075582814553
According to the type of the transmission line of the microwave filter, the performance index and design method of various microwave RF filters are introduced in detail according to this classification method.
With the development of modern microwave communication, especially satellite communication and mobile communication, the system has become more and more selective in the channel, which puts forward higher requirements for the design of microwave filters. As an important part of the communication system, the performance of the microwave filter often determines the quality of the entire communication system.
A microwave filter is a kind of lossless two-port network, which is widely used in microwave communication, radar, electronic countermeasures, and microwave measuring instruments. It is used in the system to control the frequency response of the signal so that the useful signal frequency component is almost attenuated. Through the filter, the transmission of unnecessary signal frequency components is blocked. The main technical indicators of the filter are center frequency, passband bandwidth, in-band insertion loss, out-of-band suppression, passband ripple, etc.
There are many classification methods for microwave filters. According to the different passbands, microwave filters can be divided into low-pass, band-pass, band-stop, and high-pass filters; according to the frequency response characteristics of the filter's insertion attenuation, it can be divided into the flattest type and Equal ripple type; according to the width of the working frequency band can be divided into narrowband and broadband filters; according to the transmission line classification of the filter can be divided into microstrip filter, interdigital filter, coaxial filter, waveguide filter, comb line Cavity filters, spiral cavity filters, small lumped parameter filters, ceramic dielectric filters, SIR (step impedance resonator) filters, high-temperature superconducting materials, etc.
Main performance indicators:
Frequency range: 500MHz～6GHz
Bandwidth: 10%～30%
Insertion loss: 5dB (different with different bandwidth)
Input and output forms: SMA, N, L16, etc.
Input and output standing wave: 1.8:1
Microstrip filters mainly include parallel-coupled microstrip line filters, hairpin filters, and microstrip-like elliptic function filters.
Half-wavelength parallel-coupled microstrip line bandpass filter is a widely used form of the bandpass filter in microwave integrated circuits. Its structure is compact, the center frequency of the second parasitic passband is located at 3 times the center frequency of the main passband, the adaptable frequency range is large, and the relative bandwidth can reach 20% when it is suitable for wideband filters. The disadvantage is that the insertion loss is relatively large. At the same time, the resonator is sequentially opened in one direction, which causes the filter to occupy a large space in one direction. As shown in Figure 1:
Figure 1 Schematic diagram of the parallel coupling microstrip line filter structure
Compared with the parallel-coupled line filter structure, the hairpin filter has a compact circuit structure. It reduces the space occupied by the filter and is easy to integrate. Hairpin filters have been widely used on occasions where the circuit size has strict requirements.
Hairpin filter parameters include hairpin arm length, hairpin spacing, hairpin line width, and tap position.
The interdigital filter has a high Q value and a moderate volume. It can achieve 5%～60% bandpass filtering in the frequency range of 0.5～18GHz, which is widely used in various military and civilian electronic products. Interdigital filters are generally cut and processed from metal, with a firm structure and stable and reliable performance.
Main performance indicators:
Frequency range: 800MHz～16GHz
Bandwidth: 10%～100%, special requirements 3%～70%
Insertion loss: 0.5～2dB (different with different bandwidth)
Stopband suppression: The near-end transition band is determined by the number of filter sections, and the far-end is generally greater than 70dB
Parasitic passband: ﹥2.5×f0
Input/output impedance: 50Ω
Input/output standing wave: VSWR≤1.7:1 (≤1.5:1 on special request)
Through power: 5W (up to 100W on special request)
Temperature: -55～+85℃
Input and output forms: SMA, N, L16, etc.
The interdigital filter is an improvement to the parallel-coupled microstrip line filter, and it also reduces the volume occupied by the microstrip filter. It has the following advantages: compact structure and high reliability. Due to the large spacing between each resonator, the tolerance requirements are lower and easy to manufacture. Because the length of the resonator rod is approximately equal to 1/4λ0, the center of the second passband is above 3ω0, during which there will be no spurious response.
Because the interdigital filter can be made into a printed circuit form and a cavity structure, it can be made into self-supporting with a thicker rod instead of a medium. Therefore, interdigital filters are widely used in electronic systems, especially in communication technology and modern aerospace fields.
The working principle of the interdigital microstrip bandpass filter can be explained as follows: the two adjacent coupling line nodes of the parallel coupling microstrip filter are cut from the midpoint, and folded, and merged into a coupling line Short-circuit one end to the ground, and open the other end, and keep the coupling gap between the adjacent two-level line sections unchanged to form an interdigital structure.
Figure 2 Schematic diagram of the interdigital filter structure
The coaxial cavity filter has a small size, high Q value, and good temperature stability, which is especially suitable for narrowband applications. The achievable bandwidth is 0.5% to 3%, and it is widely used in various military and civilian electronic systems.
Main performance indicators:
Frequency range: 800MHz~16GHz
Bandwidth: 0.1%～10%
Insertion loss: 0.5～25dB (different with different bandwidth)
Input and output forms: SMA, N, L16, etc.
Input and output standing wave: 1.4:1
Temperature: -55～+85℃
Coaxial cavity filters are widely used in communication, radar, and other systems. According to the cavity structure, they are generally divided into standard coaxial and square cavity coaxial. The coaxial cavity has the characteristics of high Q value and easy realization. It is especially suitable for occasions where the passband is narrow, the band insertion loss is small, and the out-of-band suppression is high. This type of filter is very suitable for mass production, so the cost is also very low. However, when it is used above 10 GHz, the manufacturing accuracy is difficult to achieve due to its tiny physical size. Specific design methods include negative resistance line sub-network to construct a multi-cavity coupling coaxial bandpass filter circuit model; coaxial cavity filter temperature compensation method; step impedance resonator and so on.
The waveguide filter has a high Q value, small insertion loss, and good temperature stability, which is especially suitable for narrowband applications. It can achieve 0.2%～3.5% bandpass filtering in the frequency range of 1.7～26GHz. It is widely used in various military electronic products that require high performance filtering characteristics.
Main performance indicators:
Frequency range: 2～4GHz
Bandwidth: 0.1%～20%
Insertion loss: 0.5～3dB (different with different bandwidth)
Input and output forms: SMA, N, L16, etc.
Input and output standing wave: 1.3:1
Temperature: -55～+85℃
Waveguide filters are widely used in microwave and millimeter-wave communications, satellite communications and other systems due to their high Q value, low loss, and large power capacity. In recent years, the rapid development of microwave technology has put forward higher and higher requirements for the size and stop-band characteristics of this type of filter.
Waveguide bandpass filters are also used in various microwave multiplexers, but its biggest disadvantage is that the size is significantly larger than other resonators that can be applied in the microwave range.
The standard response of the comb line filter is the 0.05dB ripple Chebyshev response, which has the characteristics of small size and moderate Q value. A relative bandwidth of 0.5%-30% can be achieved in the frequency range of 0.5-12GHZ, which is widely used in various military and civilian electronic products.
Main performance indicators:
Frequency range: 500MHz～6GHz
Bandwidth: 1%～20%
Insertion loss: 0.5～2dB (different with different bandwidth)
Input/output impedance: 50 ohms
Input/output standing wave: VSWR≤1.5:1
Temperature: -50～+85 degrees Celsius
Shape: Shape size varies with frequency, bandwidth, insertion loss, and number of sections, and there is no fixed size
Input and output forms: SMA, N, L16, etc.
In order to reduce the size and make the design simple and suitable for large-scale production, a microstrip filter, namely a comb-line cavity filter, is directly fabricated on a high-dielectric constant substrate by using λ/4 resonant lines. It uses the cross-coupling method to increase the steepness of the passband edge, and at the same time uses shielded wires in the microstrip resonator to weaken the strong coupling brought about by the high dielectric constant.
The commonly used microstrip filter structure has the form of interdigital, comb, and hairpin type. The so-called "comb filter", its resonator is a number of parallel-coupled lines that are short-circuited at one end and grounded through a lumped capacitor at one end. In this filter, the coupling between the resonators is obtained by the fringe field between the parallel coupling lines.
Main performance indicators:
Frequency range: 30MHz～1.2GHz
Bandwidth: 0.1%～20%
Insertion loss: 0.5～3.5dB (different with different bandwidth)
Input and output forms: SMA, N, L16, etc.
Input and output standing wave: 1.5:1
Some filter technologies currently used, such as piezoelectric crystal resonators, whose coaxial oscillators are too bulky, are not suitable for VHF and UHF applications. In the VHF and UHF frequency bands, the spiral filter has a high Q value and small design parameters, so that the designed oscillator can be assembled from a 1/4λ coaxial resonator. Because the spiral filter has strong coupling performance and high Q value, it can withstand high power capacity. So it is widely used in the design of lower radio frequency and high power circuits. The disadvantage is that the boundary conditions of the spiral coupling structure are very complicated, and the calculation complexity and calculation amount of the electromagnetic field numerical method are very large, so it is difficult to realize the design.
Main performance indicators:
Frequency range: 10～1500MHz
Volume: Type 1: 48×19×14mm
Type 2: 41×15×12mm
Bandwidth: 10%～200%
Insertion loss: 0.5～5dB (different with different bandwidth)
Input and output form: pin, SMA, N, L16, etc.
Input and output standing wave: 1.5:1
Small lumped parameter filters are mainly used for pre-selection, post-selection, clutter suppression, and frequency conversion filtering in electronic countermeasures, electronic reconnaissance, communications, radar, and other electronic equipment. It has the advantages of small size, lightweight, stable and reliable performance, convenient processing, and easy installation. Compared with other filters, it has better temperature performance and out-of-band suppression performance. The small lumped parameter filter adopts advanced special microwave CAD software to optimize the filter circuit selection. Both narrowband and wideband filters in the range of 10-2000Ml-lz can be realized.
The multilayer ceramic microwave filter is a high-frequency multilayer ceramic microwave filter made through various processes such as electronic ceramic material tape casting technology, low-temperature stacking sintering technology, high-precision printing stacking technology, and packaging technology. It has the characteristics of high frequency, small size, small insertion loss, and large attenuation. It is widely used in mobile communications, digital home appliances, and other products.
The multilayer ceramic microwave filter is obtained by forming a distributed capacitance C and a distributed inductance L by printing metal patterns on a dielectric layer, and forming a coupling capacitor between the metal pattern layers on different dielectric layers. Its essence is to use a stripline to realize the design of the filter. After lamination, the printed metal pattern on the medium layer is equivalent to the stripline in the medium. When the metal pattern layer of different lengths and widths is designed, different L and C can be obtained. Therefore, by designing the shape of the metal pattern layer and selecting an appropriate medium, a filter that resonates at a certain frequency and satisfies the requirements of various indicators such as band insertion loss, bandwidth, and stopband can be obtained.
With the development of wireless communication, the frequency band between signals is getting narrower and narrower, and the smaller the mutual influence of signals is required, and the requirements for filters are getting higher and higher. How to realize the miniaturization, high selectivity, and wide stopband of the filter has become the main research direction of the filter.
The step impedance resonator (SIR) is a transverse electromagnetic field or quasi-transverse electromagnetic field resonator composed of two or more transmission lines with different characteristic impedances. λ/4 SIR is one of the most attractive forms. It can not only reduce the size of the filter but also can well control the spurious frequency by adjusting the impedance ratio, so as to achieve the requirements of miniaturization and wide stopband of the filter. The filter in the form of a comb line reduces the size of the resonator of the filter due to the capacitive loading at one end. Cross-coupled filters have become a research hotspot in the past 20 years. Because of their limited transmission zero points can be set arbitrarily, the maximum number of transmission zero points can be set as many as the filter order, which maximizes the filter’s out-of-band suppression ability.
The high-temperature superconducting filter mainly includes four parts: amplifying circuit, deep refrigeration system, precise control system, and vacuum insulation system.
The superconducting filter made by using high-temperature superconducting film in the microwave frequency band has the characteristics of extremely low in-band insertion loss, steep edge, rectangular coefficient close to the ideal frequency response characteristics, and very good suppression of out-of-band interference. The superconducting filter subsystem composed of a superconducting filter and low-noise amplifier can be applied to the system (transmitter or receiver) to greatly improve the system performance and has a broad application prospect in the military, especially in the field of communications.
The high-temperature superconducting filter requires deep cooling due to its low operating temperature, so there are many peripheral components and a complex structure.
Microwave filters are widely used in various circuit systems such as communications, signal processing, and radar. With the rapid development of mobile communications, electronic countermeasures, and navigation technologies, higher demands are placed on the demand for new microwave components and the improvement of the performance of existing devices. Many countries are using new materials and new technologies to improve device performance and integration, while reducing costs as much as possible, reducing device size, and reducing power consumption.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other 