You can use the parallel port of your PC and a few additional components to generate a powerful, easy-to-use arbitrary-waveform generator. By using a Visual Basic program with the circuit in Figure 1, you can generate any waveform (for example, sinusoid, triangle, amplitude- or frequency-modulated, or exponential decay) by simply entering its characteristic equation. For this circuit, the parallel port connects to four latches (IC₁, IC₂, IC₅, and IC₆). IC₅ provides control signals, IC₁ and IC₆ transfer data to the memory, and IC₂ controls a VFC (voltage-to-frequency) converter. During the load-waveform operation, the waveform data transfers from the parallel port via latches IC₁ and IC₆ to the memory chips IC₇ and IC₁₀. The binary counter, IC₉, increments the memory addresses in sequence to allow loading each memory location with a unique 16-bit binary word. Each binary word corresponds to a waveform data point. During the load-waveform operation, the memory's configuration allows writing information to it (for example, ~OE=1, ~WE=0).

During the output-waveform operation, latches IC₁ and IC₆ disconnect from the bus, and the memory delivers the stored data (for example, ~OE=0, ~WE=1). For each accessed location, one of the binary words stored during the load-waveform operation transfers to IC₈, a DAC7621. This transfer causes the DAC to deliver one output point in the waveform. The VFC causes IC₉ to clock through all possible addresses. IC₁₁ resets the counter when the memory sequences through all possible addresses. When IC₉ resets to zero, the waveform begins to repeat itself. Thus, each waveform comprises 2048 data points. The number of points, N, and the clock frequency, C, control the frequency of the arbitrary waveform: f_AWG=1/NT_C, where T_C is the period of the clock frequency.

IC₂, IC₃, and IC₄ form a circuit that adjusts the clock frequency, C, via the parallel port. The clock rate C controls the frequency of the arbitrary waveform. The output frequency of IC₄, a VFC110 VFC, is directly proportional to its input voltage. With a full-scale input of 10V, the VFC110 delivers 4 MHz. IC₃ provides a voltage output of 0 to 10V, thus providing frequency control from near 0 Hz to 4 MHz. The voltage output of IC₃ receives its programming via the parallel port, thus allowing computer control of the clock rate. Thus, the circuit provides a frequency range of 7.6 Hz (1/(2048×64 µsec)) to 125 kHz (1/(32×250 nsec)). Figure 2 shows various sample outputs of the circuit. Click here to download the software files associated with this Design Idea.

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