Operation
Signal
Connections
A probe is usually the most convenient way to connect an input signal to the
instrument. Shielded to prevent pickup of electromagnetic interference, the standard
10X probes supplied with the instrument present a high impedance to a circuit under
test. While the 10
U9,
and 11 pF of the probe are a negligible load on most circuits,
very fast circuits or very high impedance circuits may be seriously affected.
Waveform Fidelity
and
Probe Grounds
A probe ground must be used for accurate measurements and observations.
Use the shortest ground connection possible if you want good waveform
fidelity.
The standard-accessory probe is a compensated 10X voltage divider. It
appears resistive at low frequencies and capacitive for high-frequency signal
components. The probe input capacitance can interact with the inductance of
either a long signal lead or a long ground lead to form a series-resonant circuit.
This circuit can affect system bandwidth and can ring if driven by a fast step.
Always keep both the ground lead and the probe signal-input connections as
short as possible to maintain the best waveform fidelity.
In some cases, a separate ground from the unit under test to the ground
receptacle on the oscilloscope front panel can reduce interference from low-
frequency hum and noise. For rough checks of larger signals, such as
5-volt
logic, a ground lead separate from the probe or even the safety ground
connection which is shared with the unit under test may work for a signal
ground.
Fast signal transitions will be highly distorted and extraneous noise will
be induced without the probe ground connection.
Probe Compensation
Misadjustment of probe compensation is a common source of measurement
error. Due to variations in oscilloscope input characteristics, probe
compensation should be checked whenever the probe is moved from one
oscilloscope to another or between channels of a multichannel oscilloscope.
See the procedure in the "Checks and Adjustments" section of the manual.
2-16
2465B/2455B/2445B Operators
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