Electronic Design

Function Generator Uses +5 V, Ground

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.

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