TM 11-6625-3053-14
External FM modulation is accomplished by applying the modulating signal to the klystron repeller through coupling
capacitor C2 of the A4A1 tracking board (Item H ). The external modulating signal enters the FM front panel jack (Item
I ). The MODULATION switch S3 (Item J is placed in the EXT FM position. The incoming signal goes through the
switch, into a resistor/capacitor circuit (Item K of the A2A3 card assy and then back to the MODULATION switch from
which it goes to the repeller.
The oscillator cavity is coupled through three probes. The attenuator probe (Item B has a variable amount of insertion
that determines the rf power level delivered to the RF OUTPUT connector. The fixed rf monitor probe (Item C supplies
a power sample for automatic frequency stabilization. The power set probe (Item D has a variable amount of insertion
that supplies power to the monitoring circuit for attenuator calibration and leveling.
Power from the klystron cavity is coupled through two symmetrical pipes into two matched waveguide- beyond-cutoff
attenuators. The POWER SET control (Item A ) positions the power set probe (Item B in its pipe. The power set
probe couples rf power from the klystron cavity to the crystal detector and the power monitor meter and also moves the
hairline of the attenuator dial. When the POWER SET control is adjusted for a power monitor meter indication of SET
(Item C ), the power set probe is coupled to ap- proximately 1 milliwatt power level. Should the energy level within the
cavity change when the cavity is tuned to a different frequency, the power set probe must be reset to obtain the 1
milliwatt level of power absorption which will bring the power monitor meter back to its reference SET position. At the
same time, the attenuator hairline (Item D is repositioned to a new 0 dBm reference.
Once the POWER SET adjustment is set for SET, by turning the attenuator control (Item E the attenuator probe (Item
F ) can be adjusted for 0 dBm output at the same time the attenuator dial (Item G ) is being set for 0 dBm behind the
hairline. In this way, as frequency is varied, and the power monitor meter is set for SET, by placing the attenuator dial at
0 dBm behind the hairline, the output (Item H ) will be 0 dBm within the specification accuracy. In order to obtain other
levels of power, the ATTENUATOR control is adjusted so that the desired level is behind the hairline of the attenuator
dial. This adjustment varies the position of the attenuator probe in its waveguide-beyond-cutoff pipe. The rate of
attenuation within the pipe follows a rigid physical law and is extremely accurate, thus the rate of attenuation is translated
onto the dial.
Leveling of the rf output is done through the use of a pin diode attenuator (A-5 Assy). The pin diode attenuator is
composed of identical sections, one attenuator AT2 (Item A ) is in series with the power set probe (Item B, and the
second attenuator AT1 (Item C) is in series with the rf output probe (Item D ). The power output of AT2 is detected and
fed as a -DC feedback voltage to the inverting input of leveling amplified A3AR4 (Item E . A DC reference voltage is
also present at the inverting input of A3AR4. The reference voltage is developed across A3CR2 (Item F ) and
connected to the front panel ALC SET control (Item G ). A portion of this reference voltage is summed by A3AR4 with
the detected negative voltage. A proportional DC output is developed at the output of A3AR4 and divided between AT1
and AT2 by balance control A3R24 (Item H ). The pin diode attenuator is configured in such a manner that with a
positive voltage applied to control input, the pin diodes are turned on therefore shunt- ing to ground some amount of the
rf, proportional to the DC applied. The amount of attenuation in both halves of the attenuator is identical when properly
adjusted by the balance control A3R24. Therefore, if the power at the power set probe increases with a change in
frequency, the power at the rf output probe also increases by exactly the same amount. The increase is detected at the
set output and fed to amplifier A3AR4. The amplifier inverts the detected change and produces a proportional change at
the output which is fed to both halves of the attenuator, thus returning the output to the predetermined level.
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