Generate swept sine/cosine waveforms with two filters
Phase difference holds between 89° and 90°.
John R Ambrose, Mixed Signal Integration, San Jose, CA; Edited by Martin Rowe and Fran Granville -- EDN, June 23, 2011
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.
Click to enlarge
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. A square wave at CMOS
levels would saturate the filter, so the
circuit uses resistor dividers R1 through
R4 to reduce the signal’s amplitude.
Figure 2 shows the output of the two
filters at 20 kHz with a system clock of
2 MHz. Note that the phase reading on
the scope is at −89.85°. When swept in
frequency, the phase varies from −89 to
−91°. Figure 3 shows a 20-kHz Lissajous
pattern.
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