Operator's Checks and Adjustments—2445 Operators
ASTIGMATISM ADJUSTMENT
1. Preset instrument controls and obtain a baseline
trace as described in "Initial Setup." Set 20 MHz BW
LIMIT On (in)
2. Connect a 10X probe to the CH 1 OR X input
connector and connect the probe tip to the CALIBRATOR
output.
3. Adjust the Channel 1 POSITION control to center
the display on the screen.
4. Set A and B SEC/DIV controls at 1 ps.
5. Slowly adjust the FOCUS control to its optimum
setting (best-defined display).
NOTE
If the AST/G adjustment is correctly set already,
all portions o f the trace will come into sharpest focus
at the same position o f the FOCUS control.
6. If focusing is not uniform over the entire graticule
area, use a small-bladed screwdriver to adjust the ASTIG
control (see Figure 3-1).
7. Since the ASTIG and FOCUS adjustments interact,
repeat steps 5 and 6 until the best-defined display over the
entire graticule area is obtained.
NOTE
Once it is set, the ASTIG adjustment should be
correct for any display. However, it may be necessary
to reset the FOCUS control slightly when the
INTENSITY control setting is changed.
AUTO DC BALANCE ADJUSTMENT
The 2445 can automatically perform a dc-balance
adjustment of Channel 1 and Channel 2. This adjustment
assures that the trace shifts associated with attenuator
stepping, changing the variable volts per division setting,
and switching Channel 2 between noninverted and inverted
operation are within nominal limits. The dc balance
attained by the Auto DC Balance adjustment remains
valid as long as the instrument is operating within 5°C of
the ambient temperature at which the adjustment was
performed provided a 20-minute warm-up period is allowed
before performing the adjustment. To initiate the adjust
ment, set both the Channel 1 and Channel 2 input coupling
switches to AC. Then simultaneously push up on both
switches. An alternate method of entering the auto
adjustment mode is possible with only one of the input
coupling switches set to AC. Press up and hold the input
coupling switch that is not set to AC, then press up the
other input coupling switch. With either method, the
instrument will enter an auto-adjustment mode for about
ten seconds. When the Auto DC Balance adjustment cycle
is complete, the instrument will return to the normal
operating mode.
NOTE
If a circuit defect prevents accurate dc balance, the
routine halts and LIMIT is displayed. Push the Trigger
COUPLING switch up to continue balancing the
remainder of the circuitry.
If power to the instrument is interrupted before the
balancing cycle is completed, an error will be detected
by the next power-on self test. Press A/B TRIG to
exit the diagnostic monitor and restart the Auto
DC Balance adjustment to allow the cycle to be
completed.
PROBE LOW-FREQUENCY
COMPENSATION
Misadjustment of probe compensation is one of the
sources of measurement error. The attenuator probes
are equipped with compensation adjustments. To ensure
optimum measurement accuracy, always check probe
compensation before making measurements. Probe low-
frequency compensation is accomplished as follows:
1. Preset instrument controls and obtain a baseline trace
as described in "Initial Setup." Set 20 MHz BW LIMIT
On (in).
2. Connect the two 10X probes (supplied with the
instrument) to the CH 1 OR X and the CH 2 input con
nectors. Observe that the CHANNEL 1 SCALE FACTOR
on the readout display changes from 10 mV to 100 mV
when the 10X probe is attached.
5-2
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