An oscilloscope is just a piece of a system that indicates how accurately users analyze and display signals. To connect the device under test (DUT) into the oscilloscopes, users will have to use probes, which are critical in terms of signal integrity. Thus, a users with a 1 GHz oscilloscope and using a probe that supports 500 MHz bandwidth, is not fully utilizing the oscilloscope’s bandwidth.
No probes can perfectly reproduce signals since the moment probes are connected to a circuit, it becomes part of the circuit, allowing portions of the circuit’s electrical energy to flow through the oscilloscope probe. This event is known as loading. There are three kinds of loading — capacitive, resistive and inductive.
Capacitive loading can slow down rise time as well as reduce bandwidth. To effectively reduce capacitive loading, users should choose a probe that has at least five times the bandwidth of the signal.
Resistive loading, on the other hand, causes the amplitude of the displayed signal to be incorrect. It can also make a malfunctioning circuit to start working whenever the probe is connected. Users should ensure that the resistance of the probe is ten times more than the resistance of the source, so that it can achieve an amplitude reduction of below ten percent.
Inductive loading appears as ringing in the signal, primarily attributed to the probe ground lead’s inductive effects. Hence, users are advised to utilize the shortest possible lead.
Meanwhile, probes come in various forms, such as active probes, passive probes and current probes.
In operating an active probe, users should note that the probe will need a power supply to powerup the active devices within the probe. The power supply is often supported by an external box or with a USB cable connection. It can also be supplied by the oscilloscope’s mainframe itself. Active probes utilize active components to condition or amplify a signal. These probes also have the ability to support much higher signal bandwidths, making them the probe of choice by most engineers in higher performance applications.
Aside from being more costly than passive probes, active probes are also less rugged and offer a much heavier probe tip. Despite all these, active probes deliver the best overall combination of capacitive and resistive loading while allowing users to test much higher-frequency signals.
Furthermore, passive probes feature passive components and eliminate the need for power supply during operation. This type of probe is useful for probing signals with less than 60 MHz bandwidth. When this frequency is surpassed, a different type of probe is needed, that is an active probe.
Passive probes are not only accurate and versatile — they are also rugged, inexpensive and easy to use. They commonly produce high capacitive loading as well as low resistive loading.
Different types of passive probes include compensated, high-resistance passive divider probes, low-impedance resistor-divider probes and high-voltage probes.
Usually utilized to measure the current flowing through a circuit, current probes are commonly large and feature limited bandwidth (100 MHz).
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