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Lumped Elements for RF and Microwave Circuits phần 6

Đối với các chất nền gốm sứ và GaAs, những giới hạn này là 12 và 6 m, tương ứng. Nó cũng được biết rằng bước sóng của tín hiệu là tỉ lệ nghịch với căn bậc hai của hằng số điện môi của môi trường, trong đó tín hiệu truyền. Do đó, tăng hằng số điện môi của môi trường một trăm lần sẽ làm giảm kích thước thành phần | Interdigital Capacitors 239 Figure 7.7 Interdigitated capacitor s S111 and S211 responses. 7.2 Design Considerations In this section we discuss several design considerations such as compact size high-voltage operation multilayer structure and voltage tunable capacitor. 7.2.1 Compact Size The capacitor size can be reduced by reducing the dimensions of the structure or by using a high dielectric constant value substrate. The achievable Q-value and fabrication photoetching limit on the minimum line width and separation dictate the size of the capacitor. For ceramic and GaAs substrates these limits are about 12 and 6 mm respectively. It is well known that the wavelength of a signal is inversely proportional to the square root of the dielectric constant of the medium in which the signal propagates. Hence increasing the dielectric constant of the medium a hundred-fold will reduce the component dimensions 240 Lumped Elements for RF and Microwave Circuits Frequency GHz Figure 7.8 Interdigitated capacitor s zS11 and zS21 responses. Table 7.2 Physical Dimensions and Equivalent Model Values for Interdigital Capacitors Physical Dimensions INDIG80 INDIG180 INDIG300 INDIG400 UNITS Finger length 80 180 300 400 2 m Finger width W 12 12 12 12 2 m Finger spacing side s 8 8 8 8 2m Finger spacing end s 12 12 12 12 2m Finger thickness t 5 5 5 5 2m Number of fingers N 20 20 20 20 2m Substrate thickness h 125 125 125 125 2m Capacitance C 0.126 0.252 0.405 0.527 pF Inductance L 0.001 0.025 0.064 0.101 nH Resistance Rdc 1.89 0.850 0.500 0.441 V Shunt capacitance Cs 0.028 0.052 0.080 0.104 pF Interdigital Capacitors 241 0.2 0.5 1 2 a bj Figure 7.9 The measured performance of an interdigital capacitor compared with the present model and Touchstone model a reflection coefficient and b transmission coefficient. by a factor of 10. This simple concept is being exploited extensively as distributed circuit technology is being adopted at RF and lower microwave frequencies. 7.2.2 Multilayer .