HP 8340b User Manual

ALC
LOOP
The
ALC
loop
is
a
feedback
control
system
that
monitors
RF
power
and
maintains
that
power
at
a
set
level.
The
power
can
be
monitored
at
a
point
inside
the
instrument
(internal
leveling), or
at
some
point
in
the
test setup
(external
leveling). Three assemblies
provide
control
circuitry
for
the
ALC
loop:
•
The
A25 ALC
detector assembly
•
The
A26
linear
modulator
driver
assembly
•
The
A27
level
control assembly
Several
RF
microcircuits
are
included
in
the
ALC
loop,
providing
the
input
voltages
for
the
loop
assemblies.
Leveling
Loop
Description
A
voltage
derived
from the
power sensor
(internal
crystal
detector,
external crystal
detector,
or
exter¬
nal
power
meter)
is
compared
to
a
reference voltage
at
the
loop
summing
point
(see
Figure
H-1).
If
the
resulting
currents
at
the
summing
node
do
not
cancel,
the
loop
integrator
output voltage
changes,
causing
the
modulator
to
vary
the
RF
output
power.
The
power changes
until the
voltage
representing
RF
power
cancels
the
reference,
at
which
point
the
integrator
output stops
changing and
the
power
remains
constant.
The
feedback
loop
reduces
the
current
to
the
integrator
to
zero.
Because
there
is
one
detector output voltage
that
causes
zero
integrator
current
for
any
given
reference voltage,
the
loop
controls
the
detector
output
voltage.
ATTENUATOR
T-^-
OUTPUT
<J3
MODULATOR
OSCILLATOR
16dB
DIRECTIONAL
_'
COUPLER
-wNr-
OETECTOR
AMP
^
I,-I,
\
\
LOOP
SUMMING
POINT
REFERENCE
VOLTAGE
LOOP
INTEGRATOR
Figure
H-1.
Leveling
Loop
Simplified
Block
Diagram
A
16
dB
directional
coupler samples
the
RF
output
power.
Its
coupled
arm produces
a
signal
16 dB
smaller than
the
level of the
RF
output
power.
Any
RF
power
coming
Into the
instrument
from
the
outside
is
ideally
not
coupled
to
the
coupled
arm
at
all,
but
because
the
16
dB
coupling
factor
is
not
perfectly
flat
(constant as
a
function
of
RF
frequency),
the
detector
connected
to
the
coupled
arm
does
couple minimal
reverse
power
into
the
detector.
HP 8340B/41B
RF
Section
Theory
of
Operation

HP 8340b Specifications

General

Brand	HP
Model	8340b
Category	Inverter
Language	English