Harris DX 25U, Техническое руководство

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comparator output will go LOW. Diodes CR11 and CR13 protect
U1 from transient voltages.
H.2.2.5.2
R-C Network
A VSWR condition may last for only a few microseconds.
Because the transmitter output is turned off very rapidly by a
logic signal which goes directly to the Modulation Encoder, this
may not be enough time for fault and overload logic to act.
Capacitor-resistor network C14/R38 holds the comparator out-
put low for about 20 microseconds or more after the phase angle
detector output returns to normal.
H.2.2.5.3
Manual VSWR Trip
Manual VSWR Trip switch S4 simulates a VSWR fault by
pulling the phase angle detector output to +5 VDC.
H.2.2.5.4
VSWR Loop Self Test
The transmitter includes a self-test feature. Each time the trans-
mitter low voltage supply is turned on, the VSWR trip circuits
and logic are tested. The result of the “self-test” is indicated by
the VSWR Sensor “STATUS” LED on the ColorStat™ panel.
The LED will pulse red and then turn green if the test is success-
ful, but will remain red if the test fails.
VSWR self-test logic circuits are described in SECTION Q, LED
Board. The logic generates a Self-Test Logic LOW pulse, and
evaluates the results of the test, whenever any of three conditions
occurs:
a. Transmitter low voltage is applied (either after shut-down
for maintenance or after a power failure).
b. The VSWR Sensor “MANUAL TEST” button on the
ColorStat™ panel is depressed.
c. A remote VSWR “Manual Test” command is given,
through the External Interface.
On the Output Monitor, the logic LOW self-test pulse turns on
transistors Q5 and Q6, pulling the non-inverting inputs of both
VSWR trip comparators to +5 VDC (Logic High) and simulating
a VSWR fault.
H.2.2.6
“VSWR Trip” Logic
The output of U3-11 goes to monostable multivibrator U6-1 and
also to AND gate U5-9.
H.2.2.7 AND Gate U5
If U5-9 or U5-10 goes LOW, U5-8 also goes LOW. This output
goes directly to the Modulation Encoder to immediately turn all
PA modules OFF.
H.2.2.8
Monostable Multivibrators
Each time a VSWR condition is detected by one of the phase angle
detector circuits, dual retriggerable monostable multivibrator U6 is
triggered by the falling (negative going) edge of VSWR trip com-
parators U2-11 or U3-11. The U6 output LOW pulses go to the
VSWR fault and overload logic on the LED Board.
Section U6-4 is the output to the “Antenna VSWR trip” and
section U6-12 is the output for the “Bandpass Filter VSWR trip.”
The output LOW pulse width for each trip pulse is determined
by a resistor-capacitor network. For the “Antenna VSWR trip”
C48 and R51 at U6-15 set a pulse width of 14 milliseconds. For
the “Bandpass VSWR trip” C49 and R50 at U6-7 provide a 19
millisecond pulse width.
Switch S5 prevents U6 from generating a pulse during phase
angle detector circuit adjustments.
H.2.2.9 Directional Coupler Circuit Description
A voltage proportional to RF current from the Output Sample
Board enters the board at J1-3 and J1-5 and is fed to the anodes
of CR28 and CR33. The voltages are taken from opposite sides
of the RF current transformer, so they are 180° out of phase.
Voltage samples are taken from two capacitive dividers on the
Output Sample Board and enter the board at J1-15 and J1-17
The “Forward Balance” adjustment C6 is in parallel with the
voltage divider capacitor on the Output Sample Board.
Under normal operation, P1 and P3 are connected between 1-3.
The voltage and current samples on the Anode and Cathode of
CR33 are 180° out of phase. The DC current flow through R18
establishes a voltage proportional to the current flow and the
square root of forward power. RF choke L1 and capacitor C4
form a filter to remove the RF component and series resistor R20
isolates the coupler from load variations. Resistor R18 and
capacitor C26 also form a low-pass filter to remove audio-fre-
quency variations due to modulation from the coupler’s output.
For the forward power coupler, reversing jumper plugs P1 and
P3 changes the coupler to read reflected power for calibration.
The reflected coupler operates the same way as the forward
coupler, except that the current sample is 180° out of phase.
During VSWR conditions, the phase/voltage relationships at the
ends of CR28 change such that current will begin to flow through
R19. The voltage established through R19 will be proportional
to the square root of the reflected power. Variable capacitor C40
is a balance adjustment, low-pass filter L4 and C13 remove the
RF component and R22 and C22 form a low-pass filter to remove
audio-frequency components. Resistor R22 also isolates the
coupler from load variations. For the “reflected power” coupler,
reversing jumper plugs P1 and P3 changes the coupler to read
forward power to calibrate the reflected power meter.
H.2.2.9.1
Other Power Metering Components
The outputs of the directional coupler go through the LED Board
to voltage followers on the Controller. The voltage follower
outputs drive the power meter on Switch Board/Meter Panel and
the forward and reflected power outputs at the External Interface.
Forward and reflected power calibration controls are located on
the Switch Board/Meter Panel.
H.2.2.10 Detected Audio
Transformer T2 and Diodes CR6 and CR10 form an audio
detection circuit. The output is available at J4 and can be used to
monitor the audio signal.
H.2.2.11
Modulation Monitor Sample
The Modulation Monitor Sample circuit includes relays and
adjustments to provide the same RF output level to the modula-
tion monitor at any power level.
The from adjustable tapped inductor L7, in the Output Network
Compartment, enters the board at J7-1. The signal to the modu-
Section H - Output Sample (2A26) & Output Monitor (1A27)
Rev. R: 11-11-96 888-2297-002 H-3
10/14/1999 WARNING: Disconnect primary power prior to servicing.