ELEC 242 Lab

Experiment 6.1

Filters and Transfer Function

Equipment

Components

In this experiment we will examine the frequency domain behavior of the same RC circuit we used in Part 2 of Experiment 5.1.

Part 1: Measuring Frequency Response Manually

We have to make the measurement at a number of frequencies, and we must measure phase as well as amplitude.


Step 1:

 Select a 2.2 k resistor and a .33  F capacitor from your parts kit.

Step 2:

 Using these components, wire the following circuit:


Step 3:

 Connect the function generator to and the oscilloscope to .

Step 4:

 Using the technique described in the previous section, measure the frequency response of the circuit at the following frequencies: 20 Hz, 50 Hz, 100 Hz, 200 Hz, 500 Hz, 1 kHz, 2 kHz, and 5 kHz.

Step 5:

 Plot the magnitude of the transfer function vs. frequency on loglog axes and the phase on semilog axes.

Question 1:

 Using Matlab, compute and plot the expected frequency response for the circuit you built. How well does this compare with what you measured?

Part 2: Measuring Frequency Response Automatically

Measuring a frequency response manually is time consuming and error prone. Since we have two more systems whose frequency response we want to measure today, it behooves us to find a quicker way.


Step 1:

 Disconnect the function generator from the circuit. Leave the scope connected.

Step 2:

 Connect the 50 pin ribbon cable from the DAQ card to P12 on the back of the interface board. Be sure that it goes all the way in so that the ejector latches snap into place.

Step 3:

 Connect D/A output 1 (pin 11 on the interface board socket strip) to . Also connect A/D input 4 (pin 5) to .

Step 4:

 Connect A/D input 5 (pin 6 on the interface board socket strip) to .

Step 5:

 Load the "Frequency Response" program from the Start menu by following the path Programs -> Labview -> 242 -> Frequency Response. It should look like this:

Set the parameters as follows:


Step 6:

 Start the program by pressing the Run button or CTRL-R.

Step 7:

 If all is well, the program will cycle through the frequencies to be measured, displaying frequency, magnitude, and phase at each step. However, the graph will not be drawn until all measurements are complete.

Step 8:

 When finished the program will display a plot of the magnitude and phase of the frequency response. How do these compare with the plots you made in Part1?

Question 2:

 From the frequency response plot, estimate , the frequency at which the gain has fallen to 0.707 (-3dB) of its low frequency value. From this value, determine the time constant. How does this value for compare with what you measured in Experiment 5.1?