Specifications

which trips the comparator thereby simulating a VSWR condi-
tion. Upon transmitter foldback to extremely low power levels,
these switches have no effect because the samples have been
lowered enough not to trip the detectors.
H.6.13
VSWR Inhibit
Push-button switch S21 shunts the inputs of the VSWR Compa-
rators to ground through diodes. This keeps the VSWR Compa-
rators from being able to detect a VSWR fault. It should only be
used when testing the VSWR Self Test.
H.6.14
Directional Coupler
The Directional Coupler, like the VSWR Detectors, depends on
phase and amplitude relationships of voltageand current samples
to provide a dc output voltage. In the Directional Coupler, the
current and voltage samples are applied to opposite ends of a
diode.
H.6.14.1 Forward Power
For forward power, the current and voltage samples are 180° out
of phase with each other. The voltage across the diode and
corresponding dc current output depends on the magnitude of the
samples. As RF power output is raised, the sample voltages
increase and the dc output becomes greater.
Jumper plugs JP4 and JP5 allow reversing the current sample
phase so that the forward coupler reads “reflected power” and
the coupler can be balanced by adjustment of the Forward
Balance capacitors.
Diode CR41 is the forward power coupler’s diode. An RF choke
and capacitor form a filter to remove the RF component, while a
resistor and capacitor create a low-pass filter to remove audio-
frequency variations due to modulation from the coupler’s out-
put.
The voltage follower U9 drives the Forward Power Meter on
Switch Board/Meter Panel and the Remote Metering amplifier
on the External Interface board. The Forward Power calibration
adjustment is located on the Switch Board/Meter Panel.
H.6.14.2 Reflected Power
The reflected coupler operates in the same way as the forward
coupler, except that the samples are in phase with each other.
When sample amplitudes are the same, no voltage appears across
the diode. No current will flow through the diode, and there will
be no dc voltage at the output. When a VSWR fault occurs, the
phase and/or voltage relationship at the ends of the diode change,
so that current begins flowing through the diode and the output
voltage will increase.
The voltage follower U9 drives the Reflected Power Meter on
Switch Board/Meter Panel and the Remote Metering amplifier
on the External Interface board. The Reflected Power calibration
adjustment is located on the Switch Board/Meter Panel.
H.6.15
ARC Detectors
There are two detectors, a Lower ARC Detector and a Upper
ARC Detector. Both detector outputs are treated exactly as a
Output Network VSWR Fault.
H.6.15.1 Lower ARC Detector
The Lower ARC Detector 2A5 monitors the Bandpass Filter at
the bottom of the Output Network. A photocell is connected from
+15VDC to the input of a comparator. The Lower Detector
Threshold control R60 sets the threshold voltage for the compa-
rator. If an ARC occurs, the voltage at the (-) input will be greater
that the (+) input and the output will go logic low. This inverted
output is connected to the Network VSWR Test line that will trip
the Output Network VSWR Comparator. This output will also
latch the DS5 Lower ARC indicator RED for a visual indication,
until manually reset by the ARC Fault Indicator Reset switch
S22. If the cable at J9 is disconnected, a ground for the interlock
is removed, and the fault will be generated.
H.6.15.2 Upper ARC Detector
The Upper ARC Detector 2A4 monitors the Pi Matching Net-
work at the top of the Output Network. Two photocells are used
because more ambient light enters from the top.
H.6.15.2.1 Reference Sensor
The reference sensor photocell output is connected to the input
of differential amplifier U16. The Sensor Balance control R97 is
used to adjust this voltage at TP15, ARC Reference.
H.6.15.2.2 ARC Sensor
The actual ARC sensor photocell output is also connected to the
input of differential amplifier U16. This voltage can be measured
at TP14, Upper ARC Sense.
H.6.15.2.3 Differential Amplifier
U16 is a differential amplifier that has the outputs of both sensors
connected to its inputs. The Sensor Balance control is adjusted
so that the reference sensor voltage is the same as the actual
sensor output, and therefore the output of the differential ampli-
fier, at TP25 ARC Level, will be 0VDC.
H.6.15.2.4 Compensated Comparator
Should an actual ARC occur, the output of the differential am-
plifier will increase and the voltage at the (-) input of the
comparator will be greater than the (+) input, and the comparator
output will go logic high. This output will perform the same
functions as the Lower ARC Detector.
H.6.15.2.5 Absolute Comparator
If the amount of ambient light entering through the top of the
Output Network is too great, ARC detection might not be possi-
ble. Under these conditions, an Absolute Comparator will gen-
erate the same fault.
H.6.16
Modulation Monitor Sample Equalization
The modulation monitor sample input at J3 is taken from a tapped
inductor in the output network compartment. This input is buff-
ered by U2 and connected to the Modulation Monitor Sample
output at J4. A light dependent resistor (LDR) CR11 is connected
from the buffer input to ground. The resistance of this device will
change due to the AGC circuit from the Audio Demodulator,
which will maintain a constant RF output over the entire fre-
quency and power range of the transmitter. If the AGC function
is undesirable or defective, the circuit may be bypassed, by
moving JP1 from 1-2 to 2-3.
H-6 888-2339-002 02/17/04
WARNING: Disconnect primary power prior to servicing.