18-723 RFIC Design and Implementation

Tutorial on Cadence Design Tools

Ruifeng Sun

In 18-723, we will use Cadence tools to do transistor-level circuit design and simulation. Cadence provides a powerful integrated working environment for analog/digital/mixed-signal integrated circuit design, simulation, layout and verification. For more information, please refer to its official website: http://www.cadence.com. For the purpose of this course, we will only focus on those tools relevant to analog and RF integrated circuit design.

This tutorial will cover most of the features that is sufficient for you to finish 18-723 homework assignments and design projects. It is divided into two parts:

· Part I: Setting up your environment in ECE machine. Please carefully follow this section to start Cadence correctly, since various versions of Cadence currently co-exist in ECE machines.

· Part II: We will go through an example to simulate the noise figure of a passive bridge-T attenuator, which is actually a question in homework #1. We will also talk about how to get more help from Cadence

Even though you are already familiar with Cadence to do RF IC design and simulation, you will still need to read Part I.

Part I. Environment Setup

1. First of all, you need to have an ECE account. If this is not true, please contact ECE computing service: gripe@ece.cmu.edu.

2. If you are using a Windows workstation, you need to make sure you have an X-window server ready (most likely, the software is X-win32); and after logging in to an ECE server, you should setup a display environment variable with this command:

>setenv DISPLAY the name of the machine you are using:0.0

3. Create a directory where you want to save your work, for example, ece723:

>mkdir ece723

4. Enter the directory you just created:

>cd ece723

5. Run this command:

>/afs/ece/class/ece723/cadence/cds723

6. Wait for a while, and the CIW window of Cadence should pop up. If not, please check if you have followed the above procedures.

7. Next time to start Cadence, enter the directory where you saved your old designs (i.e., ece723 in above example), and type and run this command:

>./cds723

You can find all devices (transistors, resistors, capacitors and inductors etc.) needed in a standard device library called “analogLib”, which is provided by Cadence. For transistors, you will need device models. The models we are going to use in this class come from TSMC-0.18μm CMOS process. (Under the Non-Disclosure Agreement, you are not permitted to use this file outside of CMU.) The model file is “tsmc018.scs”, which should already be copied to your working directory (i.e., ece723 in above example) if you follow above procedures correctly. It contains several sections to account for different process corners. We will use the typical process parameters, so you need to specify section “tt” when you set up model libraries for the simulation later on in Cadence.

Part II. An Example

In this section, we will find the noise figure of a bridge-T passive attenuator, and briefly introduce how to get help from Cadence documentations.

1. As Cadence is running, in the CIW (Command Interpreter Window), click “File > Open…” in the menu bar.

Figure 1. Cadence CIW window

2. The “Open File” window will pop up. Select “Library Name” as “test”, and choose “bridge-T” as “Cell Name” from the list at the right side. Make sure “View Name” is “schematic”. Click “OK”.

Figure 2. Open File Window from CIW

3. The “Virtuoso Schematic Editing” window will appear, and you will see a bridge-T passive attenuator, which simply consists of three resistors. There are also two additional instances “PORT0” and “PORT1”.

Figure 3. Virtuoso Schematic Editing Window

4. Here are some basic schematic editing shortcuts you should know: To select an instance, simply point and click on it. To insert a new instance, press “i” and you will see a new window named “Add Instance” (Figure 4). Fill in proper “Library”, “Cell” and “View” names, and click on the schematic window, you finish to insert a new instance, and you can press “Esc” to close “Add Instance” window. To delete the instance, first select it and then press “d”. To change the properties of the instance, first select it and then press “q”; an “Edit Object Properties” window (Figure 5) will open, and you can modify the properties of the instance. To copy an instance, first select it, press “c”, and then click in a blank area in the window. To rotate an instance, first press “r” and then click it. To flip an instance, first select it, press “m”, and then press “R”. To wire the devices together, press “w” and click the two points you want to connect. To undo what you have done, press “u”. All of these commands can be found in the “Edit” menu. These shortcuts are summarized in Table 1.

Figure 4. Add Instance Window

Table 1. Commonly-used Shortcuts for Schematic Editing

Shortcut (keyboard)	Action
i	insert new instance
d	delete instance
q	modify the properties of an instance
c	copy an instance
r	rotate an instance
m + R	flip an instance
w	connect two nodes
u	undo

Figure 5. Edit Object Properties Window from Virtuoso

5. Let’s go back to the “Virtuoso Schematic Editing” window. In the menu bar, click “Tools > Analog Environment”, and a new window called “Cadence Analog Design Environment” will appear.

Figure 6. Cadence Analog Design Environment Window

6. In the menu bar, click “Setup > Model Libraries …”. Another window called “Model Library Setup” pops up. Click “browse” button, and you will see a new window similar to a file browser. Choose “tsmc018.scs”, and click “OK”. The browser closes and we return to the “Model Library Setup” window. Don’t forget to fill “tt” in the “Section (opt.)” field! Click “Add”, and then click “OK”. The “Model Library Setup” window closes and we return to “Cadence Analog Design Environment” window. By the way, we actually don’t need any device model for this example, since we only use ideal resistors and ports. However, you certainly will need this step in most of your home assignments and design projects.

Figure 7. Model Library Setup Window

7. Now we are in “Cadence Analog Design Environment” window. Click “Analyses > Choose …” in the menu bar, and “Choosing Analyses” window will appear. Check “sp” in the “Analysis” list. You will see the content of the lower half window changes as you choose different analyses. Now, we are going to setup the parameters for an “S-Parameter Analysis”. Click “Select” button in the “Ports” field, and then click on instances “PORT0” and “PORT1” in the “Virtuoso Schematic Editing” window. Press “Esc” before you return to “Choosing Analyses” window. You now should see the “Ports” has been filled with “/Port0 /Port1” as shown in Figure 8. Select “Sweep Variable” as “Frequency”. Select “Sweep Range” as “Start-Stop”, and fill “Start” and “Stop” with “1k” and “1G”, respectively. This means we will sweep the frequency range from 1-kHz to 1-GHz. Choose “Sweep Type” as “Logarithmic” from the drop-list, check “Points Per Decade”, and fill the blank field with “100”. Check “yes” for “Do Noise”. Select “Output port” by first clicking “Select” button and then in the schematic window clicking “PORT1”. Do the same to select “Input port” as “PORT0”. The finished “Choosing Analysis” window will look as that shown in Figure 8. Finally, click “OK”.

Figure 8. Choosing Analysis Window

8. Now, we return to “Cadence Analog Design Environment” window. At the right side of the window, you will find a column of buttons. Click the one like a traffic signal with green light. The function of the button will show up as you place your mouse on it. So, as you can see, we are going to “Netlist and Run” the simulation. Wait for a second, Cadence will first generate the netlist of the circuit and then run the simulation. As the simulation is running, a new window (Figure 9) pops up and let you know the progress of the simulation. It won’t take a long time to finish, and you can look for such a sentence in the CIW window:

“simulation completed successfully.”

Figure 9. Output Log Window

9. Going to “Cadence Analog Design Environment” window, in the menu bar, choose “Results > Direct Plot > Main From …”. A “Direct Plot Form” window will pop up. Check “NF” from the list of the “Function”. Check “dB10” as the “Modifier”. Click “Plot” button. A new “Waveform” window will appear, and the noise figure of the bridge-T attenuator against the frequency will be plot in this window.

Figure 10. Direct Plot Form Window

Now, we finish our simple example. In fact, all of above procedures can be found in Cadence help documentation. There are two ways to open the help documentation: (1) type and run “cdsdoc &” in your command line; (2) in the CIW window, click “help > Cadence documentation”. In either way, after waiting for a while, you will see a new window. Choose “Docs by Family” in the second pop-up menu. Double click on “Analog & Mixed-Signal Design”. Find and double click on “Spectre RF User Guide”. Double click on “Table of Contents” and a web-browser will open. Look for the chapter of “Simulating Low noise amplifier”, and look at the section of “Linear Two-Port Noise Analysis with S-Parameters”. You will find what we just discussed to do a noise-figure analysis. “Spectre RF User Guide” contains all aspects of RF IC simulation, and the best of all, it has step-by-step examples to help you to simulate LNA, Mixed, VCO etc. You are suggested to spend some time on it to get familiar with those topics.

Good luck!