Fluke 6060B - Synthesizer Troubleshooting

MAINTENANCE
TROUBLESHOOTING
AND REPAIR
4D-3S.
SYNTHESIZER
TROUBLESHOOTING
NOTE
A llfrequencies
mentioned
are
synthesized; hence they
are exact
(coherent
with
the
10-MHz
reference),
unless noted
as
approximate.
If
the Generator
has
the
Low-Rate
FM (Option -651),
set A2A9S 1
for
normal
operation
(See Table
651-1)
while
troubleshooting.
If the
Generator
level is
inaccurate or an
unleveled
condition
exists,
then the
A2A4
Output
assembly is
probably
at fault.
If
an
unlock
condition
exists, the
problem is in
the
synthesizer. If
the output
frequency is in
error or
erratic, there is
likely a
problem
with the
Synthesizer
assembly.
However,
if
the
unlocked
condition
only occurs
when using an
external
reference, the
problem is
probably
in the
Sub-Harmonic
Reference
circuitry.
If the
unlocked
condition
exists
with REF
INT/EXT set to
INT, be sure
no
signal is
applied
to the REF IN
connector. An
external signal
applied (while
operating on
internal
reference) can
cause
the main loop
to unlock.
Next,
check
to see
if the
Generator
frequency
is
stuck
high
or
low. A
good way to
do
this is
to check
the dc
voltage
at TP44. If it
is
around
2V,
go to
the
Reference Circuitry
Check
in
the
following
paragraphs.
If
the
voltage is
around
25V, the
problem is associated with the main
PLL, i.e., VCO,
UHF
binary
divider,
buffer
amplifier, SSB
mixer,
triple-modulus prescaler,
or
N-
Divider.
Table
4D-10 shows the
characteristics of the
signals at the various
test
points on
the
Synthesizer
PCA. The
range
of the signal and the
expected value for a
typical
instrument
state are
given. The values
in the TYPICAL
column
are for the UUT
programmed to
160.11999
MHz, INT FM
on at 1 kHz, and 99.9
kHz deviation.
4D-36.
Reference
Circuitry
Check
There should
be
a 10-MHz
square wave at U55
pin 12. If
there is no
signal
at this
point,
the
problem
is in the
internal 10-MHz
crystal
oscillator.
The
frequency
should
change as
R230
is
adjusted.
The DC
voltage at TP 57
should vary
from 4 to 6
volts as R230
is
adjusted.
If there is
an external
signal
connected,
with
the switch
set to EXT,
there should
be
a TTL
signal at U67 pin
1 1. The same
signal should
appear at U68
pin 3. If either
of
the
enhanced
stability
options is present,
with the
switch
set to INT,
there should
also
be a 10
MHz TTL
signal
at U68 pin 3. If
the loop is
locked, there
should be a
stable
DC
voltage
at
the
output
of the loop
amp, U69 pin 6.
Table
4D-I I
shows the
relationship
between
various reference
frequency
configurations
and
the
control
of the
reference
circuitry.
4D-37.
Main
Phase Lock
Loop
If
the
voltage
at
TP44 is
around
25V, connect a
variable power
supply
to TP41.
This
allows the
frequency of
the VCO to be
controlled directly.
Use
a Spectrum
Analyzer
or
Counter
to
monitor
the Generator
output.
Program
the
UUT
to
640
MHz. If the
power supply can be
adjusted
to obtain
an
output
frequency
of
about 640 MHz, the
VCO
is probably OK;
proceed to the next
paragraph. If
the
power
supply
cannot be
adjusted to obtain
about 640
MHz
output
frequency,
troubleshoot the
VCO or the circuitry
between TP41 and
TP44.
4D-13