The Optimize dialog box
Run transient analysis and select Transient / Optimize or press CTRL + F11. This loads the Optimize dialog box, which looks like this.
|
Figure 28-2 The Optimizer dialog box |
The syntax of the optimization process is: |
Find |
{Parameter value} |
That |
{Maximizes, minimizes, equates} {Performance function} To {Value} |
Using |
{Standard Powell or Stepping Powell Method} |
While |
{Boolean Constraints} |
The user supplies the items in braces {}.
The dialog box offers these options:
Find:
Parameter: This is where you select the parameter to be optimized. Click on the Get button to select a parameter. The choice of parameters is the same as in the Stepping dialog box.
Low: This is the lower limit on the parameter value.
High: This is the upper limit on the parameter value.
Step: This is the amount by which the Stepping Powell method will step the parameter value between local optimizations.
Current: The current value of the parameter is displayed here during the optimization process.
Optimized: The most optimal value of the parameter found so far is displayed here during the optimization process.
That:
Maximize, Minimize, Equate list box: This is where you select one or more optimizing criteria. You can maximize or minimize the chosen performance function. You can also match a curve by using the equate option together with the Y-Level performance function.
-: This removes the optimizing criterion for the current row.
+: This adds a new optimizing criterion at the end of the list. You can have multiple maximize or minimize criteria, or multiple equate criteria, but you cannot mix maximize / minimize criteria with equate criteria. Each criterion carries equal weight.
Get: This selects the performance function and its parameters.
To: The optimizer will try to match the selected performance function to this value if equate is selected. If Y_Level is used, the optimizer tries to match the Y expression value to the value in this field.
Current: The current value of the performance function is displayed here during the optimization process.
Optimized: The most optimal value of the performance function found so far is displayed here during the optimization process.
Error: This shows a measure of the error for the row criteria.
Method:
Standard Powell: This is the standard Powell direction-set optimizer.
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Stepping Powell: This method steps the parameters from Low to High using steps of Step. At each step, the standard Powell method is used to find the local optimum. If the local optimum is better than the current best value, it is retained, and the next step is taken. This is the method to use where you want to get the best of many local optima. It is also the slowest since the optimizer does N standard Powell optimizations, where
N =number of steps for parameter 1 * number of steps for parameter 2 *
...
number of steps for last parameter
N can be very large so use this method judiciously.
Total Error:
This shows the sum of all criteria errors. In equate optimization, it shows the square root of the sum of the squares of the differences between the target and actual values.
Constraints:
There are four fields for constraints. Each constraint is entered as a Boolean statement. Here are some examples.
PD(R1)<=100m
V(OUT)>=1.2
VCE(Q1)*IC(Q1)<=200m
The buttons at the bottom of the dialog box give these options:
Optimize:
Starts the optimizer.
Stop:
Stops the optimizer.
Apply:
Modifies the circuit by changing its parameters to the optimized values.
Format:
Lets you choose the numeric format of the displayed values.
Close:
Quits the optimizer.
The settings for the OPT1 circuit determine the value of the resistor R2 that maximizes the power dissipated in R2. It finds the value of R2 that maximizes
Y_Level(PD(R2),1,1,0)
PD(R2) is the power dissipated in resistor R2.
Y_Level(PD(R2),1,1,0) is the Y expression value of the curve PD(R2) at the X expression (T) value of 0, which is the DC operating point value.
Click on the Optimize button. After a few seconds the optimizer finishes and presents the optimal value of R2 = 732.13 ohms.
We could have guessed this value from the simplicity of the circuit. Maximum real power is delivered when the load impedance equals the conjugate of the source impedance. In this case maximum power is delivered when R1 and R2 are the same value, 732.13 ohms.
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Optimizing low frequency gain
Load the circuit file OPT2. It looks like this:
Figure 28-3 The OPT2 circuit
Select AC analysis and press F2. Press F8. Note that the gain at F=10KHz is about 55.6 dB. The standard run looks like this:
Initial gain at 10KHz is about 55.6 dB.
Figure 28-4 AC analysis before optimization
Select AC/ Optimize or press CTRL + F11. This loads the Optimize dialog box for AC analysis. It looks like this:
Figure 28-5 The Optimizer dialog box for OPT2
These settings for the OPT2 circuit determine the value of the model parameter R in the resistor model RMOD that maximizes the 10KHz gain of the circuit. RMOD is the model used by the two 5K load resistors R2 and R5. The optimization choices find the following:
The value of RES RMOD(R) that maximizes
Y_Level(db(V(OUTA)),1,1,1e+004)
db(V(OUTA)) is the dB value of the output voltage of the diffamp circuit.
Y_Level(db(V(OUTA)),1,1,1e+004) is the value of the curve db(V(OUTA)) at the X expression (F) value of 1E4, which is the lowest frequency in the AC analysis run.
Click on the Optimize button. After a few seconds the optimizer finds the optimal value of R = 3.305. Since R multiplies the nominal resistance, this means that the value of the load resistors R2 and R5 that maximizes db(V(OUTA)) is:
3.305*5k = 17K
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We used Standard Powell here. Most circuit optimizations problems have simple local minima like this and are best served with this method.
Click on the Apply and Close buttons, then press F2. When the run is over, press F8. The Apply and Close buttons copy the optimized values to the circuit and F2 produces a new run with the optimized R model parameter. F8 puts us in Cursor mode so we can readily read the new values. The display should look like this:
Optimized10kHzgain is about 65.9 dB.
Figure 28-6 AC analysis before optimization
The gain at F=1E4 is now about 65.9 dB.
When the Apply button is clicked the old model name is replaced by a new one with the newly optimized parameter value. The old model statement with the old name is left unchanged in the text area of the circuit. All parts that used the old model name are changed to use the new model name and thus the new optimized parameter value.
Optimizing matching networks
Load the circuit file OPT3. It looks like this:
Figure 28-7 The OPT3 circuit
Select AC analysis and press F2. Press F8. Note that the RL power, V(RL)*I(RL) peaks at about F=1.3GHz. The standard run looks like this:
RL power peaks at about 1.3GHz.
Figure 28-8 AC analysis before optimization
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Select AC/ Optimize or press CTRL + F11. This loads the Optimize dialog box for AC analysis. It looks like this:
Figure 28-9 The Optimizer dialog box for OPT3
These settings find the values of the C3 and C4 network-matching capacitors that maximize the AC power, V(RL)*I(RL), delivered to the load, RL at 4GHz. To summarize, the optimizer finds the following:
The value of C3 and C4 that maximizes
Y_Level(V(RL)*I(RL),1,1,4e+009)
V(RL)*I(RL) is the AC power delivered to the load RL.
Y_Level(V(RL)*I(RL),1,1,4e+009) is the value of the curve V(RL)*I(RL) at the X expression (F) value of 4GHxz, which is the frequency at which we want the matching network to deliver peak power.
Click on the Optimize button. After a few seconds the optimizer finds the optimal value of C3 = 3.3pF and C4 = 1.894pF. These values deliver about 5mW to the load at 4GHz.
Click on the Apply and Close buttons, then press F2. When the run is over, press F8. The Apply and Close buttons copy the optimized values of C3 and C4 to the circuit and F2 produces a new run with the optimized values. The display should look like this, after locating and tagging the peak.
RL power peaks at 5mw at 4GHz.
Figure 28-10 AC analysis after optimization
Note that the Smith chart is plotting 2*VIN-1 which, for this circuit, is equivalent to plotting the scattering parameter, S11. Note also that S11 goes through the Smith chart origin (1,0) at 4 GHz, another indication that the matching capacitor network has been optimized to deliver maximum AC power to the load.
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