Generate swept sine/cosine waveforms with two filters

摘要

Demodulators, directional circuits, and other electronics applications often need two sine waves with a 90° difference in phase-a sine wave and its cosine wave. Engineers typically use analog filters to create the phase shift. This approach, however, offers a limited frequency range. Using the circuit in Figure 1, you can make a swept sine/cosine pair at frequencies of less than 1 Hz to 25 kHz. The Mixed Signal Integration MSFS5 selectable lowpass/bandpass switched-capacitor filter removes the harmonics from a square wave you apply to its inputs. The clock for the MSFS5 is 100 times the input square wave. The 74HC390 and 74HC74 form a divide-by-25 and a divide-by-two circuit. The Q outputs from the 74HC74 connect to the two divide-by-two circuits in the 74HC390A, which produces square waves that are 1/100 of the filter clock's frequency and are 90° out of phase from each other.