Parallel port controls arbitrary-waveform generator
Art Kay, Texas Instruments Inc, Tucson, AZ - June 13, 2002
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: fAWG=1/NTC, where TC 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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