Project 8:The Low-Pass Filter

Objective

The objective of this project is to determine how Vout changes as the frequency of the input signal changes for a low-pass filter.

General Instructions

After the circuit is set up, measure Vout for each frequency. You also calculate XC for each frequency value to show the relationship between the output voltage and the reactance of the capacitor.

Parts List

• You need the following equipment and supplies:
• One 1 kΩ, 0.25-watt resistor. (You can use the same resistor that you used in Project 7.)
• One 0.016 μF capacitor. (You can use the same capacitor that you used in Project 7.)
• One function generator.
• One oscilloscope. (You can substitute a multimeter and measure Vout in rms voltage rather than peak-to-peak voltage.)
• One breadboard.

Step-by-Step Instructions

Set up the circuit shown in Figure 6.19. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help building the circuit, look at the photos of the completed circuit in the “Expected Results” section.

Figure 6.19

Carefully check your circuit against the diagram.
After you have checked your circuit, follow these steps, and record your measurements in the blank table following the steps:
Connect the oscilloscope probe for channel 2 to a jumper wire connected to Vin, and connect the ground clip to a jumper wire attached to the ground bus.
Connect the oscilloscope probe for channel 1 to a jumper wire connected to Vout, and connect the ground clip to a jumper wire attached to the ground bus.
Set the function generator to generate a 10 Vpp, 25 Hz sine wave.
Measure and record Vout.
Adjust the function generator to the frequency shown in the next row of the table.
Measure and record Vout.
Repeat steps 5 and 6 until you have recorded Vout for the last row of the table.
Enter the values of XC for each row in the table. (Because you used the same capacitor and resistor in Project 7, you can take the values XC from the table in Project 7.)

In the blank graph shown in Figure 6.20, plot Vout versus fin with the voltage on the vertical axis and the frequency on the X axis. The
curve should have the same shape as the curve shown in Figure 6.18.

Figure 6.20

Expected Results

Figure 6.21 shows the breadboarded circuit for this project.

Figure 6.21

Figure 6.22 shows a function generator and oscilloscope attached to the circuit.

Figure 6.22

The input signal is represented by the upper sine wave, as shown in Figure 6.23, and the output signal is represented by the lower sine wave. Reading the number of divisions for the peak-to-peak output sine wave and multiplying it by the corresponding VOLTS/DIV setting allows to you measure Vout.

Figure 6.23

As you change fin adjustments in the TIME/DIV control, the VOLTS/DIV and vertical POSITION controls for channel 1 may be needed. The controls shown in Figure 6.24 are adjusted to measure Vout when fin = 20 kHz.

Figure 6.24

Your values should be close to those shown in the following table, and the curve should be similar to Figure 6.25.

Figure 6.25

Notice the relationship between XC and Vout in this circuit. Low values of Vout (The voltage drop across the capacitor in this circuit.) occur at
frequencies for which XC is also low. When XC is low, more voltage is dropped across the resistor and less across the capacitor.
(Remember that XC changes with frequency, whereas the value of the resistor stays constant.) Similarly, when XC is high, less voltage is
dropped across the resistor, and more voltage is dropped across the capacitor, resulting in a higher Vout.