2-7
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.
