A simple PWM generator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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A simple modulated generator is proposed, which can be used to form and process various signals in amateur radio devices.

To begin with, consider the circuit of a rectangular pulse generator (Fig. 1), which is made on two RS-flip-flops from the logic elements of a MOS or CMOS microcircuit.

The generator works as follows. When the power is turned on, the input parasitic capacitances of each element - conventionally shown in the diagram as C1 and C2 - are discharged. The initial state of inputs 1 and 5 of the first trigger corresponds to the log. 0, and at its outputs 3 and 6 - log. 1. The second trigger is randomly set in one of two states: suppose that the output is 10 - log. 1, output 13 - log. 0. In this case, the diode VD1 is closed, and VD2 opens and charges C2 quickly enough. Log is set at input 5. 1, and at the output 6 - log. 0, and the second trigger switches to a different state, respectively opening the diode VD1 and closing VD2. Capacitance C1 is charged through the diode VD1, and a log appears at input 1. 1.

The triggers will remain in this state until the log level appears at input 1. 0. This time is determined by the input capacitance C2, the input leakage current * and the difference between the voltage log. 1 (approximately equal to Upit) and the threshold voltage of the microcircuit (approximately half of Upit): t = C2-(Upit·Uthr)·Iut.

After the capacitance C2 is discharged to the threshold voltage, the second trigger will switch again, C2 will be charged again and the discharge of C1 will begin. Upon reaching the threshold voltage on it, the second trigger will switch again; further processes are repeated.

As can be seen from the above formula, with practically unchanged leakage current and threshold voltage, the discharge time of parasitic capacitance depends on its value. When a hand was brought nearer to the mock-up sample of the generator, a change in the frequency and duty cycle of the pulses was observed. To reduce the effect of the reverse current of the diodes, they are chosen with the lowest possible leakage current (type KD102A).

The duration of the pulses in such a generator can be controlled by changing the discharge current of the input capacitances of the logic elements. Based on this principle, a pulse-width modulated oscillator can be built.

Let's consider this modulation option in more detail. To the inputs 1 and 6 of the DD1 elements, we connect two current sources controlled by a modulated signal (Fig. 2). When the input signal changes, the current of one source increases by ∆I, while the current of the other source decreases by ∆I.

Accordingly, one period will be: T \u1d t2 + t1 \u2d CXNUMX X Upor / (I + ΔI) + CXNUMX x X Upor / (I - ΔI).

As can be seen from the formula, the greater the discharge current of the input capacitances, the shorter the period and, accordingly, the higher the modulator frequency.

Restoration of the original (modulating) signal is possible using a simple integrating circuit, at the output of which, at a constant pulse amplitude (Uamp), the output voltage will be: Uout = Uamp x t1(t1+t2). It is easy to conclude that with ΔI = 0, the same input capacitances and threshold voltages of the logic element inputs, a voltage close in value to half the supply voltage will operate at the output of the integrating circuit. The change in the output voltage and the transmission coefficient for the modulating signal correspond to the expressions: ΔUout = Uamp X ΔI/2I; K \u2d ΔUout / ΔUin \u2d (Uamp / 26I) ∙ (300I / Ut) \uXNUMXd Uamp / Ut, where Ut is the temperature voltage equal to XNUMX mV at a temperature of XNUMX k.

One more note. Under the action of the input signal, both the pulse duration and the pause duration change. The pulse frequency also changes somewhat: as the input signal increases, it decreases. This determines a sufficiently large dynamic range of the device. The practical scheme of the generator is shown in fig. 3. Its elements are selected for reasons of their availability and repeatability of parameters.

The input differential stage (VT1, VT2) is made on bipolar transistors KT315 (with any letter index), preferably with similar base current transfer coefficients. KD102 with low reverse current was used as diodes. To increase the stability of the generator, negative feedback was introduced into the circuit from output 4 through a low-frequency filter from resistor R5, capacitor C2 and resistor R4 with a cutoff frequency of about 16 Hz.

The generator is tuned by selecting the resistor R3 for the required modulation frequency.

Author: V. Gorbatykh, Ulan-Ude

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