A simple PWM modulator. Scheme, description

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

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

It is regrettable to see how the domestic market is filled mainly with overseas equipment, which tramples on our native manufacturer of electronic equipment. However, I have no doubt that there are many talents in our country that will not yield to foreign luminaries, but there are no conditions for these talents to show themselves. Therefore, I am pleased to present one of my developments to our radio amateurs.

Simple PWM modulator

Let's start with a little theory. Consider the circuit (Fig. 1) of a pulse generator based on a K176LA7 MOS logic chip and two diodes. The generator is made on a double RS - trigger.

Simple PWM modulator
Fig. 1

The generator works like this:

When the power is turned on, C1 and C2 - the input parasitic capacitances of each element - are discharged. Accordingly, at the inputs 1 and 5 the state of logical zero, and at the outputs 3 and 6 - a logical unit. The second trigger is randomly set to some stable state. Suppose output 10 is a logical one, output 13 is a logical zero. At the same time, VD1 is closed, VD2 opens and charges C2 fairly quickly. At input 5, a logical unit is set, and at output 6 - logical 0, which switches the second trigger to another state (output 10 - logical 0, output 13 - logical 1), respectively opening VD1 and closing VD2. Through VD1, C1 is charged and a logical 1 appears at input 1.

In this state, the double trigger will remain until the logic 1 level appears at input 0. This time is determined by the input capacitance C2, the input leakage current and the difference between the logic 1 voltage (approximately Upit) and the threshold voltage of the microcircuit (approximately Upit / 2 ).

t=C2*(Upit-Uthr)/Iut After C2 is discharged to the threshold voltage, the second trigger switches again, C2 is charged and C1 is discharged. Upon reaching the threshold voltage on it, the second trigger switches and the process is repeated in the future.

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. A model sample of such a generator showed a change in the frequency and duty cycle of the pulses when a hand was brought closer to the generator. To reduce the effect of the reverse current of the diodes, they are chosen with the lowest possible leakage current (type KD102).

Modulator based on a two-trigger oscillator

The pulse duration in such a generator can be modulated by changing the capacitance connected in parallel to the input or by controlling the discharge current of the input capacitances. Consider a variant with control of the discharge current of the input capacitances. At inputs 1 and 6, we turn on two current sources controlled by a modulated signal (Fig. 2).

Modulator based on a two-trigger oscillator
Fig. 2

Moreover, when the input signal changes, the current of one source increases by ?I, and the current of the other decreases by ?I.

Accordingly, one period will be:

T=t1+t2=C1*Uthr/(I+??I)+C2*Uthr/(I-??I);

whence it can be seen that the greater the discharge current of the input capacitances, the shorter the period and, accordingly, the higher the frequency of the modulator. The original signal is restored using an integrating circuit, at the output of which, with a constant amplitude of the output pulses (Uam), the voltage will be:

Uout=Uam*t1/(t1+t2)

it is easy to deduce that for the same input capacitances, threshold voltages, and ?I=0, Uout=Uam/2.

And the change in the output voltage and the transfer coefficient:

?U=??I*Uam/2I;

K= Uam/2I

Thus, by reducing the discharge current of the input capacitances and increasing the amplitude of the output pulses of the modulator, it is possible to obtain, in addition to modulation, an amplification of the input signal. And one more note: since when the input signal changes, both the pulse duration and the duration of its absence change, the modulation frequency changes, and as the input signal increases, it decreases. This also determines a rather large dynamic range of the modulator.

The practical scheme of the modulator is shown in Fig. 3. Parts of the modulator were chosen for reasons of accessibility and easy repeatability of the circuit.

The input differential stage is made on bipolar transistors KT315 with any letter, preferably with similar current gains. KD102 with a low reverse current was chosen as diodes.

Modulator based on a two-trigger oscillator
Fig. 3

To increase the stability of the modulator, negative feedback was introduced into the circuit from output 4 through a low-frequency filter, a 12 k resistor, a 1.0 μF capacitor, and a 24 k resistor with a cutoff frequency of about 16 Hz.

The modulator is tuned by selecting a 110 k resistor for the required modulation frequency.

Author: Vladimir Alekseevich Gorbatykh, Ulan-Ude; Publication: N. Bolshakov, rf.atnn.ru

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