Radar receiver (part II)
Найдите в словаре и запишите в тетрадь транскрипцию данных слов. Прочитайте их вслух несколько раз.
Echo, sufficient, accept, dynamically, measure, furthermore, ahead, enough, dial, easily
Words to be learnt
sufficiently- достаточно, в достаточной мере
pulse envelope- огибающая импульса
to feed- поставлять, снабжать, обеспечивать
to accept- принимать
intermediate frequency- промежуточная частота
clutter level- уровень шума (помех)
magnitude- величина
cell- элемент, секция
abrupt- внезапный, неожиданный
to destroy- разрушать
to affect- оказывать влияние на…, воздействовать
to process- обрабатывать
frequency-band- частотный диапазон
unwanted signal- посторонний сигнал
bandwidth- полоса частот, полоса пропускания, пропускная способность
to pick up a signal- поймать, уловить сигнал
image frequency- частота зеркального канала
swept gain- рабочий коэффициент усиления
Text
The function of the receiver is to take the weak echoes from the antenna system, amplify them sufficiently, detect the pulse envelope, amplify the pulses, and feed them to the indicator. The receivers used in radars are capable of accepting weak echoes and increasing their amplitudes by a factor of 20 or 30 million. Since radar frequencies are not easily amplified, a superheterodyne receiver changes the radio frequency to an intermediate frequency for amplification.
Local clutter levels dictate the magnitude of swept gain and different requirements for swept gain are presented as the antenna rotates. Modern systems dynamically measure clutter levels for a large number of cells within the coverage area of the radar. These measurements are slowly adjusted to take account of changing clutter levels and used to set the swept gain attenuator to an appropriate level for the range azimuth cell currently being processed. In most cases, the values used are a variation on the normal static law. This approach, while simple in principle, can risk reduction of MTI performance at the edges of clutter. This is due to abrupt changes in swept gain law destroying the integrity of the clutter amplitudes. Furthermore if long or compressed pulses are used, amplitude changes can affect the performance. Swept gain is generally applied to pin diodes, which are biased to provide a reasonably linear characteristic.
The superheterodyne receiver changes the RF frequency into an easier to process lower IF- frequency. This IF- frequency will be amplified and demodulated to get a video signal. The RF-carrier comes in from the antenna and is applied to a filter. The output of the filter is only the frequencies of the desired frequency-band. These frequencies are applied to the mixer stage. The mixer also receives an input from the local oscillator. These two signals are beat together to obtain the IF through the process of heterodyning. There is a fixed difference in frequency between the local oscillator and the RF signal at all times by tuning the local oscillator. This difference in frequency is the IF. This fixed difference ensures a constant IF over the frequency range of the receiver. The IF-carrier is applied to the IF-amplifier. The amplified IF is then sent to the detector. The output of the detector is the video component of the input signal.
A low-noise RF amplifier stage ahead of the converter stage provides enough selectivity to reduce the image-frequency response by rejecting these unwanted signals and adds to the sensitivity of the receiver. The borders of the bandwidth of this amplifier are chosen to eliminate the image frequencies. Many older radar receivers do not use a low-noise pre-amplifier (RF stage), they simply send the echo signal directly to a crystal mixer stage. It is possible for these receivers to receive two different stations at the same point of the dial.
The mixer stage is used to increase the received frequency to an intermediate frequency. The result is a second reception frequency as a „mirror image” around the intermediate frequency. Assuming an intermediate frequency of 60 MHz, the local oscillator will track at a frequency of 60 MHz higher than the incoming signal. For example, suppose the receiver is tuned to pick up a signal on a frequency of 1030 MHz. The local oscillator will be operating at a frequency of 1090 MHz. The received and local oscillator signals are mixed, or heterodyned, in the converter stage and one of the frequencies resulting from this mixing action is the difference between the two signals, or 60 MHz, the IF frequency. This IF frequency is then amplified in the IF stages and sent on to the detector and audio stages. Any signal at a frequency of 60 MHz that appears on the plate of the converter circuit will be accepted by the IF amplifier and passed on.
Задание I. Внимательно прочитайте текст еще раз. Подобрав части предложений, друг к другу, составьте из них целые предложения.
1. The receiver… …to eliminate the image frequencies.
2. A superheterodyne receiver… …the radio frequency to an intermediate frequency.
3. Local clutter levels… …the received and local oscillator signals are mixed.
4. Modern radar systems are capable of.. …from the local oscillator.
5. IF is amplified and demodulated… …to get a video signal.
6. The mixer receives the input signal… …the received frequency increases to an intermediate frequency.
7. The output of the detector is… …takes the weak echoes from the antenna, amplifies them, detects the pulse envelope, amplifies the pulses and feeds them to the indicator.
8. The borders of the amplifier bandwidth are chosen… …dictate the magnitude of swept gain.
9. During the mixer stage… …the video component of the input signal.
10. During the converter stage… …measuring clutter levels for a large number of cells within the coverage area of the radar.
Задание |II. Задайте вопросы к составленным предложениям.