Micro-Cap v7.1.6 / RM

Curve fitting with the optimizer

Load the circuit file OPT4. It looks like this:

Figure 28-11 The OPT4 circuit

Select AC analysis and press F2. Press F8. The standard run looks like this, after placing the cursors at 2MHz and 10MHz. Note the value of db(V(OUT)) is 1.397 at 2MHz and -9.583 at 10MHz.

Figure 28-12 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 28-13 The Optimizer dialog box for OPT4

These settings for the OPT4 circuit determine the value of R1, C1, and L1 that equate the six values of DB(V(OUT)) to specified values at different frequencies. To summarize, it finds the following:

The value of R1, C1, and L1 that:

Equates Y_Level(DB(V(OUT)),1,1,2e+006) to 2.188

Equates Y_Level(DB(V(OUT)),1,1,4e+006) to 10.449

Equates Y_Level(DB(V(OUT)),1,1,6e+006) to -1.696

Equates Y_Level(DB(V(OUT)),1,1,8e+006) to -9.103

Equates Y_Level(DB(V(OUT)),1,1,10e+006) to -13.939

Equates Y_Level(DB(V(OUT)),1,1,20e+006) to -27.134

In short, we are trying to match six data points, at frequencies ranging from 2 MHz to 20 MHz on the plot of db(v(out)).

In equate optimization, the optimizer actually minimizes the square root of the sum of the squares of the differences between the target and actual values. The Total Error field always shows this root-mean-square error measure.

668 Chapter 28: Optimizer

Click on the Optimize button. After a few seconds the optimizer finds the optimal values; R1= 255, L1=3u, and C1 = 500pF.

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 values. The display should look like this, after locating and tagging the 2MHz and 10MHz values.

Figure 28-14 The optimized plot

Checking two of the equate values, at 2Mhz and 10MHz, we find they equal the specified target values 2.188 and -13.939 respectively.

The optimizer was able to match these values exactly in this example. That is because the target values were taken originally from a circuit that was identical to the optimized circuit. We know that the target values are realizable because they came from an actual circuit. In cases like this it is sometimes possible for the optimizer to find scaled versions of the same circuit, since these have exactly the same circuit response. A scaled circuit is derived from a circuit by multiplying every resistance and inductance value by a scale factor and by dividing every capacitor value by the same factor. There are an infinite number of such scaled circuits. This is not usually a problem, only something to be aware of. You may get many optimal solutions to this kind of problem.

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*.JPG - Graphics file in JPG format. MC7 can read but not write JPG files

*.LBR - Micro-Cap binary library file *.LIB - SPICE library file

*.MAC- Micro-Cap macro file

*.MC7 - Micro-Cap usage statistics file

*.MC7 - Micro-Cap program file used for the demos or error messages *.MDLModel program data file

*.OPTResults file created when MODEL is run under batch mode *.NET - Orcad, Protel, PADS, or Accel netlist file

*.PKG - Package Library file

*.RES - Listing of resistor values used in the filter designer

*.SAN - Minimum, maximum, and default limits for model parameters

*.TMC - Transient Monte Carlo statistics *.TNO - Transient numeric output

*.WMF - Windows meta file (picture file)

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