Oscilloscope Probe Selection

By Author: trsrentelco
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One of the most common questions asked when engineers and technicians need to make oscilloscope measurements is what probe should I use? Oscilloscope probes at their most basic can be viewed as a transmission line for connecting a DUT (Device Under Test) to an oscilloscope input. There are a number of different oscilloscope probes to choose from, and they all have applications where they perform best. Based on the signal that you need to measure you may need a Passive, Active, Differential, Trimode, or Current probe. But for the purpose of this discussion I would like to limit the conversation to voltage probes.

Some of the key parameters that you should take into account when selecting an oscilloscope rental are the bandwidth (BW) or frequency, amplitude of the signal you would like to measure, and the impedance of the probe. Please keep in mind that the bandwidth of your oscilloscope probe will be limited by the bandwidth of the test equipment you select.

Passive Probes

Passive oscilloscope probes generally have the greatest dynamic range; some of them are available with a selectable divider network ...
... for 1X, 10X and 100X attenuation options. Passive oscilloscope probes are primarily constructed of a coaxial cable and a resistor and capacitor network. The simple construction of the used oscilloscope make passive probes one of the least expensive test equipment rentals on the market. There are no active components in the circuit, so there is no need to supply power to these probes. The typical bandwidth of a passive probe is 600MHz, or less. However, there are some specialty passive probes with bandwidths in the Gigahertz range.

Single-Ended Active Probes

Active oscilloscope probes contain an active amplifier (typically a FET) and they have very low capacitance as well as high input impedance. Because of their low capacitance they can be used with longer ground leads, and they have low DUT loading impact. Active FET probes would be a good choice if you need to measure low signal levels, or fast logic signals. These electronic equipment rentals can operate at much higher bandwidths than single ended passive probes with some as high as 6 GHz.

Differential Probes

A differential oscilloscope probe has two inputs; one positive (non-inverting), and one negative (inverting) input. The two input signals are fed into a differential amplifier that combines for a single differential signal to input into the oscilloscope. One of the primary benefits of a Differential probe is that it has a high CMRR (Common Mode Rejection Ratio). The high CMRR is especially important when you are trying to measure small signals that are riding on a large DC offsets. Differential probes can be found with bandwidths up to 20GHz.

TriMode Probing

TriMode oscilloscope probes are presently only offered by Tektronix. This probe allows you to make one connection to your DUT and, then you can measure Differential, Single ended, and Common Mode measurements all with one setup. TriMode probes can save both set up time by only having to make one connection for multiple measurements, as well as money by only having to purchase one probe. These digital oscilloscopes would be a good choice if you need to measure multiple serial data compliance standards (HDMI, SATA, PCIe, etc.). Trimode probes come in various bandwidths up to 20GHz.

Once you have determined the oscilloscope probing requirements, you will need to plan your acquisition strategy. If your project is short-term, a used oscilloscope or rental may be your best solution. Oscilloscope rentals allow you to scale to project size and scope, minimize capital outlay and quickly replace a down unit. If your project is long-term, or repetitive in nature, a lease or purchase may be a better option.

TRS-RenTelco offers the widest variety of test equipment for general and communication test purposes including used oscilloscopes, spectrum analyzers, thermal imager rentals, fusion splicers, digital meters, and more. TRS-RenTelco supplies test equipment for rent and for purchase, and take pride in providing excellent customer service and satisfaction.