• Analog Custom Design & Analysis Examples

004_Chebyshev_LPF : Chebyshev Low Pass Filter schematic

Minimum Required Versions: Gateway 2.12.8.R, SmartSpice 3.16.11.R

Example 4 illustrates the AC magnitude response of the 2-pole Chebyshev Low Pass Filter schematic (see chebyshev_schematic.png ). The example uses the JFET-based LF356 operational amplifier. The Chebyshev magnitude response in db is shown from the waveforms screenshot ( magnitude.png ) to change as the poles of the filter are changed. There are several ways to do this in SPICE, but a common method is to use a form of parametric analysis. In this example, the .ST statement is used to modify the capacitance value of C1 which is increased from 0.8uF to 1.2uF in steps of 0.1uF. The five magnitude responses from these values can be seen in the plot.

The syntax of the .ST used to vary the pole values is: .ST LIN C1(CAP) 0.8u 1.2u 0.1u where the capacitance parameter (CAP) of C1 is swept from 0.8u to 1.2u.

The Chebyshev_LPF.ctr shows the full test bench for the example. Adding a second .ST statement to vary resistance will also affect the poles of the filter. For example, entering :

.ST LIST R1(RES) 800 1k 1.2k 1.5k into the control file yields the results seen in the parametric_waveforms.png chart in smartview.

Input Files
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