Temperature coefficient of resonant frequency

Table 5.1 Microwave properties of the most important microwave dielectrics (SC= bulk single crystals , bc = bulk ceramics , tf = thin films , r/ = temperature coefficient of resonant frequency . The materials marked with are tuneable dielectrics.

With the progress in microwave telecommunication technology, dielectric materials have come to play an important role in the miniaturization and compactness of microwave passive components. The dielectric materials available for micro-wave devices are required to have predictable properties with respect to a high dielectric constant (K), high quality factor (Qf), and small temperature coefficient of resonant frequency (TCP). Numerous microwave dielectric materials have been prepared and investigated for their microwave dielectric properties and for satisfying these requirements. In particular, complex perovskite compounds A(B,B )03... 

FIGURE 22.7 Temperature coefficient of resonant frequency (TCP) of PCCN, PCMT, PCFTN specimens with A-site and B-site bond valence. 

The dielectric constant (6r) and the quality values Q at microwave frequency were measured using the Hakki-Coleman s dielectric resonator method , and modified and improved by Courtney. A vector network analyzer (E8363, Agilent Technologies, Loveland, CO, USA) was employed in the measurement. The temperature coefficient of resonant frequency (t/) was measured in the temperature range from -25 to h-85 °C. The X/ value was defined as follows ... 

Fig. 6 Dielectric constants, Qxf /a w and temperature coefficients of resonant frequency of the Nb-ZMT ceramics as a function of the amount of added Bi203.

The required properties for dielectric resonator materials are (a) high dielectric constant (b) low dielectric loss tangent tan and (c) low temperature coefficient of resonant frequency Xj. Many kinds of dielectric resonator materials have been developed since the 1970s 128-311. Table 5.1.5 shows the dielectric properties of some materials that are commercially available now. In the table, the quality factor Q is reciprocal of dielectric loss tangent Q = If tan As tan S is proportional to frequency for ionic paraelectric materials, the product of Q and frequency is the value inherent to each material. Some materials have high Q value equal to copper cavity and some have the temperature coefficient as stable as Inver cavity. The material with lower 8j. generally has higher Q value. 

Superior temperature stability of resonant frequency is obtained by selecting the composition of each material. The temperature coefficient of resonant frequency zj is dehned by the following equation. 

The rapid progress in mobile and satellite communication system has created a high demand for the development of microwave dielectric materials with a high quality factor, an appropriate dielectric constant, and a near-zero temperature coefficient of resonant frequency. Several corundum-structured A4B2O9 ceramics have been found due to their excellent microwave dielectric properties, and... 

Figure 10. Variation of temperature coefficient of resonant frequency of Ca[(Lii/3Nb2/3)i.xMx]03.5 [M=Ti, Sn, Zrjceramics sintered at 1150 C as a function of x.

Chapterb- The perovskite family of materials is of considerable technological importance for its excellent temperature stable microwave properties for dielectric resonator based filters, oscillators and antenna applications. In this chapter author review the preparation, characterization and the microwave dielectric properties of Ca[(Lii/3A2/3)i-xMx]03.5 [A=Nb, Ta and M=Ti, Zr, Sn] dielectric ceramics. This family of perovskite materials shows relative permittivity in the range 20 to 56 with a quality factor up to 45000 GHz and temperature coefficient of resonant frequency (xf) in the range from -21 to +83 ppm/°C. The Xf can be tailored by adjusting the titanium content. The sintering temperature can be lowered below 950 °C to suit LTCC by the addition of low melting glasses. 

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