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PCBD-Feb2014

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58 The PCB Design Magazine • February 2014 FAQ: MICRoWAVE PCB MATERIALS continues plications. In the microwave range of frequen- cies (about 300 MHz to 30 GHz), the PCB circuit patterns are often the microwave circuit com- ponent. As an example, a bandpass filter might not be a component that is soldered onto the PCB, but is actually the conductor pattern of the PCB. An edge-coupled series of conductors and space can act as a bandpass filter at microwave frequencies. A simple microstrip edge-coupled filter is shown in Figure 1 and its function is based on wavelength which is related to the di- electric constant of the material. Wavelength is a property of an electromag- netic wave that propagates on the circuit. As the name implies, the wavelength is the physi- cal length of the wave and this length is depen- dent on the dielectric constant of the material and the frequency. The size of many microwave components is based on a fraction of the wave- length; often ½ or ¼ wavelength is used to de- fine circuit features. In Figure 1, there are line segments with small spaces between them. It may be hard to believe but electrical energy will pass through this circuit very effectively, at the right frequen- cy. The length of each pair of conductors (with a space between them) is ¼ the wavelength at the intended frequency. With conductor lengths at ¼ of the wavelength, there will be a lot of electric energy radiated on one of these elements and that energy couples to its neigh- boring element (line segment). At the right frequency, the energy of the propagating wave will jump from one element to another, going down this circuit. However, when energy is in- troduced into this circuit pattern at a different frequency, the wavelength will be different and the energy will not couple from one element to another. Therefore, no electrical energy will propagate. The physical size of this bandpass filter is re- lated to wavelength at the frequency of interest. If the circuit material dielectric constant is much higher, the size of the circuit would shrink in order to maintain the same wavelength proper- ties. Having a high dielectric constant material can reduce the size of a microwave circuit. This is only one example of many, but in general, microwave circuitry is very dependent on the dielectric constant of the material because the technology uses wavelength properties to pro- duce many different types of circuit functions. Wavelength is very much related to dielectric constant, and a higher dielectric constant yields shorter wavelength. Q: Why is insertion loss getting so much at- tention on new applications? A: Actually, the microwave industry has been concerned with insertion loss for decades. However, now more digital applications are fo- cusing on this concept. Insertion loss is a com- plicated subject, so I can only give a simple overview here. But in general, insertion loss is the total electrical loss of a high-frequency/ high-speed circuit. Insertion loss is made up of a combination of many losses and one of them is dielectric loss. Usually dielectric loss is related to the dis- sipation factor (tan-delta, loss tangent) of the circuit material. As the name implies, dielectric loss is the property of a circuit where losses oc- cur due to the substrate. Conductor losses are Figure 1. Microstrip edge-coupled bandpass filter as part of the PCB pattern, and a wavelength illustration. lightning speed laminates

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