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04-06-2010

# Rectangular-waveform generator produces 25 and 75% duty cycles

Use two ICs to produce both waveforms.

Marián Stofka

Test applications may call for a rectangular waveform having a precision duty cycle higher or lower than 50%. The circuit in Figure 1 is a free-running generator using just two ICs that produces rectangular-waveform duty cycles of both 25 and 75%. It holds the duty-cycle accuracy regardless of the duty-cycle accuracy from the signal source, an oscillator circuit comprising a Schmitt-trigger input NAND gate, IC1A. Flip-flop IC2A divides the oscillator’s frequency by two at its 1 and Q1 outputs. Flip-flop IC2B functions as a modulo-two divider clocked from the Q1 output of IC2A. Thus, IC2A and IC2B divide the oscillator’s output by four.

 Figure 1. This circuit uses two flip-flops and three NAND gates to generate waveforms with 25 and 75% duty cycles.

NAND gates IC1B and IC1C generate the output waveform from the 1 and Q2 signals. Figure 2 shows the output from NAND gate IC1B. You can generate the 25% duty cycle by simply replacing the waveform that IC1B outputs with the one that gate IC1C outputs. If the active level is low instead of high, you can simply interchange the outputs of IC1B and IC1C.

 Figure 2. NANDing the 1 and Q2 logic signals gives a glitch-free output.

The repetition frequency, 1/TREP, of the oscillator employing IC1A is almost independent of the supply voltage within the range of 3 to 5V because both the positive and the negative thresholds of the input CMOS Schmitt trigger are roughly proportional to the supply voltage. Rough analysis gives a repetition frequency of approximately 2.7/π, where π=RC, the time constant of the RC circuit around gate IC1A. Further, the oscillator’s waveform duty cycle is approximately 46.3%.

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