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High frequency board: what are the common RF microwave transmission lines
24Nov
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High frequency board: what are the common RF microwave transmission lines

high frequency board: what are the common RF mICrowave transmission lines


circuit board manufacturing, circuit board design and PCBA processing manufacturers explain high-frequency boards: what are the common RF microwave transmission lines


RF signal of HF board can be transmitted through wireless channel, i.e. free space transmission, or through wired channel, i.e. transmission line transmission. Microwave transmission line plays a very important role in RF system. By using microwave transmission lines, not only microwave signals can be effectively transmitted, but also various microwave devices, (high-frequency boards) microwave circuits and antennas can be formed through proper design. Common guided wave systems include regular transmission lines (such as parallel double lines and coaxial lines), metal waveguides (such as rectangular waveguides and circular waveguides) and surface wave guides (such as microstrip lines).


Most practical waveguide structures are single-mode transmission. Therefore, according to the presence or absence of the longitudinal field components Ez and Hz, high-frequency (high-frequency plate) electromagnetic waves propagating in the waveguide can be classified as follows:

Transverse wave, also known as TE wave: Ez=0, Hz ≠ 0

Transverse magnetic wave, also known as TM wave: Ez ≠ 0, Hz=0

Transverse electromagnetic wave, also known as TEM wave: Ez=0, Hz=0

Mixed wave: Ez ≠ 0, Hz ≠ 0


The transmission lines can also be classified according to different polarization modes, as shown in the figure below. TEM wave is the main transmission mode of coaxial line and parallel plate transmission line. TE and TM waves usually appear in rectangular, circular, elliptical or ridged waveguides. In addition to TEM, TE or TM modes, the mixed wave also contains all six components of the electric and magnetic fields (Ex, Ey, Ez, Hx, Hy, Hz). The mixed wave is mainly transmitted in microstrip (MS) transmission lines, slot lines (SL) and coplanar waveguide (CPW) structures.


Among them, the three most commonly used transmission lines in modern microwave circuits and systems: coaxial lines, rectangular waveguides and microstrip lines have different advantages and disadvantages, as shown in Table 1. It can be seen that waveguides are more suitable for radar and SIMilar high frequency/high power application scenarios. In these applications, their physical dimensions will not become an obstacle; In contrast, the loss of coaxial line and microstrip line at high frequency is relatively high; When the integration and size are strictly required, such as mobile phones, handheld receivers, and some solid-state and high-power amplification modules, the (high-frequency board) microstrip line is the first choice.


Printed circuit board


Here is a brief introduction to the working principles and types of these typical transmission lines:

Coaxial transmission line

The coaxial line is the most commonly used equipment for transmitting signals from the source to the terminal. It is used to connect cables, signal sources and terminals together with connectors during transmission. In RF coaxial cable, the propagation mode of electromagnetic wave is TEM mode, that is, the direction of electric field and magnetic field is perpendicular to the propagation direction.

Within the cut-off frequency, the signal propagates in the form of TEM wave. It can be seen from the cut-off frequency calculation formula that the cut-off frequency is determined by the inner and outer conductor size of the coaxial line. In general, the smaller the size (axial) of the coaxial line, the higher the transmission frequency.

Another important parameter of the coaxial line is the characteristic impedance. In fact, the impedance of coaxial cable is 25 Ω, 50 Ω, 75 Ω, 93 Ω, etc., but in most cases, 50 Ω is selected as the standard. Why?

For air medium coaxial cable, to minimize the loss, the optimal ratio of inner diameter of outer conductor to outer diameter of inner conductor is required to be 3.6, and the corresponding impedance Z0 is 77 Ω. At the maximum power capacity, the ratio of inner diameter of outer conductor to inner diameter of conductor shall be 1.65, and the corresponding impedance Z0 shall be 30 Ω. Therefore, the impedance standard of 50 Ω is a comprehensive consideration, that is, the compromise between the minimum loss and the maximum power capacity of the coaxial cable.

The invention of high-frequency plate RF and high-frequency plate microwave coaxial connectors closely connected with the coaxial line dates back to the 1940s during the Second World War. With the birth of high-frequency plate radar and various radio communication equipment, N and BNC connectors were the first to emerge. As shown in the following table, the main electrical parameters of commonly used RF coaxial connectors are listed.

Various TM and TE modes, as well as linear combinations, can appear in rectangular guided waves. When the operating wavelength is less than the cut-off wavelength of various modes, or the operating frequency is greater than the cut-off frequency of various modes, these modes are transmission modes, so the waveguide can form multi-mode transmission.

The cut-off frequency of rectangular waveguide of high frequency board (High Frequency Circuit Board/high frequency circuit board) is:

The cut-off frequency depends not only on the wave type and waveguide size, but also on the medium filLED in the high-frequency plate waveguide.

The advantages of waveguide are small conductor loss and dielectric loss; Large power capacity; No radiation loss; The structure is simple and easy to manufacture. From the perspective of application, the characteristic parameters of high-frequency plate waveguides are as follows:

Dispersion characteristics: the dispersion characteristics represent the relationship between the longitudinal propagation constant of the waveguide and the frequency. Different from ordinary transmission lines, TEM modes cannot be transmitted in waveguides. There is a serious dispersion phenomenon in the transmission of electromagnetic waves, which indicates that the propagation speed of electromagnetic waves is related to frequency.

Characteristic impedance: the characteristic impedance Z reflects the ratio of the transverse electric field to the transverse magnetic field of the waveguide on the amplitude. When different waveguides are connected, the closer the characteristic impedance is, the smaller the reflection at the connection is. The characteristic impedance of waveguide is a very useful parameter to measure the reflection of electromagnetic energy at the junction.

Loss: loss is the main factor limiting the long distance transmission of electromagnetic wave by waveguide.

Field distribution: A possible field distribution that meets the boundary conditions of the waveguide cross-section is called the waveguide mode. Different modes have different field structures. They all meet the boundary conditions of the waveguide cross-section and can exist independently.

Waveguide transmission line

In electromagnetics and communication engineering, the term waveguide can refer to any linear structure that transmits electromagnetic waves between its endpoints. But the original and most common meaning refers to hollow metal tubes used to transmit radio waves. This waveguide is mainly used as the transmission line of microwave frequency, and is used to connect the microwave transmitter and receiver with their antennas in microwave ovens, radars, communication satellites and microwave radio link equipment.

Generally speaking, there are two main types of waveguides: metal waveguides and dielectric waveguides. Metal waveguides generally refer to hollow metal tubes, which can be divided into rectangular waveguides, circular waveguides, etc. according to the different cross section shapes of waveguides. At present, rectangular waveguide and circular waveguide are still the two main waveguide modes in practical applications.

Various TM and TE modes, as well as linear combinations, can appear in the rectangular guided wave of high-frequency plate. When the operating wavelength is less than the cut-off wavelength of various modes, or the operating frequency is greater than the cut-off frequency of various modes, these modes are transmission modes, so the waveguide can form multi-mode transmission.

The cut-off frequency of rectangular waveguide is:

The cut-off frequency depends not only on the wave type and waveguide size, but also on the medium filled in the waveguide.

The advantages of waveguide are small conductor loss and dielectric loss; Large power capacity; No radiation loss; The structure is simple and easy to manufacture. From the application point of view, the characteristic parameters describing the waveguide have the following four points:

Dispersion characteristics: the dispersion characteristics represent the relationship between the longitudinal propagation constant of the waveguide and the frequency. Different from ordinary transmission lines, TEM modes cannot be transmitted in waveguides. There is a serious dispersion phenomenon in the transmission of electromagnetic waves, which indicates that the propagation speed of electromagnetic waves is related to frequency.

Characteristic impedance: the characteristic impedance Z reflects the ratio of the transverse electric field to the transverse magnetic field of the waveguide on the amplitude. When different waveguides are connected, the closer the characteristic impedance is, the smaller the reflection at the connection is. The characteristic impedance of waveguide is a very useful parameter to measure the reflection of electromagnetic energy at the junction.

Loss: loss is the main factor limiting the long distance transmission of electromagnetic wave by waveguide.

Field distribution: A possible field distribution that meets the boundary conditions of the waveguide cross-section is called the waveguide mode. Different modes have different field structures. They all meet the boundary conditions of the waveguide cross-section and can exist independently.

Microstrip transmission line

The high frequency plate microstrip transmission line is a high frequency plate microwave transmission line composed of a single conductor strip supported on a dielectric substrate. A planar transmission line suitable for making microwave integrated circuits. Compared with metal waveguide, it has the advantages of small volume, light weight, wide bandwidth, high reliability and low manufacturing cost; But its disadvantage is that the loss is slightly large and the power capacity is small. In the early 1960s, due to the development of microwave low loss dielectric materials and microwave semiconductor devices, microwave integrated circuits were formed, making microstrip lines widely used, and various types of microstrip lines appeared one after another.

The structure of the coplanar waveguide of the high-frequency plate is similar to the microstrip line, which is to add a grounding plate on both sides of the metal wire of the microstrip line, while there is no grounding plate on the ground of the dielectric substrate

As a kind of microwave planar transmission line with superior performance and convenient processing, the coplanar waveguide of high frequency board (high frequency circuit board/high frequency circuit board) is playing a more and more important role in MMIC circuit, especially in the millimeter wave band, the coplanar waveguide of high frequency board has more performance advantages than microstrip line. Compared with conventional microstrip transmission lines, coplanar waveguides have the advantages of easy fabrication, easy realization of series connection and parallel connection of passive and active components in microwave circuits (no need to punch holes on the substrate), and easy improvement of circuit density.

summary

The coaxial line is suitable for occasions where the transmission signal power is not large and the transmission line loss is not required.

The advantages of waveguide transmitting RF signal are large power capacity and low loss, which is especially suitable for the wave band with the wavelength above 10 cm. Its disadvantages are large volume and weight, which is not conducive to integration.

Microstrip lines can be fabricated by photolithography. Their advantages, such as small size, light weight, wide bandwidth, high reliability and low manufacturing cost, especially the advantages of easy integration, make them widely used in printed circuit boards. Its disadvantages are also obvious, with slightly large loss and small power capacity. PCB manufacturers, PCB designers and PCBA manufacturers will explain high-frequency boards: what are common RF microwave transmission lines

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