Radar Receiver (part III)

Найдите в словаре и запишите в тетрадь транскрипцию данных слов. Прочитайте их вслух несколько раз.

Rather, through, circuit, interference, high, excite, value, either, neither, actual.

Words to be learnt

to get through- проходить, проводить

jack- разъем, гнездо

bandpass- полоса пропускания

to reject- отклонять, отвергать

actual signal- фактический, текущий сигнал

to match- подбирать, выравнивать, согласовывать

variable- переменный, непостоянный

indicating device- сигнальное устройство

frequency response- частотная характеристика

emitter follower- эмиттерный повторитель

low- impedance- низкоомный

to couple- соединять, сцеплять

to excite- возбуждать, вызывать (напр. ток)

both…and…- и…и…

frequency shift- уход, сдвиг частоты

either…or…- или…или…, либо…либо…

ahead of…- перед…

Text

There are receivers with no RF amplifier. In them the input to the converter is rather broadly tuned and some signals, other than the desired signal, will get through to the input jack of the converter stage. Normally these other signals will mix with the local oscillator signal and produce frequencies that are outside the bandpass of the 60 MHz IF amplifiers and will be rejected. However, if there is a station operating on a frequency of 1150 MHz, and this signal passes through the rather broad tuned input circuit and appears on the input jack of the converter stage, it also will mix with the local oscillator and produce a frequency of 60 MHz.

This signal will also be accepted by the IF amplifier stage and passed on, thus both signals will be indicated on the screen. This is known as image-frequency interference.

IF-Filter must filter the desired intermediate frequency out from the mixture frequencies arisen in the mixer stage. It is designed as one or more bandpasses. Normally, the bandpass is as narrow as possible without affecting the actual signal energy. When a selection of pulse widths is available, such as short and long pulses, the bandpass must be able to match the bandwidth of the two different signals.

The IF amplifier has the capability to vary both the bandpass and the gain of a receiver. After conversion to the intermediate frequency, the signal is amplified in several IF- amplifier stages. Most of the gain of the receiver is developed in the IF amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF stages. Gain must be variable to provide a constant voltage output for input signals of different amplitudes.

The detector in a microwave receiver serves to convert the IF pulses into video pulses. The simplest form of detector is the diode detector. The video amplifier receives pulses from the detector and amplifies these pulses for application to the indicating device. A video amplifier is fundamentally an RC coupled amplifier that uses high-gain transistors. However, a video amplifier must be capable of a relatively wide frequency response. The output stage of the receiver is normally an emitter follower. The low-impedance output of the emitter follower matches the impedance of the cable. The video pulses are coupled through the cable to the indicator for video display on the CRT.

The local oscillator excites a frequency for mixing with the incoming signal to get the intermediate frequency. Most radar receivers use megahertz intermediate frequency (IF) with a value between 30 and 75 megahertz. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. For example, if the local oscillator frequency is 3,000 megahertz, a frequency change of 0.1 percent will produce a frequency shift of 3 megahertz. This is equal to the bandwidth of most receivers and would greatly decrease receiver gain. The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power. The local oscillator output frequency must be tunable over a range of several megahertz in

The 4,000-megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 or 75 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable. The exiting frequency is either higher or lower than the incoming frequency. An 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.

Задание I. Найдите в тексте ответы на данные вопросы. Поработайте в парах: постарайтесь проговорить вопросы и ответы, не глядя в текст.

1. What occurs with frequencies that are outside the bandpass of the 60 MHz IF amplifiers?

2. What is known as image- frequency interference?

3. What is the function of the IF-Filter?

4. What capability does the IF amplifier have?

5. What is the overall bandwidth of the receiver determined by?

6. Must the gain be variable? Why?

7. What is the function of the detector in a microwave receiver?

8. What capability must a video amplifier have?

9. What frequency do most radar receivers have?

10. What capability must the local oscillator have?

Задание II. Приготовьтесь побеседовать по изученной теме в группе и с преподавателем.