Verilog-AMS Tutorials using SMASH from CMOSedu.com (Return)  

Tutorial 3 – Performing AC simulations

We should preface this tutorial by stating that to perform an AC analysis in SMASH you must have a circuit that generates an AC signal. This is crucial since if there is no AC signal being generate SMASH will not even let you attempt to access the AC analysis function. To avoid any unnecessary frustration make sure you are using components that can and are generating AC values (0 does not count as an AC stimulus)

1. To test our AC simulation we first need to create a circuit that provides an AC stimulus. For this tutorial we will again use a RC circuit but this source will be a sinusoidal (SIN) voltage source. The first step is to create a circuit file. Let’s name this one basic_sim3. Once again we are using the same components as the previous tutorial so make sure you are adding the proper library (.LIB) component files. You can also type the .ELABORATE command if you have already decided on what the module name will be (this is a quick trick if you are constantly using the same file. It also bypasses the step of the software asking you to select a top level file.

2. Next we will build the RC circuit. To do this we need to create a top level module. Just like in tutorial 2, we can create a new module in a file that already contains others. Select FileàOpen and change the file type to Verilog Source files. Then locate the file basic_sim_top.va. If you wish to build a new file that is fine just refer to the image below for the proper format of the module.

3. The setup is nearly identical to tutorial 2 except we are using a sinusoidal source. The parameters needed for this source are the frequency, amplitude and AC signal. You will notice several of the parameters passed are 0. This is because parameters enter on a first come, first served basis. The first parameter passed automatically applies to the first parameter in the component module. In this case the AC component is the last parameter so we pass 0 to make sure we do not accidentally pass values to the other parameters (whose default values are 0).

4. Now go back to the circuit file. Make sure that the module has been set as the top level (.elaborate) and select AnalysisàSmall SignalàParameters

5. The new window that opens up is where you set the specifications for your AC simulation. Just like in the transient simulation you have the option to Run it immediately or Apply it so it generates the code equivalent in your circuit file. For this tutorial, let’s generate a logarithmic (decade) simulation that range from 1 to 10k with 100 points. Select Run

6. Just like in the transient simulation a window will appear prompting you for the desired signals to simulate. Remember for current select the nodes in the purple squares (expand them so see where current flows through) and for voltage select the signals on the bottom (has waveform next to its name). Once you have highlighted the desired signal press the Add button to add it to the current simulation. Select Close once you are finished.

7. By default the data shown will be in dB. However, it is possible to switch to degrees without running the simulation again. RIGHT-CLICK in the area under the signal and select View Complex As. A list will appear show the different magnitudes that you can view the data in. Using this tool you can view the signal in multiple formats.

8. Generally in AC simulations you want to plot vout/vin. In this tutorial our Vin = 1 so it was not necessary. However, future cases may not be the same. To plot Vout/vin you need to use the Trace Formula window. Select ProbesàTrace Formula. This will open a small window on the bottom of the screen. From here you can mathematically modify your signals and display them. To create vout/vin drag the vout symbol into the box, add a division symbol, and then drag the vin symbol into the box after it. To plot the vout/vin signal you must give it a name in the white box directly above it. To the right of the box is the type of analysis you are performing. Make sure it matches the type of analysis you just performed. If not change it (in the tutorial it should be small signal). Lastly to the left of the white box is a button that will plot your created formula. Press it and it will print to the screen.

IMPORTANT: SMASH will not simulate complex formulas. For you to plot vout/vin you must do it in terms of dB.  To accomplish this surround the equation  with the dB  conversion  as seen above.

9. One last useful trick for AC simulations. Since you may need to view the data in multiple formats you can create multiple simulation of the same signal and then edit each of them individually. For example, in this tutorial you may want to view magnitude and phase on the same window. To achieve this just drag and drop the signals on the left to the space under the current graph (below the purple line).

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