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 The hard tube modulator has been replaced by the line-pulsed modulator in most cases.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).