TM 11-6625-828-45
is applied through C145 and R185 to V109, a pentode,
the RF output tank coil L114. Attenuator Z105 permits
which serves as the second doubler an modulated output
continuous control of the RF output available at J104,
tube. The tank circuit comprising L114, and either
between 1 and 10,000 microvolts across a 51-ohm load.
C149A or C149B as selected, by S105C, is tuned in the
same way as the firs doubler. The setting of R119
piston of the attenuator is moved along the axis of the
determines the value of screen voltage applied to V109.
attenuator tube by means of a rack and pinion drive. A
Varying the screen voltage applied to V109 by means of
fixed output level of 1 volt is available at 1 VOLT jack
RF LEVEL SET control R119 controls the RF output
J105. To keep the output impedance of the AN/ARM-5A
voltage. This controlled RF output volt age is used as
constant, 51-ohm resistor R191 is placed in series with
the reference value for zero-setting RF meter M101,
the internal pickup loop of the attenuator to match the
before reading the percent modulation on %M meter
nominal 51-ohm RG-58/U cable generally used for
M102. The RF signal i modulated by a voltage supplied
external output connections. The 51-ohm cap P103
by V102, modulation being effected by impressing the
containing R192, or its equivalent, must be in place on
modulation voltage from V102 on the screen grid of
J105 to make the attenuator reading correct. The
V109. The output of V109 is inductively coupled to the
attenuator is calibrated to read microvolts across a 51-
RF output connectors through L115. The RP output may
ohm termination.
u. RF Meter Circuit (fig. 1-14). RF meter
be taken at a fixed 1-volt level from 1 VOLT jack J105 or
at a continuously adjustable calibrated level, controlled
M101 is a microammeter which, with its voltage divider
by RF attenuator Z105, between 1 and 10,000 microvolts
consisting of R207, R208, and R164, measures the de
(iv) from ATTEN jack J104.
voltage developed by the detector circuit (Vl10) of Z104.
r. Demodulator Circuits (fig. 1-13). The de
This dc voltage consists of a fixed bias of approximately
modulator or detector circuit (V110A) is connected
16 volts plus a voltage which is proportional to the RF
across the plate tank circuit of V10( and operates as an
voltage appearing across RF coil L114. Since both
infinite impedance detector The output of V110OA is
M101 and %M meter M102 (v below) are connected in
coupled through an RF filter to cathode follower V11OB.
the cathode circuits of vacuum tubes, a source of
The RF filter consists of C151, L117, and C153. The
bucking voltage is required to make them read zero
audio output of the demodulator circuit is the same in
when no RF or modulation voltages are present. Such a
waveform and phase as the modulation envelope of the
voltage is developed by monitor bridge arm V115B, and
RF output of RF unit Z104. Cathode follower V110lOB is
is more suitable than if bled from a dc supply since it is
less subject to the same variations with filament and
thereby helping to maintain good audiofrequency fidelity
plate voltage as is V11OB. Monitor bridge arm V115B is
of the demodulator circuit.
biased by resistors R216 and R217, to produce a-current
s. RF Unit Filter Circuit (fig. 1-13). All the
flow in the cathode-resistor net- work consisting of R218,
R219, R222, R221, and R223. A portion of this voltage
is used to cancel the fixed de voltage present in the
doublers, and detector circuits, is contained within an
demodulator output. Exact zero settings of M101 and
electrical shield. The demodulator out put and all supply
M102 are accomplished by adjustment of R218 and
leads are brought out through two-section filters to
R219, when the RF output has been reduced to zero;
prevent RF leakage out side of this shield.
The
that is, when the related ZERO SET knob is depressed,
demodulated output is brought out through L119, C156,
operating switch S108, and reducing the output from RF
C157, and L120 to terminal 3. The filament circuit
unit Z104 to zero. The sensitivity of M101 is such that
connection is made at terminal 4 and filtered through
the redline (LEVEL SET) indication corresponds to a 1-
L108 L111, C134, and C136. The modulating voltage
volt output across 51 ohms at J106. At this level,
circuit which enters at terminal 2, and the plate supply at
provided J105 is terminated in 61 ohms, the proper input
terminal 1, are filtered by L118 C155, L116, and C154,
is fed to RF attenuator Z106. Such a 51-ohm termination
and C133, L107, C135 and L110, respectively.
is provided by cap P108
output of the AN/ARM-5A is controlled by a piston-type
attenuator Z105 which is inductively coupled by L115 to
1-6