10. Переведите предложения с английского языка на русский:
1. It is possible to use efficient RF amplifiers with frequency modulated signals. 2. The spectrum of an FM wave consists of a carrier wave with a series of pairs of sidebands. 3. Ionospheric refraction doesn't appreciably affect FM or TV signals because of their higher frequency. 4. The unmodulated frequency of a FM signal is called its center frequency. 5. The amount of "swing" in the transmitter's frequency in any direction above or below the center frequency is called its deviation. 6. The total frequency space occupied by a FM signal is twice its deviation. 7. FM is often encountered by the general public in the form of FM radio broadcasting, which supports high fidelity sound and stereophonic reproduction.
11. Переведите текст с русского языка на английский
ЧМ- вид модуляции, при которой частота сигнала несущей изменяется в соответствии с аналоговым информационным сигналом. Наиболее распространенный вид модуляции радиосвязи в КВ (SW) и УКВ диапазонах. По сравнению с АМ менее подвержен зашумлению и помехам. Есть два подвида - узкая и широкая частотная модуляция. Все известные приемники способны принимать оба подвида.
TEXT 2. Amplitude Modulation
12. Выучите следующие слова и выражения:
accurately ['ækjərətlɪ] - точно, безошибочно, аккуратно
interfere [ˌɪntə'fɪə] - 1. интерферировать, пересекаться , скрещиваться
2) (interfere with) служить препятствием, мешать, быть помехой (чему-л.)
superimpose [ˌs(j)uːp(ə)rɪm'pəuz] (on) - накладывать (одно на другое)
sum [sʌm] - сумма, количество, величина, итог, совокупность.
reference ['ref(ə)r(ə)n(t)s] - справка; справочная информация
predominant [prɪ'dɔmɪnənt] - господствующий, преобладающий
vestigial sideband signal [ves'tɪʤɪəl] - сигнал с частично подавленной боковой полосой
13. Прочитайте и устно переведите текст:
Amplitude modulation (AM) is one of the earliest and easiest radio modulation techniques. In this case the radio frequency signal has a representation of the data (a sound wave) superimposed in it.
Fig. 5. Amplitude Modulation
In AM, the carrier itself does not fluctuate in amplitude. It is found that when a carrier is amplitude modulated, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the main carrier. These components are called sidebands. The sideband power accounts for the variations in the overall amplitude of the signal. To see how this happens, let’s take the example of a carrier on a frequency of 1 MHz which is modulated by a steady tone of 1 kHz.
The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz (upper sideband), and a difference frequency is produced at 1 MHz - 1 kHz (lower sideband).
Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information.
A variant of amplitude modulation when only one sideband is transmitted is called single-sideband transmission (SSB). In this case, the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency. Since the sidebands are mirror images, the choice which sideband to use is a matter of convention.
The problem is that the more of the sideband you filter out, the more distortion you get from the detected waveform. So there is a practical limit to how much you can reduce the bandwidth of the signal. It's about 50% reduction of one sideband. SSB modulation is used for long-distance telephony (such as in the amateur radio bands) and telegraphy over land and submarine cables.
An alternative technique used for signal power reduction is to remove the remaining carrier signal from the AM signal; the signal produced is a double-sideband suppressed carrier (DSSC) signal. A compromise between the DSBSC modulation and the SSB modulation is known as Vestigial Side Band (VSB) modulation. VSB is used in television transmission to save RF spectrum space. If the carrier signal is not completely removed, the signal is called a double-sideband reduced carrier (DSRC) signal.
However, as there now are more efficient and convenient methods of modulating a signal, the use of AM is declining.