Assignment 6, Part 3: Identifying unknown filter circuits

Identify unknown filter circuits

In this part of the lab, you will measure the transfer function of four filter circuits made out of resistors and capacitors. The circuits will be hidden inside blue boxes marked A through D. Your goal is to figure out what type of circuit is inside of each "black box."

All of the boxes have two BNC connectors. You will use a function generator to drive sine waves of various frequencies into one connector and an oscilloscope to measure the resulting output on the other connector. Input and output are marked on the box.

Measure each box at each decade of frequency between 10 Hz and 1 MHz. You may want to measure choose additional frequencies in transition regions. The transfer function consists of two plots: one that shows the magnitude of the output divided by the magnitude of the input as a function of frequency; and another that shows the phase difference between the input and the output. The former is the Bode gain plot like that for your RC filter. The latter is the Bode phase plot. From those plots, you can derive the topology of the circuits inside and determine the cutoff frequencies of each filter.

1. Use BNC cables to connect the function generator and oscilloscope to a box.
2. Set the scope to trigger from the input channel.
3. Measure the input, output, and phase difference at a range of frequencies.
   • Make sure to take enough measurements to completely identify the transfer function.
   • Take extra measurements in the vicinity of the cutoff frequency or frequencies.
4. Generate the Bode gain plot and the Bode phase plot for each of the four black (blue) boxes: A through D.
5. Draw the circuit inside each box.

Turn in your Bode gain and phase plots and the corresponding circuit diagram for each of the four circuits A through D. Optional: In Matlab, use nlinfit to fit parameters of a transfer function for each circuit.