Microwave circuit PCB

Microwave circuit pcb is a special circuit board dedicated to high-frequency signal transmission, the frequency range is usually located between 300MHz and 300GHz. It is widely used in the fields of communication, radar and radio, etc. It is characterized by good high-frequency performance, compact size and high reliability.

Microwave circuit is a circuit that works in the microwave band and millimeter wave band, and is integrated on a substrate by microwave passive components, active components, transmission lines and interconnections, and has a certain function.

Microwave circuit are divided into hybrid microwave circuit and monolithic microwave circuit. Hybrid microwave circuit is a functional block that uses thin film or thick film technology to fabricate passive microwave circuit on a substrate suitable for transmitting microwave signals. The circuit is designed and manufactured according to the needs of the system. Commonly used hybrid microwave circuit include various broadband microwave circuit such as microstrip mixers, microwave low-noise amplifiers, power amplifiers, frequency multipliers, and phased array units. Monolithic microwave circuit are functional blocks that use planar technology to directly manufacture components, transmission lines, and interconnect lines on a semiconductor substrate. Gallium arsenide is the most commonly used substrate material. Microwave circuit started in the 1950s. An important reason why the microwave circuit technology consists of coaxial lines, waveguide components and their systems turned to planar circuit is the development of microwave solid-state devices. In the 1960s and 1970s, alumina substrates and thick film technology were used; monolithic integrated circuit began to be available in the 1980s.

Hybrid microwave circuit use thick film technology or thin film technology to fabricate various microwave functional circuit on a medium suitable for transmitting microwave signals, and then install discrete active components in corresponding positions to form a microwave circuit. The medium used in microwave circuit includes high alumina porcelain, sapphire, quartz, high-value ceramics and organic medium. There are two types of circuit: distributed parameter microstrip circuit and lumped parameter circuit. Active devices use packaged microwave devices, or directly use chips. The main feature of microwave circuit is that they are designed and manufactured according to the requirements of the complete microwave machine and the division of microwave bands. Most of the integrated circuit used are dedicated. Commonly used are microstrip mixers, microwave low noise amplifiers, microwave integrated power amplifiers, microwave integrated oscillators, integrated frequency multipliers, microstrip switches, integrated phased array units and various broadband circuit.

Monolithic microwave circuit is an integrated circuit in which a microwave functional circuit is fabricated on a chip made of gallium arsenide material or other semiconductor materials by a semiconductor process. Microwave circuit made of silicon materials work in the 300-3000 GHz frequency band, which can be regarded as an extension of silicon linear integrated circuit and is not included in monolithic microwave circuit.

The manufacturing process of GaAs monolithic microwave circuit is to use epitaxial growth or ion implantation of silicon to form an active layer on a semi-insulating GaAs single wafer; implant oxygen or protons to generate an isolation layer (or other ions suitable for generating an isolation layer) ;Inject beryllium or zinc to form a PN junction; make metal-semiconductor barriers by electron beam evaporation; make active devices (such as diodes, field-effect transistors) and no Source components (inductors, capacitors, resistors and microstrip component couplers, filters, loads, etc.) and circuit patterns. Circuit design is also divided into two forms: lumped parameters and distributed parameters. Distributed parameters are mainly used in power circuit and millimeter wave circuit. Millimeter wave circuit refer to integrated circuit operating in the range of 30 to 300 gigahertz.

Gallium arsenide is more suitable than silicon for making monolithic microwave circuit (including ultra-high-speed circuit) mainly because: 1. The resistivity of semi-insulating gallium arsenide substrate is as high as 107～109 ohm·cm, and the microwave transmission loss is small; 2. Arsenide The electron mobility of gallium is about 5 times higher than that of silicon, the operating frequency is high, and the speed is fast; 3. The key active device gallium arsenide metal-semiconductor field-effect transistor is a multifunctional device with good radiation resistance, so gallium arsenide single Chip microwave circuit have broad application prospects in solid-state phased array radars, electronic countermeasures equipment, tactical missiles, television satellite reception, microwave communications, ultra-high-speed computers, and large-capacity information processing.

The monolithic microwave circuit that have been successfully developed and gradually applied include: monolithic microwave integrated low-noise amplifier, monolithic TV satellite receiver front end, monolithic microwave power amplifier, monolithic microwave voltage-controlled oscillator, etc. The design of this circuit mainly revolves around the generation, amplification, control, and information processing functions of microwave signals. Most of the circuit are designed according to the requirements of different complete machines and the characteristics of microwave frequency bands, and they are very specific.

Microwave circuit PCB

Microwave Circuit Board Materials and Design

Material selection and design of microwave circuit boards is a key element to ensure stable device performance during high frequency operation. Selection of appropriate materials and structures can significantly improve the efficiency and reliability of signal transmission.

1. Material Characterization

Microwave circuit boards typically use materials with specific characteristics, including dielectric constant (Dk) and loss angle tangent (Df). The dielectric constant indicates a material's ability to store electrical energy, while the loss angle tangent characterizes the energy loss that is converted to heat during signal transmission. In general, a lower loss angle tangent helps to minimize signal loss and increase the efficiency of the material in high frequency applications .

2. RF Material Selection

When selecting microwave circuit board materials, factors such as environmental humidity, thermal stability and chemical resistance should be considered to ensure reliable performance under different operating conditions. Commonly used microwave circuit board materials include FR-4, PTFE (polytetrafluoroethylene) and ceramic materials, etc. Among them, PTFE is widely used in microwave applications requiring high performance due to its excellent high frequency characteristics .

3. Design Structure

Common structures used in the design of microwave circuits include microstrip lines and ribbon lines. A microstrip line consists of a conductive tape located on a dielectric substrate with a ground plane underneath and is suitable for high frequency signal transmission. Ribbons consist of a conductive tape sandwiched between two ground planes to provide better shielding and are suitable for applications requiring higher signal integrity.

4. Impedance Matching

Impedance matching is an important factor in microwave PCB design to ensure signal integrity. Good impedance matching can reduce signal reflection and optimize power transfer efficiency. The design should consider the use of appropriate transmission line layout, materials, and often need to be analyzed and adjusted with the help of electromagnetic simulation technology to improve signal quality.

5. Thermal design

When the microwave circuit board is in operation, some high-power components will generate significant heat, so reasonable heat dissipation design is crucial. This includes optimizing the layout of high-power components to enhance thermal contact, using thermal vias to improve temperature uniformity, and selecting materials with good thermal conductivity to promote heat dissipation.

The creation of microwave circuit

"Microwave circuit" has always been synonymous with "waveguide circuit". As early as the early 1930s, people realized that waveguides are a very useful transmission structure for microwave frequencies. Researchers have long discovered that a small section of the waveguide after proper modification can be used as a radiator or an electrical antigen piece. Such as resonant cavity and horn antenna. In the development of modern waveguide circuit, from the very beginning, efforts were made to effectively transmit microwave power from the microwave source to the waveguide transmission line, and to be effectively recovered at the receiving end. This puts forward changes to the corresponding original transmitter and receiver originals. High demands. Therefore, it led to the appearance of components such as traveling wave detectors, wavelength meters, and terminal loads.

The development and application of microwave technology has formed the basis of microwave circuit. From the initial discovery of the discontinuous multiple reflection principle and the corresponding cavity resonance principle, to the use of these principles to match the microwave power source with the waveguide, and then to match the waveguide with the receiver (such as a crystal detector), And use these devices to make a certain frequency signal through the circuit.

One of the basic characteristics of microwave circuit is to adjust or tune their characteristics based on experience through the screws and diaphragms inside the waveguide (and even the compressed size). At first, this was only a trial and error method, and later developed into the so-called "waveguide engineering". For a long time, it was also one of the most commonly used methods in microwave engineering.

Current status of microwave circuit

The microwave circuit started from the three-dimensional microwave circuit used in the 1940s. It is composed of a waveguide transmission line, a waveguide element, a resonant cavity and a microwave tube. In the 1960s, a new generation of microwave integrated circuit with semiconductor devices, thin film deposition technology and photolithography technology emerged. Due to its small size, light weight, and convenient use, it is fully utilized in weapons, aerospace, and satellites.

Two basic transmissions are often used in microwave circuit, namely waveguide and TEM mode coaxial line. The waveguide is characterized by high power and low loss. The latter feature led to the emergence of high-Q resonant cavities. The coaxial line has inherent broadband characteristics because there is no dispersion effect. In addition, the concept of impedance can also be easily explained in the coaxial line, which simplifies the design process of the component. These two transmission structures have developed into important microwave circuit components, and the use of the two together can achieve unexpected results.

The strip line transmission structure is used in the microwave circuit. The form is the same as that used today. It is composed of two dielectric plates with metal on the outside and a thin strip conductor. With the advent of copper-clad laminates, stripline has developed into a precision process whose performance can be calculated in advance. The most important feature of the stripline transmission structure is that its characteristic impedance is controlled by the width of the center strip conductor. The two-bit characteristic of the stripline circuit structure makes it possible to realize the interconnection of many components without destroying the shielding layer of the outer conductor, which also brings great flexibility to the input and output positions. Due to the inherent coupling characteristics when two strip conductors are close together, the strip line is very convenient to be used in parallel line couplers.

Since 1974, Plessey of the United States has used GaAs FETs as active devices and GaAs semi-insulating substrates as carriers to successfully develop the world’s first MMIC amplifier. It has been used in military applications (including smart weapons, radar, communications and electronic warfare, etc.). Under the impetus of MMIC, the development of MMIC is very rapid. It is the advent of GaAs technology and the characteristics of GaAs materials that have contributed to the transition from microwave circuit to monolithic microwave circuit (MMIC). Compared with the second-generation microwave hybrid circuit HMIC, MMIC has the advantages of smaller size, longer life, high reliability, low noise, low power consumption, and higher operating limit frequency. Therefore, it has received extensive attention.

The emergence of monolithic microwave circuit has made the realization of various microwave circuit possible. Therefore, various MMIC devices have achieved unprecedented development, such as MMIC power amplifiers, low noise amplifiers (LNA), mixers, upconverters, voltage controlled oscillators (VCO), filters, etc., up to the MMIC front end and the entire transceiver system . Monolithic microwave integrated circuit have broad application prospects in solid-state phased array radars, electronic countermeasures equipment, tactical missiles, television satellite reception, microwave communications, ultra-high-speed computers, and large-capacity information processing.

With the further improvement of MMIC technology and the progress of multi-layer integrated circuit technology, the three-dimensional multi-layer microwave structure that uses the multi-layer substrate to realize almost all passive devices and chip interconnection networks has received more and more attention. And the MCM (Multi-Chip Module) technology built on the multilayer interconnection substrate will make the size of the microwave millimeter wave system smaller.

Microwave circuit PCB

The development trend of microwave circuit

1.Interconnection and manufacturing technology of microwave circuit

Microwave technology and microwave circuit interconnection and manufacturing technology using frequencies above 1 GHz have developed rapidly and are widely used. In modern information systems and military electronic equipment such as radar, navigation and communication equipment, microwave circuit are the "aorta" of high-speed information. Therefore, microwave circuit and their interconnection and manufacturing technology is a major key technology in the development and production of information systems and military electronic equipment. Microwave circuit interconnection and manufacturing technology includes: microwave circuit substrate materials and manufacturing technology, microwave circuit design and manufacturing technology, packaging and assembly technology of microwave devices or components, interconnection and debugging technology of microwave components or systems. It involves many disciplines such as microelectronics, materials science, computer application technology, electronic mechanical engineering, etc.; it is a multi-disciplinary and comprehensive science and technology. It has the characteristics of high technological content, high technical difficulty, fast development speed, wide application area and great effect in information systems and military electronic equipment.

With the rapid progress of science and technology such as microelectronics technology, component technology, material science, computer-aided design and manufacturing, etc., new technologies and technologies for interconnection and manufacturing of microwave circuit are constantly emerging. For example, multi-layer microwave integrated circuit and three-dimensional microwave integrated circuit (3DMMIC), low-loss transmission lines and shielding membrane microstrip (SMM) circuit, multi-chip microwave modules, microwave circuit, micro-electromechanical systems (MEMS) interconnection and manufacturing technology, new Resin microwave PCB technology, new microwave circuit protective coating technology, as well as three-dimensional circuit simulation technology applied to microwave circuit design, microwave circuit CAD and optimization technology based on intelligent methods, etc.

2.The photonic band gap structure of microwave circuit

In 1987, Yablonovitch proposed the sub-band gap (PBG) structure, which was originally applied in the optical field, and has been introduced into the microwave band in recent years, which has attracted widespread attention. When electromagnetic waves propagate in materials with periodic structures, they will be modulated to produce a photonic band gap. When the operating frequency of electromagnetic waves falls within the band gap, there is no transmission state. The sub-band gap structure is applied to the microwave band, which can prevent electromagnetic waves in a specific frequency band from propagating in it at all. At the same time, the photonic band gap structure will also change the propagation constant in the passband, which is a slow-wave structure. Due to the above characteristics of the photonic band gap structure, it is widely used in band rejection, suppression of high-order harmonics, improvement of efficiency, increase of bandwidth, and reduction of size. The photonic band gap structure can adopt metal, dielectric, ferromagnetic or ferroelectric substance implanted in the substrate material, or directly form a periodic arrangement of various materials. There are many kinds of microwave photonic bandgap structures proposed at home and abroad, and the current development from three-dimensional structures to one-dimensional and two-dimensional structures. Due to the ease of implementation and integration, the research of photonic bandgap structures has been developed into the fields of electronics and communications. At present, the unit shape of the photonic bandgap structure, periodic conditions, the combination of various periodic structure deformation bodies and the development of materials are all research hotspots worthy of attention.

Sub-crystals are artificial crystals formed by periodic arrangement of one medium in another medium. The basic feature of photonic crystals is that they have a photonic band gap. Electromagnetic waves whose frequencies fall in the band gap are prohibited from propagating. The unique characteristics of photonic crystals were first used in the field of optics, and then quickly expanded to other fields, and now they are also researched and applied in the microwave frequency band. At present, a variety of microwave photonic band gap structures have been proposed at home and abroad. The original microwave photonic band gap structure is composed of three-dimensional medium periodic arrangement. Because the processing and analysis of the three-dimensional structure are very complicated, the research and production of microwave photonic band gap structures are concentrated. On the plane structure. The appearance of the planar photonic band gap structure has changed the traditional design method, provided a new way for the design of high-performance, high-integration circuit, and brought a revolution in the design thinking of microwave integrated circuit. Because the one-dimensional and two-dimensional planar bandgap structures are flexible, easy to implement and easy to integrate, they have been widely used in microwave circuit, and they have brought faster development of microwave integrated circuit.

3.MEMS switches for microwave circuit

According to the latest definition of MEMS, it is a miniaturized device or device array that combines electrical and mechanical components, and can be manufactured in batches using IC technology. Although the traditional IC manufacturing process and MEMS manufacturing process have great similarities, the former is a planar technology, and the latter is a three-dimensional technology. Currently widely used MEMS manufacturing technologies include: bulk micromachining technology, surface micromachining technology, bonding micromachining technology and LIGA technology (lithography electroforming technology).

The switch is the key element of microwave signal conversion. Compared with traditional p2i2n diode switches and FET switches, current RFMEMS switches have superior microwave characteristics and inherent advantages such as light weight, small size, and low power consumption. With the development of MEMS manufacturing technology and process theory, after overcoming the shortcomings of MEMS switches such as short working life and low switching speed, RFMEMS switches will surely achieve greater development in microwave systems. Currently, RFMEMS switches have been used in the front-end circuit, digital capacitor banks and phase shifting networks of some microwave systems.

4.Lumped componentization of microwave circuit

Another trend in microstrip circuit is to use lumped components. In the past, because the size of lumped elements was comparable to microwave wavelengths, they could not be used for microwave frequencies. With the development of photolithography and thin film technology, the size of lumped components (capacitors, inductors, etc.) has been greatly reduced, so that the J-band can be used all the time. Assembling the lumped element on the dielectric substrate with the semiconductor device in the form of a chip is a brand-new method for microwave integrated circuit. In addition to reducing the size, another advantage of lumped components is that some very useful techniques and optimization techniques in low-frequency circuit can now be directly used in the microwave field.

5.Two-dimensional planarization of microwave circuit

In addition to lumped elements and one-dimensional transmission line elements, some people have also proposed two-dimensional planar elements for microwave circuit. Such components are compatible with strip lines and microstrip lines, which provides a very useful alternative for the design of microwave circuit.

At present, there are three main ways to realize a two-dimensional planar circuit: a three-element structure, an open structure, and a cavity structure. Compared with the strip line circuit, it has the advantages of large degree of freedom and low input resistance. Compared with the waveguide circuit, it is easier to analyze and design. With the help of the powerful computing power of the high-speed computer, it can deal with any shape according to the requirements. The planar circuit is analyzed, which greatly improves the work efficiency. I believe that in the near future, its application will become more and more extensive.

6.A new generation of MIC

A new generation of MIC may be a monolithic microwave integrated circuit on a semiconductor substrate. The semiconductor substrate used is high-resistivity silicon, high-resistivity gallium arsenide, and low-resistivity silicon with a silicon dioxide layer. There are two technical difficulties. The first is that there is no universal manufacturing method for the various microwave semiconductor devices used in it, and the second is that passive distributed components (transmission line segments) require large-area substrates. However, recent trends indicate that the GaAs process is the key to microwave monolithic integrated circuit. In analog amplifiers with gigahertz bandwidth and digital integrated circuit with gigabit rates, gallium arsenide metal-semiconductor field effect transistors (MESFETs) will dominate. Whether it is a hybrid or monolithic microwave integrated circuit, its advantages are basically the same as that of a low-frequency integrated circuit, that is, the system has high reliability and reduced volume and weight. Where a large number of standardized components are required, this will eventually lead to a reduction in cost. Like low-frequency integrated circuit, MIC has great potential in expanding existing markets and opening up many new uses, including a large number of civilian projects.

Microwave circuits are developing at an unprecedented speed. With the popularity of various integrated circuits, the development of microwave circuits is bound to have a bright future. iPCB Circuit Company specializes in manufacturing microwave circuit PCB. If you have any questions, please consult iPCB.