Multisim/Ultiboard for Low-Frequency Simulation and Layout
Introduction
RF and microwave circuit designers choose Microwave Office® software because of its complete range of powerful circuit design capabilities, from design capture, to harmonic balance and time-domain simulation, to layout, synthesis, and optimization, and even electromagnetic (EM) analysis and post-layout verification.
The Microwave Office design environment is well suited for higher-frequency designs, and, when combined with National Instrument’s Multisim circuit simulation software through AWR Connected™, low-frequency circuit functionalities are added for an even more complete circuit design flow (DC up through microwave/RF). Moreover, NI’s Ultiboard printed circuit board (PCB) layout software is capable of integrating high-frequency circuit layouts with Multisim, enabling designers to rapidly create a layout of a complete prototype, add mechanical and graphical shapes to the design, create 3D views of the final design, and export standard Gerber files, as well as other formats, for fabrication.
With the flexible routing capabilities of Ultiboard, all the particular details of the RF circuit affecting the frequency response are preserved (such as the transmission line bends, matching networks, and ground vias), while all the complementary low-frequency circuits are quickly routed around it.
Getting Started in Microwave Office
In the AWR Design Environment™, go to File>Open Example and select the LNA_10GHz.emp project. This project shows the design of a 10GHz low noise amplifier (LNA) built on 10-mil alumina using an NEC 76038 gallium arsenide (GaAs) metal semiconductor field effect transistor (MESFET). Run the analysis and it can be seen that the LNA has 7.7dB gain and 2.15dB NF at 10GHz.
Designing this amplifier in Microwave Office enables users to take advantage of the software’s real-time tuning and frequency subset Smith Chart plots, as well as AWR’s proprietary X-models to account for discontinuities, so that performance can be optimized and validated before committing to fabrication.
Next, let’s take DC biasing into consideration and turn to NI’s Multisim/Ultiboard software.
Create a Multisim Component for the LNA
DXF Export
To create a biasing circuit in Multisim, export the relevant DXF file from the Microwave Office environment by going to Layout>Export and saving the design file as a DXF file on your computer. Remember that the project units were in mils.
Now, go to Ultiboard and open a blank design. Go to Tools>Database>Database Manager and click on Create a New Part, then select New PCB Footprint in the pop-up dialog box.
Figure 1 shows the DXF import dialog. In your new footprint design file, go to File>Import>DXF and import the DXF file that you exported from the Microwave Office software. Make sure you only import the copper layer (merge it with the copper top layer) and ignore the rest of the layers. Remember to set the units to mils.
Select the imported shapes and right-click to go to their properties. Under the copper area tab, make sure you change the fill style to a solid fill style.
Adding Pins
You now need to create 10 pins to connect the LNA to its DC circuitry: one pin each for RF input, RF output, drain bias, and gate bias and six ground pins that connect the through vias to the ground. To add the pins, go to Place>Pins. The LNA layout after adding the vias, copper fill, and ports in Ultiboard is shown in Figure 2.
Adding the LNA to the Ultiboard Database
Once you have finished editing the layout, go to File> Save to Database as and save the new component to the user database. The final layout of the LNA component should look similar to that shown in Figure 3.
This LNA design can now be used in Ultiboard, where DC biasing and other low-frequency circuitry can be built around it. At this stage, where distributed high-frequency effects no longer need to be considered in the PCB layout, designers benefit from the easy-to-use PCB prototyping capabilities of Ultiboard.
Conclusion
NI Multisim/Ultiboard and AWR Microwave Office software are complementary and together offer capabilities that enable designers to create circuits spanning from DC to microwave frequencies. Complex tasks such as harmonic balance simulation and SPICE analysis can now be accomplished with a high degree of accuracy, and RF layouts can be readily combined with rapid prototyping tools inclusive of both low and high-frequency circuitry.
To evaluate NI Multisim circuit simulation and Ultiboard PCB layout software, visit www.ni.com/multisim/try
AWR would like to thank Mahmoud Wahby for his contributions to this application note.
Mahmoud Wahby, M.Sc Product Marketing Engineer National Instruments Toronto www.ni.com.
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