Operating Instructions-2225 Service
Misadjustment of probe compensation is a common
source of measurement error. Due to variations in
oscilloscope input characteristics, probe compen-
sation should be checked and adjusted, if neces-
sary, whenever the probe is moved from one
oscilloscope to another or between channels. See
the Probe Compensation procedure in Operator's
Checks and Adjustments, or consult the instructions
supplied with the probe.
Coaxial Cables
Coaxial cables may also be used to connect signals
to the vertical input connectors, but they may have
considerable effect on the accuracy of a displayed
waveform. To maintain the original frequency
characteristics of an applied signal, only high-
quality, low-loss coaxial cables should be used.
Coaxial cables should be terminated at both ends in
their characteristic impedance. If this is not pos-
sible, use suitable impedance-matching devices.
INPUT-COUPLING CAPACITOR
PRECHARGING
When the Input Coupling switch is set to the GND
position, the input signal is connected to ground
through the input-coupling capacitor and a high
value resistance. This series combination forms a
precharging circuit that allows the input-coupling
capacitor to charge to the average dc voltage level
of the signal applied to the input connector. Thus,
any large voltage transients that may accidentally be
generated are not applied to the vertical amplifier
when the input coupling is switched from GND to
AC.
The precharging network also provides a measure of
protection to the external circuitry by reducing the
current level that is drawn from the external circuitry
while the input-coupling capacitor is charging.
If AC input coupling is in use, the following proce-
dure should be followed whenever the probe tip is
connected to a signal source having a different dc
level than that previously applied. This procedure
becomes especially useful if the dc-level difference
is more than ten times the
VOLTS/DIV switch
setting.
1.
Set the AC-GND-DC (input coupling) switch to
GND before connecting the probe tip to a signal
source.
2.
Touch the probe tip to the oscilloscope ground
(A)
connector.
3.
Wait several seconds for the input-coupling
capacitor to discharge.
4.
Connect the probe tip to the signal source.
5.
Wait several seconds for the input-coupling
capacitor to charge to the dc level of the signal
source.
6.
Set the AC-GND-DC switch to AC. A signal with
a large dc component can now be vertically
positioned within the graticule area, and the ac
component of the signal can be measured in
the normal manner.
Scans
by
ARTEK
MEDL4
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