Signal Generator Experiments

On this page you find initial experiments to get a working circuit.

A741

The circuit consists of a hysteresis switch and an integrator.

LM318

Basically the same circuit, but implemented using LM318 Operational Amplifiers. Unlike A741 the LM318 has a strong urge to oscillate so th power supply must be connected as depicted on the side. Also a lead compensation has been added.

Note: When the LM318 oscillates uncontrolled it can actually destroy itself at higher supply voltages. So if you are uncertain wether a circuit is stable, start with +/-5V if possible.

The circuit delivers good results at 40KHz, but the absolute maximum is 350KHz. After that the triangle will be badly distorted because the minimum output voltage is reached - due to the ever growing amplitude.

Also the square flanks loose their steepness because the slew rate is limiting at 50V/s. The flawed square input causes the integrator to produce a triangle signal with rounded edges.

74LS00

This version utilizes two NAND gates of a 74LS00 TTL chip as active part of the hysteresis switch. The gates are likely to produce much steeper square flanks. Two gates are used to better shape a square signal from non TTL input. The circuit can only be operated at +/- 5 Volts.

Symmetrical supply is needed because the LM318 output has to be negative to reach TTL low level at gate inputs when the switch is turned on. After the switch turned off, the gate input will be below ground. This might seem severe at first, but the LS00 inputs feature builtin schottky diodes connected to ground that will keep the input level within the limits.

To work with TTL levels as input the integrator now has an adjustable zero level. This allows to correct or modify output signal symmetry.

Adding 100p at the second gate's output will reduce overswing (see below) at the price of longer switching times.

The LS00's output has steep flanks but overswings. The steepness also appears to cause oscillation within the integrator that is strong enough to still be clearly visible despite all the compensation that has been added.