2-39
Pulse modulation will play a major part in your electronics career. In one way or another, you will
encounter it in some form. The function of the particular system may involve many variations of the
characteristics presented here. We will now look at some specific applications of pulse modulation in
radar and communications systems.
Q-14.
Overmodulating an rf carrier in amplitude modulation produces a waveform which is similar to
what modulated waveform?
Q-15.
What is prt?
Q-16.
What is nonpulse time?
Q-17.
What is average power in a pulsed system?
RADAR MODULATION
Radio frequency energy in radar is transmitted in short pulses with time durations that may vary
from 1 to 50 microseconds or more. If the transmitter is cut off before any reflected energy returns from a
target, the receiver can distinguish between the transmitted pulse and the reflected pulse. After all
reflections have returned, the transmitter can again be cut on and the process repeated. The receiver
output is applied to an indicator which measures the time interval between the transmission of energy and
its return as a reflection. Since the energy travels at a constant velocity, the time interval becomes a
measure of the distance traveled (RANGE). Since this method does not depend on the relative frequency
of the returned signal, or on the motion of the target, difficulties experienced in cw or fm methods are not
encountered. The pulse modulation method is used in many military radar applications.
Most radar oscillators operate at pulse voltages between 5 and 20 kilovolts. They require currents of
several amperes during the actual pulse which places severe requirements on the modulator. The function
of the high-vacuum tube modulator is to act as a switch to turn a pulse ON and OFF at the transmitter in
response to a control signal. The best device for this purpose is one which requires the least signal power
for control and allows the transfer of power from the transmitter power source to the oscillator with the
least loss. The pulse modulator circuits discussed in this section are typical pulse modulators used in radar
equipment.
Spark-Gap Modulator
The SPARK-GAP MODULATOR consists of a circuit for storing energy, a circuit for rapidly
discharging the storage circuit (spark gap), a pulse transformer, and an ac power source. The circuit for
storing energy is essentially a short section of artificial transmission line which is known as the
PULSE-FORMING NETWORK (pfn). The pulse-forming network is discharged by a spark gap. Two
types of spark gaps are used: FIXED GAPS and ROTARY GAPS. The fixed gap, discussed in this
section, uses a trigger pulse to ionize the air between the contacts of the spark gap and to initiate the
discharge of the pulse-forming network. The rotary gap is similar to a mechanically driven switch.
A typical fixed, spark-gap modulator circuit is shown in figure 2-38. Between trigger pulses the
spark gap is an open circuit. Current flows through the pulse transformer (T1), the pulse-forming network
(C1, C2, C3, C4, and L2), the diode (V1), and the inductor (L1) to the plate supply voltage (Eb). These
components form the charging circuit for the pulse-forming network.
