Section H
Output Sample Board (A26) and
Output Monitor (A27)
H.1
Introduction
This section includes circuit descriptions and troubleshooting
information for the Output Sample Board and Output Monitor.
The Output Sample Board contains circuits to sample RF voltage
and RF current. The outputs of these sample circuits are fed to
the Output Monitor. The Output Sample Board is located in the
Output Network Compartment.
The Output Monitor includes circuits for VSWR protection,
power metering, and modulation monitoring. The Output Moni-
tor is located on the upper left side of the Center Control
Compartment.
H.2
Circuit Description
Refer to schematic 839-7930-018 and schematic 843-5400-101
in the Drawing Package.
Refer to SECTION V, Maintenance, for adjustment procedures
and pc board maintenance procedures.
H.2.1
Output Sample Board
The Output Sample Board contains RF voltage samples and RF
current samples from the 50 Ohm point in the output network.
These are sent to the Output Monitor for VSWR protection and
Forward/Reflected power monitoring.
H.2.1.1
Current Samples
The RF output conductor between 2C4 and the output connector
J2 passes through toroidal transformers T1 and T2. These trans-
formers pick up samples of RF current.
•
A voltage proportional to the current through T1 is devel-
oped across resistors R1 through R4 for the Antenna VSWR
circuit and sent to the Output Monitor at J1-1.
•
Voltages proportional to the current through T2 are devel-
oped across resistors R10 through R17 for forward/re-
flected current samples to the directional coupler. The
forward sample is sent to the Output Monitor at J1-3 and
the reflected sample is sent to the Output Monitor at J1-5.
These voltages are 180° out of phase.
H.2.1.2
Voltage Samples
A copper strap connects the RF output conductor to E1 and then
to capacitor C7 for RF voltage samples.
•
Capacitive voltage divider C11/C14/C15 develops a volt-
age sample for the forward power directional coupler. This
sample is sent to the Output Monitor at J1-13.
•
Capacitive voltage divider C10/C13/C16 develops a volt-
age sample for the reflected power directional coupler. This
sample is sent to the Output Monitor at J1-15.
•
Capacitive voltage divider C9/C12/C17 develops a voltage
sample for the Antenna VSWR circuit at J1-11.
•
A voltage sample for the Network VSWR circuit from 2C2
is fed to the board at E2 and leaves the board on J1-1.
H.2.1.3
Calibration Jumpers
Plugs P1 and P2 and jumper JPR4 are used to calibrate the VSWR
circuits on the Output Monitor.
H.2.2
Output Monitor
The Output Monitor contains circuits to detect a VSWR condi-
tion when arcs, faults, or impedance changes occur in the trans-
mitter bandpass filter/output network or in the antenna system or
load connected to the transmitter output. If a VSWR fault is
detected, the PA modules are immediately turned off and the
“Oscillator Sync” circuit is activated. This will protect the PA
module transistors during a VSWR shut-down. The VSWR logic
on the LED Board will return the transmitter to normal operation
within approximately 20 milliseconds unless a number of VSWR
trips occur in quick succession.
Directional Coupler circuits to detect Forward/Reflected power
and Modulation Monitor sample adjustment circuits are also on
this board.
H.2.2.1
Phase Angle Detector, Theory Of Operation
This description of Phase Angle Detector circuit operation ap-
plies to both the Antenna VSWR and Bandpass Filter VSWR
phase angle detectors. Refer to the Simplified Schematic Dia-
gram (Phase Angle Detectors), Figure H-1, for the following
discussion.
When a transmission line is terminated with a resistive load, the
VSWR will be 1.0 and voltage and current in the line will be in
phase and will have amplitudes determined by Ohm’s law
(E=IR). If the load RESISTANCE changes, the current and
voltage AMPLITUDE relationship will change. If the load RE-
ACTANCE changes, the current and voltage PHASE relation-
ship will change. The phase angle detectors used in the
transmitter are balanced or “nulled” for the phase and amplitude
relationships that exist when the output network is properly tuned
into a 50 + j0 Ohms. Any VSWR condition will cause the RF
current and voltage phase/amplitude to change. This will produce
a voltage at the output of one or both phase angle detectors.
a. CURRENT SAMPLE: The current sample for the phase
angle detector is a current transformer. The primary, a
copper tube or rod carrying the RF current, passes through
the secondary, a ferrite core inductor. Resistors are con-
nected across the secondary to load the winding and to
convert the current sample to a voltage sample for the
phase angle detector.
b. VOLTAGE SAMPLE: A capacitive voltage divider pro-
vides an RF voltage sample for the phase angle detector.
The RF voltage sample and the RF current sample are applied to
opposite ends of the primary winding of the phase angle detector
transformer (T1 or T3). When the samples are in phase and have
the same amplitudes, there will be no RF current flow through
the transformer. If the phase and/or amplitude of either sample
changes, current will flow through the transformer primary
Rev. R: 11-11-96
888-2297-002
H-1
10/14/1999
WARNING: Disconnect primary power prior to servicing.
