HP 8340b User Manual

ALC
Loop
Assembly-Level
Troubleshooting
To
troubleshoot
the
ALC
loop,
you can
break
the
loop
and
inject
a
signal
that
you
can
trace
around
the
loop.
In
the
ALC
loop
troubleshooting
mode,
FET
switches
break the
connection
from
the
A25
ALC
detector
assembly
to
the
A26
linear
modulator assembly).
A
resistor
(A26R17)
is
switched
across
the
integrator
capacitor on
the
A26
linear
modulator
assembly,
converting
this
stage
to
an
inverting
amplifier.
In
this
way,
the
integrator output
can
be
controlled via
the
reference
DAC,
and
the
signal traced from
there
to
the
modulator driver,
RF
output,
detector
output,
logger
output,
X5
amplifier,
through
the
sample
and
hold,
level
amplifier,
and
to
the
analog-to-digital converter
driving
the
level
meter.
1.
To
enter
the
test
mode,
press
[INSTR PRESET] [CW]
[SHIFT]
[METER].
The
UNLEVELED
annunciator
should be
on.
The
POWER
dBm display
should indicate
the
approximate
RF
output
power.
The
ENTRY
display
should indicate
ATN:
-00,
MOD:
0.0 dB.
The
modulators
are
controlled
by
the
control voltage
MODLVL,
on
the
A26
assembly.
This
voltage
is
programmed
by
the
A27
level control
assembly
ALC
reference
generator
DAC.
The
DAC
can
be
controlled
using
the
RPG.
2.
Rotate
the
front
panel
knob
CCW
and
note
that
the
ENTRY
display
MOD
level
changes.
3.
Continue
turning
the
front
panel
knob
CCW until
the
POWER
dBm
display
begins
to
change.
The
absolute
MOD
level
indication
is
not
important,
but
a
further
decrease
in
MOD
level
should
generate
a
corresponding
decrease
in
the
POWER dBm
display,
and the
RF
output
power.
For
example,
a
10
dB
MOD
level
change
should
generate
a
10
dB
±3
dB
power
change.
4.
Isolate
a
defective
assembly
by
tracing
the
voltage from
the
MODLVL
output
to
the
modulators, or
by
tracing
the
voltage
from
the
high
and
low
band
detectors
to the
level
meter
amplifier
output
on
the
A25
ALC
detector
assembly.
The
AM
signal
path
(on
the
A26
assembly)
must
be checked
separately.
Refer
to Figure
H-7.
5.
Attach
an oscilloscope
or
DVM
to
the
MODLVL
test point
(A26TP5)
and
vary
the
front
panel
knob.
The
voltage
level
should
vary
between
+0.5
(ENTRY
display
MOD
level
at
0.0
dB)
and
—2.50
Vdc
(ENTRY
display
MOD
level
at
-100
dB).
6.
Attach
an
oscilloscope
or
DVM
to
the
MOD
test point
(A26TP5)
and
vary
the front
panel
knob.
The
voltage
level
should
vary
between
—0.25
(ENTRY
display
MOD
level
at
0.0
dB)
and
+1.00
Vdc
(ENTRY
display
MOD
level
at
—100
dB).
The
signal
should
vary
exponentially
with
the
front
panel
knob.
7.
If
either
MODLVL
or
MOD
are
incorrect,
the
A26
linear
modulator assembly
is
the
most
probable
cause.
You
can
measure
RF
levels
at
various
points
in
the
RF
path,
and
power
levels
should
vary
with the
front
panel
knob.
If
the
A26
assembly signals
(MODLVL
and
MOD)
are
good,
and
the
RF
measurements
are
not
correct
(as
indicated
by
the
RF
power
display
on
the
front
panel),
refer
to the
RF
path
trou¬
bleshooting
procedures
for
microcircuit
assembly
isolation.
HP
8340B/41B
ALC
Loop Assembly-Level Troubleshooting
H-21

HP 8340b Specifications

General

Brand	HP
Model	8340b
Category	Inverter
Language	English