Tektronix 2445, Instruction Manual

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Operating Information—2445 Service
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O I U I V M L I I V M V O
Probes
Generally, probes offer the most convenient means of
connecting an input signal to the instrument. Shielded
to prevent pickup of electromagnetic interference, the
standard 10X probes supplied w ith this instrument offer
a high input impedance that minimizes circuit loading. This
allows the circuit under test to operate w ith a minimum o f
change from the normal, unloaded condition. Also, the
subminiature body of these probes has been designed fo r
ease of use either when probing circuitry containing close
lead spacing or when probing in a confined space.
Both the probe itself and the probe accessories should
be handled carefully at all times to prevent damage. Avoid
dropping the probe body. Striking a hard surface can cause
damage to both the probe body and the probe tip. Exercise
care to prevent the cable from being crushed or kinked.
Do not place excessive strain on the cable by pulling.
The standard-accessory probe is a compensated 10X
voltage divider. It is a resistive voltage divider for low
frequencies and a capacitive voltage divider fo r high-
frequency signal components. Inductance introduced by
either a long signal or ground lead forms a series-resonant
circuit. This circuit w ill affect system bandwidth and w ill
ring if driven by a signal containing significant frequency
components at or near the circuit's resonant frequency.
Oscillations (ringing) can then appear on the oscilloscope
display and distort the true signal waveform. Always keep
both the ground lead and the probe signal-input connec­
tions as short as possible to maintain the best waveform
fidelity.
Misadjustment o f probe compensation is a common
source o f measurement error. Due to variations in oscillo­
scope input characteristics, probe compensation should
be checked and adjusted, if necessary, whenever the probe
is moved from one oscilloscope to another or between
channels of a multichannel oscilloscope. See the procedure
in the “ Operator's Checks and Adjustments" part o f this
section or consult the probe instruction manual.
Coaxial Cables
Cables may also be used to connect signals to the input
connectors, but they may have considerable effect on the
accuracy of a displayed waveform. To maintain the original
quality, low-loss coaxial cables should be used. Coaxial
cables should be terminated at both ends in their char­
acteristic impedance. If this is not possible, use suitable
impedance-matching devices.
INPUT-COUPLING CAPACITOR
PRECHARGING
When the input coupling switch is set to GND, the input
signal is connected to ground through the input-coupling
capacitor in series w ith a 1-M£2 resistor to form a pre­
charging network. This network allows the input-coupling
capacitor to charge to the average dc voltage level o f the
signal applied to the probe. Thus, any large voltage tran­
sients that may accidentally be generated w ill not be
applied to the amplifier input when 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 levels that can be drawn from the
external circuitry during capacitor charging.
EXTERNAL TRIGGERING
Both the A and the B trigger signals may be independ­
ently obtained from any of the four vertical input channels.
When viewing signals that require a trigger source different
from one o f the displayed vertical signals (traditionally
referred to as "external triggering"), any free vertical
channel may be used to input a trigger signal. The signal
can be viewed on the crt to aid in setting the trigger circuit
controls by selecting that respective channel fo r the vertical
display (replaces the usual "trigger view" feature). A fter
establishing the correct triggering, the trigger signal display
can then be removed from the vertical signal display or
allowed to remain, at the operator's descretion.
Channel 1 and Channel 2 can condition a wide range o f
signals to produce triggers—over the full vertical deflection
range o f the channel from m illivolts to hundreds of volts
in amplitude. Channel 3 and Channel 4 inputs have a much
more limited choice o f vertical deflection ranges available
(0.1 volt and 0.5 volt per division w ithout external attenu­
ation) and are more useful for digital signal amplitudes.
However, signals much larger can be processed, provided
they do not exceed the maximum-rated signal amplitude
fo r the input.
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