Fluke 6060B - ALC, Detector, and RPP Control

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
A2A5A2
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 TPI.
It can be
reset by
4D-21