c. Perhaps at this time, you are wondering why hydrogen gas is used
instead of some other gas, such as argon or mercury vapor. Hydrogen gas is
used because it has a short deionization time compared to the other gases.
Also hydrogen allows high currents to pass through the tube without causing
damage to the cathode.
4.
Brief review.
a. The switch discharges the
pulse-forming
network
at
the
pulse-
repetition frequency of the radar.
b. Early radar sets used fixed and rotary spark gaps as switches to
discharge the pulse-forming network.
c. Modern
radar
sets
use
the
HYDROGEN
THYRATRON
as
an
electronic
switch.
d. The HYDROGEN THYRATRON is small, light, and highly efficient.
Also,
it can be triggered accurately.
e. The HYDROGEN THYRATRON has a positive control-grid characteristic.
Grid current must flow first before the tube will ionize.
f. Hydrogen gas is used because it will deionize quickly and allow high
current flow without damage to the cathode.
Learning Event 5:
CHARGING IMPEDANCE
1. Charging impedance isolates the high-voltage power supply from the
closed switch while the PFN is discharging.
In other words, the charging
impedance prevents the closed switch from short-circuiting the source
voltage.
Isolation, then, is one of the primary purposes of the charging
impedance. However, the charging impedance has another important use, that
of increasing the amplitude of the modulator pulse.
a. For example, recall that the amplitude of the pulse developed across
the load is equal to half the voltage to which the PFN was charged. This
means that if the PFN charges to 6,000 volts, then, upon discharging, the
PFN will develop a 3,000-volt pulse across the load. Suppose, however, a
pulse of 6,000 volts is needed across the load.
One way of getting the
additional voltage is to use a 12,000-volt power supply.
The PFN, then,
charges to 12,000 volts and, upon discharging, the PFN develops a 6,000-volt
pulse across the load. However, this method is costly and inefficient.
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