Different Probe Types And Their Benefits - Tektronix PRIMER P6101B Manual

Primer
Also, there are many cases where the signal voltages are
differential. That is, the signal exists across two points or two
wires, neither of which is at ground or common potential
(see Figure 2.2). Such differential signals are common in
telephone voice circuits, computer disk read channels, and
multi-phase power circuits. Measuring these signals requires
yet another class of probes referred to as differential probes.
And then there are many cases, particularly in power
applications, where current is of as much or more interest than
voltage. Such applications are best served with yet another
class of probes that sense current rather than voltage.
Current probes and differential probes are just two special
classes of probes among the many different types of available
probes. The rest of this chapter covers some of the more
common types of probes and their special benefits.

Different Probe Types and Their Benefits

As a preface to discussing various common probe types,
it's important to realize that there's often overlap in types.
Certainly a voltage probe senses voltage exclusively, but a
voltage probe can be a passive probe or an active probe.
Similarly, differential probes are a special type of voltage probe,
and differential probes can also be active or passive probes.
Where appropriate these overlapping relationships will be
pointed out.
Passive Voltage Probes
Passive probes are constructed of wires and connectors, and
when needed for compensation or attenuation, resistors and
capacitors. There are no active components – transistors or
amplifiers – in the probe, and thus no need to supply power to
the probe.
Because of their relative simplicity, passive probes tend to be
the most rugged and economical of probes. They are easy to
use and are also the most widely used type of probe.
Passive voltage probes are available with various attenuation
factors – 1X, 10X, and 100X – for different voltage ranges. Of
these, the 10X passive voltage probe is the most commonly
used probe, and is the type of probe typically supplied as a
standard accessory with oscilloscopes.
www.tektronix.com/accessories
16
For applications where signal amplitudes are one-volt peak-
to-peak or less, a 1X probe may be more appropriate or
even necessary. Where there's a mix of low amplitude and
moderate amplitude signals (tens of millivolts to tens of volts),
a switchable 1X/10X probe can be a great convenience. It
should be kept in mind, however, that a switchable 1X/10X
probe is essentially two different probes in one. Not only are
their attenuation factors different, but their bandwidth, rise
time, and impedance (R and C) characteristics are different
as well. As a result, these probes will not exactly match
the oscilloscope's input and will not provide the optimum
performance achieved with a standard 10X probe.
Most passive probes are designed for use with general
purpose oscilloscopes. As such, their bandwidths typically
range from less than 100 MHz to 500 MHz or more.
There is, however, a special category of passive probes
that provide much higher bandwidths. They are referred to
variously as 50 Ω probes, Z
probes. These probes are designed for use in 50 Ω
environments, which typically are high-speed device
characterization, microwave communication, and time domain
reflectometry (TDR). A typical 50 Ω probe for such applications
has a bandwidth of several gigaHertz and a rise time of 100
picoseconds or faster.
Active Voltage Probes
Active probes contain or rely on active components, such as
transistors, for their operation. Most often, the active device is
a field-effect transistor (FET).
The advantage of a FET input is that it provides a very low
input capacitance, typically a few picoFarads (pF) down to less
than 1 pF. Such ultra-low capacitance has several desirable
effects.
First, recall that a low value of capacitance, C, translates to a
high value of capacitive reactance, X
the formula for X , which is:
c
probes, and voltage divider
o
. This can be seen from
c