RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-9
01 November 2003
2.2.6.3
Monitor's +15V (A) Supply
: The B+
voltage is applied through J3-1, resistors R1/R4 and
across 15-volt zener diode CR3. The three
components provide the
+15V (A)
supply voltage for
the monitor circuits, which is required to be active
when the +15V supply is inhibited.
2.2.6.4
Low Carrier Level (-3dB) Detector
: The
low carrier level detector circuit is comprised of
comparator U3D,
LOW CARRIER
potentiometer R10
and their associated components. Under normal
operating conditions, a portion of the forward power
from the output buffer U1B-7 is applied to the wiper
of potentiometer R10. Potentiometer R10 is normally
set for the desired
minimum
carrier level (normally to
detect a 3.0 dB drop on the
intended
carrier level).
The DC voltage on the wiper of potentiometer R10 is
applied through resistor R18, across capacitor C5
and passed to non-inverting input of U3D-11.
Capacitor C5 filters out the modulation component of
the RF carrier and charges to the mean DC level
which is representative of the RF carrier level. The
voltage on U3D-11 represents the
carrier threshold
level
that is compared to the voltage level, established
by resistors R22/R23, on the inverting gate of U3D-
10. Normally, the output on U3D-13 will be high
impedance and have no effect on the status of
transistors Q4 or Q6. If the output on U3D-13
switches to low impedance (drop of 3dB on the
carrier level), the
low
potential will be applied to the
base of transistors Q4 and Q6. Transistors Q4 and
Q6 will be reversed biased and turn off.
THRESH
lamp DS1 will turn on.
2.2.6.5
High Carrier Level (+2dB) Detector
:
The high carrier level detector circuit is comprised of
comparator U3B,
HIGH CARRIER
potentiometer R89
and their associated components. Under normal
operating conditions, a portion of the forward power
from the output buffer U1B-7 is applied to the wiper
of potentiometer R89. Potentiometer R89 is normally
set for the desired
maximum
carrier level (normally to
detect a 2.0 dB increase in the
intended
carrier level).
The DC voltage on the wiper of potentiometer R89 is
applied through resistor R90, across capacitor C33
and passed to inverting input of U3B-6.
Capacitor C33 filters out the modulation component
of the RF carrier and charges to the mean DC level
which is representative of the RF carrier level. The
voltage on U3B-6 represents the
carrier threshold
level
that is compared to the voltage level, established
by resistors R91/R92, on the non-inverting gate of
U3B-7. Normally, the output on U3B-1 will be high
impedance and have no effect on the status of
transistors Q4 or Q6. If the output on U3B-1
switches to low impedance (increase of 2dB on the
carrier level), the
low
potential will be applied to the
base of transistors Q4 and Q6. Transistors Q4 and
Q6 will be reversed biased and turn off.
THRESH
lamp DS1 will turn on.
2.2.6.6
Low Modulation Depth Detector
: The
low modulation depth detector circuit consists of
operational amplifier U1C,
MOD THRESH
potentiometer R8, a synchronous detector circuit
comprised of transistor Q1 and FET Q2, comparator
U3A and associated components. A sample of the
forward power on J1-3 is applied through the wiper
of
MOD THRESH
potentiometer R8 and passed to the
non-inverting input of U1C-10.
MOD THRESH
potentiometer R8 is normally set for the desired
minimum
modulation level (normally to detect a
4.0dB drop on the intended modulation depth).
FET Q2 and transistor Q1 detect signals that are in
phase with the keyed tone input to transistor Q1.
Under normal operating conditions the AC output of
U1C is applied to the inverting gate of U3A where it
is compared to a bias voltage, formed by resistors
R24/R25, on the non-inverting gate of U3A. The
output on U3A-2 is applied across a differentiating
circuit consisting of resistor R33 and capacitor C7.
The differentiating circuit produces a positive pulse,
during each transition, which is applied to the base of
transistor Q6.
THRESH
lamp DS1 will normally be flashing on and
off at the keyed tone rate. When the positive pulses
being applied to the base of transistor Q4 are
removed, Q4 will be reversed biased and turned off.
Transistor Q5 will be forward biased and turned on.
THRESH
lamp DS1 will turn on. If
THRESH
lamp
DS1 remains turned on for a period longer than the
shutdown time delay, the subject transmitter will shut
down (shutdown time delay is dependent on setting of
DELAY
potentiometer R41; adjustable between 20
and 80 seconds).
