Calculating characteristic impedance: when taking measurements or doing simulations, it's good to anticipate your results ahead of time.QUESTION FROM SCOTT IN TUCSON, AZ: Everyone says that if you bring a grounded trace next to a signal line, the impedance impedance, in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. of the signal line will drop. This is because of the proximity of the ground next to the signal line. If this is true, is there a rule of thumb to use to estimate how the impedance of a line is affected by the proximity of an adjacent signal line? Answer: I think this is another myth. And while there is always a basis of truth in each myth, it is dangerous to apply sweeping generalizations. After all, the most common answer to every signal integrity question is, "It depends," and the only way to know if it depends in your specific case is to plug in the numbers. In this example, we will put in numbers in numbered parts; as, a book published in numbers. See also: Number using the built-in 2D boundary element field solver in Agilent's ADS. We can use this tool, as well as other 2D field solver tools, to calculate the characteristic impedance This article is about impedance in electronics. For characteristic acoustic impedance, see acoustic impedance. The characteristic impedance or surge impedance of a uniform transmission line, usually written of one transmission line in almost any geometrical configuration. Let's start with a simple microstrip with a line width of 10 mils, dielectric constant dielectric constant n. See permittivity. of 4.5 and thin metallization Met`al`li`za´tion n. 1. The act or process of metallizing. . Using ADS, we can sweep the dielectric dielectric (dī'ĭlĕk`trĭk), material that does not conduct electricity readily, i.e., an insulator (see insulation). A good dielectric should also have other properties: It must resist breakdown under high voltages; it should not thickness, plot the extracted characteristic impedance and find the thickness for a 50-[OMEGA 1. (programming) Omega - A prototype-based object-oriented language from Austria. ["Type-Safe Object-Oriented Programming with Prototypes - The Concept of Omega", G. Blaschek, Structured Programming 12:217-225, 1991]. 2. ] microstrip (FIGURE 1). [FIGURE 1 OMITTED] With this optimized dielectric thickness of about 5.3 mils, we gradually bring a second trace, also 10 mils wide and grounded on the ends, in proximity and calculate the characteristic impedance of the first trace as the separation changes. As a general rule, you should never perform a measurement or simulation without first anticipating what you expect to see. If you are right, you have a good intuitive feel for the problem. If you are wrong, either your intuition is off or the problem is not configured con·fig·ure tr.v. con·fig·ured, con·fig·ur·ing, con·fig·ures To design, arrange, set up, or shape with a view to specific applications or uses: as you think. Either way, you will learn something new and advance along the learning curve. And when you do this a lot, you find this continual testing of your intuition is fun. So, what would you expect to see? When the grounded trace is far away, there should be no impact on the impedance of the first trace. As they are brought in proximity, the grounded trace should increase the capacitance capacitance, in electricity, capability of a body, system, circuit, or device for storing electric charge. Capacitance is expressed as the ratio of stored charge in coulombs to the impressed potential difference in volts. to ground of the signal trace, thereby lowering its impedance. But by how much? When the traces are far apart, there are fringe fields from the first trace to the ground plane. When the adjacent ground trace is brought close, it will intercept intercept in mathematical terms the points at which a curve cuts the two axes of a graph. these fringe fields, but they will also go to ground (FIGURE 2). [FIGURE 2 OMITTED] The proximity of the adjacent ground trace should not have a strong impact on the signal trace. A quick way to find out is with a 2D field solver. The calculated impedance of the trace, as we move the ground trace closer, is shown in FIGURE 3. [FIGURE 3 OMITTED] The proximity of the grounded second trace really does Warren Trotter, better known as Really Doe, is an American rapper from Chicago, Illinois. He is affiliated with Kanye West and his G.O.O.D. Music family and label. Discography Songs
In most applications, an adjacent ground trace is at least 1 line width apart. This analysis suggests a simple rule of thumb: There is no impact on the impedance of a 50-[OMEGA] microstrip signal line as a grounded trace is brought adjacent to it. This rule of thumb also applies to stripline. This and other topics are covered in the public classes Eric teaches. Check his Web site for the schedule: BeTheSignal.com. Send questions for the Signal Doctor to signaldoctor@ upmediagroup.com. DR. ERIC BOGATIN is president of Bogatin Enterprises. |
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