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17.1
DESCRIPTION
The RF Input module
comprises
four
sections:
1)
the
Wattmeter board
(A17A1),
2)
the Wide band Amplifier board
(A17
A2),
3) the
Duplex
Generator board or
Enhanced Duplex
Generator (A17
A3),
and
4) the
mechanical
assembly.
The
chassis
uses feed-through filters
and capacitors
to
interconnect the
three
printed circuit boards. The
mechanical
assembly includes
a
0
to 130-dB rotary
step
attenuator with associated
co-axial cables,
Antenna,
RF In/Out and Duplex
Generator
ports, and the
Duplex
Generator
On/
Off
switch.
Electrically,
the
RF Input module provides five pri-
mary RF
functions:
1) High-level
generate
(GENERATE ANTENNA
PORT)
provides
an
RF
-output signal
(AM,FM,
or
CW;
10kHz
to
1 GHz)
adjustable over a
range
of
+
13 dBm to
-135
dBm;
2)
Low-level
generate (GENERATOR
RF IN/OUT
PORT) provides an RF -output signal
(as above,
for
GENERATE
ANTENNA PORT), adjustable
over
a
range
of -17
dBm to
-165
dBm;
3)
High-level
power monitor
(MONITOR
RF IN/
OUT PORT) provides
a
means
of
measuring RF
power (1 MHz
to
1
GHz;
1
to
125W
at
±
10
per-
cent
accuracy);
4) Sensitive
monitor
(MONITOR
ANTENNA
PORT) provides
off-the-air
reception;
5)
Duplex
Generator (DUPLEX
GEN) provides an
RF
-output
signal,
offset
in frequency
(0 to
10 MHz
or fixed
45 MHz) from the
rec!lived
signal.
6) Enhanced
Duplex Generator (DUPLEX
GEN)
provides
an
RF -Output
signal, offset
in
frequency
(0 to
10
MHz
or fixed 39
MHz,
45
MHz
or 55
MHz)
from the received signal.
Enhanced Duplex
Gen-
erator
is required for
the 900
MHz
trunking
option.
A
wirelist of the
RF Input module
is
shown at
the
end of the section
in
Table 17-3, a block
diagram in
Figure
17-1,
and an
interconnect diagram in
Figure
17-
2.
Following
that,
Figure
17-3 coordinates
a
parts list
with
the
assembly of
the RF Input
module and with
the
printed
wiring
board
assembly
of the
RF Input Flex
Standard.
17.2
THEORY OF OPERATION
17.2.1 WATTMETER BOARD (A17A1)
17-1
SECTION
17.
RF INPUT MODULE (A17)
17
.2.1.1 General
RF
power to and from the System
Analyzer
passes
through the
Wattmeter board
to one of two
ports,
Antenna or
RF In/Out. In
the sensitive
monitor
mode
(less
.than
100
milliwatts) or in
high-level generate
mode,
the signal
is
either
transmitted or received
through the
Antenna
port.
In
the
high-level RF-input
mode, power
enters
the module through
the
RF In/Out
port.
This
port is
-
30
dB below
the
Antenna
port.
T
his
board
also
contains
the
circuitry
for the wattmeter.
A block
diagram
of
the Wattmeter board
is shown at
the end of the section in
Figure
17-4,
a schematic in
Figure
17-5
and the
printed
wiring
board
assembly
and
parts list in Figure
17-6.
17.2.1.2
Wattmeter
In
the
monitor-RF-power
mode,
the
signal applied
to
the
RF In/Out
port (J1)
is
first attenuated
by
a
14-dB,
125W
attenuator
(AT2).
This
reduced
power
level
is
applied to 50-ohm
load
AT3.
Diode
CR1
detects
the
peak of the
voltage
produced
across the
load,
and
the
resulting
de
level
is applied to a three-stage,
instrumentation-style amplifier (U1A, U1B,
and
U1C).
Thermistor RT2 compensates for
changes
in the
internal temperature of the
compartment.
Diode CR3
and operational amplifier U1A
provide temperature
compensation for
diode
CRl. U1B
amplifies
the
detected
signal,
and
then
U1C differentially
sums
U1B's output with
the output
of the
reference
ampli-
fier,
U1A.
The
signal at the
output
of U1C
is
con-
nected
to
the
Scope/DVM
Control
board (A7), where
it
is
routed
to the
DVM. Because
of
diode detector
CR1, this
output
is directly
proportional to the square
root of the power applied (Power in
watts
=
1.25*V
2 )
.
Potentiometer R20
provides
offset
adjustment, while
R7
provides gain
adjustment.
For 125W
of input
power, 50-ohm
load
AT3 dissipates
only
5W,
with
the
balance absorbed
by
the
14-dB,
125W
attenuator,
AT2.
17.2.1.3
Over-Temperature
Protection
The
14-dB,
125W
attenuator (AT2) can
dissipate
125
watts for approximately
1.5 minutes,
at which
time
the
temperature
of the
device exceeds
the
tempera-
ture
rating
of
100°C.
Thermistor
RT1,
recessed
into
the
flange
that
AT2 is
soldered
to, decreases
in resist
-
ance
to
10K
ohm, causing
the
output
of comparator
U2A to go
high.
This
signal
causes the
processor
to
activate the System
Analyzer's
audible alarm and to
flash a
warning
signal
on
the
CRT until
the
applied
RF
power
is
removed and
AT2 has
had ample time to
cool
off.
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