Input Amplifiers - Fluke PM6690 Service Manual

Input Amplifiers

Check the power supply voltages according to the instructions
on page 7-13 before proceeding to the next step.
All adjustments on the input amplifiers must be made in the
specified order.
The input amplifiers are enclosed in an RF shield consisting
of a metal lid that is soldered to two of the shield clips on the
main circuit board. Without these solder joints there is a risk
of the lid coming off, should the instrument be subjected to
heavy bumps.
There are two alternative ways to adjust the input amplifiers.
One is to remove the shield to gain access to all trimmers and
test points on the top side of the main circuit board. Don't for-
get to put the shield back and restore the solder joints. The
other way is to access the trimmers via the holes in the shield
lid and to use the test points on the bottom side of the main cir-
cuit board. Place the instrument on end to get access to both
sides of the main circuit board.
Step Response X1
n
Setup
– Do as described under Preparations above.
– Select default settings by keying in
®
Save/Recall Recall Setup
– Select for Input A of the timer/counter: Manual Trig,
DC, 50 W, Att x1. Select Single Period as measurement
function.
– Connect a symmetrical pulse signal with fast rise/fall
time, 100 ms period time and 5 V amplitude from a
pulse generator to a power splitter.
– Connect one side of the power splitter to Input A on the
timer/counter and connect the other side of the power
splitter to Input A on the oscilloscope.
– Connect a 10X 500 W probe to Input B on the oscillo-
scope.
– Set the oscilloscope to 10 ms/div and Input A to 50 W,
DC, 0.5 V/div and Input B to 50 W, DC, 100 mV/div
(including probe).
n
Adjustment
– Connect the probe tip to the center hole of J7. The sur-
rounding holes are suitable ground connections. See Fig-
ure 7-2.
– Adjust C106 and C496 until the pulses on the oscillo-
scope look the same. Use Temex 4192 adjustment tool.
Repeat all steps above for Input B on the timer/counter. Con-
nect the probe tip to J9. Adjust C137 and C497.
7-12 Input Amplifiers
®
USER OPT
®
Default
Figure 7-3
Linearity compensation.
Step Response X10
n
Setup
– Keep the settings on the timer/counter and the oscillo-
scope from Step Response X1.
– Select Att X10 for Input A on the timer/counter.
– Reconnect the signal from the power splitter to Input A
on the timer/counter.
– Set Input B on the oscilloscope to 20 mV/div (including
probe).
n
Adjustment
– Connect the probe tip to the center hole of J7. The sur-
rounding holes are suitable ground connections. See Fig-
ure 7-2.
– Adjust C111 and C116 until the pulses on the oscillo-
scope look the same. Use a screwdriver type of adjust-
ment tool.
Repeat all steps above for Input B on the timer/counter. Con-
nect the probe tip to J9. Adjust C143 and C148.
Linearity
n
Setup
– Keep the settings on the timer/counter and the oscillo-
scope from Step Response X10.
– Select Att X1 for Input A on the timer/counter.
– Change the period time of the pulse signal from the
pulse generator to 2 ms.
– Reconnect the signal from the power splitter to Input A
on the timer/counter.
– Set the oscilloscope to 200 ms/div and Input B to
100 mV/div (including probe).
Overcompensation
Correct compensation
Undercompensation
Input signal