How to design RF and digital-analog circuits on the same PCB?
The popularity of handheld wireless communication devices and remote control devices is driving a significant increase in demand for analog, digital, and RF hybrid designs. Handheld devices, base stations, remote controls, Bluetooth devices, computer wireless communication capabilities, numerous consumer appliances, and military/aerospace systems now require RF technology.
For years, RF design required professional designers to use specialized design and analysis tools. Typically, the RF portion of the PCB is designed by the RF professional in a separate environment and then merged with the rest of the hybrid technology PCB. This process is inefficient and, in order to integrate with hybrid technology, it often requires repeated design and multiple unrelated databases.
In the past, design functions were performed and repeated in two design environments and connected via a non-intelligent ASCII interface (Figure 1(a)). PCB system design and RF-specific design systems in both environments have their own libraries, RF design databases, and design archives. This requires design data (schematics and layouts) and libraries in both environments to be managed and synchronized via a cumbersome ASCII interface.
Under this old approach, RF designers isolated the rest of the PCB system design for RF circuit development. The RF circuit is then translated into the overall PCB design using an ASCII file to create a schematic and physical implementation on the main PCB. If there is a problem with the RF circuit, the design must be corrected in a separate RF solution and then re-translated into the main PCB.
The RF simulator only simulates the ideal RF circuit. There are many fragmented formations, ground-to-space and adjacent RF circuits in the actual hybrid system implementation, which makes analysis very difficult, and anyone knows that these additional shapes will have a lasting impact on RF circuit operation.
This old method has been successfully used in mixed-signal circuit board design for many years, but as the RF circuit content in the product increases, the problems caused by two independent design systems have begun to affect the designer's productivity, time-to-market and products. the quality of.
To solve these problems, Mentor Graphics has developed a dynamic linking technology that integrates PCB schematic and layout tools with RF design and simulation tools to create a new solution that overcomes the traditional Disadvantages of RF design.
RF aware PCB design
To maintain the design intent between the PCB and the RF design, the RF design tool must understand the layer-oriented structure of the PCB layout, and the PCB system must also understand the parametric planar microwave components used in the RF design environment.
Another key issue is that the PCB system builds the layout of the RF circuit into a short circuit, which prevents proper design rule checking (DRC) of the design. For today's complex RF system designs, functional RF-aware DRC is necessary to ensure that the design methodology is correct.
All of this helps to keep the design intent. Maintaining design intent is critical because it is the basis for implementing multiple round trips of design data between toolsets without losing information.
RF design is an iterative process that requires many steps to adjust and optimize the design. In the past, RF design was very difficult in the context of real PCB design. When implementing an optimized RF module on a PCB, there is still no guarantee that it will still work optimally. As a verification, electromagnetic field analysis (EM) is required for PCB implementation.
There are several problems with this design process. First, the circuit is simulated as a simple metal layer geometry, so the RF tool cannot modify the metal layer and cannot return the optimized results to the PCB design and still have a good RF circuit. Second, the EM solution is time consuming.
In the new process, because PCB tools and RF tools have a common understanding of design intent, circuits can be sent between tool sets without losing design intent. This means that the circuit simulation (very fast) and EM analysis (when needed) can be repeated and the results of each circuit modification can be compared. All this is done in a real PCB environment, including ground planes, layouts of RF circuits, wires, vias, and other components.
RF PCB design bottleneck
The RF PCB design bottlenecks are mainly the following. First, since each RF module on the PCB may have been designed by a separate RF design team, and each module can be independently upgraded, evolved, and reused, managing the entire circuit as a whole becomes It is essential, but these modules are still accessed as separate circuit components at all times. To solve this problem, the schematic and layout tools must be extended to support hierarchical grouping circuits. In this way, even if an RF circuit is already laid out on the PCB, it can be placed as an RF circuit with other modules and connected to the appropriate RF design team for analysis.
The next obstacle is how to design the ground plane. In the traditional design process, RF metal is used as a black box metal block, and the spacing from the ground is done manually, because the empty space passes through each formation. When the RF circuit is updated (this is a frequent operation), the cut portion must be manually modified to correspond to the new circuit. For some designs, this editing process alone can take several weeks.
New integrated design process
The integration between RF design tools and PCB design tools has been based on bidirectional conversion of ASCII IFF files. Although this format can handle some of the design data, it is far from seamless and repetitive. The lack of library synchronization is a fatal cause.
This design requirement has spawned a network-based tool communication that provides a dynamic two-way link between RF design and system-level PCB design (Figure 1(b)). To support parallel engineering processing, multiple PCB engineers can use the same design database at the same time, and each person can link one or more analog parts. RF design tools can now be used to design RF modules and integrate them into system-level schematics and PCBs at the right time, rather than just an elusive black box circuit as in the past. At this stage, you can upgrade your circuit and simulate its effects in either environment.
Think of each RF circuit as a set of objects to help maintain traceability, version management, and design issues. Because the design intent is preserved, any number of design iterations can be implemented without time cost. In addition, because the RF module can be simulated in a real system-level PCB environment, its functionality should be verified in more detail to help shorten the design cycle.
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