Test & Measurement World, July/August 2012

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TESTDIGEST INSTRUMENTATION Active probes are worth the extra cost In an article on the "Test & Measurement Designline" portion of the EE Times Web- site, Jae-Yong Chang, product manager and planner for Agilent Technologies' oscillo- scope product line in the Oscilloscope Products Division, provides guidance on the optimal probe to use for different ap- plications. He points out that most low-to- mid-range oscilloscopes come with one high-impedance passive probe per channel. But what about active probes? Chang notes that active probes are more expen- sive, but they offer better performance that may make a difference in applications that require high-bandwidth, high-signal fidel- ity performance. The article explains the issues with a) The amplitude of the signal measured with the 100:1 resistor divider probe is decreased to 1.65 V due to the resistive loading of the low-impedance probe. "probe loading," or when the probe be- comes a part of the circuit, introduces loading, and changes the measurement. Chang provides a graph that compares the input impedance of a general-purpose pas- sive probe with 10-MΩ resistance and 4-pF capacitance with that of an active probe with 1-MΩ resistance and 1-pF ca- pacitance as they are used across increasing frequencies. He notes that although the 10:1 passive probe comes with higher input impedance at low frequency ranges, the input-loading characteristics of the active probe are usu- WIRELESS TEST Testing E911 In the event of an emergency, we have been trained since childhood in the US to dial 911 from the closest available phone, which back when almost every household had plain- old-telephone service, enabled the emergency operator to look up the exact location of the phone line used to originate the call and coordinate a response. Today, with the majority of 911 calls originating from mobile devices, pinpointing the exact location of callers is not so simple. More than 10 years ago, the FCC introduced its E911 Phase II location requirements for US wireless network operators and mobile devices. E911 (enhanced 911) requires that caller location be provided to public-safety answering points with 50-m accu- racy for 67% of calls and with 100-m accuracy for 95% of calls (see figure). The FCC is also investigating ways to further improve the performance of locating 911 callers who use mobile devices. A typical E911 call from a mobile phone can pinpoint the call- er's location to within 50 m, translating to about 27 houses in this type of neighborhood. Test & Measurement World | JULY/AUGUST 2012 | –14– b) The output measured with an active probe with 1-MΩ input impedance makes the amplitude measurement correctly. ally better at high frequencies because of the probe's lower input capacitance. Next, Chang outlines the benefits of differential active probes as well as high-voltage differential probes. He explains how they work and that they provide better signal integrity due to very low impedance grounding and higher input impedance. Giving equal time, he drills down into detail on the Z0 passive probe (also known as a 50-Ω passive probe). Chang ex- plains that if you select a Z0 probe as a low-cost alternative to a higher- priced active probe, you should be careful with the resistive loading ef- fect, because it may alter the mea- sured amplitude of the signal as well as the bias point. Finally, Chang steps through a de- tailed comparison of a measurement taken with a 100:1 resistor divider probe and a measurement taken with an active probe with 1-MΩ input im- pedance (see figure). You can link to his article, "Active probes: why they are worth buying," from the online version of this article at Janine Love, Senior Editor

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