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Oscilloscope manufacturers are looking to supply more intelligent probes as engineers seek greater measurement accuracy

Connecting a probe to a device under test can prove to be a time consuming activity, especially in the case of the increasingly complex devices that are now entering the market. While the ideal probe should offer ease of connection, convenience, absolute signal fidelity, zero signal source loading and complete noise immunity, there is currently no ideal probe size or configuration.

From the engineers’ perspective, there is growing demand for probes that can measure higher bandwidths, as well as performing other functions. So, rather than intelligence and calibration being applied in the oscilloscope, the latest probes are being equipped with more intelligence.

Probe technology is, as a result, undergoing profound change and companies are now not only having to focus on the ease of use and avoiding damage to the device under test, but are also having to meet the need for more intelligent probes and accessories.

“Oscilloscope probes are similar to the tools you might find in your garage,” said Joel Woodward, strategic oscilloscope planner with Rohde & Schwarz. “A crescent wrench is a great tool,” he said, “but there’s typically a better wrench for a specific task. The equivalent of that wrench in the oscilloscope world is the generic 10:1 passive probe shipped as standard with most instruments. These probes can perform a large number of measurement tasks, but have specifications that limit their measurement accuracy.”

Many products today have much tighter tolerances on their DC power rails than was the case in previous generations of products. As a result, design engineers need probes that are capable of displaying mV sensitivity when they are looking to measure noise, ripple and transients on their DC power rails.

According to Woodward, a growing number of oscilloscope manufacturers are now offering power rail probes; designed for accurate power integrity measurements and capable of addressing the rapidly changing needs of engineers, whether that is measuring periodic and random disturbances, static and dynamic load response, programmable power rail response or similar power integrity measurements.

“Smaller DC rail values and tighter tolerances have made it extremely difficult to accurately measure ripple and noise. For example, measuring the peak-to-peak voltages of a 1V rail with a 1% tolerance requires a 10mV measurement accuracy. Most legacy scopes and probes have a noise floor that exceeds this value. That is why oscilloscope manufacturers are now supplying probes that have been designed specifically to measure AC characteristics of DC signals. The parameters associated with power rail probes provide insight into why they work so well for the task,” Woodward explained.

Power rail probes support a number of different connections, including a 2.5mm browser, SMA connection and a surface mount device clip. A solder-in 50ohm coax can be attached to a bypass capacitor

Generic probes with a 10:1 attenuation factor will obviously attenuate signals by a factor of 10 before presenting them to the front end of the oscilloscope. The scope then compensates for this by multiplying the input signal by a factor of 10. This means, however, the noise floor is also multiplied by that same factor of 10. As a result, there is simply too much noise for a power integrity measurement to be made accurately.

“Good power integrity probes have a low-noise 1:1 attenuation ratio for more accurate measurements,” said Woodward.

Most oscilloscopes natively don’t have enough offset to centre power rails and then zoom in to see signal detail. For example, a scope may have a 1.4V offset at 10mV/div. For measuring ripple and noise on a rail larger than this value, say 1.8V or 3.3V, users can’t centre the signal and zoom; they are forced to use a less sensitive vertical setting.

“As a result,” Woodward said, “the user will have to settle for a larger vertical scale, which means that the measurement will have more noise. Because the user can’t zoom in on the signal, the scope is only using a portion of its A/D converter’s dynamic range, contributing to measurement inaccuracy. Power rail probes include a large integrated offset to allow users to centre and zoom on a wide range of DC rails.”

When it comes to how much bandwidth is needed to test DC rails, Woodward said: “It depends. Bandwidth needs are often dictated by harmonics, transients and coupled signals that appear on the power rail. A quick look using a scope’s FFT function will often help to uncover coupled signals. This is why power integrity probes generally have a bandwidth in excess of 1GHz.”

Power rail impedance values are typically in the mΩ range. When users attach the rail to the scope’s 50Ω path, they can quickly find that the DC rail voltage has dropped as a result of the resistor divider network. For this reason, power integrity probes have a high DC input impedance – typically 50kΩ. This higher value ensures there is minimal change to the DC value when a power rail probe is attached to a DC rail. Power rail probes, like other probes, have a frequency response that rolls off quickly. At higher frequencies, the input impedance goes to 50Ω to match the power rail SMA and pigtail coax impedance and prevent reflections.

“Power rail probes are not only useful for detecting small AC perturbations on DC rails, but also for determining the DC value of the rail,” according to Woodward. “More advanced power rail probes incorporate an integrated DC meter. This can measure DC values, even if the signal is not displayed on the probe screen and this can be useful for a quick determination of rail values, as well as for knowing what offset values need to be entered on the scope in order to centre signals.”

Another important specification for power rail probes that limits their use outside of measuring DC rails is their dynamic range. This value defines the maximum peak-to-peak amplitude that the probe can measure. For most power rail probes, this value is often as little as 850mV. This means the probe cannot be used for applications where peak-to-peak voltages exceed this value.

“Special probes for making oscilloscope power integrity measurements are gaining in popularity,” Woodward suggested. “Probes such as the ZPR20 offer lower noise, integrated offset, higher bandwidth, better DC input impedance and some other features – for example, DC voltage readouts and AC coupling – that are not available with probes that have historically been used for larger noise and ripple measurements.”

Neil Tyler

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