Theory: oscillators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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General Principles of Oscillation Generation

We know that nothing comes from nothing. In order to produce any action in nature, say, to create movement, it is necessary to expend a certain amount of energy. Oscillations, including electrical ones, are one of the types of motion. Muscle energy is needed to swing the swing, the energy of steam or water accumulated in front of the dam is needed to spin the turbine and generate an industrial frequency current (50 Hz). In the same way, the energy of the power supply allows you to excite the radio frequency generator, which, in fact, is a converter of DC energy into high-frequency vibration energy - these can be amplified and brought to the radio transmitter antenna.

In the very first radio transmitters, for example, the functions of generating and amplifying oscillations were combined in one device, made on a powerful radio tube (and even earlier, on a spark or arc gap or a high-frequency machine). Subsequently, it turned out to be more expedient to generate oscillations of relatively low power (but highly stable), and then amplify them to the desired level. Generators in which oscillations occur independently are called self-excited, or self-oscillators, and power amplifiers of high-frequency oscillations are often called generators with extraneous excitation.

Low-power generators - local oscillators - are available in almost every broadcasting and television receiver. They are part of a frequency converter - a device that serves to transfer the signal from the received frequency to the so-called intermediate frequency, at which the main amplification, filtering and signal processing take place. Such a receiver is called superheterodyne.

The oscillator usually contains an amplifying element, the output of which is connected to the input by a feedback circuit (OS), as shown in Fig. 44.

The polarity of the oscillations entering the OS circuit must be such as to maintain the oscillations already existing in the system, increasing their amplitude. Such an OS is called positive (POS). When the loop transfer coefficient of the amplifying element - the OS circuit is more than one, the slightest push, even thermal fluctuations, is enough to cause oscillations in the oscillator. Their amplitude will increase until some restraining mechanism is activated that reduces the gain, for example, until the amplitude is limited in the amplifying element.

Relaxation generators

If a broadband amplifier and an OS circuit are used in the oscillator (broadband means transmitting a wide frequency band, from the lowest to quite high), a relaxation oscillator will be obtained. The process of self-excitation in it occurs so quickly that even one cycle (period) of oscillations does not have time to go through, as the amplifying element is in saturation (limitation) mode. After that, the device must "rest" for some time (relax - rest) to return to its original state, after which the process will repeat.

Relaxation generators produce non-sinusoidal oscillations. On their basis, generators of short pulses of rectangular, triangular or some other voltage of a special shape are created. They are used, for example, to generate a sweep voltage in TVs. Most often, there are no inductors in relaxation generators (an exception is a transformer in a blocking generator), and the frequency or period of oscillation is determined by the duration of the charge-discharge of capacitors through resistors, i.e., the time constant of RC circuits (t = RC).

One of the simplest relaxation oscillators is usually performed on a Schmitt trigger (Fig. 45, a) - a device whose output voltage can take two values ​​- high (say, 5 V) and low (3 V). If the voltage at the trigger input increases, then at a certain value (for example, 2 V), the output voltage becomes low, and when the input voltage drops below another threshold (for example, XNUMX V), it becomes high. Thus, the transmission characteristic of the Schmitt trigger has the form of a rectangular hysteresis loop, which is indicated by the figure on its symbol. The fact that the output voltage is inverted, i.e., has the opposite polarity relative to the input, is indicated by a circle on the output pin of the microcircuit. Ready-made Schmitt triggers are available in various series of digital microcircuits produced by the industry.

This generator works like this. After it is turned on, the capacitor C1 is discharged, the voltage at the output DD1 is high. Capacitor C1 begins to charge through resistor R1, and after a while the voltage across it reaches the upper trigger switching threshold (3 V). The output voltage abruptly drops to zero, and the capacitor begins to discharge through the same resistor. When the voltage on it drops to the lower switching threshold (2 V), the output voltage jumps r. This process will be repeated periodically - self-oscillations will occur. The voltage shape on the capacitor is close to triangular (Fig. 45,b), and at the generator output it is rectangular (Fig. 45,c).

Let's consider another widely used relaxation oscillator made on discrete elements - a multivibrator (Fig. 46).

In fact, this is a two-stage transistor amplifier with communication between the stages through a decoupling capacitor C1. Capacitor C2 connects the output of the amplifier to the input, creating an OS. Because each stage inverts the signal, after two stages the signal is non-inverted and the feedback is positive. R1 and R4 are stage load resistors, and R2 and R3 are bias resistors that set some initial base current to saturate the transistors.

On the collectors of transistors, antiphase pulses are formed, which are close to rectangular in shape. If the values ​​​​of resistors and capacitors are the same, the pulses will be of the same duration - such a multivibrator is called symmetrical. With different denominations of parts, the pulses will become asymmetric - one half-cycle is shorter, the other is longer. The multivibrator becomes asymmetrical.

There are many circuits of relaxation oscillators, you can get acquainted with them in the radio engineering literature devoted to pulse technology. Today, such devices are in most cases performed on digital microcircuits, which is simpler, more convenient and more reliable.

Author: V.Polyakov, Moscow

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