TM 11-6625-1633-45
i. For this mode of transmission, the rate of attenua-
temperature, audio oscillator output, and the magnitude
tion in the attenuator tube is constant. This mode of
of rf power coupled to the thermistor.
transmission is known as the TMol mode. The position
of the probe assembly, controlled by the ATTENUA-
(2) The thermistor is dc biased, at room tempera-
TOR control knob through a gear train, determines the
ture, to a resistance of approximately 130 ohms. This
output power from the rf oscillator. The output is indi-
causes an initial unbalance in the self-balancing bridge
cated from -7 decibel meter (dbm) to -127 dbm below 1
and an audio output appears between pins 5 and 6 of
millivolt into 50 ohms on the MICROVOLTS/ - DBM
transformer T4, and ground. This voltage is amplified
dial scale. The output power can be read directly from
by transistors Q5 and Q6 and coupled back through
the scale within the tolerance limits of the signal gen-
transformer T4 to the thermistor, and thereby balances
erator only if the POWER SET control knob is adjusted
the bridge.
for a POWER SET indication on power set-zero set me-
ter M2.
(3) METER SENSITIVITY potentiometer R67 is
factory adjusted so that power set-zero set meter M2
indicates zero with 1 milliwatt ((mw)) of audio power
2-6. Analysis of Power Monitor Bridge Cir-
and zero rf power across the thermistor. When MODU-
LATION selector switch S2 is rotated from OFF to CW,
rf power is substituted for audio power in the thermistor.
a. The power monitor bridge circuit consists of an
This substitution is caused by the self-balancing feature
audio oscillator, an output voltage monitor, and a
of the bridge. Meter M2 now reads up scale. When the
transformer-coupled self-balancing bridge. The active
POWER SET control knob is adjusted, the rf power ab-
elements in the self-balancing bridge are thermistors
sorbed by the thermistor is changed. The POWER SET
RT2 and RT3 that are located in the other two tubes
control knob is adjusted until power set-zero set meter
projecting from the klystron cavity (para 2-5c).
M2 indicates POWER SET. This indication corresponds
to a dissipation of 0.3 mw of rf power and 0.7 mw of
audio power in the thermistor. The POWER SET con-
and Q6 and associated components. The transistors are
trol knob is mechanically gauged to a hair line indicator
coupled together through a series resonant circuit, com-
which sets the reference for the ATTENUATOR control
prised of capacitor C50 and coil L3, that determines the
knob.
frequency of the oscillator.
c. The output voltage monitor consists of fullwave
2-7. Analysis of Reflector Power Supply
rectifier CR24 through CR27 and power set-zero set
meter M2. METER SENSITIVITY potentiometer R67
controls the meter sensitivity. Resistors R64 and R66
The reflector power supply is a regulated power supply
determine the null point of the meter when no audio
that furnishes the reflector voltage for the klystron. It is
voltage is present.
referenced to the beam power supply in order to main-
tain the reflector negative with respect to the cathode
d. The self-balancing bridge network consists of the
center-tapped secondary winding of coupling trans-
CR9 provide a full-wave rectified output to a pi type
former T4, resistor R49, capacitor C46, and thermistor
filter, consisting of capacitors C35, C36, and choke
RT2 or RT3 (which is used to cover the low and high
L1B. The output of the filter is applied to series regula-
frequency ranges, respectively). The dc current flowing
tor tube A3V5. Dual triode A3V6 functions both as a
through the selected thermistor is controlled by ZERO
control tube and an amplifier whose input is determined
SET potentiometers R51 and R52. Power monitoring in
by voltage regulator tubes A3V7, A3V8, resistor A3R4,
the AN/URM-149 is achieved as follows:
REFL-650 potentiometer A3R8, and resistors A3R9 and
A3R19. The circuit functions as follows:
(1) The power monitor bridge measures power by
a. The alternating-current (ac) voltage from the sec-
the use of variable resistance thermistors in the self-
ondary winding of transformer T2 is rectified by full-
balancing bridge circuit. The thermistor resistance var-
ies inversely with temperature. Changes of thermistor
rectified voltage is filtered by a pi filter consisting of
resistance can be caused by three variables: environ-
mental
2-4
