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2.3.2
FREQUENCY
STANDARD
To provide
a
10-MHz
signal
to the
EXTERNAL
10
MHz OUTPUT
and
to
the
RF Synthesizer
modu
le
(A9),
the Frequency-Standard Interface board (A16)
contains a
10-MHz
crystal
oscillator with buffering and
switching.
An
external10-MHz
standard
signal
can be
used by switching the IN/OUT
switch
on
the bottom
panel
to IN.
The
signal applied to the 10-MHz STD
input/output
port
on
the
back is
then
routed
through
the
Al6
board
to the A9 module.
2.3.3
RF SYNTHESIZER
The RF
Synthesizer
uses the
10-MH
z standard sig-
nal for
its
phase-locked-loop
reference
frequencies and
also converts
the 10-MHz
standard
signal
to a
TTL
level
for system
use.
The
RF
Synthesizer consists of
a
fixed 640-M Hz
loop and a programmable GHz
loop.
The programming
of the
GHz
loop
is provided
by
the
RF CONTROL BUS
from
the processor. The
Synthe-
sizer
Output board chooses one
of three
possible
out-
puts for the
SYNTH
RF output
signal. The
first
is
directly
from
the GHz
loop.
The
second
is the
output
of the
divide-by-two
on
the G
Hz
loop, which provides
frequencies
from
250 to
500 MHz. For outputs below
250
MHz,
the output of
the GHz loop is mixed
with
the
fixed 640-MHz signal,
and the difference
signal
is used
for the
output.
To
obtain
outputs from
10
kHz to 250
MHz,
the processor programs the GHz
loop for
fre-
quencies between
640.01
and 890 MHz,
respectively.
FM and
sweep
modulation
is
also
implemented in
the
G Hz
loop
by modulating its 60.5 MHz reference.
The Audio
Synthesizer
board (A10)
supplies
the FM
modulation
signal,
which provides modulation fre-
quencies
from 5
Hz to
20
kHz. FM
capability
is
100kHz
peak deviation
.
The Scope/DVM Control
board (A7)
supplies
the sweep-modulation signal.
The
sweep
capability,
10 MHz,
is used
for both
the sweep-gener-
ator and
the spectrum-analyzer
functions.
2.3.4
WIDEBAND
AMPLIFIER
The
SYNTH
RF signal is
amplified and
leveled in
the
RF
Input
module
(A17)
by
the
Wideband Ampli-
tier
(Al7
A2).
The signal level at
the
output
of
the
Wideband Amplifier is detected
and compared
to the
AM MOD
+
DC REF
signal
from the
front
panel's
level
control. If there
is a difference between
the
two
signal levels,
the ALC
am
plifier
provides an error
volt-
age.
This
error voltage
controls
the
attenuation of
the
voltage-controlled
attenuator
(VCA)
in the direction
that will
make
the
detected RF
output
equal
to
the
AM
MOD
-
DC
REF signal. There are
two
possible
VCAs
for the output
leveling.
The
VCA on
the Wideband
Amplifier board
is
used for
frequencies
from
1
to
1000
MHz.
For
frequencies below 1
MHz, the
VCA on
the
Processor
Interface
board
(All)
is
set
to
minimum
2-4
attenuation, and the VCA on the
RF
Synthesizer
module
(A9)
does
the
leveling.
Amplitude modulation
is
incorporated by summing the
modulation
signal with
the de reference
signal,
thus forcing
the leveling
loop
to vary the output level in
proportion to
the
modulat-
ing signal.
The
signal from the
RF-level detector
(CARRIER+
MOD
LVL)
is
used
by the
processor
to
determine
the
RF
output
level
and the percent
AM.
The
leveled
output
range
of
the Wideband Amplifier
is
from
-
3
dBm to+
13
dBm
(0.16 to
1.0
Vrms).
T he
leveled
output
from
the
Wideband Amplifier
is
applied
to the
Generate/
Monitor
switch.
For
generat-
ing AM,
FM, and CW
signals,
the
switch connects the
Amplifier
output to the step attenuator.
For double
sideband-suppressed
carrier (DSBSC) generation, the
Wideband Amplifier
output
is
connected to the
local-
oscillator (L.O.)
port
on
the
receive
mixer, and the
attenuator
is connected
to the
RF
port.
The
DSBSC
modulation
signal
then
drives
the
IF port
of
the mixer,
giving
a DSBSC signal
at
the
RF port and
thus
at the
step
attenuator.
For monitor
mode,
the
Wideband
Amplifier
output
is
connected
to
the
L.O.
port
on
the
receive
mixer,
and
the
attenuator is
connected
to
the
RF
port.
The
difference-frequency
output of the
receive
mixer
is sent
to
the
Receiver board
through the
10.7
MHz
IF
output.
2.3.5
STEP A TTENUATOR
Level
control
in 10-dB increments
is
provided by
the
step
attenuator.
The total range
of the
attenuator is
from
0 to 130
dB.
For the basic System
Analyzer,
the
step attenuator
is
controlled
directly
by
a knob
on the
front panel.
Wit
h
the
IEEE
control option (A13), the
step
attenuator is
electrically programmable and con-
trolled
by
the
processor.
In
this case, the knob on the
front panel is
connected only to
a
rotary
switch
which
directs
the
processor
in
setting the attenuation
level.
Under
IEEE
control, commands coming via
the
IEEE
bus
dete
rmine the
attenuator
setting.
2.3.6
INPUT/
OUTPUT
The
step attenuator
is connected to
the output
port
relay,
which chooses
between
the Antenna or
RF
In/
Out port.
There
is a
30-dB
difference
between these
two
ports.
The Antenna port
is for
receiving
low-level
signals (less
than
100
mW)
and for generating
high-
level
signals (
+
13
dBm
).
The RF In/Out port
is for
receiving high-level
signals,
with a
maximum
of
125W.
2.4
WATTMETER
Input
power
measurements
from 1 to
125W are made
only
through the
RF
In/Out
port.
(To measure the
power
of signals
under
l
W,
see paragraph 2.5,
MON-
ITOR
MODE.)
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