Microwave RF filter classification and performance indicators

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Microwave RF filter classification and performance indicators [Copy link]

Microwave filters are classified according to their transmission line types, and the performance indicators and design methods of various microwave 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 to channels, which puts higher requirements on the design of microwave filters. As an important part of the communication system, the performance of microwave filters 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 kind of lossless two-port network, which is widely used in microwave communication, radar, electronic countermeasure and microwave measuring instruments. It is used to control the frequency response of the signal in the system, so that the useful signal frequency component passes through the filter almost without attenuation, while blocking the transmission of useless signal frequency components. 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 ways to classify 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 insertion attenuation frequency response characteristics of the filter, it can be divided into the flattest type and the equiripple type; according to the width of the working frequency band, it can be divided into narrowband and broadband filters; according to the transmission line classification of the filter, it can be divided into microstrip filters, interdigital filters, coaxial filters, waveguide filters, combline 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

Key performance indicators:

● Frequency range: 500MHz～6GHz

● Bandwidth: 10% to 30%

● Insertion loss: 5dB (varies with bandwidth)

● Input and output form: 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 elliptic function filters.

The half-wavelength parallel coupled microstrip line bandpass filter is a widely used bandpass filter in microwave integrated circuits. It has a compact structure, the center frequency of the second parasitic passband is located at 3 times the center frequency of the main passband, the adaptability frequency range is large, and the relative bandwidth can reach 20% when it is suitable for broadband filters. Its disadvantages are large insertion loss. At the same time, the resonators are spread out in one direction, causing the filter to occupy a large space in one direction. As shown in Figure 1:

Figure 1 Schematic diagram of the structure of parallel coupled microstrip line filter

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

The hairpin filter is formed by the hairpin resonators arranged side by side and coupled. It is a deformation structure of the half-wavelength coupled microstrip filter. It is formed by folding the half-wavelength coupled resonator into a U shape. Therefore, compared with other microwave filter structures such as interdigital type and comb line type, its circuit structure is more compact, with small size, open circuit of microstrip line terminal without via grounding, easy to manufacture and other advantages. The hairpin filter coupling topology belongs to cross coupling. The essence of cross coupling is that there is more than one coupling path from the signal source to the load end, including the main coupling path and the relatively weak auxiliary coupling path. Coupling can be generated between any two resonators. Compared with cascade coupling, the biggest advantage of cross coupling is that it can generate transmission zeros at a limited frequency near the passband, so the out-of-band suppression ability of the filter will be greatly improved. The resonator filter using cross coupling has better frequency selectivity than the ordinary cascade filter, and can reduce the number of required resonators.

The parameters of the hairpin filter 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 within the band, but have poor out-of-band suppression characteristics. Microstrip elliptic function filters can significantly improve the out-of-band characteristics of the filter by introducing attenuation poles outside the band, and have better electrical characteristics than parallel coupled line filters and interdigital filters. In addition, microstrip elliptic function filters have a smaller volume. At the same time, in the superconducting state, due to the high unloaded Q value of the conductor film, this type of filter will have high selectivity and low insertion loss, and has a good application prospect.

2. Interdigital Filter

The interdigital filter has a high Q value and a moderate size. It can achieve 5% to 60% bandpass filtering in the frequency range of 0.5 to 18 GHz and is widely used in various military and civilian electronic products. The interdigital filter is generally made of metal cutting and processing, with a solid structure and stable and reliable performance.

Key performance indicators:

● Frequency range: 800MHz～16GHz

● Bandwidth: 10% to 100%, 3% to 70% for special requirements

● Insertion loss: 0.5～2dB (varies with 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 upon special request)

● Through power: 5W (up to 100W on special request)

● Temperature: -55～+85℃

● Input and output form: SMA, N, L16, etc.

The interdigital filter is an improvement on the parallel coupled microstrip line filter, which 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 requirement is low and it is easy to manufacture; because the length of the resonant rod is approximately equal to 1/4λ0, the center of the second passband is above 3ω0, and there will be no parasitic response in between.

Since interdigital filters can be made into printed circuits or cavity structures, and are self-supported by thicker rods without dielectrics, they 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 coupled wire nodes of the parallel coupled microstrip filter are cut off from the midpoint, folded up, and merged into a coupled wire node, one end of which is short-circuited to ground, and the other end is open, and the coupling gap between the two adjacent wire nodes is kept unchanged to form an interdigital structure. As shown in Figure 2

Figure 2 Schematic diagram of the interdigital filter structure

3. Coaxial filter

The coaxial cavity filter is small in size, has a high Q value, and has good temperature stability, making it particularly suitable for narrowband applications. It can achieve a bandwidth of 0.5% to 3%, and is widely used in various military and civilian electronic systems.

Key performance indicators:

● Frequency range: 800MHz~16GHz

● Bandwidth: 0.1%～10%

● Insertion loss: 0.5～25dB (varies with bandwidth)

● Input and output form: 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 different cavity structures, they are generally divided into standard coaxial, square cavity coaxial, etc. Coaxial cavities have the characteristics of high Q value and easy implementation. They are particularly suitable for narrow passband, small in-band insertion loss, and high out-of-band suppression. This type of filter is very suitable for large-scale production, so the cost is also very low. However, when used above 10 GHz, due to its tiny physical size, it is difficult to achieve manufacturing accuracy. Specific designs include methods such as negative resistance line sub-network to construct a multi-cavity coupled coaxial bandpass filter circuit model; coaxial cavity filter temperature compensation method; step impedance resonator, etc.

4. Waveguide Filter

Waveguide filters have high Q value, low insertion loss, good temperature stability, and are particularly suitable for narrowband applications. They can achieve 0.2% to 3.5% bandpass filtering in the frequency range of 1.7 to 26 GHz, and are widely used in various military electronic products that require high-performance filtering characteristics.

Key performance indicators:

● Frequency range: 2～4GHz

● Bandwidth: 0.1%～20%

● Insertion loss: 0.5～3dB (varies with bandwidth)

● Input and output form: 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 communication, satellite communication and other systems due to their high Q value, low loss and large power capacity. The rapid development of microwave technology in recent years has put forward higher and higher requirements on the size, stopband characteristics and other indicators of such filters.

通常可用直接耦合半波长谐振腔结构来构造波导型滤波器,但由于高次模的影响,这种类型的滤波器第二通带很近,频率高端阻带性能较差。采用1/4波长传输线耦合谐振膜片结构,可对此进行改善。通过选择合适的膜片尺寸,使各谐振膜片谐振在同一频率上,但具有不同的Q值,可使其第二通带位置变远,从而显著提高其阻带特性。另外,1/4波长传输线耦合谐振膜片型(以下简称谐振膜片型)滤波器还具有尺寸小的优点,其总长度比直接耦合半波长谐振腔型(以下简称半波长型)缩短近40%。与半波长型相比较,谐振膜片型带通滤波器的尺寸缩短了38.4%,且具有更宽的阻带。

Waveguide bandpass filters are also used in various microwave multiplexers, but their biggest disadvantage is that their size is significantly larger than other resonators that can be used in the microwave band.

5. Comb Line Cavity Filter

The standard response of the combline filter is 0.05dB ripple Chebyshev response, with the characteristics of small size and moderate Q value. It can achieve a relative bandwidth of 0.5%-30% in the frequency range of 0.5-12GHZ, and is widely used in various military and civilian electronic products.

Key performance indicators:

● Frequency range: 500MHz～6GHz

● Bandwidth: 1% to 20%

● Insertion loss: 0.5～2dB (varies with bandwidth)

● Input/output impedance: 50 ohms

● Input/output standing wave: VSWR≤1.5:1

● Temperature: -50～+85 degrees Celsius

Appearance: The appearance size varies with frequency, bandwidth, insertion loss, and number of nodes. There is no fixed size.

Input and output form: 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 made on a high dielectric constant substrate using a λ/4 resonant line. It uses a cross-coupling method to increase the steepness of the passband edge, and at the same time uses shielding lines in the microstrip resonator to weaken the strong coupling caused by the high dielectric constant.

Commonly used microstrip line filter structures include interdigital, comb and hairpin types. The so-called "comb line filter" has a structure in which the resonator is composed of a number of parallel coupled lines with one end short-circuited and the other end grounded through a lumped capacitor. In this filter, the coupling between the resonators is obtained by the fringe field between the parallel coupled lines.

However, combline filters suffer from temperature drift.

6. Spiral Cavity Filter

Key performance indicators:

● Frequency range: 30MHz～1.2GHz

● Bandwidth: 0.1%～20%

● Insertion loss: 0.5～3.5dB (varies with bandwidth)

● Input and output form: SMA, N, L16, etc.

● Input and output standing wave: 1.5:1

Some filter technologies currently used, such as piezoelectric crystal resonators, have coaxial oscillators that are too large and are not suitable for applications in VHF and UHF bands. In the VHF and UHF bands, spiral filters have high Q values and smaller design parameters, allowing the designed oscillator to be assembled from a 1/4λ coaxial resonator. Since spiral filters have strong coupling performance and high Q values and can withstand high power capacity, they are widely used in low-frequency high-power circuit designs. The disadvantage is that the boundary conditions of the spiral coupling structure are very complex, and the complexity and amount of calculations using electromagnetic field numerical methods are very large, so it is difficult to implement the design.

7. Small Lumped Parameter Filter

Key performance indicators:

● Frequency range: 10～1500MHz

● Volume: Type 1: 48×19×14mm

● Type 2: 41 x 15 x 12 mm

● Bandwidth: 10% to 200%

● Insertion loss: 0.5～5dB (varies with 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, radars and other electronic equipment. It has the advantages of small size, light weight, stable and reliable performance, convenient processing and installation. It has better temperature performance and out-of-band suppression performance than other filters. Small lumped parameter filters use advanced dedicated microwave CAD software to optimize the filter circuit. It can realize narrowband and broadband filters in the range of 10-2000Ml-lz.

8. Ceramic dielectric filter

Multilayer ceramic microwave filter is a high-frequency multilayer ceramic microwave filter made through a variety of process flows such as electronic ceramic material tape casting, low-temperature lamination sintering technology, high-precision printing lamination technology and packaging technology. It has the characteristics of high frequency, small size, low insertion loss and high attenuation, and is widely used in mobile communications, digital home appliances and other products.

The multilayer ceramic microwave filter is obtained by forming distributed capacitance C and distributed inductance L by printing metal patterns on the dielectric layer, and forming coupling capacitance between metal pattern layers on different dielectric layers. Its essence is to use strip lines to realize the design of the filter. After lamination, the printed metal pattern on the dielectric layer is equivalent to the strip line in the dielectric. When designing metal pattern layers of different lengths and widths, different L and C can be obtained. Therefore, by designing the shape of the metal pattern layer and selecting the appropriate dielectric, a filter that resonates at a certain frequency and meets the requirements of various indicators such as in-band insertion loss, bandwidth and stop band can be obtained.

9. SIR (Step Impedance Resonator) Filter

With the development of wireless communication, the frequency band between signals is getting narrower and narrower, requiring the signals to have less mutual influence, and the requirements for filters are getting higher and higher. How to achieve miniaturization, high selectivity, and wide stopband of filters has become the main research direction of filters.

A stepped impedance resonator (SIR) is a transverse electromagnetic field or quasi-transverse electromagnetic field composed of two or more transmission lines with different characteristic impedances.

The λ/4 type SIR is the most attractive form. It can not only reduce the size of the filter, but also control the spurious frequency well by adjusting the impedance ratio, so as to achieve the requirements of miniaturization and wide stopband of the filter. The comb line filter shortens the size of the resonator of the filter due to the capacitive loading at one end. The cross-coupled filter has become a research hotspot in the past 20 years. Since the transmission zero point at its limited position can be set arbitrarily, the maximum number of transmission zero points can be set as many as the filter order, which maximizes the out-of-band suppression ability of the filter.

10. High-temperature superconducting materials

High-temperature superconducting filters mainly include four parts: amplifier circuit, deep refrigeration system, precise control system and vacuum insulation system.

The superconducting filter made by using the extremely low microwave surface resistance of high-temperature superconducting film in the microwave frequency band has the characteristics of extremely low in-band insertion loss, steep edges, close to ideal frequency response characteristics of rectangular coefficient and very good suppression of out-of-band interference. The superconducting filter subsystem composed of superconducting filter and low-noise amplifier can be applied to the system (transmitter or receiver) to greatly improve the system performance and has broad application prospects in the military, especially in the field of communication. In foreign countries, the superconducting filter system cooled by refrigerators has been used in communication systems, which greatly improves the quality of communication systems: improve call quality, increase call capacity, increase the coverage area of base stations, enhance the anti-interference ability of base stations, reduce the transmission power of mobile phones and reduce the call drop rate, etc. In modern information warfare weapons and equipment (such as early warning aircraft, radar, electronic warfare equipment, missile guidance components, etc.), replacing ordinary filters with high-temperature superconducting filters can greatly improve the sensitivity and selectivity of receivers, enhance anti-interference ability, improve communication distance and quality, extend warning time, reduce transmitter power, improve guidance accuracy, and increase terminal guidance distance. Application in satellite communication systems can greatly improve the utilization of space frequency resources, effectively reduce the satellite's payload, and provide a new way to develop and utilize space frequency resources.

High-temperature superconducting filters have low operating temperatures and require deep refrigeration, so they have more peripheral components and a more complex structure.

Summarize

Microwave filters are widely used in various circuit systems such as communications, signal processing, radar, etc. With the rapid development of mobile communications, electronic countermeasures and navigation technology, higher requirements are put forward for the demand for new microwave components and the improvement of the performance of existing devices. Developed countries are using new materials and new technologies to improve device performance and integration, while reducing costs, device size and power consumption as much as possible. Compared with foreign countries, the development of microwave filters in my country still has a certain gap, so we should grasp the development direction of microwave filters and strive to catch up with the world's advanced level.