The Realities Of Probes - Tektronix PRIMER P6101B Manual

An ideal probe causes zero signal source loading. In other
words, it doesn't draw any signal current from the signal
source. This means that, for zero current draw, the probe must
have infinite impedance, essentially presenting an open circuit
to the test point.
In practice, a probe with zero signal source loading cannot
be achieved. This is because a probe must draw some small
amount of signal current in order to develop a signal voltage
at the oscilloscope input. Consequently, some signal source
loading is to be expected when using a probe. The goal,
however, should always be to minimize the amount of loading
through appropriate probe selection.
Complete Noise Immunity
Fluorescent lights and fan motors are just two of the many
electrical noise sources in our environment. These sources
can induce their noise onto nearby electrical cables and
circuitry, causing the noise to be added to signals. Because of
susceptibility to induced noise, a simple piece of wire is a less
than ideal choice for an oscilloscope probe.
The ideal oscilloscope probe is completely immune to all noise
sources. As a result, the signal delivered to the oscilloscope
has no more noise on it than what appeared on the signal at
the test point.
In practice, use of shielding allows probes to achieve a high
level of noise immunity for most common signal levels. Noise,
however, can still be a problem for certain low-level signals.
In particular, common mode noise can present a problem for
differential measurements, as will be discussed later.

The Realities of Probes

The preceding discussion of The Ideal Probe mentioned
several realities that keep practical probes from reaching the
ideal. To understand how this can affect your oscilloscope
measurements, we need to explore the realities of probes
further.
First, it's important to realize that a probe, even if it's just a
simple piece of wire, is potentially a very complex circuit.
Figure 1.4. Probes are circuits composed of distributed resistance, inductance, and
capacitance (R, L, and C).
For DC signals (0 Hz frequency), a probe appears as a simple
conductor pair with some series resistance and a terminating
resistance (Figure 1.4a). However, for AC signals, the picture
changes dramatically as signal frequencies increase (Figure
1.4b).
The picture changes for AC signals because any piece of
wire has distributed inductance (L), and any wire pair has
distributed capacitance (C). The distributed inductance
reacts to AC signals by increasingly impeding AC current flow
as signal frequency increases. The distributed capacitance
reacts to AC signals with decreasing impedance to AC current
flow as signal frequency increases. The interaction of these
reactive elements (L and C), along with the resistive elements
(R), produces a total probe impedance that varies with signal
frequency. Through good probe design, the R, L, and C
elements of a probe can be controlled to provide desired
degrees of signal fidelity, attenuation, and source loading over
specified frequency ranges. Even with good design, probes
are limited by the nature of their circuitry. It's important to be
aware of these limitations and their effects when selecting and
using probes.
ABCs of Probes
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