microwave are two different categories, not a single category, of printed circuits.

For RF, click on the Glossary tab above (the Glossary will open in a new window).

The Macmillan Dictionary for Students defines microwave as "high-frequency electromagnetic wave having a wavelength in the range of from about one millimeter to thirty centimeters, used in radar and for such purposes as long-distance transmission of television signals through the air."

Go to the Glossary window you just opened, scroll down to the OneLook Dictionaries search window, enter the words "microwave frequency" and click the "LOOK IT UP" button.

A printed circuit operating or controlling frequencies in the range of over a gigahertz is a microwave printed circuit. RF would be radio frequencies below 1 gigahertz.

"How is RF / Microwave Printed Circuit different from Conventional Printed Circuit?"

It is more difficult to get printed circuits to work well at higher frequencies. A circuit tested and proven in wire-wrap may not work when implemented as a printed circuit. This becomes more true the higher one goes in frequency ranges.

"What are the key applications of RF / Microwave Printed Circuit?"

Go back to the definition of Microwave you found and go from there for an answer. The applications are many. Radar comes to mind for microwave, including radar jammers.

"Which are there any company manufacturing, supplying or have the technical knowledge on RF / Microwave Printed Circuits?"

There are such companies. When I have worked on RF and on microwave layouts, I listened carefully to the electrical engineer and did what he wanted. There are engineers who mark their specialty in part by frequency ranges they work in.

There are also many related printed circuit design topics to RF and microwave, such as cross talk, parasitics, reactance and controlled impedance, which are design problems that can affect non-RF circuits as well.

Terms to be added to the Glossary:

"cross talk"--Interference between adjacent PCB tracks.
Types of cross talk:
Z-axis parallelism--cross talk occurs when tracks follow the same path but on adjacent layers, and causes the most problems in printed boards. Separate overlapping traces in order to diminish potential adjacent layer cross talk. When crossing over a trace on an adjacent layer, cross it at a right angle.
X-Y Parallelism--cross talk occurs between tracks in the same layer running adjacent to each other over a long distance.
X-Y Noise Coupling and Z-axis noise coupling--According to David Price of DFM (http://www.dfmpcb.com) "Noise Coupling is an increasingly stubborn problem in PCBs. The amount of noise coupling is NOT a function of clock speed. It is a function of edge rates--the rise time of the driving signal. It is fast edge rates that create the cross-talk. Today, many electronic parts are being manufactured with sub-nanosecond rise times. This can result in a new assembly of an old board, with new parts, suddenly and inexplicably not working."

"parasitics"--The RLC (resistance, inductance, capacitance) qualities of a conductor.
parasitics in a printed circuit design can be simulated, as by The Quantic Engine, a suite of signal integrity (SI) and electromagnetic compatibility (EMC) tools by Quantic Laboratories of Winnepeg, Canada ( http://www.quantic-emc.com/ ). Thus, parasitics can be reduced or eliminated by automated techniques.

"controlled impedance" is the design of conductors done in such a way as to limit the impedance to a specific range. This is implemented by calculating the requirements of r trace thicknesses and widths based on parametric equations, the results being tested in the manufactured product with a TDR (see Glossary).

"impedance" is defined in the Macmillan Dictionary for Students as "opposition that an electrical circuit offers to the flow of an alternating current."

"reactance" is defined in the Macmillan Dictionary for Students as "opposition to an alternating current in a circuit, caused by capacitance and inductance."


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