A zero-cost, real-time, Windows signal laboratory

International Journal of Electrical Engineering Education, Jan 2009 by Leis, John, Snook, Chris

Whilst the original motivation for the project was to prepare students for laboratory work (especially those from varying cultural backgrounds, who sometimes have inhibitions in working in typical group-based laboratory situations), it became ap parent during the development process that the concept of a simultaneous timefrequency display would have significant pedagogical advantages. This is illustrated in Fig. 2, where a synthesized waveform is shown on the time-domain display (left), with the corresponding frequency analysis (right).

Capabilities

The following sections describe the salient features of the program, and Figs 1-5 illustrate various waveforms. This section is not intended to be an exhaustive discussion of capabilities and features, merely an introduction - the interested reader is referred to the online tutorial at http://www.usq.edu.au/users/leis/software/

Waveform display

The sampled waveform is displayed on the left-hand panel (Fig. 1). The display parameters such as sweep per graticule division, vertical gain, and d.c. offset may be set in a manner similar to a conventional instrument. The trigger is simply a zero-crossing detector - this is adequate for the purpose, and more importantly does not overcomplicate the display for new users. Note that the amplitude scaling is necessarily relative, and not calibrated as such. This is one aspect which should be borne in mind if using Win-eLab as an instructional tool.

Spectral analysis display

The amplitude spectrum of the sampled waveform is displayed on the right-hand panel. Figs 1-3 show sampled audio, sinusoidal and sawtooth waveforms, respectively. The frequency components are calculated using a Fast Fourier Transform (FFT) and displayed with a fixed update time. The update time was chosen so as to give fast frequency display whilst not overburdening the CPU. The display parameters such as span per graticule division, vertical gain, and start frequency may be set in a manner similar to a conventional instrument. The start frequency may be from the left-hand edge, or the center, of the display panel. Linear vertical scaling is the default, although logarithmic scaling may also be selected.

Waveform synthesis

The lower control panel interfaces to the signal synthesis module. Standard waveforms able to be generated are sine, random, square, triangle, and sawtooth (ramp). The frequency and amplitude may be set for each. In addition, amplitude modulated and frequency modulated waveforms may be selected. In either case, the modulation parameters are then able to be set. Note that in this figure, the 'loopback' mode of operation is selected: the output buffer is simply copied to the input buffer in this mode.

The AM and FM waveforms require additional parameters to be specified. When these are selected, two additional controls, 'fm' and 'm', become visible, these being interpreted as the modulation frequency and modulation index, respectively. In either case, the main frequency control becomes the carrier frequency.