Tuning and Optimizing Designs

This chapter provides information on tuning and optimizing designs using the Advanced Design System tuning and optimization capabilities.

Tuning Cadence Instance Parameters and Design Variables

The ADS tuning capability enables you to change one or more design parameter values and see its effect on the output without re-simulating the entire design again from the beginning. Dynamic Link extends the ADS tuning function to instance parameters and design variables in Cadence subcircuits. For more information on using the ADS tuning feature, refer to Tuning in the["../optstat/wwhelp.htm" Tuning, Optimization, and Statistical Design] documentation.

This section uses the PowerAmp example used in Getting Started Tutorial for demonstrating tuning of Cadence instance parameters and design variables.

A Dynamic Link For Cadence Tuning Example

1. Follow the steps listed in Performing an S-parameter Simulation in the Getting Started Tutorial. Plot S(2,1) in the ADS Data Display window after ADS S-parameter simulation is completed. The S(2,1) plot should resemble the following figure.
   
   

   Data Display with S(2,1) of the PowerAmp_test Design.

2. In the ADS Schematic window, choose Simulate > Tuning or click the Tune Parameters icon (tuning fork) in the toolbar.
3. Wait for the initial analysis to complete. The Tune Parameters dialog box appears as shown in the following figure.
   
   

   The Initial ADS Tune Parameters Dialog Box.

   Note that the prompt at the bottom of the Cadence schematic window says:

   ADS Tune Mode: Click an instance>

4. Move your cursor over resistor R7 in the Cadence schematic window and click the left mouse button (left click). A slider control for R7.R immediately appears in the ADS Tune Parameters dialog box. Note that only integer and floating point parameters are tunable. R7 contains only one tunable parameter R ( r in Cadence spectre), the resistance.
5. Left click resistor R0 in the Cadence schematic window. A menu pops up beneath R0 as illustrated in the following figure. The popup menu is displayed because there is more than one tunable parameter in R0 , R and Tnom . Cadence instance parameters are sent to ADS one at a time.
   
   

   Select Tunable Parameter Pop-up Menu in the Cadence Virtuoso Schematic Window.

6. Select examples_PowerAmp_schematic.R0.R(10) in the pop-up menu. This creates a slider control for R0.R in the ADS Tune Parameters dialog box.
7. In the ADS Schematic window, click the design variable Remitout in the VAR1 block.
   The following figure shows the ADS Tune Parameters dialog box with VAR1.Remitout , R7.R , and R0.R being selected. Recall that VAR1.Remitout is a design variable originated from the Cadence subcircuit.
   
   

   ADS Tune Parameters Dialog Box

   Notice the distinction that a Cadence design variable is selected directly from the VAR1 block in the ADS Schematic window, while an instance parameter is selected from the Cadence Schematic window.

8. In the ADS Tune Parameters dialog box, select a tune analysis mode from the Simulate drop-down list. This tells ADS when you want tuning to occur. For this example, choose After each change if not already selected.
   After you finish with all of the steps in this example, try each tuning analysis method ( After Each Change , After Pressing Tune , While Slider Moves ) to see which one works best for you.
9. Drag the slider for examples_PowerAmp_schematic.R7.R to 300 (Ohms). You also can change the tunable parameters by doing the following:
   • Click the up or down arrows.
   • Type the value directly into the box.
     Observe the results in Data Display window each time you release the mouse button after dragging a slider or changing a value in the Tune Parameters dialog box.
10. Drag the slider for examples_PowerAmp_schematic.R0.R to 5 (Ohms).
11. Finally, enter a value for PowerAmp_test.VAR1.Remitout of 120 (Ohms). The Tune Parameters dialog box should now appear similar to the following figure.
    
    

    ADS Tune Parameters Dialog Box (With VAR1.Remitout , R7.R , and R0.R values Changed).

    The next figure shows the resultant S(2,1) curve displayed in the Data Display window after changing VAR1.Remitout , R7.R , and R0.R in the Tune Parameters dialog box. You can compare this to the initial results in the figure Data Display with S(2,1) of the PowerAmp_test Design.

12. You can click the Reset Values button to restore all the sliders to their original values. The Update Schematic button in the Tune Parameters dialog box enables you to write the instance parameter values currently displayed in the dialog box into the Cadence Schematic window. For Cadence design variables, such as VAR1.remitout , you still need to select DynamicLink > Design Variables > Update Design Variables to Cellviews in the ADS schematic window and then follow the instruction in the Confirmation Message form to complete the update. Because there is no undo function for the Update operation, do not click the Update Schematic button if you do not want to change values in the Cadence subcircuit.
    
    

    Results of S(2,1)

    (as R7.R , R0.R and VAR1.Remitout are changed in the Tune Parameters dialog box)

13. Click the Close button in the Tune Parameters dialog box to end the tuning session. The prompt at the bottom of the Cadence Virtuoso schematic window returns to its default greater than sign ( > ).

    Note For more information on using the ADS tuning feature, refer to Tuning in Advanced Design System in Tuning, Optimization, and Statistical Design.

This example demonstrated three types of tuning operations in Dynamic Link:

• Clicking a Cadence instance with a single tunable parameter causes that parameter to be sent to ADS for tuning.
• Clicking a Cadence instance with multiple tunable parameters results in a pop-up menu being displayed. Selecting an item from the pop-up menu causes the parameter associated with that menu item to be sent to ADS for tuning.
• After obtaining Cadence design variables by selecting DynamicLink > Design Variables > Get Design Variables in the ADS schematic window, clicking a Cadence design variable in the ADS VAR1 block sends that variable for tuning.

All the above three types of operation act like a toggle switch. Selecting an item already on the Tune Parameters dialog box removes it from the dialog box.

You can descend down a Cadence design hierarchy to find an instance parameter for tuning. You can then return to higher level Cadence design hierarchy to select another instance parameter.

Note During the tuning operation, the left mouse button in the Cadence Virtuoso schematic window is mapped to a Dynamic Link SKILL procedure. Do not bind any function to the left mouse button during this period. Any bindkey function previously mapped to the left mouse button will not work until the tune mode ends.

If you have a problem while tuning and need to discontinue the operation, enter IdfMpsTuneEnd in the Cadence CIW input area. This will end the Dynamic Link Tune Mode operation.

Optimizing Designs

Performance optimization enables you to specify a range of device or component values. The software can then automatically compute the nominal values that best meet your performance goals or specifications. A family of optimizers come with ADS, each with a different mathematical effect or use. For more information on performance optimization, refer to Performing Nominal Optimization in the["../optstat/wwhelp.htm" Tuning, Optimization, and Statistical Design] documentation.
To optimize a design in the Advanced Design System:

1. In the Schematic window containing the design you want to optimize, choose Optim/Stat/Yield/DOE from the component palette. ** The Optim/Stat/Yield/DOE palette is displayed.
   
   

   The Optim/Stat/Yield/DOE Component Palette

2. Set the options ( Goal, Nominal Optimization, etc.) as desired.When a Nominal Optimization component is first added, you need to enable the output to the dataset. Click the Nominal Optimization component and choose Edit > Component > Edit Component Parameters . ** This brings up a Nominal Optimization dialog box. Select the Parameters tab and ensure that the Analysis outputs and Optimization variables are checked. Click OK .
   
3. Proceed with the Advanced Design System optimization.
   
   

   An Optimization Setup

   Statistical Variations

   Both the Monte Carlo and Yield simulation controllers include options for specifying statistical variations. Note that process and mismatch variations are only used for ADS RFIC Dynamic Link.
   
   

   Statistical Variation Options

   option	Description
   stat{...}	ADS syntax for statistical variations.
   process †	For process only analysis, select the process checkbox and unselect mismatch checkbox. For process and mismatch analysis, select both the process and mismatch checkboxes.
   mismatch †	For mismatch only analysis, unselect the process checkbox and select mismatch checkbox. For process and mismatch analysis, select both the process and mismatch checkboxes.
   † These options use Spectre syntax and are provided only for use with RFIC Dynamic Link. For more information on process and mismatch variations, refer to the section on Monte Carlo Analysis in Chapter 6 of the Cadence _Virtuoso Spectre Circuit Simulator User Guide, Product Version 5.1.41_ .

Updating the Cadence Cellview

Once the optimum value of a variable is computed by an ADS simulation, you can update the value to the Cadence cellview. To update the Cadence cellview:

1. In the Schematic window, choose DynamicLink > Design Variables > Update Design Variables to Cellviews .

   Note Only the nominal values of variables get updated to Cadence; any range values are ignored. Variables to be optimized should not be assigned a value in Cadence; they may be assigned a nominal value and a range in ADS.