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Nuclear Fundamentals


TM 11-5825-271-34
the NDB transmitter occurs and an alarm is generated.
of the antenna loss resistance, the ATU coil loss
A visual alarm is provided on the control panel of the
resistance, and the antenna radiation resistance.
e. The change-over unit is used only in dual
NDB transmitter and there are provisions to route the
alarm condition to a remote site.  In dual transmitter
transmitter systems. In the event of a shutdown of the
configuration, a shutdown of the main NDB transmitter
main NDB transmitter, the change-over unit auto-
causes the standby NDB transmitter to be switched on.
matically switches on the standby NDB transmitter. The
Shutdown occurs 20 to 35 seconds after the abnormal
rf outputs of the two NDB transmitters are connected to
operation condition is detected.
the change-over unit and the switched rf output from the
d. The ATU matches the antenna impedance to the
change-over unit is fed to the ATU.
f.  The NDB transmitter operates from a 115/230
50-ohm output impedance of the NDB transmitter While
the  system  is  operating,  the  ATU  automatically
Vac, 60/50 Hz primary power source.  Provisions are
compensates for the variations in antenna reactance
included for connecting a 48V backup battery supply, If
caused by environmental changes. The power radiated
required. Power for the ATU and the change-over unit is
from the antenna is dependent upon the relative values
provided by the NDB transmitter.
150 milliseconds, normally set at 125 milliseconds.
2-3. General
b. The carrier frequency produced by exciter as-
This section provides a functional description of the NDB
sembly A1A2 is derived from a crystal-controlled
transmitter and ATU. The description is supported by a
oscillator operating at 10 times the operating carrier
block schematic diagram (fig. FO-1) which identifies all
frequency. The carrier can be produced without tone or
major  assemblies  within  each  unit  and  their
voice modulation, with continuous tone modulation, or
interconnections.  The overall functional description of
modulated with the keyed beacon signal. The external
the various assemblies is provided in the following
audio modulation input (TB2-5/6, fig. FO-1), if used, is
paragraphs For detailed functional description, refer to
unswitched and can be operated with or without the
keyed beacon code.
Refer to figures FO-1 and FO-2 for the
2-6. Switched Regulator Driver A1A3
description that follows.
Switched regulator driver A1A3 is turned on and off by
the input from exciter assembly A1A2. This arrangement
2-4. Power Supply A1A1 and Transformer Chassis
prevents over-dissipation in the pa module A1A4, should
a  fault  condition  cause  the  mark/space  signal
Although physically located on the transformer chassis
(identification code) to assume a steady mark condition.
assembly, the power transformer T1 and choke L1 are
If this occurs, the rf output remains off until the NDB
functionally an integral part of the power supply A1A1.
transmitter is reset by switching the POWER switch S1
Together, they provide the regulated and nonregulated
to OFF, then back to ON again.
dc voltages (+24 Vdc and +50 Vdc) for operation of all
circuitry within the NDB transmitter, and the ATU. Power
2-7. Pa Module A1A4
supply A1A1 contains the power switching arrangement
The output transistors in the pa module A1A4 are hard-
and adjustment controls for the output voltage levels. An
switched on/off by the rf drive signal from exciter
adjustable trickle charging circuit for an external 48V
assembly A1A2. The dc supply level for the pa module
battery is also contained in the power supply A1A1.
A1A4 is varied sinusoidally by the modulation signal and
regulated by the negative feedback to the modulator.
2-5. Exciter Assembly A1A2
a. A keyer code matrix in the exciter assembly
2-8. Rf Filter A1A5
A1A2 contains solder-in type links to provide selection of
The rf output from the pa module A1A4 is fed to a tuned
any two or three letter sequence for the appropriate call
filter which removes harmonics resulting from the hard-
sign (station identification code). The code length is 48
switching action of pa module A1A4 The harmonic
bits (dot lengths) for two letter codes or 64 bits for three
attenuation, relative to carrier level, is 60 dB. Five differ-
letter codes. The dot length is adjustable from 100 to

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