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.

Using a PC-based oscilloscope to monitor outputs from an electronic keyboard.

Figure 1. Using a PC-based oscilloscope to monitor outputs from an electronic keyboard.

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.

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

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.