A circuit that can generate pulse waves without excitation. Rectangular pulses can be generated by a self-excited multivibrator with transistors and field effect transistors as switches and capacitors or inductors as inert elements. Circuit types include collector-base coupling, emitter-coupled multivibrator and so on. Intermittent oscillator with pulse transformer as feedback element can produce narrow pulse with small duty cycle. Use negative resistance devices, such as tunnel diodes, single junction transistors, thyristors, etc. And can also form a self-excited multivibrator circuit. With the development of integrated circuits, this kind of discrete component circuit has been rarely used because of the trouble of design adjustment.

The self-excited multivibrator composed of integrated logic gate and RC charge-discharge circuit is relatively simple. Figs. 3(a) and 3 (b) show symmetric and asymmetric multivibrator circuits composed of positive feedback circuits composed of two NOT gates and capacitors, and their oscillation frequencies are determined by RC. When the stability index of oscillation frequency is required, the synchronous crystal can be connected in series with the coupling capacitor in the above circuit to form a synchronous crystal multivibrator. The oscillation frequency of the circuit is equal to the resonant frequency of the synchronous crystal. The ring oscillator circuit with RC delay link is shown in Figure 3(c), and its oscillation frequency is also related to RC. If the resistor R in the figure is replaced by a potentiometer, a rectangular wave generator with adjustable frequency range can be formed.

Rectangular pulses can also be generated by multivibrators composed of integrated circuits, such as operational amplifiers, Schmitt triggers or 555 timers.

3. Pulse waveform transformation

The process of converting a given input signal waveform into a specific output waveform through the system. Include linear transformation and nonlinear transformation.

The nonlinear transformation circuit of pulse waveform is a circuit that limits, clamps, shapes and divides the input signal. The simplest nonlinear conversion circuit is the limiter and clamp circuit realized by switching elements such as diodes and triodes. The former is to cut off the amplitude of the pulse signal and "cut off" the waveform part whose amplitude is higher or lower than a certain level. The latter is to fix a certain level of the pulse signal to a specified value, while the waveform remains unchanged, so that the signal maintains a fixed DC level.

Common linear conversion circuits include differential circuit and integral circuit composed of resistor R and capacitor C. If a square wave signal is added to the input end of the differential circuit, the jumping edge of the pulse voltage can be taken out and the sharp wave as shown in figure 1(c) can be obtained. This kind of pulse can be used as the trigger pulse of the circuit. If the square wave signal is added to the input of the integrating circuit, the triangular wave can be obtained.