Upon examination of several IC function generators, it becomes evident that
none can operate from +5 V and ground. This circuit does that, and also includes
an independent duty cycle and frequency control with very low phase jitter (see
the figure).
The function generator is based on the LT1016 high-speed comparator. Three
features make it useful for this particular application: inverting and noninverting
outputs; TTL output rise and fall times that are under 5 ns; and the device
is fully specified for 5-V performance.
The circuit functions as follows: Assume that LT1016's noninverting output
is high (≈4 V) and that the integrating capacitor, C1,
is charging through diodes D1 and R5. A2 inverts the resulting ramp
from A1 and drives the negative input of the LT1016. The positive input for
the LT1016 is at a reference set by voltage divider R1 and R2.
Resistor R2 is connected to the noninverting output (≈4 V), and R1 is connected to the inverting output (≈0.4 V). Therefore, the positive input is at 2.8 V. A2 will continue
to drive the negative input with a positive-going ramp until the 2.8-V threshold
is reached and the LT1016's outputs flip. Now the LT1016 positive input reference
is switched to 1.6 V, providing hysteresis, and capacitor C1 discharges
through diode D2 and resistor R6. That causes the circuit to oscillate.
Resistors R3 and R4 ensure that reference inputs to
A1 and A2 will always be centered between the TTL output levels. R5
and R\[subscript\]6 set the duty cycle. Resistor R8 can be replaced
with a voltage-controlled variable resistor to convert the function generator
into a VCO.
The maximum frequency is limited by the notoriously slow performance of single-supply-optimized
op amps (slew rates under 1 V/µs).
This function generator has a very low phase jitter because of the fast switching
of the LT1016 combined with the hysteresis. Thus, it can be used as a VCO for
such applications as CRT displays and hard-disk drives.
