Vol. DIY Electronics Projects
Chapter 4 AC Circuit Projects

# AC Lab - Frequency Analysis

## In this hands-on AC electronics experiment, use the frequency spectrum display from an oscilloscope program to observe waveforms from a musical keyboard and learn about spectral analysis.

### Project Overview

Oscilloscopes typically provide an additional feature besides the time-domain display, for example, a frequency-domain display, which plots signal amplitude on the vertical axis over frequency on the horizontal axis. An oscilloscope’s time-domain display plots amplitude (vertical) over time (horizontal), which is fine for displaying waveshape. However, when it is desirable to see the harmonic constituency of a complex wave, a frequency-domain plot is the best tool.

This project again uses the keyboard and plug arrangement described in the previous experiment for using a musical keyboard as a signal generator to generate input signals to a computer sound card for display on the PC-based oscilloscope, as illustrated in Figure 1.

### Parts and Materials

• IBM-compatible personal computer with a sound card running Windows 3.1 or better
• Winscope software, which can be downloaded free from the internet
• Electronic keyboard (musical)
• Mono (not stereo) headphone-type plug for the keyboard
• Mono (not stereo) headphone-type plug for computer sound card microphone input, with wires for connecting to voltage sources
• 10 kΩ potentiometer

The parts and equipment for this experiment are identical to those required for the PC oscilloscope experiment.

### Learning Objectives

• To understand the difference between time-domain and frequency-domain plots
• To develop a qualitative sense of Fourier analysis

### Instructions

Step 1: Connect the keyboard output to the outer terminals of a 10 kΩ potentiometer, as illustrated in Figures 1 and 2.

##### Figure 2. The schematic diagram for connecting the keyboard output to the PC sound card input with potentiometer adjustable gain.

Step 2: If using Winscope, click on the rainbow icon to switch to frequency-domain mode.

Step 3: Generate a sine-wave signal using the musical keyboard (panflute or flute voice). You should see a single spike on the display, corresponding to the amplitude of the single-frequency signal. Moving the mouse cursor beneath the peak should result in the frequency being displayed numerically at the bottom of the screen.

Step 4: Play two notes simultaneously on the musical keyboard. The frequency plot should show two distinct peaks, each one corresponding to a particular note (frequency).

Step 5: Try basic chords (three notes) to produce three spikes on the frequency-domain plot, and so on.

Step 6: Contrast the three-note chord frequency response with a normal oscilloscope (time-domain) plot by clicking once again on the rainbow icon. A musical chord displayed in time-domain format is a very complex waveform but is quite simple to resolve into constituent notes (frequencies) on a frequency-domain display.

Step 7: Experiment with different instrument voices—instrument types—on the musical keyboard, correlating the time-domain plot with the frequency-domain plot. Waveforms that are symmetrical above and below their centerlines contain only odd-numbered harmonics (odd-integer multiples of the base or fundamental frequency). While nonsymmetrical waveforms contain even-numbered harmonics as well. Use the cursor to locate the specific frequency of each peak on the plot and a calculator to determine whether each peak is even- or odd-numbered.

### Related Content

Learn more about the fundamentals behind this project in the resources below.

Textbook:

Worksheets: