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
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
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.