Tektronix 2445, Instruction Manual

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Operating Inform ation-2445 Service
ACTimiATjOli A ■ ■ IP T A A C A IT
H O I IV J IV I/-A I i o i v i H U J U O I I V I C I V I
1. Preset instrument controls and obtain a baseline
trace as described in "In itia l Setup." Set 20 MHz BW
L IM IT 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 jus.
n no ro tin n
\A/i + hin nominal limitc Tho Ho halanno
attained by the Auto DC Balance adjustment remains
valid as long as the instrument is operating w ithin 5°C o f
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 o f entering the auto­
adjustment mode is possible w ith 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 w ill enter an auto-adjustment mode fo r about
ten seconds. When the Auto DC Balance adjustment cycle
is complete, the instrument w ill return to the normal
operating mode.
5. Slowly adjust the FOCUS control to its optimum
setting (best-defined display).
NOTE
I f the AST/G adjustment is correctly set already,
a ll portions o f the trace w ill 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 2-2).
NOTE
I f a circu it defect prevents accurate dc balance, the
routine halts and LIM IT is displayed. Push the Trigger
COUPLING switch up to continue balancing the
remainder o f the circuitry.
I f power to the instrument is interrupted before the
balancing cycle is completed, an error w ill be detected
by the next power-on self test. Press A /B TRIG to
e xit the diagnostic m onitor and restart the A uto
DC Balance adjustment to allow the cycle to be
completed.
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 AST/G adjustment should be
correct fo r any display. However, it may be necessary
to reset the FOCUS control slightly when the
IN TEN SITY control setting is changed.
AUTO DC BALANCE ADJUSTMENT
The 2445 can automatically perform a dc-balance
adjustment o f Channel 1 and Channel 2. This adjustment
assures that the trace shifts associated w ith attenuator
stepping, changing the variable volts per division setting,
and switching Channel 2 between noninverted and Inverted
PROBE LOW-FREQUENCY
COMPENSATION
Misadjustment of probe compensation is one of the
sources o f measurement error. The attenuator probes
are equipped w ith 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 "In itia l Setup." Set 20 MHz BW LIM IT
On (in).
2. Connect the two 10X probes (supplied w ith 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 p robe is attached.
2-18