Block Diagram Description

TM 11-5825-270-23

h. The carrier monitor circuit continuously checks

b. In the receiver, the rf signal is fed to a switch-

the dc output from the envelope detector (indicative of

selectable attenuator capable of providing up to 80 dB of

carrier level). As long as this carrier-derived dc voltage

attenuation in 20 dB steps, i.e., the incoming signal may

is greater than a predetermined threshold, the carrier

be subjected to 0, 20, 40, 60 or 80 dB attenuation. The

level is assumed to be acceptable. Similarly, the

attenuator setting, determined by the signal strength at

modulation monitor continuously checks the ac

the antenna, is carried out at the time of installation and

component at the output of the envelope detector

need not be readjusted during normal operation.

c. The tuning circuit following the attenuator

(indicative of modulation presence), and the operation is

considered to be normal as long as a keyed ac

permits tuning of the receiver to the NBD transmitter

component is present.

frequency. This tuning is performed once at the time of

i. In the event of carrier or keyed-modulation loss

installation and does not require readjustment during

(or both), the or-gate triggers a delay circuit which can

normal operation.

d. The output from the tuning circuit is amplified by

be preset between 0-60 seconds at the time of

installation. The purpose of the delay is to prevent false

a fixed gain rf amplifier and then mixed with a local

alarm condition during the time gap between two

oscillator signal in a balanced mixer. The local oscillator

successive code transmissions. If the malfunction (loss

signal is generated by a crystal-controlled oscillator and

of carrier and/or modulation) dears itself within the

its frequency is always 4.4 MHz minus the signal

duration of the preset delay, the delay circuit is

frequency. Thus, the output from the balanced mixer is

automatically reset and the receiver does not produce an

always at 4.4 MHz, the receiver if.

e. From the balanced mixer, the if. signal is passed

alarm condition. However, should the malfunction still

be present at the end of the delay period, the following

through a 4.4 MHz crystal filter which rejects all

events are initiated to indicate an alarm condition:

unwanted frequencies (image frequency, local oscillator,

(1) Front panel ALARM (red) light comes on

and the signal frequency) and produces an output of 4.4

and NORMAL (green) light goes off.

MHz if. The amplitude of this if. signal is proportional to

(2) A 3 kHz oscillator is activated and its output

the signal strength at the antenna.

f. The if amplifier provides approximately 80 dB

drives the loudspeaker through the power amplifier. The

volume of this loud 3 kHz alarm tone cannot be

gain to the if. input and its output is fed to an envelope

controlled by the front panel AUDIO LEVEL control.

detector which produces a dc output proportional to the

(3) The 3 kHz alarm tone may be silenced by

carrier level. Superimposed on this dc output is an ac

the front panel ALARM SIL switch. However, the

component proportional to the modulation level of the

ALARM light will remain on as long as the alarm

received signal. The envelope detector output is applied

condition is present.

to three circuits: the line amplifier circuit, modulation

(4) The normally open and normally closed

monitor circuit, and the carrier monitor circuit.

g. The line amplifier is an adjustable gain stage

remote alarm contacts change state, providing alarm

indication at the remote monitoring site.

which provides a 600 ohm balanced output of the

j. When the malfunction is corrected (i.e., both

detected audio (modulation signal). The output from the

carrier and modulation are present, the receiver

line amplifier (250 mW max.) is normally sent to a

automatically reverts to normal operation. Note that

remote monitoring site over a 600 ohm line. A second

alarm delay is by passed when reverting to normal

output from the line amplifier is used to drive the audio

operation.

amplifier which presents the modulation tones on the

k. The power supply for the receiver generates +12

loudspeaker. The volume of the modulation tones can

V and +28 V used by the receiver circuitry. Both outputs

be adjusted by the front panel AUDIO LEVEL control.

are regulated and overload protected.

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