According to the type of the transmission line of the microwave filter, the performance index and design method of various microwave filters are introduced in detail according to this classification method.

With the development of modern microwave communications, especially satellite communications and mobile communications, the system's channel selectivity is getting higher and higher, which puts higher requirements on the design of microwave filters, and microwave filters are used as a For the important part, the quality of its performance often determines the quality of the entire communication system. Therefore, it is of great significance to study the performance indicators and design methods of microwave filters.

Microwave filter is a type 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 useful signal frequency components are 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, band interpolation, out-of-band suppression, passband ripple, etc.

There are many classification methods for microwave filters. According to the different pass bands, microwave filters can be divided into low-pass, band-pass, band-stop, and high-pass filters; according to the filter's insertion attenuation and frequency response characteristics, it can be divided into the flattest 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. This article analyzes the main characteristics of various microwave filters in detail according to the classification of transmission lines.

1.Microstrip filter

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 bandpass filter in microwave integrated circuits. Its structure is compact, the center frequency of the second parasitic passband is located at 3 times of 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.

Compared with the parallel coupled line filter structure, the hairpin filter has a compact circuit structure, which reduces the space occupied by the filter, is easy to integrate, and reduces the cost. Hairpin filters have been widely used in occasions where the circuit size has strict requirements.

The hairpin filter is formed by arranging and coupling hairpin resonators side by side. It is a modified structure of half-wavelength coupled microstrip filter. It is formed by folding half-wavelength coupled resonators into a U-shape. Compared with other microwave filter structures such as finger type and comb line type, its circuit structure is more compact, has the advantages of small size, open circuit of the microstrip line terminal without via hole grounding, and easy manufacture. The hairpin filter coupling topology is cross-coupling. 

The essence of cross-coupling is that there is more than one coupling path from the signal source to the load, including the main coupling path and the relatively weak auxiliary coupling path, which can be generated between any two resonators. coupling. Compared with cascade coupling, the biggest advantage of cross-coupling is that it can generate transmission zeros at a finite frequency near the passband. Therefore, the out-of-band suppression capability of the filter will be greatly improved. The connected filter has better frequency selectivity and can reduce the number of resonators required.

Hairpin filter parameters include: hairpin arm length, hairpin spacing, hairpin line width and tap position.

Parallel coupled line filters, interdigital filters, etc., obtain relatively flat amplitude-frequency characteristics in the band, but poor out-of-band suppression characteristics. Microstrip-like elliptic function filters can significantly improve the out-of-band characteristics of the filter by introducing attenuation poles out of the band, and have better electrical characteristics than parallel coupled line filters and interdigital filters. And the microstrip-like elliptic function filter has a small volume. At the same time, in the superconducting state, due to the high unloaded Q value of the conductive film, this kind of filter will have a higher selectivity and a lower Insertion loss has good application prospects.

Two, interdigital filter

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 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.

T

he 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 interval between each resonator, the tolerance requirements are low and easy to manufacture; because the length of the resonator rod is approximately equal to 1/4λ0, the second passband center 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.

Three, coaxial filter

The coaxial cavity filter has small size, high Q value and good temperature stability, which is especially suitable for narrowband applications. The achievable bandwidth is 0.5% to 3%, which 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 with narrow passband, small band insertion loss, and high out-of-band suppression. This type of filter is very suitable for mass production, so the cost is also very low. But when it is used above 10 GHz, it is difficult to achieve the production accuracy 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.

Four, waveguide filter

The waveguide filter has 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, and 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.

Usually, a direct coupling half-wavelength resonator structure can be used to construct a waveguide filter. However, due to the influence of higher-order modes, this type of filter has a very close second passband and poor stopband performance at the high frequency. This can be improved by using a 1/4 wavelength transmission line coupled with a resonant diaphragm structure. By selecting the appropriate diaphragm size, the resonant diaphragms can resonate at the same frequency but have different Q values, which can make the second passband position farther, thereby significantly improving its stopband characteristics. In addition, the 1/4 wavelength transmission line coupled resonant diaphragm type (hereinafter referred to as the resonant diaphragm type) filter also has the advantage of small size, and its total length is shorter than the direct coupling half-wavelength resonant cavity type (hereinafter referred to as the half-wavelength type) by nearly 40 %. Compared with the half-wavelength type, the size of the resonant diaphragm bandpass filter is shortened by 38.4%, and it has a wider stop band.

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.

Five, comb line cavity filter

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 caused by the high dielectric constant.

The commonly used microstrip line filter structure has the form of interdigital, comb and hairpin type. The so-called "comb line 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. The structure composed. In this filter, the coupling between the resonators is obtained by the fringe field between the parallel coupling lines.

However, the comb line filter has the disadvantage of temperature drift.

Six, spiral cavity filter

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 by a 1/4λ coaxial resonator. Because of its strong coupling performance and high Q value, the spiral filter 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.

Seven, small lumped parameter filter

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 in electronic countermeasures, electronic reconnaissance, communications, radar and other electronic equipment for pre-selection, post-selection, clutter suppression and frequency conversion filtering. It has the advantages of small size, light weight, 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. Both narrowband and wideband filters in the range of 10-2000Ml-lz can be realized.

8. Ceramic dielectric filter

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, and 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 stripline to realize the design of the filter. After lamination, the printed metal pattern on the medium layer is equivalent to the strip line in the medium. When the metal pattern layer of different length and width 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.

Nine, SIR (step impedance resonator) filter

With the development of wireless communication, the frequency band between signals is getting narrower and narrower, the smaller the mutual influence of signals is required, and the requirements for filters are getting 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 combination of two or more transmission lines with different characteristic impedances.

Resonator in electromagnetic field mode. λ/4 SIR is one of the most attractive forms. It can not only reduce the size of the filter, but also control the spurious well by adjusting the impedance ratio