Reverse engineering of an existing circuit is a task that may be carried out for a number of reasons. One can learn a great deal about circuit design by looking at circuits that someone else (hopefully with experience) created. Sometimes schematics and specifications of a circuit get lost and need to be recreated. Sometimes it is necessary to replace a legacy circuit by a newer design. The goal of this lab is to analyze a passive filter design and to replace it with a cascade of first and second order active circuit modules.
The passive filter circuit to be analyzed in this lab is shown in the following figure.
The blue and grey plastic cubes are capacitors and the black cylinders are inductors. The little wire loops with black, red, white and yellow color codings are test points in the circuit. The filter is a 2-port that was designed to be used with a source resistor RS = 680 ohms and a load resistor RL = 680 ohms as shown in the following circuit diagram.
The system function of interest is H(s) = VO(s)/VS(s).
Note: The instructions for this lab are intentionally kept to a minimum to give you an opportunity to be creative and try out different ideas. You are encouraged to ask questions and ask for additional information and ideas for carrying out the reverse engineering process and the implementation of the filter using active first and second order building blocks.
E1. Initial Analysis of Circuit. Things you might want to do here are measuring the frequency response (magnitude and phase) of the passive filter circuit (with appropriate source and load resistors). You will probably also want to draw a schematic of the circuit and determine the type and order of the filter.
E2. Strategies to Determine Element Values. A good way to analyze the existing filter is to measure the values of the components from which the filter is made as accurately as possible without disassembling the circuit. Some ideas to consider are to use jumpers to bypass sections of the circuit and/or using different values for the source and load resistors. You can also inject a signal from the waveform generator anywhere in the circuit and measure the responses at some of the test points. If you think you have found the right component values, use PSpice to verify that the circuit with these component values performs the same way as the given filter circuit.
E3. Finding Poles and Zeros of Filter. Once you have found the correct schematic and element values of the passive filter circuit, you can compute the poles and zeros of the system function H(s) of the existing design. Consider using Matlab (to plot the magnitude and the phase of H(jw)) to verify that your pole and zero locations are correct.
E4. Active Circuit Implementation of Filter. The final step is to design a cascade of active (i.e., using OpAmps for isolation and/or transfer function implementation) first and second order modules that has the same overall system function as the original passive filter design. Implement this circuit, either using actual hardware and your breadboard, or using a PSpice simulation. In both cases verify that your design has the same input-output behavior as the original passive filter.
Lab worksheet in PDF format: lws06.pdf
Note: Each student needs to turn in a lab worksheet. The raw measured data for each student in a lab team will be the same, but the conclusions drawn from it are individual to each partner in the team.
©2003-2007, P. Mathys. Last revised: 4-24-07, PM.