Digital Sinusoidal Signal Synthesizer with the 4018 (CB4629E)

   The 5 outputs of the internal 4018 integrated flop flops can be used simultaneously for the synthesis of sine signals from a rectangular input signal. The counter setting is then set to 10 and the flip-flop outputs synthesize each voltage level in sequence to form a sinusoid. The proportions by which each output should contribute to the sine signal level is given by the following relationship:

     Q1 = 1.618

     Q2 = 1,000

     Q3 = 1,000

     Q4 = 1.618

   This way, when output Q1 is activated, we have a voltage level of 1,000 at the output. When outputs Q1 and Q2 are activated we have 2,618. When the 4 outputs are activated we have the 5,236 which corresponds to the maximum voltage level or positive peak. From there, with the deactivation of output 1 and keeping outputs Q2, Q3 and Q4 activated we have 3,618 and in a next step we reach again 3,618 and then with only Q4 activated we return to 1,000. The successive waveforms synthesized by this circuit are shown in figure 1.

Figure 1 - Generating a sinusoidal signal digitally. | Clique na imagem para ampliar |

   Of course, the waveform obtained at the output of the integrated is formed by steps. If we want a perfect sinusoid we need to pass this signal through a low pass filter that removes the smooth transitions of the smoothing signal. The complete circuit including the filter is shown in figure 2.

Figure 2 - Full circuit of the digital sinusoidal signal synthesizer. | Clique na imagem para ampliar |

    This circuit can operate satisfactorily with signals in the 500 to 2500 Hz range. Changes in component values allow operation in other frequency ranges. The output signal strength is 0.8 Vpp for a 5 V supply voltage. Sinoids with fewer synthesis points can be obtained by dividing by 6 or 8 as shown in the circuit of Figure 3.

Figure 3 - Circuit to synthesize sinusoidal signals with less precision.