Text 2 (c) Basic Receivers and Transmitters

A much simplified transmitter block-diagram is shown in Fig. 1. This could be called the all-purpose block-diagram since it could be easily converted to a low-level or high-level amplitude modulation transmitter, a single sideband transmitter, or a frequency modulation transmitter. Obviously in the example, the circuit details would vary greatly, particularly in the modulators, and if detailed block diagrams were drawn the underlying similarities in structure would be less obvious.

ARIAL

R.F.

OSCILLATOR

MODULATOR

MODULATING

SIGNAL

Fig.1. Basic radio transmitter block-diagram

The basic туре of a receiver is a tuned radio frequency, however, this is rarely used. The standard receiver configuration is the superheterodyne (superhet) shown in Fig.2.

MIXER

DETECTOR

OUTPUT

LOCAL OSCILLATOR

Fig.2. Basic superheterodyne receiver block-diagram

The desired radio frequency (r.f.) is converted to a constant intermediate frequency by taking the difference frequency after mixing the received signal with the output from a local oscillator. Since most of the amplification and selectivity is provided by constant frequency and bandwidth stages, the design problem is eased.

In both the transmitter and the receiver, r.f. oscillators have to be tuned to different frequencies. In the transmitter it is the master oscillator, while in the receiver it is the local oscillator. Modern practice is to use a frequency synthesizer with a single crystal to provide stability and accuracy.

Text 2 (d) Current and Future Developments of Radio Systems

Changes in aircraft radio systems occur more and more frequently. The first airborne radio equipment¹ used thermionic devices², cat’s whisker detectors³ and large parallel plate tuning capacitors4; power, weight and size were restrictions on the development of such equipment. In the 1950s transistorized equipment began to appear although not completely transistorized. Even now thermionic devices are still with us in the shape of the magnetron and the cathode-ray tube. Claims concerning all solid-state weather radars were made about mid-1979, and commercially available equipment appeared in 1980. The cathode-ray tube will remain with us for many years but will eventually be replaced by a matrix of electroluminescent elements.

Transistorized equipment is, of course, still marketed, but many of the transistors, diodes and resistors now appear on integrated circuits. The emergence of integrated circuits has revolutionized the design of air radio systems. The use of IC techniques to produce microprocessors has opened up a new world. Increased safety, increased payload, increased reliability and improvement in performance allow flights to be made in conditions where previously aircraft have to be grounded. Completely new systems do not appear very frequently, although when they do, it is often because the improvement in the state of the art made the impossible possible.

NOTES: ¹airborne radio systems – бортовое радиооборудование; ²thermionic devices - термоэлектронные устройства; ³cat’s whisker detectors – детекторы (датчики) с нитевидным кристаллом; parallel plate tuning capacitors – конденсатор настройки с параллельными анодами