General Test & Measurement Accessories

Introduction

The standard laboratory oscilloscope provides voltage measurements relative to the oscilloscope ground, usually the 3rd prong on the AC socket. This is true for the O-Scope and Pico units as well where the computer or an attached peripheral is plugged into the AC wall socket. A few, primarily LCD handheld scopes offer isolated channels providing isolation up to 600VAC.

Some dual channel units offer the ability to combine channels into a single differential measurement at fairly low voltages but such measurements are usually a little difficult to setup properly.

Using the right probe is more than just convenience. It can also be a matter of safety and accuracy. Voltages beyond just a few tens of Volts can be hazardous to equipment and can be deadly to users.

There are a variety of probe accessories which are useful in many applications. Two of the most common are high voltage probes and differential probes. High voltage probes are useful for measuring voltages beyond the standard input range for the oscilloscope. Differential probes offer the ability to measure voltages between two points that are relative and not tied to ground.

ATC will be glad to special order TPI and API probe products for its customers. If you need a probe which is not described here, contact us with the details and we will be glad to check with our suppliers as to price and availability.

Probe voltage specifications are given in the maximum voltages allowable. This maximum voltage can consist of both a DC (direct current) part and an AC (alternating current) voltage. Some signals have only AC or only DC values while others have both, an AC signal riding on top of a DC voltage. For the most common type of AC signal (a sine wave), the peak voltage is 1.414 times the Vrms (Vrms is the normal way to describe AC voltage). That means for a 100 VAC (rms) signal, the peak voltage is actually 141 Volts.

Attenuation

Probes are attenuating devices. They are usually rated in terms of attenuation by using the value Xn, where n may be 1, 10, 20, 100, 200, or 1000. An X1 probe means no attenuation, like a piece of wire. The most common attenuating probe is the X10 which divides the signal seen at the tip by 10 by the time it reaches the BNC connector at the oscilloscope. Most probes are switchable between X1 and X10.

There are three important and different reasons to use an attenuating probe. The first is to reduce the voltage to a level that can be handled by the oscilloscope. An O-Scope II which can handle 30 Volts peak using an X1 probe can view up to 300 Volts peak when using an X10 attenuating probe. The second reason is signal loading. While an oscilloscope has a rather high input impedance of 1 MegOhm, there are signals which are also of very high impedance where the 1 MegOhm load added to the circuit can affect the operation of the circuit as well as the signal being viewed. An X10 probe reduces the loading by increasing the impedance of the oscilloscope input by 10 times from 1 MegOhm to 10 MegOhms (and the capacitive AC load of the oscilloscope is also reduced). The third reason for attenuating probes is the expansion of the usable frequency range (from reducing the capacitive AC load). Most probes are useful only up to a maximum of 6 to 20 MHz in the X1 mode.

Frequency Response

Probe frequency response is normally specified for the maximum attenuation of the probe. That means that for a switchable X1/X10 probe, it is specified for X10. Most X1 probes are useful only up to a maximum of 6 to 20 MHz. In the X10 mode the same probes may be good for 60 to 350 MHz. X100 probes are seldom switchable and are usually X100 only. Most of these probes are good for use to 100 MHz or even up to 250 MHz depending on the model.

For frequencies beyond 200-300 MHz, it is necessary to use other types of devices and approaches. Active FET probes may be used or simply going to 50 Ohm inputs and cabling can be used.

Most oscilloscope probes can handle up to an absolute peak of 600 Volts. These are virtually always intended to measure voltages relative to ground (usually a 3rd prong AC wall socket).

Measuring high voltages, usually beyond 110VAC single phase is usually best handled with specialized high voltage probes.