, .. , . , , . .
To be able to
to be able to can, . . .
He is able to help you. .
He was able to help you. .
He will be able to help you. .
To be allowed to
To be permitted to
to be allowed to to be permitted to may.
I am allowed to use this device. ( ) .
He will be allowed to use this device. .
To have to
to have to must, .. . ", ".
It was very dark and we had to stay at home. , .
I don't have to stay here. .
To be to
to be to must, . . .
I was to meet her at 3 o'clock. 3 .
They are to begin this work at once. .
Should
should , must.
You s hould see a doctor. .
Ought to
a ought to must .
You ought to help your parents. .
Exercise. . .
1. Jack and Mike are playing tennis tomorrow. Mike is a very good player but I think Jack (can) win.
2. You (must) read this book. It is very interesting.
3. Nobody (may) take photos in this secret laboratory.
4. Yesterday I (must) leave the meeting because I (must) go to the airport to meet my mother.
5. You (must) send all the documents now. We need them very much.
6. My friends (must) arrive next week.
7. The fire was great but fortunately everybody (can) escape.
8. Im not working tomorrow so I (must not) get up early.
9. Tomorrows conference is very important for your future work. You (must) come.
10. You (must not) believe everything you read in newspapers.
Antenna
.
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Oscillation, diligent, viable, subsequent, concisely, either, essential, aperture, grind, accept, view, rewrite, surface
Words to be learnt
to conduct -
viable source
resonant antenna
fully yet concisely ,
essential part
to emerge ,
a new breed
the origin ,
plane subreflector
grinding -
surface -
equation -
to predict -
to cause ,
to accept ,
to trace back to ,
society -
to recognize
to simplify -
to rewrite
Text
Antennas have been used for 100 years since Hertz conducted his experiments in the 1880s. First, he had to develop a source of very rapid electrical oscillations. After very careful and diligent development, he had a viable source and detector which formed the basis of the apparatus used in subsequent experiments. Hertz discovered the principles of a resonant antenna. Hertz was not only a brilliant experimentalist but also an extremely good writer. He wrote all his work up into papers which describe fully yet concisely the details of all his experiments. The main papers were collected together and published as a book in 1893.
An antenna is a wire or metal conductor used either to radiate energy from a transmitter or to pick up energy at a receiver. It is insulated from the ground and may be situated vertically or horizontally. Antennas are an essential part of every radio system and the steady growth of radio communications has increased the demand for antennas. The 100year period can be broadly divided into two halves. From the start to the 1930s, the story of antenna development follows in a single chronological path, from Hertz work to the early microwave period to long wave communications and then to short wave communications. Most antenna development in this period was empirical. The 1930s represents a clear break point in the history of radio communications and hence antennas. The uses of radio expanded rapidly and separate subject areas emerged. Radar started, microwaves returned, broadcasting expanded and radio astronomy started. A new breed of antenna engineers used rigorous theory to analyze and design antennas. Each of the areas has its own story to tell and the antennas for each area developed to some extent separately with a common theory linking all types.
Hertz was the first person to develop and use antennas in order to verify the existence of radio waves. But microwave engineers now often design antennas using optical principles. Thus the origin of the large aperture antenna can be traced back to the optical telescope of Newton and others. The first reflecting telescope was proposed by the Scottish mathematician James Gregory in 1663, but it was Isaac Newton who built the first instrument in 1672. This used a spherical main reflector and a plane subreflector to produce a focal point on one side of the telescope. In the same year that Isaac Newton proposed his telescope to the Royal society, George Gassegram proposed a reflecting mirror and a subreflector to refocus the light to a point behind the main reflector. However neither the Gassegram nor the Gregorian telescope was practical until James Short developed a method of grinding nonspherical surfaces in 1740.
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The formulation of Maxwell`s Equations and hence the start of antennas theory also came before the classical experiments of Hertz. The research of James Clerk Maxwell was founded on the experimental work of Michael Faraday who discovered electromagnetic induction. Maxwell was a brilliant theoretician who predicted that all changes in electrical and magnetic fields cause waves to be propagated in space and that light was just another form of electromagnetic wave. This was a revolutionary suggestion at that time because the generally accepted view was that the ether behaved as a fluid and Newtonian physics of action-at-a-distance applied. Maxwell first published his ideas in 1862 and subsequently expanded them into a book in 1873. The book is recognized as a classical but it was and still is extremely difficult to follow. It was Hertz and Heaviside who simplified and rewrote the Equations in the form we know today.
I. , :
1. An antenna is (what, where, when)
2. Essential part of every radio system is (what, where, why)
3. The 100year period can be divided into (what, how, where)
4. Hertz conducted his experiments (what, who, when)
5. Hertz discovered (what, who, where, when)
6. The story of antenna development started (when, where, what, why)
7. James Gregory proposed (what, when, where, who)
8. Isaac Newton built (what, who, when)
9. George Gassegram proposed(what, who, why, where)
10. A method of grinding nonspherical surfaces was developed by(what, who)
11. Electromagnetic induction was discovered by(who, what, why)
12. Maxwell first published his ideas(when, who, what, where)
13. Hertz and Heaviside simplified and rewrote(what, who, why, when, where)
II. 3-4 , ,
III. ,
Unit 14.
Grammar Revision
- , () ().
? -How many?
1 12 - . .
1- one 7- seven
2- two 8- eight
3- three 9- nine
4- four 10- ten
5- five 11- eleven
6- six 12- twelve
13 19 - teen.
13- thirteen
14- fourteen
19- nineteen
, , ty
20- twenty
30- thirty
90- ninety
:
1.
She was born on March 4, 1982.
: n March fourth nineteen eighty two on the fourth of March nineteen eighty two.
2.
o , :
I live in flat 14. / flat fourteen /.
o , :
Take bus 5 to get to the park. / bus five /.
o , :
Open the book at page 20. / page twenty /.
.
. ? ( ) - Which? (the), .
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:
- the first the third
- the second the fifth
4 20 - th.
4 - the fourth
6 - the sixth
7 - the seventh
............
19 - the nineteenth
20 - the twentieth
, , y i:
30- thirtieth
40- fortieth
50- fiftieth
90- ninetieth
.
21 - the twenty-first
22 - the twenty-second
23 - the twenty-third
...............
55 - the fifty-fifth
Exercise1. - .
1. 5.08.1900
2. page 74
3. 8 + 25 = 33
4. I live in Kirensky Street 25, flat 69
5. 1 216 square miles
6. 8.07
7. 09.03.1879
8. 649 books
Exercise2. .
1. There are ________ months in a year.
2. January is ________ month of the year.
3. May is ________ month of the year.
4. There are ________ months in winter.
5. December is ________ month of the year and ________ month of winter.
6. There are ________ days in a week: ________ one is Monday, ________ one is Tuesday, ________one is Wednesday, ________ one is Thursday, ________ one is Friday, ________ one is Saturday and ________ one is Sunday.
7. Sunday is ________ day of the week in England and ________ one in Russia.
8. Monday is ________ day in Russia and ________ in Great Britain.
9. There are ________ hours in a day, ________ minutes in an hour and ________ seconds in a minute.
10. September, April, June and November have ________ days. All the rest have ________ except February.
11. There are ________ days in February except the leap year. It's the time when February has ________ days.
Radar Antenna
. .
Azimuth, accuracy, equate, weigh, ratio, rectangular, desired, aperture, key, major, vary.
Words to be learnt
to perform- ,
essential- , ,
distribution- ,
to apply- ,
accurate-
in the case of-
revolution rate-
in terms of- ,
to take into account- ,
gain-
to compare-
field- ,
radiation pattern-
axis-
intersection- ,
beam- ,
lobe-
to eliminate- ,
appropriate- ,
(un) desirable- ()
Text
The antenna is one of the most critical parts of a radar system. It performs the following essential functions:
It transfers the transmitter energy to signals in space with the required distribution and efficiency. This process is applied in an identical way on reception.
It ensures that the signal has the required pattern in space. Generally this has to be sufficiently narrow in azimuth to provide the required azimuth resolution and accuracy.
It has to provide the required frequency of target position updates. In the case of a mechanically scanned antenna this equates to the revolution rate. A high revolution rate can be a significant mechanical problem given that a radar antenna in certain frequency bands can have a reflector with immense dimensions and can weigh several tons.
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It must measure the pointing direction with a high degree of accuracy.
The antenna structure must maintain the operating characteristics under all environmental conditions. The basic performance of radar can be shown to be proportional to the product of the antenna area or aperture and the mean transmitted power. Investment in the antenna therefore brings direct results in terms of system performance. Taking into account these functions and the required efficiency of a radar antenna, two arrangements are generally applied:
the parabolic dish antenna
the array antenna.
Independently of the use of a given antenna for transmitting or receiving, an important characteristic of this antenna is the gain. Some antennas are highly directional; that is, more energy is propagated in certain directions than in others. The ratio between the amount of energy propagated in these directions compared to the energy that would be propagated if the antenna were not directional (Isotropic Radiation) is known as its gain. When a transmitting antenna with a certain gain is used as a receiving antenna, it will also have the same gain for receiving.
Most radiators emit (radiate) stronger radiation in one direction than in another. A radiator such as this is referred to as anisotropic. However, a standard method allows the positions around a source to be marked so that one radiation
pattern can easily be compared with another. The energy radiated from an antenna forms a field having a definite radiation pattern. A radiation pattern is a way of plotting the radiated energy from an antenna. This energy is measured at various angles at a constant distance from the antenna. The shape of this pattern depends on the type of antenna used. To plot this pattern, two different types of graphs, rectangular-and polar-coordinate graphs are used. The polar-coordinated graph has proved to be of great use in studying radiation patterns. In the polar-coordinate graph, points are located by projection along a rotating axis (radius) to an intersection with one of several concentric, equally-spaced circles.
The main beam (or main lobe) is the region around the direction of maximum radiation (usually the region that is within 3 dB of the peak of the main beam). The sidelobes are smaller beams that are away from the main beam. These sidelobes are usually radiation in undesired directions which can never be completely eliminated. The sidelobe level (or sidelobe ratio) is an important parameter used to characterize radiation patterns. It is the maximum value of the sidelobes away from the main beam and is expressed in Decibels. One sidelobe is called backlobe. This is the portion of radiation pattern that is directed opposing the main beam direction.
For the analysis of an antenna pattern the following simplifications are used:
Beam Width
The angular range of the antenna pattern in which at least half of the maximum power is still emitted is described as a Beam With. Bordering points of this major lobe are therefore the points at which the field strength has fallen in the room around 3 dB regarding the maximum field strength. This angle is then described as beam width or aperture angle or half power (- 3 dB) angle.
Aperture
The effective aperture of an antenna Ae is the area presented to the radiated or received signal. It is a key parameter, which governs the performance of the antenna.The aperture efficiency depends on the distribution of the illumination across the aperture.
Major and Side Lobes (Minor Lobes)
The radiation intensity in one lobe is considerably stronger than in the other. The strongest lobe is called major lobe; the others are (minor) side lobes. Since the complex radiation patterns associated with arrays frequently contain several lobes of
varying intensity, you should learn to use appropriate terminology. In general, major lobes are those in which the greatest amount of radiation occurs. Side or minor lobes are those in which the radiation intensity is least.
Front-to-Back Ratio
The front-to-back ratio of an antenna is the proportion of energy radiated in the principal direction of radiation to the energy radiated in the opposite direction. A high front-to-back ratio is desirable because this means that a minimum amount of energy is radiated in the undesired direction.
I. . . , .
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1. What are the essential functions of a radar antenna?
2. What are the main types of a radar antenna?
3. Why is the antenna structure very important?
4. What are the main characteristics of an antenna?
5. What is antenna gain?
6. What is an anisotropic radiator?
7. What is the main beam of the antenna?
8. What is beam width?
9. What is aperture?
10. What is front-to-back ratio of the antenna?
II. .
Unit 15.
Grammar Revision
, .. , , . .
( to):
to read- Read! - ! (!)
do not. to be to have:
Do not (dont) wait. - .
Dont be late! - !
Dont have meal at night. - .
do :
Do help him! - !
, , let, ( ) :
Let me do it myself. - .
Let us (Lets) ask John. - c .
Let him ( Jim) work. - () .
Let her (Mary) speak! - () !
Let them (students) use dictionaries. - () .
Dont let him go there! - !
Exercise 1. , , . .
1. .out late. (not to go)
2. Please, ready in 15 minutes. (to be)
3. about that. (not to worry)
4. ..me! (they, to ask)
5. .careful not to fall. (to be)
6. ..everything you hear! (not to believe)
7. Always both ways before crossing the street. (to look)
8. ..here. (he, to wait)
9. ..your own business! (to mind)
10.. a letter if you have no time (not to send)
Radar Receiver (part I)
. .
Temperature, automatic, purpose, synthesize, pseudo, nearby, attenuator, bias, proportional, simultaneous.
Words to be learnt
to tune-
external signal-
oscillator-
to retain - ,
reference frequency - , ,
wiring - .
instead of-
pulse repetition period -
bias -
to decrease - , ,
to prevent -
leakage - ,
to saturate - ,
instantaneous - ,
average - (. )
weak signal -
strong signal - c,
the number of - ( )
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 transmitters frequency readjusts the receiver; the receivers 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.
I. , , .
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. (Whatfor?)
5. The output discriminator produces an output voltage. (Whatfor?)
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. (Whatby?)
11. A large signal doesnt saturate the logarithmic amplifier. (Does?)
II. , .
Unit 16.
Grammar Revision
, , , , . (), ? ?:
To read ,
To write ,
To buy ,
To sell ,
4 (Active) 2 (Passive)
Active Passive
Indefinite to ask to be asked
Continuous to be asking ----------
Perfect to have asked to have been asked
Perfect Continuous to have been asking --------------
:
1.
2.
To operate the complex device is rather difficult.
()
3.
The metal to be used in our experiment is to be hard.
, (, ) , .
, Indefinite Active Passive, .
Indefinite Infinitive Active Passive .
not :
not to ask
not to be asked
Exercise.
1.To train highly qualified scientific workers is extremely important for the development of science.
2. To study this phenomenon requires much knowledge.
3. Our task is to obtain a new mixture with new properties.
4. The engineer must know the condition under the new material is to be utilized.
5. They hope to be sent to the conference.
6. The engineer was asked to design a transistor device which will regulate the temperature in the laboratory.
7. To increase the productivity of the machine tool one should know the characteristics of the material which is being machined.
8. In order to break this glass and great amount of force must be applied.
9. This method is accurate enough to give reliable results.
10. This problem is too complex to be solved.
11. The process to be analyzed in this article is known as ionization.
12. The famous Russian scientist Lebedev was the first to solve the problem of synthetic rubber.
13. The laboratory assistant will be the last to leave the classroom.
14. The problem to find a more economical way of production is to be solved soon.
Radar receiver (part II)
. .
Echo, sufficient, accept, dynamically, measure, furthermore, ahead, enough, dial, easily
Words to be learnt
sufficiently- ,
pulse envelope-
to feed- , ,
to accept-
intermediate frequency-
clutter level- ()
magnitude-
cell- ,
abrupt- ,
to destroy-
to affect- ,
to process-
frequency-band-
unwanted signal-
bandwidth- , ,
to pick up a signal- ,
image frequency-
swept gain-
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.
I. . , , .
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.
|II. .
Unit 17.
Grammar Revision
, , . . .
2 (Active)
2 (Passive)
Active Passive
Indefinite reading being read
Perfect having read having been read
:
Reading is his favorite occupation.---- .
He finished reading the book.---- .
On coming home he began to read.---- , .
I remember seeing her there.---- , .
Besides being clever he is very industrious.---- , , .
She reproached herself for having said it.---- ,
.
Exercise.
1. Maintaining constant temperature and pressure during the test was absolutely necessary.
2. The students taking part in the research was of great help to the whole laboratory.
3. His offering new temperature conditions for the system will give greater efficiency.
4. The task of the factory was producing corrosion-resistant polymers in far larger quantities.
5. Their aim is finding new ways of utilizing this first-class polymer in light industry.
6. The experimentalist suggested purifying the solution by a new method.
7. The researcher took great interest in our improving the properties of the rubber.
8. We know of silver and cooper being very good conductors of electricity.
9. The new heating and lighting installations supply the shops of our plant with heat and light.
10. At present scientists take great interest in the methods of turning the light and heat of the sun directly into electricity.
11. By subjecting air to very great pressure and cooling it is possible to transform it to the liquid state.
12. One cannot transform water into steam without heating it.
13. In converting water into ice we do not change its composition.
14. Physical changes are those which influence the condition or state of matter without changing its composition.
Radar Receiver (part III)
. .
Rather, through, circuit, interference, high, excite, value, either, neither, actual.
Words to be learnt
to get through- ,
jack- ,
bandpass-
to reject- ,
actual signal- ,
to match- , ,
variable- ,
indicating device-
frequency response-
emitter follower-
low- impedance-
to couple- ,
to excite- , (. )
bothand-
frequency shift- ,
eitheror- ,
ahead of-
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.
I. . : , .
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?
II. .
Unit 18.
Grammar Revision
- , , .
2 (Active)
3 (Passive)
Active Passive
Present asking being asked
Past ---- asked
Perfect having asked having been asked
1. Present Participle Active ing ( to)
to read reading ,
to build building ,
2. Past Participle Active Passive ed
to ask , asked ,
to order , ordered ,
Past Participle III
To give , given ,
To send , sent ,
To buy , bought ,
