At low frequencies, the transmission lines are a simple connection; however, at microwave frequencies they are no longer just simple connections and their operation becomes a complicated distributed circuit element. There are various types of transmission lines in microwave integrated circuits; some common examples are waveguides, coaxial, and microstrip lines. Figure 1. Although there are special cases of microwave integrated circuits that are composed of coaxial lines and waveguides, in most cases the microwave integrated circuits are formed using planar transmission lines. Therefore, the content of this book is restricted to microwave integrated circuits formed using planar transmission lines, examples of which are microstrip, slot line, and co-planar waveguide CPW , as shown in Figure 1.
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At low frequencies, the transmission lines are a simple connection; however, at microwave frequencies they are no longer just simple connections and their operation becomes a complicated distributed circuit element. There are various types of transmission lines in microwave integrated circuits; some common examples are waveguides, coaxial, and microstrip lines. Figure 1. Although there are special cases of microwave integrated circuits that are composed of coaxial lines and waveguides, in most cases the microwave integrated circuits are formed using planar transmission lines.
Therefore, the content of this book is restricted to microwave integrated circuits formed using planar transmission lines, examples of which are microstrip, slot line, and co-planar waveguide CPW , as shown in Figure 1. These planar transmission lines are frequently used in the large-scale production of microwave circuits and generally form the basic transmission lines for microwave circuits.
They are explained in Chapter 3. The implementation of planar transmission lines on substrates can be classified into two basic groups: monolithic and hybrid integrated circuits. In monolithic integration, the active and passive devices as well as the planar transmission lines are grown in situ on one planar substrate that is usually made from a semiconductor material called a wafer.
The monolithic microwave integrated circuit in Figure 1. The monolithic integration provides a compactsized circuit and eliminates a significant amount of assembly when building a component or a system. Especially because size is of critical importance in most recent RF systems, monolithic integration is frequently employed to provide a compact component.
An advantage of monolithic integration is that it is well suited for large-scale production, which results in lower costs. A disadvantage is that monolithic integration takes a long time to develop and fabricate, and small-scale production results in highly prohibitive costs. Hybrid integration is a fabrication method in which the transmission lines are implemented by conductor patterns on a selected substrate with either printing or etching, and active and passive devices are assembled on the patterned substrate by either soldering or wire bonding.
When implementing transmission lines by conductor patterns on a substrate, careful consideration must be given to the substrate material and the conductor material for the transmission lines because these materials can have significant effects on the characteristics of transmission lines.
Hybrid integration is thus classified into three types based on the method by which the lines are formed on the substrate: a printed circuit board PCB , a thick-film substrate, and a thin-film substrate.
Both sides of the dielectric material are attached with copper cladding that is then etched to obtain the desired conductor patterns. FR4 substrate a kind of epoxy fiberglass can be used from lower frequencies to approximately 4 GHz, while teflon or duroid can be used up to the millimeter wave frequencies, depending on their formation. Generally, all these materials lend themselves to soldering while wire bonding for an integrated circuit assembly is typically not widely used.
Furthermore, compared with other methods that will be explained later, a PCB can result in lower costs; its fabrication is easy and requires less time to produce. In addition, production on a small scale is possible without the use of expensive assembly machines; it is easy to fix and could also be used in large-scale production, and is thus widely used. The power amplifier is implemented in a separate block.
Thick-film substrates are produced by screen-printing techniques in which conductor patterns are formed by pushing conductive paste on a ceramic substrate through a patterned screen and then firing printed conductor patterns. The substrate is called thick film because the patterns formed by such techniques are generally much thicker than those formed using thin-film techniques.
As a benefit of using screen-printing techniques, multiple screen printings are possible. Dielectric or resistor patterns can also be formed by similar screen-printing techniques using dielectric or resistor pastes. Using an appropriate order of multiple screen printings, it is also possible to form capacitors and resistors on the ceramic substrate.
Since the ceramic substrate is more tolerant of heat, it is easy to assemble active devices in the form of chips. On the other hand, considering the lines and patterns formed by this process, the pattern accuracy of thick film is somewhat inferior compared to that of thin film. The costs and development time, on a case-by-case basis, are somewhere between those of the PCB and thin-film processes.
Recently, however, the integration based on thick-film technology has become rare because its cost and pattern accuracy are between the PCB and thin-film technology, while thick film is widely used to build multifunction components. A typical example is the package based on LTCC low-temperature co-fired ceramics technology.
Identical circuits can be arrayed for efficient production. This circuit is for the mobile communication VCO presented in Chapter The thin-film technique is very widely used in the fabrication of microwave circuits for military and microwave communication systems. In the case of the thin-film process, a similar ceramic substrate material used in thick film is employed, but compared to the thick-film substrate, a fine surface-finish substrate is used. Other substrates such as fused silica, quartz, and so on are possible for conductor-pattern generation based on thin-film technologies.
The pattern formation on the substrate is created with a photolithographic process that can produce fine tracks of conductor patterns similar to those in a semiconductor process. Since the thin-film substrate is also alumina as in the case of a thick-film substrate, the assembly of semiconductor chips using wire bonding is possible. Thin film compared with PCB and thick film is more expensive, and due to the requirement of fine tracks, a mask fabrication is necessary and the process generally takes longer.
Passive components such as resistors and air-bridge capacitors can be implemented using this process. In addition, integrated circuits produced by the thin-film process require special wire bonders and microwelding equipment for assembly. Compared to the monolithic integration process, the thin-film process tends to be cheaper in terms of cost, but compared to MMIC, the assembled circuit using the thin-film patterned substrate is difficult to characterize precisely because of unknown or poorly described parasitic circuit elements associated with the assembly methods such as wire bonding and die attach.
Before the emergence of MMICs monolithic microwave integrated circuits , thin-film technology was the conventional method for building microwave-integrated circuits MICs. The choice of integration method depends on the application and situation, taking into account several factors mentioned previously, such as the operating frequency of the integrated circuit, the types of semiconductor components chip or packaged , the forms of the passive components, large-scale fabrication costs, and method of assembly.
These factors should all be considered when selecting the optimum method of integration. For a description of microwave-patterned substrate fabrication, assembly with wire bonding and soldering, and packaging, see reference 1 at the end of this chapter.
The book provides general information about microwave-circuit fabrications. Table 1.
Microwave integrated circuits
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Microwave Integrated Circuits,
MICROWAVE INTEGRATED CIRCUITS BY K.C.GUPTA PDF
Microwave Integrated Circuits