Section II. FUNCTIONAL DESCRIPTION

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.

Section II. FUNCTIONAL DESCRIPTION

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

section III.

keyed beacon code.

NOTE

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

Assembly

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

2-3