Functional Description; Level; Amplitude Modulation - Fluke 6060B Instruction Manual

THEORY OF OPERATION
3-2
The power
supply
is
a
linear design providing two +
1
5
V, -15V, +5 V, +37 V, +
1
8
V,
and
6
V
ac to the
Generator.
All the
power
supplies are series-pass regulated
except the
6V
ac
filament
supply and
the +
1
8
V
supply,
which
provides
power
to the
Attenuator
relays
and
Reverse
Power
Protector
(RPP)
relays.
A
fuse/
filter/
line-voltage selector allows the
Generator
to
operate
from any
common
supply voltage.
The
dc fan
is
powered from
the
unregulated
+5V
supply.
The
fan operates only
when
line
power
is
available
and
the front panel
POWER
switch
is
ON,
The
Option -130 High-Stability Reference operates whenever
the
instrument
is
plugged
into
an
active ac outlet, regardless
of
the position of the instrument
POWER
switch.
3-6.
FUNCTIONAL
DESCRIPTION
The
following
paragraphs
describe the
key output parameters
of
the Generator:
level,
amplitude modulation, frequency,
and
frequency modulation.
3-7. Level
Level control
is
provided
by two
separate
circuits,
a
step attenuator
and
a
vernier
level
DAC.
The
6.02
dB
per step Attenuator/
RPP, (A2A5)
provides coarse control.
Fine
level
control
is
provided by
a vernier
level
DAC
that varies the
automatic
level
control voltage
(ALC). The
microprocessor automatically controls the step attenuator
and
the vernier
level
DAC.
The
microprocessor
also applies level
correction
to
compensate
for the
Generator frequency
response.
Each Generator
has
level
correction data for the
Output and Attenuator
assemblies,
stored
in
the
Output and
Attenuator calibration
EPROMs.
The
EPROMs
are located
on
the
Controller assembly.
The
correction data
is
based
on measurements
of each assembly
during
calibration of the
Generator
at the factory.
This microprocessor
level
correction
data
is
applied only to the vernier
level
DAC;
it
does
not
affect
the coarse
level
control
provided by
the Attenuator, In other words,
all
Generators have
the
same
attenuator
pads
inserted at a selected
level
even though
the
correction data
is
different for
each Generator.
To
improve
level
accuracy
in
relation to
temperature,
the
Generator
uses
a software
temperature compensation
technique. This technique
uses
data
that
is
the
same
for
all
Generators regardless
of
the options
installed.
3-8.
Amplitude Modulation
The
output of
the level
DAC
is
the
ALC
loop
control
voltage.
The
Generator output
signal
is
amplitude modulated by varying
this
control voltage with the modulating
signal.
A
I
V
peak modulating
signal
from
the
internal
modulation
oscillator
or
from
the external
MOD
INPUT
connector
is
applied
to
the
AM
DAC,
a multiplying
D-to-A
Converter.
The
multiplying factor of
this
DAC,
corresponding to the
programmed
percentage of
modulation,
is
factored by
the Controller.
The
modulation
signal
from
the
AM
DAC
is
summed
with
a fixed
dc
reference voltage.
The
composite signal
(dc plus
modulation)
is
applied
to
the
LEVEL
DAC,
a
level
control-
multiplying
DAC,
The
multiplying
factor for
this
DAC
is
also
handled by
the Controller
and
corresponds
to the
programmed
signal
level.
The
multipling factor also includes the
level
correction information
stored in the calibration
EPROMs,
The
operation of the
ALC
loop causes
the
amplitude
of the
RF
signal to
conform
to
this
varying control voltage, thus amplitude modulating
the
Generator
output.