It’s not always easy to design a
ramp generator whose ramping frequency
can be changed without manipulation
of capacitors or inductors. Many ramp circuit
designs are available, but they may
not be flexible when it comes to varying
the rise time. The digital ramp generator
circuit in the figure, however, can vary the
ramp frequency from less than 1 Hz to
about 30 kHz just by varying the input
clock from 100 Hz to about 6 MHz.
The output was measured at a voltage
of 5 V peak, but it can be adjusted as
well, by varying R1 and R6. Make sure
the negative voltage driving the DAC08
8-bit high-speed multiplying digital-toanalog
converter is greater than -8.5 V
dc. If not, the rail-to-rail ramp doesn’t go
from 0 to 5 V.
This circuit can be modified to start at
0 and ramp up to any given voltage by
adjusting the reset of the CD4520 dual
up-counter. This can be useful in voltage-controlled-oscillator tracking circuits,
where the rise time and input voltage
are critical in the locking performance of
a phase-locked loop.