Theory: radio transmitting devices. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Theory: radio transmitting devices. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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With the development of radio engineering, a huge number of different radio transmitting devices appeared - from powerful broadcasting and radar, generating megawatts of high-frequency power, to miniature pocket ones, with a power of milliwatts, used to radio control models or turn on a car security alarm. They operate at a wide variety of frequencies from tens of kilohertz (very long waves) to tens of gigahertz (millimeter waves). Nevertheless, all such devices have much in common, which makes it possible to distinguish them into a separate class of radio engineering devices.

Now, single-stage radio transmitters are rarely used, which are a self-oscillator connected to the antenna. These can be either the simplest micropower transmitters of radio control signals, or unique microwave transmitters, such as radar. Most radio transmitters are built according to the master oscillator - power amplifier scheme. In this case, the functions of excitation of oscillations and their amplification to the required power level turn out to be separated, which makes it possible to construct these cascades in an optimal way.

Consider the most common and interesting for radio amateurs LW, MW and HF transmitters, i.e. operating in the bands that are reserved for broadcasting with amplitude modulation (AM). Historically, this is the oldest broadcasting system, which has many shortcomings, but it cannot be abandoned. The fact is that the waves of these ranges propagate over long distances, and hundreds of millions of radio receivers are used in the world, designed specifically for receiving AM signals. Therefore, there are a great many AM transmitters in the world. Their joint work on the air is impossible without a clear organization, primarily related to the distribution of frequencies. Each radio station has its own operating frequency, and the frequency grid is set to a multiple of 9 kHz - on the LW and MW and 5 kHz - on the HF.

The frequency stability requirements of broadcast transmitters are very high, and now only frequency synthesizers are used in their master oscillators. Moreover, reference frequencies for synthesizers are "tied" to national time and frequency standards. In a number of cases, the carrier of a powerful LW station serves as such a standard, such as, for example, the carrier of the Droitwich radio station in England.

In Russia, they act a little differently: the reference signal received from the atomic frequency standard is emitted by special radio stations in the Moscow region at a frequency of 66. (6) kHz and in Irkutsk at a frequency of 50 kHz. Each radio center has a special reference frequency receiver (RF) and a frequency comparison device that allows you to adjust the reference frequency of the synthesizer to it (Fig. 56). The relative instability of the frequency of broadcasting stations can be only 10-12 ... 10-15. Clocks synchronized with such accuracy would "gone" somewhere by a second in a million years! By the way, the industry is already starting to produce electronic watches with adjustment according to reference frequency signals.

Theory: radio transmitting devices

So, highly stable carrier frequency oscillations are received from the master oscillator, they are amplified by the intermediate stages of the transmitter and fed to the final, powerful stage, in which modulation is carried out simultaneously with amplification. The question may arise: why not modulate the signal at a low level and then amplify the modulated oscillations? This is due to the desire to obtain the maximum efficiency of the transmitter - after all, we are talking about powers of tens and hundreds of kilowatts.

Anode modulation in class B mode with high efficiency has become most widespread. A simplified diagram of the final stage of the transmitter with a modulator is shown in fig. 57. High-frequency carrier oscillations through the coupling coil L1 enter the grid circuit L2C1 of the output stage of the transmitter, assembled on a powerful tetrode VL1. The automatic bias circuit R1C2 creates (due to the flow of grid current) such a negative bias on the control grid that the operating point is at the lower bend of the lamp characteristic. In this case, the anode current pulses have the form of half-cycles of sinusoidal oscillations.

Theory: radio transmitting devices

The anode circuit L3C4 restores the sinusoidal shape of the carrier oscillations, and their amplitude is almost equal to the anode supply voltage Ua, and the power corresponds to the rated power of the transmitter. Through the coupling coil L4, the amplified oscillations enter the antenna. The screen grid of the generator lamp is powered from a separate source with a voltage Ue, less than the anode one.

The modulator is a conventional push-pull audio frequency amplifier, made on powerful triodes VL2 and VL3, also operating in class B mode. The output power of the modulator reaches half the carrier power. The secondary winding of the modulation transformer T2 is connected to the anode circuit of the generator lamp in series with the power source.

With a modulation depth of 100%, the anode voltage of the generator lamp changes almost from zero to 2Ua, and the amplitude of high-frequency oscillations in the anode circuit changes accordingly, as the oscillograms show. The industrial efficiency (the ratio of radiated power to the power consumed from the power network) reaches 60...70% for the described transmitter at a radiated power of about 100 kW.

To operate at such high powers, special generator lamps with forced air or water cooling of the anode have been developed. The oscillatory circuits and other elements also use unique designs: large-diameter coils wound with a copper tube on ceramic insulators, air-dielectric capacitors with a large distance between the plates to prevent high-frequency breakdown, etc. It is not surprising that the output circuit of a powerful transmitter occupies, for example , at the radio center a separate room.

Author: V.Polyakov, Moscow

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