Pulse generator (electronics)

A pulse generator is a special type of signal generator in electronics that emits either a single pulse or repeatedly as a pulse with a defined shape and duration. Depending on the pulse generator type rectangular pulses can be generated, this is at a square-wave generator is the case, as well as various pulses course in so-called english Arbitrary wave generators.

construction

Circuit of a pulse generator for square-wave pulses with an IC

Time course of the pulse trains from the above circuit

The circuit in the adjacent picture shows a simple way of producing very short pulses with a specified duration and repetition frequency. The oscillator is a relaxation oscillator that generates a square wave voltage (voltage curve A). The period T can be approximated with the adapted size equation

T ≈ 0.7 · C 1 · 500 Ω

to calculate. For a frequency of 20 kHz one therefore needs C 1 = 150 nF.

This square wave voltage is fed directly to a NAND gate on one path (signal A) and delayed and inverted on another path (signal B) to the other input of the NAND gate. Without delay, the output voltage of the inverting NAND gate would always be +5 V. Because of the short delay caused by the RC element and the running time of the inverting AND gate , however, very short times of a few nanoseconds occur during which signal A is already at +5 V and signal B has not yet reached 0 V. During this time the output voltage X is briefly 0 V. If reverse polarity is desired, another inverter can be used to reverse the signal curve X. The voltage Y is then almost constantly at 0 V and briefly jumps to +5 V.

The duration of the generated pulses can be set with the capacitor C 2 . The context applies ${\ displaystyle T_ {p}}$

{\ displaystyle T_ {p} \ approx 0 {,} 7 \ ​​cdot {C2} \ cdot 500 \; \ Omega +15 \; \ mathrm {ns}}

The last summand is the signal transit time between the input and output of the logic gate and can be found in the data sheet. If C 2 = 22 pF is chosen, each pulse lasts 23 ns. If gates from faster logic families such as Advanced-TTL are used, shorter pulse durations can be achieved.

A frequency doubling of square waves in the frequency range up to a few 100 MHz can be achieved with an exclusive-OR gate if one input is fed directly and the other with a slightly delayed signal (RC element). The resulting needle pulses are phase-bound and about as short as the time constant of the RC element. Since this method does not use a resonance filter, the input signal can have any duty cycle or be heavily frequency-modulated .

Another pulse circuit is the monoflop ; it only generates a single pulse when triggered. By interconnecting two monoflops, pulse trains can be created with an adjustable ratio between pulse duration and pause.

Electronic pulse generators in device form are used for measuring and testing purposes, usually as part of function generators . Simple devices can only generate single pulses or pulse trains with a 5 V level (TTL signals). Arbitrary wave generators also make it possible to design any pulse shape that deviates from the rectangular shape and to generate it periodically in a rapid pulse sequence.

literature

Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology . 12th edition. Springer, 2002, ISBN 3-540-42849-6 .