Rectangular pulse generators on CMOS chips. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The author of this article carried out experimental work to study the characteristics of various oscillators based on CMOS microcircuits. As a result, he selected some of the most interesting, in his opinion, options for their execution, which we present to the attention of readers.

This article briefly describes several circuit designs of a rectangular pulse generator built on various K561 series microcircuits. By its structure, the article is a comparative reference. For each scheme, a list of parameters and features is given (see table), as well as graphical dependences of the consumed current and generated frequency on the supply voltage.

The generator according to the scheme in fig.	The lower limit of the resistance of the resistor R1, kOhm	The highest generation frequency, MHz	Minimum supply voltage, V	Change in frequency when heated to 85°С, %	Duty cycle of output pulses
1, and	1	2	2	-4	2
2, and	1	2	4	-	2
3, and	0,56	2	2,5	-5	>2
4, and	0,56	2	2	+2,5	<2
5, and	-	1,3	3	-	<2
6, and	1	1	1,4	-11	>2

In addition, for each generator, a formula is indicated that allows you to calculate the value of the generated frequency depending on the values ​​\uXNUMXb\uXNUMXbof the elements of the frequency-setting circuit (frequency - in hertz, resistance in ohms, capacitance - in farads, inductance - in henry; more convenient, by the way, for RC generators : frequency - in kilohertz, resistance in kiloohms, capacitance - in microfarads; for LC generators: frequency in megahertz, capacitance - in nanofarads, inductance - in millihenries). Calculation formulas for a number of generators were obtained empirically.

All the characteristics of the considered generators presented in the article were obtained as a result of experiments with specific samples of microcircuits. With other instances of microcircuits, the characteristics may be somewhat different. The formulas for calculating the frequency correspond to a supply voltage of 5 V and an ambient temperature of 25°C. The load capacity of the generators is the same as that of the elements of the K561 series microcircuits. The upper limit of the generator supply voltage is also determined by the applied series of microcircuits and is equal to 15 V, and the lower limit is indicated in the table. For practical reasons, I set the upper limit of the resistance of the resistors at 40 MΩ.

In generators with capacitive positive feedback, the amplitude of the pulses at the input of the element may exceed the supply voltage. In these cases, the input protective diodes open, and current begins to flow through them. To limit this current, it is necessary to install a resistor with a resistance of 1 ... 150 kOhm in the input circuit, as indicated in [1] and used in [2].

All generators considered in this article have soft excitation. In other words, no matter how slowly the supply voltage increases, the generator will still work.

The generator based on 2I-NOT elements (Fig. 1, a) has already become a classic and is known from a large number of publications. It remains operational when the supply voltage Upit is reduced to 2 V, while, however, the generation frequency is significantly reduced.

The duty cycle of the pulses is close to two at any supply voltage. As a result of heating the microcircuit case, the frequency decreases somewhat (by 4% at 85°C).

Such a generator can also be made on two 2OR-NOT logic elements (Fig. 2, a), on two inverters (Fig. 3, a), and also on three inverters (Fig. 4, a). Details about the operation and differences of generators on two and three inverters can be found in [3]. Note that for a generator based on 2OR-NOT elements, the generation frequency practically does not depend on the temperature of the microcircuit case, and for generators based on inverters, the frequency is very stable in the Upit = 9 ... 15 V section.

Figure 5a shows a diagram of a simple LC generator with a 2I-NOT logic element. The LC circuit shifts the phase of the output signal of the element by 180 degrees, as a result of which the generator self-excites. Such oscillators work well at high frequencies, are gently excited, and are characterized by high temperature stability [3].

As the frequency increases above 1,3 MHz, the amplitude of the output pulses begins to fall.

2OR-NOT elements can also work in the generator, and in this case it produces not rectangular pulses, but oscillations that are close to sinusoidal in shape.

For stable operation of the generator, the impedance of the LC circuit should not be less than 2 kOhm. The generation frequency practically coincides with the resonant frequency of the LC circuit. The advantage of the generator is the high temperature stability of the frequency.

Generators similar in structure can be implemented on a single Schmitt trigger element (Fig. 6a). With a supply voltage close to the maximum, they are very stable in frequency. In addition, they are extremely economical - at a supply voltage of less than 6 V, they consume a current of only a few tens of microamperes.

Literature

1. Biryukov S. A. Digital devices based on MOS integrated circuits, vol. 1132, p. 60-65; issue 1220, p. 105-111. - M.: Radio and communication, 1990; 1996 (MRB).
2. Nechaev I. Logic probe without power source. - Radio, 1990, No. 10, pp. 83,84.
3. Biryukov S. Generators and pulse shapers on CMOS microcircuits. Radio, 1995, No. 7, pp. 36,37.
4. Kiverin N. LC generator based on logic elements. - Radio, 1990, No. 7, p.55.

Author: S. Elimov, Cheboksary

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