Module 18—Radar Principles
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Index-1 to 3
The charge path includes the primary of the pulse transformer, the dc power supply, and the charging impedance. When the modulator switch is closed, the transmission line discharges through the series circuit. This circuit consists of the modulator switch and the primary of the pulse transformer.
The artificial transmission line is effectively an open circuit at its output end. Therefore, when the voltage wave reaches the output end of the line, it is reflected. As the reflected wave propagates from the output end back toward the input end of the line, it completely discharges each section of the line. When the reflected wave reaches the input end of the line, the line is completely discharged, and the modulator pulse ceases abruptly. If the oscillator and pulse transformer circuit impedance is properly matched to the line impedance, the voltage pulse that appears across the transformer primary equals one-half the voltage to which the line was initially charged.
The width of the pulse generated by an artificial transmission line depends on the time required for a voltage wave to travel from the input end to the output end of the line and back. Therefore, we can say the pulse width depends on the velocity of propagation along the line (determined by the inductances and capacitances of each section of the line) and the number of line sections (the length of the line).
Figure 2-6.—Modulator with an artificial transmission line for the storage element.
PULSE-FORMING NETWORKS.—A pulse-forming network is similar to an artificial transmission line in that it stores energy between pulses and produces a nearly rectangular pulse. The pulse-forming network in view B of figure 2-5 consists of inductors and capacitors so arranged that they approximate the behavior of an artificial transmission line.
Each capacitor in the artificial transmission line, shown in view A, must carry the high voltage required for the modulator pulse. Because each capacitor must be insulated for this high voltage, an artificial transmission line consisting of many sections would be bulky and cumbersome.
The pulse-forming network, shown in view B of figure 2-5, can carry high voltage but does not require bulky insulation on all of its capacitors. Only series capacitor C1 must have high-voltage insulation. Because the other capacitors are in parallel with the corresponding inductors, the modulator- pulse voltage divides nearly equally among them. Thus, except for C1, the elements of the pulse-forming network are relatively small.
Pulse-forming networks are often insulated by immersing each circuit element in oil. The network is usually enclosed in a metal box on which the pulse width, characteristic impedance, and safe operating voltage of the network are marked. If one element in such a network fails, the entire network must be replaced.
Q10. What are the two basic types of transmitters?
Q11. What controls transmitter pulse width?
Q12. In addition to a flat top, what characteristics must a modulator pulse have?
Q13. What type of modulator is most commonly used in modern radar systems?
Q14. What three types of storage elements most often are used in modulators?
Q15. What characteristic is determined by the time required for a voltage wave to travel from the input end of an artificial transmission line to the output end and back again?
Modulator Switching Devices
The voltage stored in a storage-element capacitor, artificial transmission line, or pulse-forming network must be discharged through a MODULATOR SWITCHING DEVICE. The modulator switching device conducts for the duration of the modulator pulse and is an open circuit between pulses. Thus, the modulator switch must perform the following four functions:
1. Close very quickly and then reach full conduction in a small fraction of a microsecond
2. Conduct large currents (tens or hundreds of amperes) and withstand large voltages (thousands of volts)
3. Cease conducting (become an open circuit) with the same speed that it starts to conduct
4. Consume only a very small fraction of the power that passes through it
These switching and conducting requirements are met best by the THYRATRON tube. The thyratron tube is normally held below cutoff by a negative grid voltage and conducts when a positive trigger pulse is applied to its grid. Once fired, the thyratron tube continues to conduct as long as the storage element (artificial transmission line or pulse-forming network) is discharging.
During discharge of the storage element, the gas in the thyratron tube is highly ionized. While the storage element discharges, the plate-to-cathode resistance of the thyratron is practically zero. When the storage element is completely discharged, current ceases to flow through the thyratron and the gases become deionized; the negative grid bias regains control, and the thyratron is cut off (the modulator switch opens).
Most radar modulators use a high-voltage, dc power supply. Typical dc power supplies for radar modulators use a half-wave rectifier, a full-wave rectifier, or a bridge rectifier.
The modulator charging impedance, shown in figure 2-7, prevents the dc power supply from becoming short-circuited when the modulator switch closes. When the modulator switch is open, the charging impedance also controls the rate at which the storage element charges. When the charging impedance is small, the storage element charges rapidly.
Figure 2-7.—Modulator charging impedance.
Many different kinds of charging impedance and charging circuits are used in radar modulators. The type of charging impedance and charging circuit used depends on the following five elements:
1. The type of power supply (ac or dc)
2. The type of storage element
3. The amount of modulator pulse voltage required
4. The pulse-repetition rate
5. The frequency of the available ac supply voltage
Q16. What type of tube best meets the requirements of a modulator switching element?
Q17. What modulator element controls the rate at which the storage element charges?
The KEYED-OSCILLATOR TRANSMITTER most often uses a MAGNETRON as the power oscillator. The following discussion is a description of a magnetron used as a keyed-oscillator radar transmitter.
Figure 2-8 shows the typical transmitter system that uses a magnetron oscillator, waveguide transmission line, and microwave antenna. The magnetron at the bottom of the figure is connected to the waveguide by a coaxial connector. High-power magnetrons, however, are usually coupled directly to the waveguide. A cutaway view of a typical waveguide-coupled magnetron is shown in figure 2-9.
Figure 2-8.—Keyed oscillator transmitter physical layout.
Figure 2-9.—Typical magnetron.
The magnetron is an electron tube in which a magnetic (H) field between the cathode and plate is perpendicular to an electric (E) field. Tuned circuits, in the form of cylindrical cavities in the plate, produce rf electric fields. Electrons interact with these fields in the space between the cathode and plate to produce an ac power output. Magnetrons function as self-excited microwave oscillators. These multicavity devices may be used in radar transmitters as either pulsed or CW oscillators at frequencies ranging from approximately 600 to 30,000 megahertz. (If you wish to review magnetron operation in more detail, refer to NEETS, Module 11, Microwave Principles.)
Let’s examine the following characteristics of a magnetron used as a pulse radar transmitter oscillator stage:
• Pulse characteristics
• The magnet
• Output coupling
In speaking of a magnetron oscillator, STABILITY usually refers to the stability of the mode of operation of the magnetron. The two main types of mode instability are MODE SKIPPING and MODE SHIFTING.
Mode skipping (or misfiring) is a condition in which the magnetron fires randomly in an undesired, interfering mode during some pulse times, but not in others. Pulse characteristics and tube noises are factors in mode skipping.
Mode shifting is a condition in which the magnetron changes from one mode to another during pulse time. This is highly undesirable and does not occur if the modulator pulse is of the proper shape, unless the cathode of the magnetron is in very poor condition.
PULSE CHARACTERISTICS are the make up of the high-voltage modulator pulse that is applied to the magnetron. The pulse should have a steep leading edge, a flat top, and a steep trailing edge. If the leading edge is not steep, the magnetron may begin to oscillate before the pulse reaches its maximum level. Since these low-power oscillations will occur in a different mode, the mode of the magnetron will be shifted as the pulse reaches maximum power. This mode shifting will result in an undesirable magnetron output. For the same reason (to prevent mode shifting), the top of the modulator pulse should be as flat as possible. Variations in the applied operating power will cause variations in the mode of operation. The trailing edge of the pulse should also be steep for the same reason--to prevent mode shifting.
The purpose of the MAGNET is to produce a fairly uniform magnetic field of the desired value over the interaction space between the cathode and plate of the magnetron. The strength of the magnet is critical for proper operation. If the magnetic field strength is too high, the magnetron will not oscillate. If the magnetic field strength is too low, the plate current will be excessive and power output will be low. Frequency of operation will also be affected.
Since the strength of the magnet is critical, you should be careful when handling the magnet. Striking the magnet, especially with a ferromagnetic object, will misalign the molecular structure of the magnet and decrease the field strength.
The OUTPUT COUPLING transfers the rf energy from the magnetron to the output transmission line (coaxial line or waveguide). A number of considerations impose restrictions upon the output circuit. The wavelength (frequency) and the power level of the magnetron output energy determine whether the transmission line to the antenna will be waveguide or coaxial line.
The coaxial output circuit consists of a length of coaxial line in which the center conductor is shaped into a loop and inserted into one of the magnetron cavities for magnetic coupling. The load side of the coupling line may feed either an external coaxial line or a waveguide. If the external line is coaxial, the connection may be direct or by means of choke joints. If the external line is a waveguide, the output circuit must include a satisfactory junction from the coaxial line to the waveguide. One type of junction used quite often is the PROBE COUPLER. The probe coupler acts as an antenna radiating into the waveguide.
The waveguide output may be fed directly by an opening (slot) into one of the magnetron cavities, as shown in figure 2-9. This opening must be covered by an iris window to maintain the vacuum seal.
The peak power ratings of magnetrons range from a few thousand watts (kilowatts) to several million watts (megawatts). The average power ratings are much lower, however, and vary from a few watts to several kilowatts. Additionally, many of the magnetrons used in modern radar systems are tunable in frequency. Typically, a tunable magnetron can vary the output frequency 5 percent about the center of its frequency band. Thus the carrier frequency of radar can be changed to obtain the best operation or avoid electronic jamming on a particular frequency.
Modulator signals of many thousands of volts are applied to the magnetron cathode during operation. These high voltage levels require large glass posts to insulate the cathode and filaments from the anode block. In some high-power magnetrons, the cathode is completely enclosed in a container filled with insulating oil.
All radar transmitters contain lethal voltages. Extreme care and strict observance of all posted safety precautions are essential when working on a radar transmitter.
Q18. What is the frequency range of magnetron oscillators?
Q19. What two forms of instability are common in magnetrons?
Q20. What is the effect on magnetron operation if the magnetic field strength is too high?
Q21. What is the typical frequency range about the center frequency of a tunable magnetron?
POWER-AMPLIFIER TRANSMITTERS are used in many recently developed radar sets. This type of transmitter was developed because of the need for more stable operation of the moving target indicator (mti). In a magnetron transmitting system, the high-power magnetron oscillator has a tendency to drift in frequency because of temperature variations, changes in the modulating pulse, and various other effects.
Frequency drift is compensated for, in part, by the use of automatic frequency control (afc) circuits designed to control the frequency of the local oscillator in the receiver system. This, however, does not completely eliminate the undesirable effects of frequency drift on mti operation.
The power-amplifier transmitter system does the same thing as the keyed-oscillator transmitter but with fewer stability problems. It generates, shapes, and amplifies pulses of rf energy for transmission.
Figure 2-10 is a block diagram of a typical power-amplifier transmitter system. In this transmitter system a multicavity klystron tube amplifies lower-powered rf pulses that have been generated and shaped in other stages. CROSSED-FIELD AMPLIFIERS (AMPLITRONS) are used in radar systems with a wide band of transmitted frequencies because they are stable over a wider frequency range. A crossed-field amplifier transmitter is discussed later in this section.
Figure 2-10.—Power amplifier transmitter block diagram.
In figure 2-10, the power-amplifier chain input signals are generated by heterodyning (mixing) two frequencies. That is, two different frequencies are fed to a mixer stage (mixer amplifier) and the resultant, either the sum or difference frequency, may be selected as the output. (The operation of mixer circuits is explained in more detail in the section on receivers.) The low-power pulse is then amplified by intermediate power amplifier stages and applied to the klystron power-amplifier. The klystron power- amplifier concentrates the rf output energy into a very narrow frequency spectrum. This concentration makes the system more sensitive to smaller targets. In addition the detection range of all targets is increased.
To examine the operation of the transmitter, we will trace the signal through the entire circuit. The local oscillator shown at the left of figure 2-10 is a very stable rf oscillator that produces two CW rf outputs. As shown, the CW output is sent to the receiver system; the CW output is also one of the two rf signals fed to the mixer amplifier by way of the two BUFFER AMPLIFIER STAGES. The buffer amplifiers raise the power level of the signal and also isolate the local oscillator.
The COHERENT OSCILLATOR (COHO) is triggered by the system trigger and produces as its output an rf pulse. This rf pulse is fed directly to the mixer amplifier.
The mixer-amplifier stage receives three signals: the coherent rf pulse, the local oscillator CW rf signal, and a dc modulating pulse from the low-voltage modulator. The coherent and local oscillator signals are mixed to produce sum and difference frequency signals. Either of these may be selected as the output. The modulator pulse serves the same purpose as in the keyed-oscillator transmitter, because it determines the pulse width and power level. The mixer stage functions only during the modulator pulse time. Thus the mixer amplifier produces an output of rf pulses in which the frequency may be either the sum or difference of the coherent and local oscillator signals.
The mixer-amplifier feeds the pulses of rf energy to an intermediate power amplifier. This amplifier stage is similar to the buffer-amplifier stage except that it is a pulsed amplifier. That is, the pulsed amplifier has operating power only during the time the modulator pulse from the low-voltage modulator is applied to the stage. The amplified output signal is fed to a second intermediate power amplifier that operates in the same manner as the first.
From the second intermediate power amplifier, the signal is fed to the KLYSTRON POWER AMPLIFIER. This stage is a multicavity power klystron. The input rf signal is used as the exciter signal for the first cavity. High-voltage modulating pulses from the high-voltage modulator are also applied to the klystron power amplifier. These high-voltage modulating pulses are stepped up across a pulse transformer before being applied to the klystron. All cavities of the klystron are tunable and are tuned for maximum output at the desired frequency.
Provisions are made in this type of transmitter to adjust the starting time of the modulating pulses applied to the coherent oscillator, mixer amplifier, intermediate power amplifiers, and klystron power- amplifier. By this means the various modulator pulses are made to occur at the same time.
This transmitter produces output rf pulses of constant power and minimum frequency modulation and ensures good performance.
Q22. What is the primary advantage of power-amplifier transmitters over keyed-oscillator transmitters?
Q23. In the power amplifier shown in figure 2-10, what two signals are mixed to produce the output signal?
Q24. What type of klystron is used as the final stage of a power-amplifier transmitter?
Figure 2-11 is a block diagram of a power-amplifier transmitter that uses a FREQUENCY SYNTHESIZER to produce the transmitted frequency rather than the heterodyning mixer. The frequency synthesizer allows the transmitter to radiate a large number of discrete frequencies over a relatively wide band. Such a system is commonly used with frequency-scan search radars that must transmit many different frequencies to achieve elevation coverage and to compensate for the roll and pitch of a ship.
Figure 2-11.—Power amplifier transmitter using crossed-field amplifiers.
A typical frequency synthesizer consists of a bank of oscillators producing different fixed frequencies. The outputs of a relatively few fixed oscillators can be mixed in various combinations to produce a wide range of frequencies. In mti systems the selected oscillator frequencies are mixed with a coherent oscillator frequency to provide a stable reference for the mti circuits. The frequency synthesizer also produces the local oscillator signals for the receiver system. Because the transmitted pulse changes frequency on each transmission, the local oscillator signal to the receiver must also change and be included in the transmitted frequency. A system of this type is frequency-programmed by select gates from the synchronizer.
The detailed operation of frequency synthesizers is beyond the scope of this manual but may be found in the technical manuals for most frequency scan radar systems.
The first rf amplifier receives the pulses of the selected frequency from the synthesizer and a modulator pulse (from the first stage modulator) at the same time. The rf pulse is usually slightly wider than the modulator pulse which prevents the amplifier tube from pulsing when no rf energy is present. Most pulsed rf amplifiers will oscillate at an undesired frequency if pulsed without an rf input. The output of the first rf amplifier is an amplified rf pulse that is the same width as the first stage modulator pulse. The second stage modulator is designed to produce a pulse slightly narrower than the first stage
modulator pulse; this also prevents the amplifier from pulsing when no rf is present. Therefore, the second stage amplifier receives a modulator pulse a short time after the first stage rf arrives at the input. As shown in figure 2-11, the same procedure is repeated in the third and final stage.
The amplifiers in this type of power-amplifier transmitter must be broad-band microwave amplifiers that amplify the input signals without frequency distortion. Typically, the first stage and the second stage are traveling-wave tubes (twt) and the final stage is a crossed-field amplifier. Recent technological
advances in the field of solid-state microwave amplifiers have produced solid-state amplifiers with enough output power to be used as the first stage in some systems. Transmitters with more than three stages usually use crossed-field amplifiers in the third and any additional stages. Both traveling-wave tubes and crossed-field amplifiers have a very flat amplification response over a relatively wide frequency range.
Crossed-field amplifiers have another advantage when used as the final stages of a transmitter; that is, the design of the crossed-field amplifier allows rf energy to pass through the tube virtually unaffected when the tube is not pulsed. When no pulse is present, the tube acts as a section of waveguide. Therefore, if less than maximum output power is desired, the final and preceding cross-field amplifier stages can be shut off as needed. This feature also allows a transmitter to operate at reduced power, even when the final crossed-field amplifier is defective.
Q25. What transmitter component allows the radiation of a large number of discrete frequencies over a wide band?
Q26. What is the result of pulsing a pulsed rf amplifier when no rf is present?
Whenever a single antenna is used for both transmitting and receiving, as in a radar system, problems arise. Switching the antenna between the transmit and receive modes presents one problem; ensuring that maximum use is made of the available energy is another. The simplest solution is to use a switch to transfer the antenna connection from the receiver to the transmitter during the transmitted pulse and back to the receiver during the return (echo) pulse. No practical mechanical switches are available that can open and close in a few microseconds. Therefore, ELECTRONIC SWITCHES must be used. Switching systems of this type are called DUPLEXERS.
BASIC DUPLEXER OPERATION
In selecting a switch for this task, you must remember that protection of the receiver input circuit is as important as are output power considerations. At frequencies where amplifiers may be used, amplifier tubes can be chosen to withstand large input powers without damage. However, the input circuit of the receiver is easily damaged by large applied signals and must be carefully protected.
An effective radar duplexing system must meet the following four requirements:
1. During the period of transmission, the switch must connect the antenna to the transmitter and disconnect it from the receiver.
2. The receiver must be thoroughly isolated from the transmitter during the transmission of the high- power pulse to avoid damage to sensitive receiver components.
3. After transmission, the switch must rapidly disconnect the transmitter and connect the receiver to the antenna. For targets close to the radar to be seen, the action of the switch must be extremely rapid.
4. The switch should absorb an absolute minimum of power both during transmission and reception. Therefore, a radar duplexer is the microwave equivalent of a fast, low-loss, single-pole, double-throw switch. The devices developed for this purpose are similar to spark gaps in which high-current microwave discharges furnish low-impedance paths. A duplexer usually contains two switching tubes
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics