PCB News - Microwave circuit and its PCB design

About CAD-aided design software and network analyzer

For high-frequency circuit design, there are already good CAD softwares. Its powerful functions are enough to overcome people’s shortcomings in design experience and tedious parameter retrieval and calculation. Coupled with powerful network analyzers, it should be Those with a little experience can complete the RF components with better quality. However, this is not the case in practice.

CAD design software relies on powerful library functions, including component parameters and basic performance indicators provided by most radio device manufacturers in the world. Many RF engineers mistakenly believe that as long as the software is used for design, there will be no major problems. But the actual results are always contrary to expectations. The reason is that they gave up the flexible application of basic concepts of high-frequency circuit design and accumulated experience in the application of basic design principles under the wrong understanding. As a result, they often made basic application errors in the application of software tools.

RF circuit design CAD software is a transparent visualization software, using its various high-frequency basic configuration model libraries to complete the simulation of the actual circuit working state. At this point, we can already understand the key link among them. There are two types of high-frequency basic configuration models, one is the component model of intensive parameter form, and the other is the partial function model in conventional design. So there are the following problems:

(1) The component model and CAD software have been interactively developed for a long time, and are becoming more and more perfect. In practice, you can basically believe the trueness of the model. However, the application environment considered in the component model (especially the electrical environment of the component application) are all typical values. In most cases, it is necessary to use experience to determine the series of application parameters, otherwise the actual results are sometimes even farther away than the design results without the aid of CAD software.

(2) The conventional high-frequency basic configuration model established in CAD software is usually limited to the predictable aspects under the current application conditions, and can only be limited to the basic functional model (otherwise, product development does not need to employ personnel, and it is only based on CAD to do it all. Products).

(3) It is particularly noteworthy that: the establishment of a typical functional model is completed by applying components in a typical way and using a typical and complete process method (including PCB construction), and its performance has also reached a high level of "typical" .

But in reality, it is completely imitated, which is far from the model state. The reason is: Although the selected components and their parameters are consistent, their combined electrical environment cannot be consistent. In low-frequency circuits or digital circuits, this small difference is not a big obstacle, but in radio frequency circuits, fatal errors often occur.

There are two types of network analyzers: scalar and vector, which are indispensable instruments for RF circuit design. The usual approach is to combine the basic RF circuit design concepts and principles to complete the circuit and PCB design (or use CAD software to complete), complete the PCB sample processing and assemble the prototype as required, and then use the network analyzer to design each link one by one. Only by network analysis can the circuit reach its best condition.

But the price of this work is the actual production of at least 3 to 5 versions of the PCB. If there is no basic PCB design principles and basic concepts, more PCB versions are required (or the design cannot be completed).

can be seen from the above:

(1) In the process of using a network analyzer to analyze radio frequency circuits, you must have a complete high-frequency circuit PCB design concept and principle, and you must be able to clearly know the design defects of the PCB through the analysis results. Only this item requires relevant engineers. Have considerable experience.

(2) In the process of analyzing the prototype network link, we must rely on proficient experimental experience and skills to construct a local functional network. Because many times, the circuit defects found by the network analyzer will have many causes at the same time, so it is necessary to use the construction of a local functional network to analyze and thoroughly investigate the causes.

This kind of experimental circuit construction must rely on clear high-frequency circuit design experience and skilled circuit PCB construction principles.