Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY

PC Oscilloscope and Spectrum Analyzer

Hsu-Chang Lu (presented by Gabe Alba)

Rutgers University

Serin Physics Laboratories

136 Frelinghuysen Rd.

Piscataway, NJ 08864

732-445-3897

hclu ‘at’ physics ‘dot’ rutgers ‘dot’ edu

Abstract:

FFTscope software, widely distributed as freeware, allows a standard PC equipped with sound card (with the inclusion of simple banana-plug interface board) to function as an oscilloscope and frequency spectrum analyzer to measure voltages and FFT amplitudes in real time.

The software toolbar

The induced voltage from a magnet moved through a coil (Scope Mode).

Phase relationship between driving and response signals in a circuit (Scope Mode).

RLC circuit response to white noise input (FFT Mode)

RLC circuit driven with square waves (Scope Mode)

440 Hz A3 Tone – almost pure sine wave (FFT Mode)

Construction of Apparatus:

The software is available as a free download. The only associated equipment suggested for construction would be a simple banana/miniplug interface board, recommended for connecting the sound card input/output to banana plug or alligator clip cables (see photo below):

Use of Apparatus:

This software is currently being used in all the instructional Physics labs of a major state university, as part of the lab curriculum. It allows a standard PC equipped with sound card (with the inclusion of simple, optional banana-plug interface board) to function as an oscilloscope, function generator (with sine, square, triangular, pulsed, swept sine, and white noise waveforms) and frequency spectrum analyzer (FFT)to measure physical phenomena.

The utility of the software/sound card combination in our lab is considerable as it has in the last few years obviated the need for a traditional oscilloscope, function generator, and spectrum analyzer, at least in the introductional lab experiments. It is currently used in the following experiments:

- Faraday’s Law

- Sound Wave Interference

- Standing/Traveling Waves

- Electrical Pulse Response (RLC resonance)

- Forced Harmonic Oscillator

- Normal Modes

- Simple Harmonic Motion

- Physics of Math & Music

- Speed of Sound

Limitations of the software/sound card combination will be briefly discussed:

- Sampling rate is limited to sound card’s capability (typically 44 kHz), therefore very high frequencies will require an expensive sound card (>$100) or a traditional oscilloscope.

- Very low frequencies are not accurately generated by the function generator component of the software/sound card; usually this is not a problem

- PC speakers may have to be modified to disable the inherent bass boost