Alc Loop Control Voltage; Detector Linearity; Rpp Control - Fluke 6060B Instruction Manual

MAINTENANCE
TROUBLESHOOTING
AND
REPAIR
4D-47.
ALC
Loop
Control Voltage
PROCEDURE
1.
Connect
a
IV peak (2V
p-p),
1-kHz
signal
source to the external
MOD
INPUT.
2.
Program
the
UUT
for
350
MHz,
7
dBm, 71%
AM
depth,
and
EXT
AM
ON.
3.
Measure
the
ac
and
the
dc voltage
at
TP7. The rms
voltage
should be nominally
50% of the dc
voltage.
4.
Program
the
UUT
for
35%
AM
depth.
The rms voltage
should be nominally
25%
of the
dc voltage.
If
the
UUT
fails this
test,
the
problem
lies
somewhere between
the
EXT
MOD
input
and
TP7 (ALC
loop-control voltage).
To
further localize
the
problem, the
same
test
can be
done by measuring
the ac voltage
at
U302 pin
8
(input
to
level
DAC).
If
the
measured ac
voltage
does not change
as
programmed
AM
depth
is
changed, either the
AM
DAC
or
its
control
is
at fault.
The
AM DAC
(A2A4U301)
is
an
8-bit
DAC
and
is
set
to
twice the
programmed
AM
depth,
e.g.,
180 for
90% AM.
If
the
UUT
passes
this test,
then the
ALC
loop control voltage
is
correct,
and
the
problem
is
in
the
ALC
loop.
A
likely
cause of excessive
AM
depth error
and harmonic
distortion
is
detector non-linearity.
The
following
test
checks
detector
linearity,
4D-48. Detector Linearity
PROCEDURE
1
.
Install
the
plate
covers
and
let
the
UUT
warm
up
at
room temperature
for
one
hour.
2.
Program
the
UUT
for
350
MHz,
12
dBm, modulation
OFF.
3.
Program [SPCL]
[8l[l]
and
[SPCL]
[9][l]to disable level
correction
and
enable
amplitude fixed range.
4.
Measure power
with a
power meter
at
the
UUT RF OUTPUT.
Note the reading.
5.
Program
the
UUT
for 2
dBm
using
the
EDIT
keys.
The
measured power should
be 10
dB
±
0.2
dB
below the
reading noted
in
step
4.
6.
Program the
UUT
for
-8
dBm
using the
EDIT
keys.
The
measured power
should be 20
dB
±
0.4
dB
below
the
noted reading.
7.
Program
the
UUT
for
[SPCL]
[0][0].
If
the
UUT
fails
this test,
the
problem
is
likely
to be
in
the detector or detector-Unearizer
circuit. If
the
UUT
passes the
test,
the
problem
is
constrained
to the other
ALC
loop
elements,
and
is
likely
to
be a
bandwidth problem associated
with the loop
amplifier or
the
modulator
or modulator-linearizer
circuit.
4D-49.
RPP
Control
.
,
• * *
When
servicing the
A2A5 A2
Attenuator/
RPP
Control
PCB,
use the three
dual-pm
test
points to aid
in
the
troubleshooting of
the assembly.
The
RPP
can be tripped
(to
the
protect position)
by momentarily shorting the
two points of
TP
I.
It
can be
reset
by
4D-21