Project 7: The High-Pass Filter

Objective

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

General Instructions

When the circuit is set up, measure Vout for each frequency; you will 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.
• One 0.016 μF capacitor. (You'll probably find 0.016 μF capacitors listed as polypropylene film capacitors. A polypropylene film capacitor
• is made with different material than the more typical ceramic capacitor but performs the same function. If your supplier doesn't carry 0.016
• μF capacitors, you can use the closest value the supplier carries. Your results will be changed slightly but will show the same effect.)
• 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 6.10. 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 6.10

1. Carefully check your circuit against the diagram.
2. After you have checked your circuit, follow these steps, and record your measurements in the blank table following the steps:
3. Connect the oscilloscope probe for channel 2 to a jumper wire connected to Vin, and connect the ground clip to a jumper wire attached
4. to the ground bus.
5. Connect the oscilloscope probe for channel 1 to a jumper wire connected to Vout, and connect the ground clip to a jumper wire attached
6. to the ground bus.
7. Set the function generator to generate a 10 Vpp, 25 Hz sine wave.
8. Measure and record Vout.
9. Adjust the function generator to the frequency shown in the next row of the table.
10. Measure and record Vout.
11. Repeat steps 5 and 6 until you have recorded Vout for the last row of the table.
12. Calculate the values of XC for each row and enter them in the table. 

In the blank graph shown in Figure 6.11, 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 6.8, but don't worry if your curve is shifted slightly to the right or left.

Figure 6.11

Expected Results

Figure 6.12 shows the breadboarded circuit for this project.

Figure 6.12

Figure 6.13 shows a function generator and oscilloscope attached to the circuit.

Figure 6.13

The input signal is represented by the upper sine wave shown in Figure 6.14, and the output signal is represented by the lower sine wave.

Figure 6.14

As you change fin, you may need to adjust the TIME/DIV, VOLTS/DIV, and vertical POSITION controls. The controls shown in Figure 6.15 are adjusted to measure Vout when fin = 7 kHz.

Figure 6.15

Your values should be close to those shown in the following table, and the curve should be similar to Figure 6.16:

Figure 6.16

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