A. About CAD auxiliary design software and network analyzer
For high frequency circuit design, the current has built up a good CAD software, and its powerful enough to overcome the lack of people in the aspect of design experience and complicated retrieval and calculation of parameters, combined with powerful network analyzer, arguably should be slight experience can be a better quality of complete rf components. In practice, however, is not the case.
CAD design software relies on a strong library function, contains most of the radio device manufacturers in the world to provide parameters of electronic components and the basic performance indicators. Many rf engineer mistakenly think that: as long as the use of the tool software to carry on the design, there will be no much problem. But the actual result is always contrary to the wishes, the reason is that they give up under the wrong understanding the flexible application of the concept of high frequency circuit design and the application of the basic design principle of the accumulation of experience, basic application results in the application of software tools to make mistakes. Rf circuit design CAD software is transparent visualization software, using its basic configuration model library to complete all kinds of high frequency simulation of the actual circuit working state. So far, we have been able to understand the key link of jujube basic configuration model there are two types of high frequency, type belongs to component model, lumped parameter form another belongs to local features in the design of conventional model. So the following problems:
(1) component model and interaction with the development of CAD software for a long time, gradually perfect, practice basic trust model can * true degrees. But components model considering the application environment (especially the components of the application of electrical environment) are typical values. In most cases, the application must experience is used to determine the series parameters, or sometimes even than its actual results with the help of CAD software design results differed.
(2) in the CAD software to establish the basic configuration of conventional high frequency model, usually limited to the current application conditions of predictable ways, the basic function and can only be limited to the model (or do not need to choose and employ persons product research and development, only by CAD and single-handedly born all kinds of products).
(3) is of particular note is that the typical function model, is a typical way of application components and structure in typical perfect process (including PCB structure), its performance will achieve higher level of "typical". But in practice, it is completely imitate, also with the model state. Reason: although the selection of components and its parameters are consistent, but they are not consistent combination of electrical environment. In the low frequency circuit or digital circuit, this kind of difference matter does not interfere with, but in rf circuits, often a fatal error.
(4) in using CAD software to design, software fault-tolerant design does not ignore whether conflict with actual situation of error parameter setting, so according to the software running paths given a ideal result, the result of practice is patchy. Can know its key error link is not using the basic principles of rf circuit design to the correct application of CAD software.
(5) CAD software belongs to auxiliary tool design, use of the real-time simulation of function, powerful components model library and its typical application function, model base and so on aspects to generate functions to simplify the complex design and calculation of working people, until now, it is far from instead of artificial intelligence in terms of specific design.
Embodied in the design of CAD software in the radio frequency (rf) PCB auxiliary power is an important aspect of the software is very popular. But in practice, many rf engineer will often "by its plot against". Cause reason is still the fault-tolerant features of parameter Settings. Often use its simulation function it is concluded that the ideal model (including each functional part), found in a to the actual debugging: it is better to use their experience to design.
So, CAD software in the PCB design, still only has basic rf design experience and skills of engineers, to help them in tedious process design, the basic principles of design).
Network analyzer is divided into two kinds of scalar and vector, is an essential part of rf circuit design instrument. Typically, combined with the basic concept and principle of rf circuit design to complete the circuit and PCB design (or using CAD software to complete), complete PCB sample processing and assembling prototype as required, and then using the network analyzer to the design of every link carry on network analysis, is likely to make optimal circuit. But the cost of such work is at least 3 ~ 5 version of the PCB actual production, and if no basic principles and basic concept of PCB design, the required PCB version will be more (or cannot complete design).
From the above:
(1) the use of network analyzer to analyze rf circuit in the process, must have a complete PCB design concept and principle of high frequency circuit, through the results of the analysis must be able to clearly know the PCB design flaws in jujube that alone requires relevant engineers have considerable experience.
(2) analyze the prototype network link in the process, must rely on skilled experiment experience and skill to construct the function of local network. Because a lot of times, through the network analyzer circuit defects found, at the same time there are many factors lead to, so must use local functional network structure to analyze, investigate causes. The experimental circuit structure must use high frequency circuit design experience and skilled circuit PCB structure principle.
B. This article in view of the category
This paper focuses on a frontier communications products category jujube microwave level of high frequency circuit and PCB design concept and design principles. Chose microwave PCB design principle of high frequency circuit, because the aspects principle has extensive significance and current applications of high-tech technology. Transition from microwave circuit PCB design concept to high-speed wireless network, including all kinds of access network engineering, also is the tang, because they are based on the basic principle of the same date double transmission line theory.
Experienced rf engineer design of digital circuit PCB circuit or relatively low frequency, a success rate is very high, because their design concept based on "distribution parameter" as the core, the concept of distributed parameter in the low frequency circuit (including digital circuits) of damage, is often ignored by people.
For a long time, many of his peers to complete electronic products (mainly for communication products) design, often fraught with problems. On the one hand, is with the principle of electrical design (including redundancy design, reliability design, etc) the necessary link of lack, but more important, is that many such problems under the people think that has been considering all the necessary links. To solve these problems, they will tend to focus on the application, the electric principle, parameter redundancy, etc on the verification of little will focus on auditing of PCB design, PCB design defect, but often it is because of performance problems, resulting in a large number of products.
PCB design principle involves many aspects, including the basic principles, antijamming, the electromagnetic compatibility and safety protection, and so on. For these aspects, especially in high frequency circuit (especially in high frequency circuit of microwave level), the lack of the related concept, often lead to the failure of the research and development of the whole project. Many people are still stay in "connect electrical principle with conductor play a role of scheduled" basis, and even think "PCB design belongs to the structure, process and improve production efficiency and so on account". Many professional rf engineer did not fully realize the link in the design of rf, should be special focus of the whole design work, and mistakenly effort in choosing high-performance components, the result is costs have risen sharply, but the improvement of the performance of the minimal.
Should be put forward here is in particular, relying on its strong anti-interference, digital circuit detection error correction and each one can be arbitrary structure intelligent link to ensure the normal function of the circuit. An ordinary digital application circuit and high additional configuration of all kinds of "ensure normal" link, is clearly not the product concept. But often think of "not worth" link, but lead to product the series of problems. Reason is this kind of product engineering point of view is not worth the function of the structure reliability to ensure link, should be based on the digital circuit itself on the working mechanism of the circuit is designed, including PCB design errors in construction, lead to circuit in an unstable state. This lead to unstable state, and high frequency circuit under the similar problems belong to the same concept of the basic application.
In digital circuits, is worth seriously has three aspects:
(1) digital signal itself belongs to a broad spectrum signal. According to the result of the Fourier function, it contains high frequency components are very abundant, so in digital IC design, fully consider the digital signal of high frequency component. But besides digital IC, the functional link between internal and signal transition area, if the arbitrary, will result in a series of problems. Especially in the digital and analog and mixed application of high frequency circuit circuit.
(2) the application of reliability design of digital circuit, the circuit in the practical application of reliability requirement and product engineering requirement, the conventional design can not be fully able to meet the requirements of circuit additional all kinds of the high cost of "security" section.
(3) digital circuit working at an unprecedented rate of development towards a high frequency (such as the current CPU, its frequency has reached 1.7 GHz jujube is far more than microwave frequencies lower limit). Although the reliability of the related devices safeguard function is synchronous, but its based on device characteristics based on the typical external and internal signal.
C. Double transmission line theory of microwave circuit design and PCB wiring principles guiding significance were reviewed
(a) concept under the dual theory of PCB
For microwave, high frequency circuit PCB every root corresponding stripline and ground plate forming microstrip line (asymmetric), for more than two layers of PCB, can form the microstrip line, and can form stripline (symmetric decay with transmission line). Different microstrip line (double sided PCB) or stripline (multilayer PCB) between each other, forming the coupled microstrip line, thus forming the four ports of all kinds of complex networks, which constitute the various characteristics of microwave circuit PCB.
Visible, the microstrip transmission line theory, microwave level is high frequency circuit PCB design basis.
For more than 800 MHZ RF - PCB design, PCB near the antenna network design, should be fully follow the microstrip theory (rather than merely to microstrip concept to improve performance of lumped parameter tool). The higher the frequency, the significant microstrip theory guiding significance is.
For lumped parameter and the distribution parameters of the circuit, while the lower the working frequency, the role of distribution parameter characteristics of the weaker, but it is always exists distribution parameters. Whether to consider the influence of the distribution parameters on the circuit characteristic, and no clear dividing line. So, the establishment of the concept of microstrip, for PCB design, digital circuit and the relative intermediate frequency circuit is also important.
The basis of the theory of microstrip concept and microwave and RF circuit and PCB design concept, is actually a application of the theory of microwave double transmission lines, for the RF - PCB wiring, each between adjacent signal lines (including adjacent) in different planes are formed to follow the dual characteristics of basic principles, to this, there will be clear in this paper, the subsequent).
Although usually microwave RF circuit are in the side of the configuration to the floor, make its microwave signal transmission lines tend to be complex of four port network, thereby directly coupled microstrip follow theory, but is still double its basic theory. So in the design practice, dual theory of guiding significance to the wider.
Generally for microwave circuit, the theory of microstrip quantitative guidance significance, belongs to the specific application of the dual theory, the theory of double has a wider range of qualitative guidance significance.
It is worth mentioning: dual theory of all the concepts, on the surface, seems to be no link some concepts and practical design work (especially the digital circuit and low frequency circuit), is an illusion. Dual theory can guide all the concepts of electronic circuit design problem, especially the meaning of the PCB circuit design concept is more outstanding.
Although dual theory was established under the precondition of high frequency microwave circuit, but it just because of the influence of the distribution parameters in high frequency circuit become significant, makes significance particularly outstanding. In digital or low frequency circuits, distributed parameter compared with lumped parameter components, to achieve negligible, dual theory become the corresponding fuzzy concept.
However, how to distinguish the high frequency and low frequency circuits, in the aspect of design practice, it is often easy to overlook. Usually the digital logic or pulse circuit what kind? The most obvious nonlinear components of low frequency circuit and the low frequency circuits, once some sensitive conditions change, it is easy to reflect some characteristics of high frequency. High-end CPU frequency has to 1.7 GHz, far more than microwave frequency limit, but still belongs to digital circuit. Because of these uncertainties, make the PCB design of important.
In many cases, a passive components in the circuit, can be equivalent to a particular specifications of the transmission line or a microstrip line, and use the double transmission line theory and its related parameters to describe.
In a word, can think of the double transmission line theory is based on the comprehensive all electronic circuit characteristics. In a strict sense, therefore, if every link of the actual design, first of all, based on principles of double transmission line theory, the concept of embodied, then the problems faced by the corresponding PCB circuit will rarely (no matter what the circuit is working under the conditions of application).
(b) of the double transmission line and the microstrip line structure
1, microwave dual form of PCB
Microstrip line is by application of the microwave double under certain conditions. Figure 1 - a. for microwave dual and its field distribution diagram. Work in microwave frequency of PCB substrate, can constitute a conventional parallel double line in different planes (as shown in figure 1 - b.) or the variation of different surface parallel double (as shown in figure 1 - c). When one of the strip line compared with another strip line can be equivalent to infinity, constitute a typical microstrip line (as shown in figure 1 - d.). From double transmission lines to microstrip, edge characteristics change, only qualitative characteristics are basically identical.
Note: in many microwave professional discourse, are simply described by conventional uniform cylindrical conductors of double transmission line, a two-line description of PCB circuit are regular rectangular strip line double transmission line.
2, double characteristics of the microstrip line
Figure 2 - a. for conventional microwave field distribution diagram of double line. Figure 2 - b. for PCB strip line field distribution diagram. Figure 2 - c. with limited ground plate
Microwave double field distribution schematic (note: one double line of figure and ground plate connected). Figure 2 - d for a relative infinity double field distribution of ground plate signal (note: One of the figure of double line and ground plate connected).
Figure 3 - a. for typical even inspire coupled microstrip line field distribution. FIG. 3 b. As the typical odd mode coupled microstrip line field distribution.
From figure 1, figure 2, figure 3 field distribution state, double line and microstrip line (including the coupled microstrip line) just for edge characteristics of different.
D. PCB electromagnetic wave transmission characteristics of parallel double
(a) distributed parameter concept and double transmission line
For lumped parameter circuit, with the improvement of working frequency, in the circuit inductance and capacitance will be reduced correspondingly, the oscillating circuit as shown in figure 4.
When the circuit inductance small to certain degree, will make the coil is equivalent to linear (FIG. 4 b.); When small capacitance to a certain extent, will be replaced by the distributed capacitance between conductor (FIG. 4 c.).
By the qualitative description to high frequency circuit design principles as follows:
When the working frequency is higher, will turn into distribution parameters of the concentrated parameter, and play a leading role. This is the main form of microwave circuit.
Along the wire in distributed parameter PCB circuit, everywhere distributed inductance, everywhere distributed capacitance between wires.
Components in high-frequency PCB circuit design, pay attention to the nominal value and the actual value of discrete difference is relative to the working frequency.
Strip double the figure shows that PCB is a simple form of distribution parameters of the circuit, in addition to the transmission of electromagnetic energy, can also be used as a resonance circuit.
(b) the PCB strip double distribution parameter of the equivalent way
Usually a double wire can be divided into many small pieces (for example, each section length 1 cm), then each pair of wires with the distributed inductance and capacitance is expressed as lumped parameter form, as shown in figure 5. B line in the figure, can be on the PCB surface in parallel with a linear or ground, also can be go of parallel lines in different planes, for ease of interpretation, this refers to two parallel lines in the air.
On the double transmission analysis, the dielectric loss often neglected (R1 < < L1, omega G1 < < omega C1), then the equivalent as shown in figure 5 "no consumption of transmission line" form (i.e., ignoring the electromagnetic wave attenuation). According to electromagnetic field theory, each 1 cm strip double transmission line inductance and capacitance respectively:
L1≈ (μ/π)ln(2D/d) (H)
C1≈πε/ln(2D/d) (F)
Type, μ = line between medium permeability (H/cm). When the medium is air, μ = μ0 = 4 x E - 5 (H/cm); ε= line between the dielectric constant. When the medium is air, ε = ε0 = 8.85 x E - 10; D = double line spacing; d = PCB line thickness or width (specific definition can be found in the subsequent instructions).
Integration of the design concept is as follows:
PCB, respectively approximate think d for copper sheet width (for inductance) or copper sheet thickness (for capacitor), premise is to pick up the floor with double. For different surface parallel double, D for PCB thickness, d for line width.
Work in high frequency condition more than two layers PCB design, not only to consider with surface line between the distribution parameters, also need to consider the distribution parameters of walk between lines in different planes, and more important (ground plate of the RF - PCB circuit is another way of analysis of jujube see later).
(c) the transmission characteristics of electromagnetic wave on the PCB strip double
PCB strips double equivalent circuit shown in figure 3, the dc power supply access, from left to right, is in turn to the adjacent capacitive charging voltage and current, and then to the spread of the process of secondary capacitor discharge form, called popular electric wave.
If the figure 6, the power supply for harmonic regular communication source, can infer that there will be a line voltage wave propagation from left to right. Along the voltage value is related to time position. This kind of voltage traveling wave, working wavelength and examined transmission line length can be compared, was more obvious.
Voltage and electric field, current and magnetic field, so along the electric field and magnetic field is the law of harmonic propagation along the transmission line. Above all, know that high frequency circuit of microwave PCB characteristics as follows:
When PCB line and working wavelength can compare to the voltage and current from one end to the other side of the form is not electromotive force under the action of current laws, but in the form of traveling wave transmission, but not into the surrounding radiation.
Traveling wave forms of energy, embodied in electromagnetic wave form, and in the conductor guided along the transmission. The higher the working frequency, electromagnetic wave form of energy, the more obvious, the general processing functions of lumped parameter device.
Must be clear: when the frequency is high enough, PCB line started from the classical ohms law, and in the form of "wave" or electromagnetic wave guide article reflects the functions in the circuit.
(d) the spread characteristic of the traveling wave
1. The incident wave and reflected wave
For the ideal of "no consumption of transmission line" (ignoring loss), under the simple harmonic function, instantaneous current on a transmission line can be launched PCB wave expression is:
i(t , z) = Acos(ωt-βz)-cos(ωt+βz)
Type, t = transmission moment; Z = positions on a transmission line (distance from starts); A and B = constant related to the excitation signal amplitude and terminal load (incident wave and reflected wave amplitude); ω = phase Angle; β = phase shift constant.The instantaneous electric current wave expression shows that under simple harmonic excitation, the PCB current on a transmission line into two harmonic current of algebra and Jane. Respectively to the type of two function diagram, clear: for the first current along the + Z direction over time (by the power supply to the load) current wave; The second is along the Z direction over time (from load to the power supply) of current wave transmission. The former is called incident wave current, the latter is called reflection wave current.
I.e., the steady state process, the PCB of current on a transmission line is the line to the superposition of two waves of opposite direction transmission on the results.
2. The relationship between constant introduction
α= attenuation constant. If consider PCB transmission line loss, α≠0.
β= phase shift constant. As the electromagnetic wave propagation along the PCB line per unit length of phase shift, with wavelength: β=2π/λ
Refer to figure 2, there is another relationship:
γ= propagation constant. PCB transmission line loss, the wave of phase shift constant and attenuation constant α β variables and, i.e., the γ=α+j
β
The Vp = "phase velocity", such as travelling wave phase velocity. Phase velocity and the relationship between β and ω: vp = ω/β. When the electromagnetic wave propagation direction is parallel to the Z direction, have the Vp = Vc (Vc says the speed of light). Can be introduced:
Is in the air medium
To sum up, can infer the high frequency circuit and PCB design principles are as follows:
Distributed parameter circuit not only reflected in the lumped parameter to the distribution parameter of the transformation, more important is the PCB circuit, signal processing and transmission are beginning to follow the intrinsic characteristics of electromagnetic wave. The higher the working frequency, this feature is more prominent.
Concept of reflected wave is fundamental concepts, enhance the efficiency of the circuit output power or otherwise will lead to inconsistent with the design of a series of problems.
Distribution parameter of the investigation, involving some of the basic physics of the electromagnetic wave theory definition, carefully grasp the physical definition and calculation way is reflected in the circuit, is one of the fundamental means of solving design.
Microwave, high frequency circuit PCB belt distribution parameters of line features, can be achieved by constant expressions as reflected in some relationship, PCB design and by the constant to achieve.
