EXERCISE 4 Read and translate the text

Apparatuses on multichannel and thin-route systems with frequency division of channels and amplitude modulation are comprised into a number of various devices commonly called units. Main units of a multichannel system with frequency division of channels and amplitude modulation are frequency converters, amplifiers, generators, electric filters and several special units – differential systems, transceiving devices, amplitude limiters, range extenders and range compressors, delineation circuits and correction circuits.

Frequency converters are one of the most important terminal units because they are the very devices to create multichannel systems with frequency division of channels and amplitude modulation. For this purpose in the converters there are used nonlinear elements: dead (semiconductor diodes) and active (transistors). There are also used frequency converters with electron tubes which help to obtain greater signal amplification.

Frequency converters are used to transfer frequency spectrum from one area into another. They are part of super heterodyne detector (a frequency converting receiver). The result of conversion is a new frequency value fcalled intermediate value. The intermediate frequency can be both higher and lower than signal frequency. In the first case the frequency conversion is upwards, in the other case it is downwards. In this process the frequency conversion principle of modulation is not broken, only fluctuation-carrier frequency is changed at the output of the converter.

To convert frequency there are used line circuits with periodically changing characteristics in radio receivers. The structural circuit consists of a converting element, heterodyne and filter. According to the type of a converting element the frequency converters are usually divided into diode, transistor and integrated converters. There is a different number of converting elements, so one can find the following types of converters: ordinary (with one converting element), balanced (two elements), and ring (4 elements).

Most SHF (super high frequency) receivers use crystal diodes as converting element. Crystals have fast response and comparatively low noises. But they have a drawback - they don’t have amplifying properties. Such converters are used in professional devices of decameter range.

Transistor converters use bipolar transistor, or unipolar transistor as converting element. Balanced converters are combination of two non-balanced converters. To increase channel isolation between a signal and a heterodyne there are used double balanced converters called ring converters. Such type converters have low noises and better amplitude characteristics. They are used in frequency range up to 100 MHz in professional decameter devices.