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17.2.1.4 Port
Selection
The Antenna
and
RF In/Out port
signal
is
routed to
the
Wide
band
Amplifier
board (A17
A2) by
relay Kl.
The
Antenna
port is selected
by
turning on
transistor
Ql.
This
path
starts
at the connector, passes through
fuse
F1 and
relay
K1,
connecting
to the Wideband
Amplifier
via t he
rotary
step
attenuator, ATL
Follow-
ing loads
AT2
and
AT3,
the
RF
In/Out
port is
simi-
larly connected to the Wideband
Amplifier via
rotary
step
attenuator ATL Both the
Antenna
and
RF In/Out
port can receive and transmit
signals
from the
Wide-
band
Amplifier.
17.2.2
WIDEBAND AMPLIFIER BOARD (A17A2)
17.2.2.1 General
The Wideband Amplifier
board
performs
three
functions: 1) amplification
and leveling
of
the RF
Syn-
thesizer's output;
2)
RF
-switching,
which
allows the
RF
Synthesizer's output
to be used
as the
system's
RF-
generator output or as the
local
oscillator (L.O.) for
the
system's
monitor function; and
3)
frequency-transla-
tion,
converting the RF input to 10.7
MHz
when the
system
is in monitor mode.
A block
diagram
of
the Wide
band
Amplifier board
is
shown
at the end
of
the section in Figure 17-7, a
schematic
in Figure
17-8, and a
printed
wiring
board
assembly and parts
list in Figure
17-9.
17.2.2.2
Generate Mode
When
the system's
FUNCTION control is
set
to
Generate
FM,
Generate CW, Generate
AM,
or SWP,
or
when the DISPLAY control is set to Spect
Ana-
lyzer,
the
MON
+
DSB/GEN
input
is
low, putting
the
Wideband Amplifier
in generate
mode.
In
this mode,
the Wideband Amplifier's output is directed through
relay
K1
and out the ATTENUATOR OUT/IN
port.
The
RF
-leveling loop
allows control of the
level
of the
RF
output.
Using
the
variable RF Level
control on the
front
panel,
the
operator can
vary
the level
at
the
Antenna
port
between
-3
dBm
and
+
13
dBm.
17.2.2.2.
1 Voltage-Controlled Attenuator
(VCA)
The
RF
Synthesizer's (A9) output drive to amplifier
U4
is
controlled by the VCA (Q2, CR1, CR2, CR3).
As
the voltage
on
the base of
Q2
is
varied,
the
currents
through
PIN diodes CR1,
CR2,
and
CR3
are
changed,
thus varying the
attenuation. Minimum
attenuation
occurs
when
the voltage
on
t
he
base
of
Q2 is at its
maximum (approximately 11
volts).
Over the full range
of
de
control
voltages, the
VCA range is
greater
than
50
dB.
17-2
17.2.2.2.2
Wideband Amplifiers
The VCA
is
followed by
three stages of amplifica-
tion
at
U4,
U5, and
Q7.
This cascade
has
a nominal gain
of about
35
dB
and
can linearly
deliver
+
16
dBm
to a
50-ohm
load.
U4 and
U5 are
biased with
constant cur-
rent
sources
Q4
and
Q5, respectively. Q7 is biased in
a
slightly
different manner by Q6. Q6
samples the
col-
lector
current
of
Q7
and adjusts the base
bias
accord-
ingly,
to achieve
a
collector
current of 57
mA.
17.2.2.2.3
Detector
and
Detector Amplifier
The
output
voltage
of the
Wideband Amplifier
is
sampled
by
detector CR10.
Accuracy at
low output
levels
is
improved
by
slightly
forward-biasing
CR10
with
resistors
R29-R31
and
R33. Operational
ampli-
fier
U7
amplifies the detector
output
and drives
the
CARRIER
+
MOD LVL
output
and
one
input
of
the
summing
amplifier. The
CARRIER
+
MOD
LVL
output is
used
to
measure the
output
level
and
percent
AM
of
the signal at
t he
generator
output.
Gain
poten-
tiometer
R34
and
offset
potentiometer
R38 are
adjusted so
that
the
CARRIER
+
MOD LVL is
8
times
the rms
voltage
at
the output of the front panel's
Antenna
port.
17.2.2.2.4 Summing Amplifier
and
ALC-Reference
Selection
As mentioned
in
the
previous paragraph, one
input
of
the
summing
amplifier,
US,
is
driven
by
the
detec-
tor amplifier.
The
other
input is driven by the
ALC-
refer
en~e
selector, U9.
The summing amplifier
com-
pares
the
detector
amplifier's
output
with the
selected
ALC reference and then adjusts the VCA's drive
to
correspond
to the
desired
output level
indicated by
the
ALC-reference
voltage.
In
generate
mode,
the AM
MOD
+
DC
REF input
is
selected as the
ALC
refer-
ence.
When
AM
is
not the selected
function, this
sig-
nal
is a de voltage,
controlled by the
variable
RF Level
control on
the front panel. The range
of this voltage
is
sufficient
to allow
an output-power range of -3
dBm
to
+
13 dBm at
the front panel's
Antenna
port.
When
AM
is the selected function, the
AM MOD
+
DC REF
input consists
of ac and de
components.
The ratio
of
the ac component to the
de
component
determines the
percent AM at the
generator output.
This ratio
and
the
type of
modulation
signal
are
controlled
by the front
panel's MODULATION
controls.
17.2.2.2.5 HI/LO
Band
Switching
To operate
over
the full band of
10
kHz to 1
GHz,
the
Wide band Amplifier is
operated in either HI-band
mode
or
LO-band
mode.
The mode
is controlled by
the
WB AMP HI/LO
input.
In HI-band mode,
the VCA
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