The Ideal Probe - Tektronix PRIMER P6101B Manual

Figure 1.2. Most probes consist of a probe head, a probe cable, and a compensation
box or other signal conditioning network.
Whatever the probe is in reality, it must provide a connection of
adequate convenience and quality between the signal source
and the oscilloscope input (Figure 1.2). The adequacy of
connection has three key defining issues – physical attachment,
impact on circuit operation, and signal transmission.
To make an oscilloscope measurement, you must first be
able to physically get the probe to the test point. To make this
possible, most probes have at least a meter or two of cable
associated with them, as indicated in Figure 1.2. This probe
cable allows the oscilloscope to be left in a stationary position
on a cart or bench top while the probe is moved from test
point to test point in the circuit being tested. There is a tradeoff
for this convenience, though. The probe cable reduces the
probe's bandwidth; the longer the cable, the greater the
reduction.
In addition to the length of cable, most probes also have a
probe head, or handle, with a probe tip. The probe head
allows you to hold the probe while you maneuver the tip to
make contact with the test point. Often, this probe tip is in the
form of a spring-loaded hook that allows you to actually attach
the probe to the test point.
Physically attaching the probe to the test point also
establishes an electrical connection between the probe tip
and the oscilloscope input. For useable measurement results,
attaching the probe to a circuit must have minimum affect on
the way the circuit operates, and the signal at the probe tip
must be transmitted with adequate fidelity through the probe
head and cable to the oscilloscope's input.
These three issues – physical attachment, minimum impact
on circuit operation, and adequate signal fidelity – encompass
most of what goes into proper selection of a probe. Because
probing effects and signal fidelity are the more complex topics,
much of this primer is devoted to those issues. However, the
issue of physical connection should never be ignored. Difficulty
in connecting a probe to a test point often leads to probing
practices that reduce fidelity.

The Ideal Probe

In an ideal world, the ideal probe would offer the following key
attributes:
Connection ease and convenience
Absolute signal fidelity
Zero signal source loading
Complete noise immunity
Connection Ease and Convenience
Making a physical connection to the test point has already
been mentioned as one of the key requirements of probing.
With the ideal probe, you should also be able to make the
physical connection with both ease and convenience.
For miniaturized circuitry, such as high-density surface
mount technology (SMT), connection ease and convenience
are promoted through subminiature probe heads and various
probe-tip adapters designed for SMT devices.
ABCs of Probes
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