Tektronix 2225, Operator'S Manual

Page: 34 / 79

Operation
CONNECTING SIGNALS
A probe is usually the most convenient way to connect an input
signal to the instrument. Shielded to prevent pickup of elec­
tromagnetic interference, the standard 10X probes supplied with
the instrument present a high impedance to a circuit under test.
While the 10 MQ and 13 pF of the probe-scope system present 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. To a circuit under test, it appears resistive for low-
frequency signals and capacitive for high-frequency compo­
nents. The probe input capacitance can interact with the induc­
tance 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 V 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.
2225 Operators
2-11