Words to be learnt. external signal- сигнал от внешних устройств

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to tune- настраивать

external signal- сигнал от внешних устройств

oscillator- излучатель

to retain - сохранять, удерживать

reference frequency - основная, эталонная частота, частота обращений

wiring - эл. проводка

instead of…- вместо…

pulse repetition period - период повторения импульсов

bias - смещение

to decrease - снижать, понижать, уменьшать

to prevent - предотвратить

leakage - утечка, рассеяние

to saturate - насыщать, переполнять

instantaneous - мгновенный, немедленный

average - средний (напр. о значении)

weak signal - слабый сигнал

strong signal - cильный, устойчивый сигнал

the number of - ряд (некоторое количество)

Text

The radar receiver requires a limited tuning range to compensate for transmitter and local oscillator frequency changes because of variations in temperature and

loading. Microwave radar receivers usually use automatic frequency control (AFC) for this purpose. AFC circuits are used in situations where you must accurately control the frequency of an oscillator by some external signal. The AFC circuit senses the difference between the actual oscillator frequency and the frequency that is desired and produces a control voltage proportional to the difference. This variant of AFC circuits is used in radio receivers, fm transmitters, and frequency synthesizers to maintain frequency stability. It requires relatively constant amplitude of the (received) input-signal. For pulse-radar sets this form isn't practicable therefore.

Automatic frequency control circuits in a non-coherent or pseudo-coherent radar set use two similar systems: the transmitter’s frequency readjusts the receiver; the receiver’s frequency readjusts the transmitter. Both systems retain a sample of the transmitted signal using a Directional Coupler fitted between the transmitter and the Duplexer. This RF-signal will be mixed with the local oscillator frequency to form an AFC-IF-signal. This signal is applied to a frequency-sensitive discriminator that produces an output voltage proportional in amplitude and polarity to any change in AFC-IF frequency. If the IF signal is at the discriminator center frequency, no discriminator output occurs. The center frequency of the discriminator is essentially a reference frequency for the IF signal. The output of the discriminator provides a control voltage to maintain the local oscillator at the correct frequency.

The Local Oscillator is adapted to the actual line frequency in this wiring. As a second variant the control circuit can control the transmitters frequency instead of the LO frequency! In this case the transmitter-frequency would regulate to the more stable LO-frequency. In radar receivers the wide variation in return signal amplitudes make adjustment of the gain difficult. The adjustment of receiver gain for best visibility of nearby target return signals is not the best adjustment for distant target return signals. Circuits used to adjust amplifier gain with time, during a single pulse repetition period, are called STC circuits, or „swept gain attenuator”.

Sensitivity time-control circuits apply a bias voltage that varies with time to the IF amplifiers to control receiver gain. When the transmitter fires, the STC circuit decreases the receiver gain to zero to prevent the amplification of any leakage energy from the transmitted pulse. At the end of the transmitted pulse, the STC voltage begins to rise, gradually increasing the receiver gain to maximum. In the ideal case the the receiver gain is proportionally to R4. The STC voltage effect on receiver gain is usually limited to approximately 50 miles. This is because close-in

targets are most likely to saturate the receiver; beyond 50 miles, STC has no affect and the receiver operates normally.

Gain control is necessary to adjust the receiver sensitivity for the best reception of signals of widely varying amplitudes. A complex form of automatic gain control (AGC) or instantaneous automatic gain control (IAGC) is used during normal operation. The simplest type of AGC adjusts the IF amplifier bias (and gain) according to the average level of the received signal. With AGC, gain is controlled by the largest received signals. When several radar signals are being received simultaneously, the weakest signal may be of greatest interest. IAGC is used more frequently because it adjusts receiver gain for each signal.

The AGC circuit is essentially a wide-band, dc amplifier. It instantaneously controls the gain of the IF amplifier as the radar return signal changes in amplitude. The effect of IAGC is to allow full amplification of weak signals and to decrease the amplification of strong signals. The range of IAGC is limited, however, by the number of IF stages in which gain is controlled. When only one IF stage is controlled, the range of IAGC is limited to approximately 20 dB. When more than one IF stage is controlled, IAGC range can be increased to approximately 40 dB.

The logarithmic amplifier is a nonsaturating amplifier that does not ordinarily use any special gain-control circuits. The output voltage of the logarithmic amplifier is a linear function of the input voltage for low-amplitude signals. It is a logarithmic function for high-amplitude signals. In other words, the range of linear amplification does not end at a definite saturation point, as is the case in normal IF amplifiers. Therefore, a large signal does not saturate the logarithmic amplifier; rather, it merely reduces the amplification of a simultaneously applied small signal.

Задание I. Задайте вопросы к данным предложениям, начиная их словами, предложенными в скобках.

1. The radar receiver requires a limited tuning range. (Why?)

2. AFC circuits are used in microwave radar receivers. (In what situations?)

3. AFC circuits are used to maintain frequency stability. (Where?)

4. RF signal is mixed with the local oscillator frequency. (What…for?)

5. The output discriminator produces an output voltage. (What…for?)

6. It is difficult to make the adjustment of the radar receiver gain. (Why?)

7. The STC voltage begins to rise, increasing the receiver gain. (When?)

8. Gain control is necessary. (Why?)

9. The weakest signal may be of the greatest interest. (When?)

10. The range of IAGC is limited. (What…by?)

11. A large signal doesn’t saturate the logarithmic amplifier. (Does…?)