HP 8340B Manual page 334

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