3rd Order Elliptic LP Filter

This example shows a study of a 3rd order elliptic low pass filter. The filter is derived from a passive RLC prototype circuit.

The run is controlled by a single CCICAP input control file, el032530.inp.

The input file uses .include records to incorporate different circuit realizations of the 3rd order filter. Each realization is described in a corresponding .ckt file

• RLC circuit description
• FDNR circuit description
• SSD circuit description

Note that the op-amp name defined in the active circuit descriptions has been parameterized as '[ oan ]'. The value of the parameter 'oan' (which is set in the .inp file) is substituted when the file is run.

The CCICAP input file controls the analysis of the three versions of the filter.

The CCICAP input file ties the circuits and models together. This file describes the analyses to be performed and the outputs to be produced. The file runs the following analyses:

• RLC circuit response overview
• Rerun RLC circuit with detail around the notch frequency
• FDNR circuit response overview (op27 model)
• Rerun FNDR circuit with detail around the notch frequency (op27 model)
• SSD circuit response overview (op27 model)
• Rerun SSD circuit with detail around the notch frequency (op27 model)
• FDNR circuit response overview (op37 model)
• Rerun FNDR circuit with detail around the notch frequency (op37 model)
• SSD circuit response overview (op37 model)
• Rerun SSD circuit with detail around the notch frequency (op37 model)

The analyis is performed using the command

$ ccicap el032530.inp

There are several output that CCICAP produces. There are messages delivered to STDIO that inform the user about the progress of the run. A text file, which defaults to the file 'CON' in the current directory, has more details about the run, listing the analysis responses, echoing the model parameters, and more.

The frequency domain results from the RLC passive circuit are shown here (both the overview and the detail around the notch frequency):

Using the FDNR based circuit with an op27 model gives:

It is clear that the op27 is not the best choice for this filter. A higher speed op-amp, the op37, improves the response noticably, especially around the notch frequency:

The SSD implementation, even using the op37, performs poorly. Here are the responses: