Genesys 2012 Tutorial - Using Momentum Analysis for Microwave Planar Circuits

Transcription

1 Genesys 2012 Tutorial - Using Momentum Analysis for Microwave Planar Circuits Create the following schematics in Figure 1 with Genesys s schematic editor, which depicts two sections of a cascaded microstrip line circuit. Add a substrate, called this Substrate1 and assign this to the microstrip SUBST property. The Substrate1 properties are shown in Figure 2, and are based on standard 62mils (1.57mm) thick FR4 substrate with 1 ounce copper layers on top and bottom sides. Since the widths of the microstrip trace sections are different, we need to set the impedance of Port_1 and Port_2 to reflect the characteristic impedance of the respective microstrip sections. The impedance can be easily obtained from standard microstrip design equations. Notice we name this schematic as Microstrip_Circuit. Port_1 ZO=73Ω Note the impedance, which correspond to the characteristic impedance of the microstrip sections TL1 W=1.4mm L=20mm SUBST='Substrate1 TL2 W=3.0mm L=20mm SUBST='Substrate1 Port_2 ZO=50Ω Figure 1 Microstrip step discontinuity schematic. Figure 2 Substrate1 properties and the Workspace Tree after creating the schematic. Now generate a layout corresponding to the schematic, there are a number of ways to do this, refer to Genesys tutorial videos, for instance by right-click on the status bar of the schematic window we can select Add Layout from the context menu. You can also invoke the command Connect Selected Parts under the Layout menu to automatically connect the components in the layout window. The layout window is shown in Figure 3A. 1

2 We can visualize our model as a 3D system, bounded by 6 faces. By default Genesys Momentum treats the side boundaries as unbounded continuation of the layer stack-up, with the top and bottom boundaries as air (see Figure 3A). We can change the top and bottom boundaries setting in the layer properties but not the sidewalls. These are treated as unbounded by default Figure 3A The layout generated from the schematic. TL1 Port_1 Step discontinuity TL2 Port_2 Figure 3B Zoomed view of the layout. 2

3 Click on the Layout Layout Properties command to invoke the LAYOUT Properties window (see Figure 4) Figure 4 The Layout Properties Tab: General. The via in Genesys layout will connect these metal layers. We are not using via so this is not important. Now we are going to edit the properties of each layer in the model to reflect the layer stacking and boundaries as shown in Figure 5. Extends to infinity Air TOP Conductor Model Domain Unbounded Planes Substrate1 Bottom Cover (PEC) Figure 5 The layer stacking from our microstrip circuit. In LAYOUT Properties window, first select the Layer tab, make sure to click the Show All checkbox. Set the property as shown in Figure 6A. Note that we are not using the BOTTOM layer, so it s Use property is unchecked. Slide to the right to see more properties, and again set the layer properties as shown in Figure 6B and Figure 6C. 3

4 Default is strip, use Slot when a port is a slot line, this affects the way line integration is performed to find voltage from E field. Check this to show all layer properties Top Cover and Bottom Cover will always be used. We will be using Air, TOP METAL, SUBSTRATE and Bottom Cover only. Figure 6A Setting layer properties part 1. Top Cover needs to be set to Open to indicate infinite air layer Height is not used if Top Cover is set to Open Bottom Cover set to Lossless to indicate it is PEC, and used as GND plane. Figure 6B Setting layer properties part 2. 4

5 Normal: Electric current flows in XY direction for thin metal. This is used with 3D EM solver. Port s potential reference refers to the nearest metal layer below TOP METAL Figure 6C Setting layer properties part 3. Now add a Momentum analysis. The settings of the Momentum analysis are shown in Figure 7A and Figure 7B. Figure 7A shows the General settings. Here we sweep from 100 MHz to 3000 MHz. Since EM analysis requires more memory and is computationally more complex than circuit analysis, the Momentum analysis engine only analysis 25 frequency points between 100 MHz and 3000 MHz. The frequency points are not uniform and are chosen using Adaptive Frequency Sweep method. Interpolation is used to estimate the results if we wish finer frequency coverage. We can disable this by unchecking the Use EM Simulation Frequencies checkbox as shown in Figure 7A, as we wish to compare the results point-by-point with circuit analysis. Note that in Figure 7B we leave the Use Box checkbox unchecked. If we set the Top and Bottom Covers in the Layer Properties as PEC or physical conductor, we need to check this checkbox. This will simulate the effect of having a shield on top or bottom of the model. 5

6 Settings for EM simulation sweep points. Check this if we need to use the sweep results from EM simulation, uncheck this if we want to make point-to-point comparison with simulated results from circuit simulation (like Linear Analysis) Figure 7A Settings for Momentum Analysis part 1. Use Box: Use the settings for Top and Bottom settings in Layer Properties as the top and bottom sidewalls. Since the Top Cover is set to Open, we will leave this unchecked to prevent error during computation. RF: Using first order approximation for the complex exponent function in the Green s Function. Use TL calculation: Instead of using the impedance Z o specific for Port_1 and Port_2 in the schematic, the Z o is computed based on the transmission line cross sections on the ports. Here it is not important as we have manually set Z o of the ports to correct values in the schematic. Via model: Lumped probably as an inductor. 1D probably use line current. 2D probably assume a few line currents in parallel. 3D current flow allows in 3 axes. Figure 7B Settings for Momentum Analysis part 2. 6

7 Figure 8 The workspace tree after adding a Momentum Analysis and running it. Figure 9 shows the mesh appears on the conducting layer after completing the Momentum Analysis. The S 21 and S 11 are plotted in Figure 10. We also perform a Linear Analysis using the schematic, and Figure 11 shows the comparison of the results between EM (e.g. Momentum) analysis and circuit theory analysis. Figure 9 The mesh on the microstrip line circuit. 7

8 Figure 10 The mesh Momentum Analysis results. Figure 11 Comparison between Momentum and Linear Analysis (in long dash). 8

9 Appendix Handling StripLine Circuits For strip line circuit, we require two conducting planes. This is illustrated in Figure A1, where we use TOP METAL and Bottom Cover as the conducting planes. The stripline is implemented in INNER METAL layer. Here we use the layout model without a corresponding schematics, since the vias interconnecting the TOP METAL and Bottom Cover are essentially a 3D structure with no equivalent schematics. There are now two substrate layers, and the layer stacking and properties are shown in Figure A2. Figure A1 The layout view of a simple strip line circuit with two ports. Use these to introduce new layer and arrange the stack-up Figure A2 The layer stacking and properties. 9

10 Furthermore since the via is suppose to connect the TOP METAL and Bottom Cover, the Default Viahole Layers are set as shown in Figure A3. Figure A3 Setting the connectivity of the model via. The Simulation Options tab in the Momentum Analysis Options is depicted in Figure A4. Take note that we check the Include TL Calculation checkbox to compute the characteristic impedance Z o of the stripline trace from EM fields of the trace cross section. The Momentum generated results are shown in Figure A5. 10

11 Take note! Figure A4 Setting the Momentum Analysis options. Resonance due to vias and planes Figure A5 The meshed layout and the Momentum produced S-parameters from 1-7 GHz. NOTE: If we did not check the the Include TL Calculation checkbox, we get the wrong S-parameters and even non-physical results, i.e. the S-matrix shows power gain for a passive circuit. 11