Specifications

21
Oscilloscope Probes
The oscilloscope is just one piece
of the system that determines
how accurately you are able to
display and analyze your signals.
Probes, which are used to connect
the oscilloscope to your device
under test (DUT), are crucial in
terms of signal integrity. If you
have a 1-GHz oscilloscope but
only have a probe that supports
a bandwidth of 500 MHz, you are
not fully utilizing the bandwidth
of your oscilloscope. This section
discusses the types of probes and
when you should use each one.
Loading
No probe is able to perfectly
reproduce your signal, because
when you connect a probe to
a circuit, the probe becomes
part of that circuit. Part of the
electrical energy in the circuit
flows through the probe. This
phenomenon is called loading.
There are three types of loading:
resistive, capacitive, and
inductive.
Resistive loading can cause the
amplitude of your displayed
signal to be incorrect. It can
also cause a circuit that is
malfunctioning to start working
when the probe is attached.
It is a good idea to make sure
the resistance of your probe
is greater than ten times the
resistance of the source in order
to get an amplitude reduction of
less than ten percent.
Capacitive loading causes
rise times to be slowed and
bandwidth to be reduced. To
reduce capacitive loading, choose
a probe with at least five times
the bandwidth of your signal.
Inductive loading appears as
ringing in your signal. It occurs
because of the inductive effects of
the probe ground lead, so use the
shortest lead possible.
Passive probes
Passive probes contain only
passive components and do
not require a power supply
for their operation. They are
useful for probing signals with
bandwidths less than 600 MHz.
Once this frequency is surpassed,
a different kind of probe is
required (an active probe).
Passive probes are typically
inexpensive, easy to use and
rugged. They are a versatile and
accurate type of probe. Types
of passive probes include low-
impedance resistor-divider
probes, compensated, high-
resistance passive divider probes,
and high-voltage probes.
Passive probes usually produce
high capacitive loading and low
resistive loading.
Active probes
To operate an active probe, you
need a power supply. Active
probes use active components
to amplify or condition a signal.
They are able to support much
higher signal bandwidths and are,
therefore, the probes of choice for
high-performance applications.
Active probes are considerably
more expensive than passive
probes. Active probes also tend
to be less rugged, and the probe
tip on active probes tends to be
heavier. However, they provide
the best overall combination of
resistive and capacitive loading
and allow you to test much
higher-frequency signals.
The Agilent InfiniiMax series
probes are high-performance
probes. They use a damping
resistor in the probe tips to
significantly reduce loading
effects. They also have very high
bandwidths.
Figure 29. A passive probe
Figure 30. An active probe
Oscilloscope Fundamentals