4.
Trouble symptom.
Modulator pulse present but distorted.
If the modulator output pulse is present, check for proper waveshape
on the synchroscope.
Examine it carefully for negative polarity, sharp
leading and trailing edges, correct amplitude, and proper pulse width. If
the modulator output pulse is distorted, isolate the trouble by making the
following checks:
(1) Measure the pulse amplitude.
If it is too low, check the
components in the charge circuit. Check the high-voltage power supply, the
charging choke, PFN, and pulse transformer. Also check the THYRATRON; its
conducting resistance may have increased.
(2) Measure the pulse width.
If the pulse width or shape is not
correct, check the PFN.
(3) Check the trailing edge of the pulse.
If there are too many
oscillations following the pulse, check the reverse-current diode circuit.
Replace the diode with a known good one.
5.
Final Summary.
a. A radar transmitter consists of two major units: modulator and
magnetron. The modulator develops rectangular high-voltage DC pulses which
cause the magnetron to generate bursts of RF energy. The modulating pulse
may be formed either by low-level modulation or high-level modulation. This
lesson describes the operation of a high-level modulation system in which
the pulse is formed at a high power level.
b. A basic high-level modulator consists of a high-voltage DC power
supply, a charging impedance, a pulse forming network, and a switch. At the
first instant, the high-voltage power supply charges the PFN to several
thousand volts through the charging impedance.
After the PFN is fully
charged, the switch closes, discharging the PFN through the load (magnetron
and its pulse transformer).
c. Practical high-level modulators use DC resonant charging to increase
the pulse amplitude and efficiency of the system. In DC resonant charging,
the PFN is charged through a charging choke and charging diode.
The
charging choke allows the PFN to charge to a voltage nearly twice the power
supply voltage. The charging diode in series with the
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