Specifications

ship between the voltage and current. This results in output from
the detector circuit.
The Antenna VSWR detector circuit is very similar to the Net-
work VSWR circuit. However, the RF samples are taken near the
output terminal of the transmitter by the Output Sample board.
H.6.2
Frequency Selection
There are 3 independent setups for the Antenna VSWR, Network
VSWR, Forward Power, and Reflected Power. These 3 setups are
used to satisfy the alignment for each of the 3 frequencies of
operation.
Refer to sheet 4 of schematic diagram 8435400851. Control
signals at J8-5, J8-7, and J8-9 determine which of the 3 frequen-
cies is selected. An active Low lights the corresponding led, and
drives the corresponding Schmitt trigger output high. Only one
input is driven Low at a time, as determined by the frequency
select control circuit on another board.
The voltage on U5 outputs 2, 4, and 6 provide control voltages
designated as “Control 1”, “Control 2”,and“Control 3”.These
voltages drive the FETs that determine which RF components
are in circuit. In the case of the Forward and Reflected metering
circuits shown on sheet 4, U5 drives Q40, Q41, and Q42 to select
the calibration components for each frequency. In the case of the
VSWR detection circuits, the control voltages are applied to the
array of switches that select which RF components are in effect.
Diodes are used to OR the voltages to the appropriate FET so
that one switch setting does not interfere with another.
H.6.2.1 Normal/Calibrate Test Switch
The Normal/Cal switch is used during factory testing to verify
proper resonating of the transformer primary circuit. When the
switch is in the Cal position, the current samples are connected
to ground. The voltage samples will apply an RF voltage to one
end of the transformer primary. With an oscilloscope connected
to the other end of the winding, the circuit is tuned to resonance
when the RF voltage indication on the oscilloscope is minimum.
H.6.3
Network VSWR Detector
The secondary winding of T1 is connected to a full wave rectifier
and filter capacitor. The output DC voltage can be measured at
TP19 Network VSWR and is connected to the Network VSWR
Comparator and the Network VSWR Buffer.
H.6.4
Network Fault Comparator
This circuit uses a very high-speed differential comparator, U7.
One input to the comparator is an adjustable positive reference
voltage from the Output Network VSWR Threshold control,
R139, and the other input is the dc signal from the detector.
Diodes protect the comparator’s input from transient voltages.
Normally, the detector output will be essentially zero volts, and
the comparator’s output will be logic low. When the detector
output exceeds the reference voltage during a VSWR fault con-
dition, the comparator output will go logic high.
H.6.5
Network VSWR Buffer
The detector output is buffered and connected to the NET NULL
position on the front panel multimeter. This is a relative reading
only and when the detector is properly balanced the meter should
read near zero.
H.6.6
Antenna VSWR Detector
The secondary winding of T3 is connected to a full wave rectifier
and filter capacitor. The output DC voltage can be measured at
TP7 Antenna VSWR and is connected to the Antenna VSWR
Comparator and the Antenna VSWR Buffer.
H.6.7
Antenna Fault Comparator
This circuit uses a very high-speed differential comparator, U10.
One input to the comparator is an adjustable positive reference
voltage from the Antenna VSWR Threshold control, R130, and
the other input is the dc signal from the detector. Diodes protect
the comparator’s input from transient voltages. Normally, the
detector output will be essentially zero volts, and the compara-
tor’s output will be logic low. When the detector output exceeds
the reference voltage during a VSWR fault condition, the com-
parator output will go logic high.
H.6.8
Antenna VSWR Buffer
The detector output is buffered and connected to the ANT NULL
position on the front panel multimeter. This is a relative reading
only and when the detector is properly balanced the meter should
read near zero.
H.6.9
VSWR Pulse Generator
When either the Antenna or Network VSWR Comparator output
goes active high, several events occur:
H.6.9.1 Local RF MUTE
The active high output turns on the transistor that generates an
active high RF MUTE Local at TP30. This serves two functions:
a. First, another transistor turns on pulling the RF MUTE line
J6-21 and TP28 low for 12mS. This action turns all the Big
Step and Binary RF Amplifiers off for 12ms.
b. Second, a third transistor is turned on for 250nS and its
output is inverted. The 250nS VSWR-H signal at TP29 and
J6-19 is sent to the Synthesizer Interface board for activat-
ing the Synthesizer Sync circuit.
H.6.9.2 VSWR One-Shot
a. At the same time, the B input of one-shot also goes logic
high, and this starts a timing cycle that lasts 0.5 seconds.
b. The Antenna VSWR or Output Network VSWR LED will
be illuminated RED for this duration to give a visual
indication of the fault.
1. The input to the VSWR PAL will also go logic high for
this duration and be counted by the PAL as 1 VSWR
trip.
NOTE
In the case of Antenna VSWR only, a inverter disables the Net-
work VSWR input to the PAL.
H.6.9.3 Power-Up Reset
When low voltage is first applied to the board, the Power-Up
Reset supplies a logic low that clears the one-shots.
H-4 888-2339-002 02/17/04
WARNING: Disconnect primary power prior to servicing.