RF and microwave PCB designs are becoming more and more complex, mainly due to the need for a higher level of circuit integration.
Typically, analog and digital circuits that coexist with conventional RF/microwave circuits on a pcb are not common.
This presents many major challenges, the most important of which is the design process approach.
Often, designers use many different CAD tools during the development phase of complex
RF pcb.
This can bring unforeseen issues into the design during integration, especially during PCBlayout.
We\'re going to demonstrate how to use simulation technology to extract high
Frequency PCB layout enable and simulate parasitic and virtual or software-
Based on the pre-manufacturing RF prototype.
We describe a real
World case study, the method was successfully used to detect and solve unexpected problems caused by layout parasitic in satellite TV tuner reference design.
In the ideal world, PCB design will be developed in the unifideda/CAD environment with all the necessary capabilities to allow designers from different disciplines-
RF/microwave, analog, digital, signal integrity, layout, mechanical, etc.
Designers will be able to develop designs effectively, validate design performance against requirements specifications, and generate detailed data packets to support the manufacturing process.
The first one produced
Time can be accelerated by designto-
Market and remove design-related fees
Rotation, both of which are important factors if the design is to be commercially successful.
Unfortunately, this utopia is not a reality today.
In the real world, it is not uncommon to find a complex design process to run, the bestin-
Class design and simulation tools to complement the best featuresin-
Enterprise PCB design process.
Unfortunately, design and simulation tools are often not integrated into the enterprise PCB process, which can lead to inefficiency and, more importantly, bugs can spread into the design.
This is especially true when RF/microwave content is incorporated into a larger PCB design for several reasons. The home-
The basic environment for many RF/microwave designers is dedicated RF/microwave design tools, not enterprise PCB flow tools.
Therefore, at some point in the cycle, the design created and simulated in the RF/microwave design tool must be redesigned
Created in the enterprise PCB flow tool.
In order to achieve this goal, a variety of different methods can be used, usually, trading
There must be a trade-off between the speed, ease, and accuracy of the design transfer between platforms.
See Figure 1 for PCB design process options. [
Figure 1 slightly]
At one end of the spectrum, manual redesign is possible
Input, component-by-component.
It\'s usually very simple, but it\'s time.
Consumption and Laborintensive.
In addition, the risk of introducing errors is high.
At the other end of the spectrum is an automatic translation of the schematic and/or layout design.
One of the main ways to pass design information between design frames is to use so-
Called intermediate file format (IFF)translation.
IFF offers the possibility of bi
Direction pattern and layout conversion between design frames.
While this greatly simplifies the process of migrating a design from one design frame to another, it comes at a cost.
Both design frameworks must contain a compatible library of components before performing IFF translations.
This is not always the case, of course.
Therefore,
In order to make the library compatible, it is necessary to invest in advance and then in-
Maintenance is required to keep the library synchronized.
A compromise option would be to use any of multiple industry standard formats such as Gerber or GDSII to incorporate RF/microwave layout details into enterprise PCB tools (Calma).
However, even if the thermal/microwave design is redesigned
Created perfectly in enterprise PCB tools, when it is integrated with other analog/digital circuits into PCBlayout, there is still the possibility of unforeseen problems due to layout parasitic.
To understand why layout parasitic causes such problems, it is necessary to have an understanding of the nature of RF/microwave signals and the design techniques used by RF/microwave designers.
The first and most important factor is that in design, RF/microwave circuit designers often use a combination of discrete and distributed components as components.
This immediately causes confusion as distributed components are basically just a piece of metal printed on the board and are not considered components;
It will not appear on the material list.
An example of a distributed element is a micro-strip transmission line that can be used to distribute RF/microwave signals around the PCB.
The line has an inductor, which is associated with the current and capacitance flowing along the line, because the line is separated by the dielectric substrate from the ground.
Classic L-
C. Model account for distributed terms.
In multi-layer pcb, these distributed elements are implemented as stri lines or micro-strip structures on the board.
Key electrical properties of these structures, such as characteristic impedance and electrical length, are determined by the physical width and length of theline and the thickness and dielectric constant of the substrate material.
In the RF/Microwave Circuit design process, the distributed components are carefully specified, usually closed-
Form an analytical model.
Or, electricity (EM)
Simulations can be used to create more detailed S-
Parameter model.
Obviously, it is important to assign the correct dimensions to distributed components when creating a physical layout.
This is an obvious area where errors may be introduced if the manual design is redesigned
The microwave design is integrated into the enterprise PCB tools by means of entry.
Of course, the RF/microwave signal is electric (EM)in nature.
Therefore, a strip line is surrounded by a magnetic field.
In the case of a micro-strip line, the EM field exists both in the air above the micro-strip line and in the dielectric strategy below the micro-strip line.
In some cases, field coupling between adjacent micro-lines may be an ideal and intentional phenomenon, such as edge-coupled micro-strip filters, in some specific designs.
However, unnecessary coupling between the strip line and the surrounding circuit can occasionally lead to unexpected problems.
When RF/microwave designs are integrated with other circuit blocks, this unwanted coupling is another source of frequently occurring problems (analog/digital).
In the past, in order to avoid unnecessary coupling, rough rules of thumb were often used to define
Walk around the line of the micro-strip/stri line.
However, this is no longer realistic for today\'s complex situation.
Multi-layer pcb signal.
There is a process that can help identify two errors introduced by transferring design data between tools or due to unexpected proximity coupling effects.
This method accurately predicts the unwanted coupling in PCB design using the electro-magnetic simulation technology and verifies whether the RF/microwave layout is appropriate.
The starting point of this exercise is the complete PCB design that can be obtained from the enterprise PCB tool source.
In this process, the ADS and momentum simulators of Agilent EEsof are used to extract the layout parasitic. Circuit/EMco-
The performance of the filled PCB is verified by simulation.
The basic steps for creating a virtual PCB are shown in figure 2.
It starts with exporting the PCB layout from the Enterprise Dynamics design tool.
Since most EDA tools support the Gerber file format extensively, use it.
Until recently, if an EM simulation is to be performed, the Gerber data is not an ideal starting point for layout generation.
This is only because Gerber intends to drive photoplotter, so many layout artifacts will be generated that need to be in the EM-simulation.
Figure 3a illustrates this.
It shows a typical layout (
Center Line View)
It consists of many overlapping Strobos and flashes.
The center line indicates the path of high exposure of the light source of the shadow painting machinecontrast film.
To solve this problem, the Gerberfile was imported into the layout using the GBRUnion tool.
GBRUnion also simplifies data by performing a joint operation to add profiles to the layout geometry (FIGURE 3b)
, Which in turn simplifies the grid created by the simulator. [FIGURES 2-3 OMITTED]
Figure 4 shows the PCB layout details of the import.
PCB is a relatively simple FR-5 board. It is 1.
The top and bottom of the plate are 55mm thick and bas metal.
Also use the hole connection through the circuit board.
Some additional settings are required before running the EM simulation on the imported layout: * define the substrate (
Dielectric layer: thickness, dielectric constant, tangent of dielectric loss).
* Map the layout layer into the baseboard definition.
* Add ports to the layout. [
Figure 4 slightly]
In this case, in addition to adding ports at the signal RF input and RFoutput locations, additional ports are added at all locations that Weld discrete components to the board during assemblyFIGURE 5).
This enables discrete components to collaborate
Simulation with layout parasitic extracted from EM.
Depending on the size and complexity of the PCB and the amount of computer memory available, it may be necessary or wise to reduce the PCB layout before running the EM simulation. [
Figure 5 Slightly]
In this particular example, by removing some non-
RF signal tracking. Any non-
Leave the RF track running near the trace of the theRF signal in order to consider the coupling effect.
Simulations were performed to characterize the layout in the DC to 5 GHz frequency range.
This requires more than 20 minutes of simulation time on a PC of normal specifications (1. 6 GB processor).
The output of the simulation is an S-parameter file--
In this case, a47-port S-parameter file. The S-
Parameter data can be easily used in any frequency domain circuit simulation tool.
In this case, ADSS-
The parameter simulator is used.
After EM simulation, the generated model can be implemented as layout appearance-
Similar components in the advertising environment.
This greatly simplifies the schematic design (FIGURE 6).
The initial simulation was run on the design to characterize the RF signal path from the RFinput connector at the edge of the PCB to the RF input pad encapsulated by the RFIC.
Short-circuit links are used to close the gaps left in tracesintended for discrete elements (
DC blocking capacitors, etc). [
Figure 6 slightly]
This simulation of the bare plate highlights resonance in the design band of about 1. 72 GHz (FIGURE 7a).
The next step is to optimize the PCB by adding all the discrete elements used in the PCBworld design (FIGURE 7b).
The simulation of the filled PCB shows approximately a second resonance. 1. 94 GHz. (FIGURE 8).
At this point, further simulations are carried out to debug the problem, just like solving the problem
The hardware was bugged.
Useful simulation tools like real-
Time tuning can adjust the value of the discrete element and its observed effect, or discrete elements can be extracted from the virtual PCB.
1 was determined by this method.
The DC bias feed network caused a resonance of 94 GHz, and a careful inspection of the layout found that the RF signal path through the PCB was connected to an isolated metal padd that resonated near 1. 72 GHz. [FIGURES 7-8 OMITTED]
With this information, a second virtual iteration of PCBlayout is generated and the verification process is repeated to confirm that the problem has been resolved (FIGURE 9).
The verification process described here can be used to help identify and debug problems in a new PCB design with some RF/microwave content.
This process is not limited to RF/
microwave pcb.
Similar processes can also be used to investigate high-
High Speed Digital circuit board design applications. [
Figure 9 omittedREFERENCES T. C.
Edwards, \"Fundamentals of the design of micro-strip circuits\" Willie, New York, 1981. L. G.
Maloratsky, \"reviewing the basics of micro-stripes\" microwave and radio frequency, March 2000. S.
DiBartolomeo Application Guide: import Gerber into ADSMomentum using GBRUnion works conversion software company.
David morris is a technical sales consultant at Agilent EEsof.
Eight years as a microwave circuit/subsystem designer and four years as a product marketing/sales role.
He can be at David Morris in Agilent. com.
