Function generators (FGs) are measurement instruments in the category of waveform sources. They grew out of both a need for a versatile source of multiple waveshapes and from some interesting analog circuit combinations that generate three waveshapes: triangle-waves, square-waves, and sine-waves.
This presentation of FGs follows the historical development, through two or three generations of preferred implementations. FGs have a generator loop in which an integrator drives a dual-level detector which switches the polarity of integration. The frequency is determined by integrator components and threshold levels.
A waveform is an electrical function of time. (This definition might be broadened to a more inclusive physical function of time.) Waveshape is a waveform property, that of the scale-invariant waveform - the waveform without regard to the values along its axes. More recently, arbitrary waveshapes are generated using digital synthesis in waveform generators. Analog FG generator loops output triangle and square waves. Sine-waves are produced with a triangle-to-sine converter subsystem. One of the three waveforms is selected and then amplified by a power amplifier for driving the FG output.
Op-Amp Integrator FG
The first-generation function generator scheme is shown below, popularized by early Wavetek FG products.
The integration of a bipolar square-wave results in a triangle-wave. The hysteresis switch, or Schmitt trigger, is a “window comparator”, with high and low input thresholds that determine the extrema, or peaks, of the triangle-wave. The hysteresis switch output is an amplitude- and time-symmetrical square-wave, described mathematically as the output of the inverting hysteresis switch:
The integrator integrates the hysteresis-switch vo to produce the triangle-wave
where R and C are components of an op-amp integrator, as shown below. The hysteresis switch can be implemented using two comparators and an RS flop. The logic levels of the integrator input must be bipolar and closely equal in amplitude, for the input voltage determines output voltage slope and affects the waveform time symmetry. Some level-shifting circuitry must precede the integrator input for single-supply logic and must be inverting for waveform confinement between the hysteresis thresholds.
In a unipolar or single-supply implementation, the thresholds of the hysteresis switch would be of the same polarity. To make the triangle-wave slopes bipolar, the integrator is offset so that its input is bipolar even though the square-wave fed back to it is not. The offset is easily accomplished by connecting the noninverting input of the integrator op-amp to a voltage midway between the hysteresis thresholds.
An actual implementation of a simple triangle-wave generator (TWG) loop is illustrated from an old RCA (and afterward, Intersil) application note, excerpted below.
In this circuit, the hysteresis switch is implemented with one op-amp, IC3, using positive feedback. The positive threshold is set by the 150 k
feedback and 39 k
input resistors, forming a voltage divider. When the input (from pin 6 of IC2) exceeds the voltage at pin 3, the output transitions to the positive range limit. For a CMOS op-amp, this is near the supply voltage and is fairly repeatable - an advantage when using CMOS op-amps such as the CA3130. In an instrument-grade design, the op-amp would be too slow as a switch and would be replaced by a comparator.