TRANSMITTERS

The TRANSMITTER produces the short duration high-power rf pulses of energy that are radiated into space by the antenna. Two main types of transmitters are now in common use. The first is the KEYED-OSCILLATOR type. In this transmitter one stage or tube, usually a magnetron, produces the rf pulse. The oscillator tube is keyed by a high-power dc pulse of energy generated by a separate unit called the MODULATOR (discussed in the following section). The second type of transmitter consists of a POWER-AMPLIFIER CHAIN. This transmitter system begins with an rf pulse of very low power. This low-level pulse is then amplified by a series (chain) of power amplifiers to the high level of power desired in a transmitter pulse. In most power-amplifier transmitters, each of the power-amplifier stages is pulse modulated in a manner similar to the oscillator in the keyed-oscillator type. Because the modulator is common to both types of transmitter systems, the operation of a typical modulator will be discussed first.

RADAR MODULATOR

The modulator controls the radar pulse width by means of a rectangular dc pulse (modulator pulse) of the required duration and amplitude. The peak power of the transmitted rf pulse depends on the amplitude of the modulator pulse.

Figure 2-3 shows the waveforms of the trigger pulse applied by the synchronizer to the modulator, the modulator pulse applied to the radar transmitter, and the transmitted rf pulse.

Figure 2-3. - Transmitter waveforms.

As you can see in the figure, the modulator pulse is applied to the transmitter the instant the modulator receives the trigger pulse from the synchronizer (T1, T2). The modulator pulse is flat on top and has very steep leading and trailing edges. These pulse characteristics are necessary for the proper operation of the transmitter and for the accurate determination of target range. The range timing circuits must be triggered the instant the leading edge of the transmitted rf pulse leaves the transmitter. In this way, the trigger pulse that controls the operation of the modulator also synchronizes the cathode-ray tube sweep circuits and range measuring circuits.

MAGNETRON OSCILLATORS are capable of generating rf pulses with very high peak power at frequencies ranging from 600 to 30,000 megahertz. However, if its cathode voltage changes, the magnetron oscillator shifts in frequency. To avoid such a frequency change, you must ensure that the amplitude of the modulator (dc) pulse remains constant for the duration of the transmitted rf pulse. That is, the modulator pulse must have a flat top. The range of cathode voltages over which a magnetron oscillates in the desired frequency spectrum is relatively small.

When a low voltage is applied to a magnetron, the magnetron produces a noise voltage output instead of oscillations. If this noise enters the receiver, it can completely mask the returning echoes. If a modulator pulse builds up and decays slowly, noise is produced at both the beginning and end of the pulse. Therefore, for efficient radar operation, a magnetron requires a modulator pulse that has a flat top and steep leading and trailing edges. An effective modulator pulse must perform in the following manner:

• Rise from zero to its maximum value almost instantaneously
• Remain at its maximum value for the duration of the transmitted rf pulse
• Fall from its maximum value to zero almost instantaneously

In radars that require accurate range measurement, the transmitted rf pulse must have a steep leading edge. The leading edge of the echo is used for range measurement. If the leading edge of the echo is not steep and clearly defined, accurate range measurement is not possible. The leading and trailing edges of echoes have the same shape as the leading and trailing edges of the transmitted rf pulse.

A transmitted rf pulse with a steep trailing edge is essential for the detection of objects at short ranges. If the magnetron output voltage drops gradually from its maximum value to zero, it contributes very little to the usable energy of the transmitted rf pulse. Furthermore, part of the magnetron output voltage enters the receiver and obscures nearby object echoes.

Types of Modulators

The two types of modulators are the LINE-PULSING MODULATOR and the HARD-TUBE MODULATOR. (A hard tube is a high-vacuum electron tube.) The line-pulsing modulator stores energy and forms pulses in the same circuit element. This element is usually the pulse-forming network. The hard-tube modulator forms the pulse in the driver; the pulse is then amplified and applied to the modulator. The hard tube modulator has been replaced by the line-pulsed modulator in most cases. This is because the hard-tube modulator has lower efficiency, its circuits are more complex, a higher power supply voltage is required, and it is more sensitive to voltage changes.

The line-pulsing modulator is easier to maintain because of its less complex circuitry. Also, for a given amount of power output, it is lighter and more compact. Because it is the principally used modulator in modern radar, it is the only type that will be discussed.

Figure 2-4 shows the basic sections of a radar modulator. They are as follows:

• The power supply.
• The storage element (a circuit element or network used to store energy).
• The charging impedance (used to control the charge time of the storage element and to prevent short-circuiting of the power supply during the modulator pulse).
• The modulator switch (used to discharge the energy stored by the storage element through the transmitter oscillator during the modulator pulse).

Figure 2-4. - Basic line-pulsing modulator block diagram.

View A of figure 2-4 shows the modulator switch open and the storage element charging. With the modulator switch open, the transmitter produces no power output, but the storage element stores a large amount of energy. View B shows the modulator switch closed and the storage element discharging through the transmitter. The energy stored by the storage element is released in the form of a high-power, dc modulator pulse. The transmitter converts the dc modulator pulse to an rf pulse, which is radiated into space by the radar antenna. Thus, the modulator switch is closed for the duration of a transmitted rf pulse, but open between pulses.

Many different kinds of components are used in radar modulators. The power supply generally produces a high-voltage output, either alternating or direct current. The charging impedance may be a resistor or an inductor. The storage element is generally a capacitor, an artificial transmission line, or a pulse-forming network. The modulator switch is usually a thyratron.

Modulator Storage Element

Capacitor storage elements are used only in modulators that have a dc power supply and an electron-tube modulator switch.

The capacitor storage element is charged to a high voltage by the dc power supply. It releases only a small part of its stored energy to the transmitter. The electron-tube modulator switch controls the charging and discharging of the capacitor storage element.

The artificial transmission line storage element, shown in view A of figure 2-5, consists of identical capacitors (C) and inductors (L) arranged to simulate sections of a transmission line. The artificial transmission line serves two purposes: (1) to store energy when the modulator switch is open (between transmitted rf pulses) and (2) to discharge and form a rectangular dc pulse (modulator pulse) of the required duration when the modulator switch is closed.

Figure 2-5A. - Modulator storage elements.

Figure 2-5B. - Modulator storage elements.

The duration of the modulator pulse depends on the values of inductance and capacitance in each LC section of the artificial transmission line in view A and the number of LC sections used. Other arrangements of capacitors and inductors (such as the pulse-forming network shown in view B) are very similar in operation to artificial transmission lines.

ARTIFICIAL TRANSMISSION LINES and PULSE-FORMING NETWORKS (pfn) are used more often than the capacitor-type storage elements.

ARTIFICIAL TRANSMISSION LINE. - Figure 2-6 shows a radar modulator that uses an artificial transmission line as its storage element. A modulator switch controls the pulse-repetition rate. When the modulator switch is open (between transmitted rf pulses), the transmission line charges.

Figure 2-6. - Modulator with an artificial transmission line for the storage element.

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).

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.

Q.10 What are the two basic types of transmitters?
Q.11 What controls transmitter pulse width?
Q.12 In addition to a flat top, what characteristics must a modulator pulse have?
Q.13 What type of modulator is most commonly used in modern radar systems?
Q.14 What three types of storage elements most often are used in modulators?
Q.15 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?