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FEATURE TEST & MEASUREMENT


Low-noise probe for accurate power integrity measurements


A new type of probe makes it possible to perform low-noise, high bandwidth measurements on DC power distribution networks, as Joel Woodward, Rohde & Schwarz, explains


T


he quality of the power supply is a key factor in the functionality and


performance of electronic circuits. In extreme cases, today’s advanced electronic designs can contain hundreds of power distribution networks. Each network must be tested to ensure that the pins of ICs are supplied with the necessary DC voltage within defined tolerances and the required level of quality. Oscilloscopes remain the tool of choice for these types of measurements. They can measure not only the DC voltage level, but also its quality. Traditional oscilloscopes and probes have difficulties with these measurements because their measurement accuracy is influenced by factors such as inherent noise and insufficient offset. Power rail voltages and tolerances


required by today’s electronic components, such as FPGAs, ASICs and DDR memories, continue to decrease. While any oscilloscope could make measurements on conventional 5V rails within the given 10 per cent measurement tolerance, a 1V power rail with a tolerance of two per cent corresponds to just 20mV, a value that lies within the inherent noise of many oscilloscopes and probe systems, making it impossible to accurately measure such small amplitudes.


POWER RAIL PROBE The R&S RT-ZPR20 active power rail probe was developed specifically for performing fast, accurate power integrity measurements on R&S RTO and R&S RTE oscilloscopes. It includes an integrated DC voltmeter, the R&S ProbeMeter. This feature can be extremely useful for quickly determining how much offset compensation may be required.


EXTREMELY LOW INHERENT NOISE Every oscilloscope frontend and every probe has inherent noise that is added to signals. For small signals, the added noise results in overstated peak-to- peak voltages, which is why it is essential to use oscilloscopes and probes with the lowest possible noise. The RT-ZRP20 power rail probe and


32 APRIL 2018 | INSTRUMENTATION


Two RT-ZPR20 power rail probes connected to an R&S RTO oscilloscope. One is connected to the circuit via a solder-in coaxial cable and the second has the browser module for fast overview measurements


the R&S RTO or R&S RTE oscilloscope have a low RMS noise voltage of only 120μV at 1GHz bandwidth and 1mV/div vertical scaling. The probe’s 1:1 attenuation factor circumvents the problem of 10:1 probes, which attenuate the input signal by a factor of ten without decreasing the inherent noise so that this noise appears ten times as large.


DC OFFSET COMPENSATION RANGE OF ±60V Oscilloscopes typically do not have enough DC offset compensation to directly measure with a high resolution the wide range of power rails in today’s electronic devices. This results in two negative factors. First, the user cannot centre the signal within the vertical dynamic range and therefore must work with a less sensitive vertical scaling. As a result, only a fraction of the available ADC resolution can be used for the measurement. Second, noise is a function of the


vertical resolution. Measuring with lower vertical resolution means more noise, and peak-to-peak values will be overrepresented. Some users add blocking capacitors


or use the oscilloscope’s AC coupling to remove DC offset. This has the disadvantage of not allowing the user to see the true DC value of the rail, and eliminates the ability to see low frequency drift as circuits turn on and off.


It is nearly impossible to qualify power distribution networks for sensitive electronic components with traditional probes. The narrow tolerance of < two per cent requires special probes with extremely low inherent noise


Probe adapter for the RT-ZPR20 power rail probe with connected


50SMA coaxial cable for soldering to the circuit


The RT-ZPR20 solves all these


problems with its large DC offset compensation range of ±60V. This allows users to centre and expand the voltages on DC rails from an extremely wide variety of standards, so that they can accurately measure and also see any DC drift that might occur. For users who exclusively want to


make ripple and noise measurements, the probe includes the ability to choose AC coupling. This allows users to move


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