Being an oscilloscope probe design engineer I get the chance to get out of the cave several times a year and talk with our users so that I can better understand the measurements they want to make and what they need from us to make their lives easier. In a typical conversation we would be discussing the next type of DDR memory or CPU (or whatever) that the users need to probe/measure and inevitably the question would come up—you got anything to measure ripple and noise on my power supplies? Initially the answer was no. These users wanted to measure mV ripple and noise riding on top of their 1.8 V, 3.3 V…24 V supplies. This is kind of a specialized measurement. To turn that answer into a yes I would need to design a specialized probe. But before I could do that I had to understand the application and measurement need better. Here is what I learned and what we came up with.
Thanks to Moore’s law, the doubling of gates on an IC every 18-24 months, the electronics that we encounter everyday are packed with more functionality in ever smaller, denser packaging. Consider your mobile phone. It wasn’t that long ago they were a brick that performed a single function—they made a phone call, and now they are elegantly thin machines that can give turn-by-turn directions, shoot high-definition video, monitor some of my biological functions and respond to my voice control. With this increased functional density comes some power related problems—power density and power supply induced jitter (PSIJ—power supply induced jitter can be the single biggest cause of clock and data jitter in a digital system). Folks learned that if they put tighter tolerances on the ripple and noise on their supplies and reduced the supply voltages where possible, they could reduce their power and jitter issues. It is not uncommon today to see supplies with tolerances of 1-3%. I saw an LPDDR with a 0.6 V supply with a 1% tolerance—this means measuring 6 mVpp ripple and noise.
Based on what our users shared with me I distilled the measurement challenges down to this: the need to measure a small AC signal riding on top of a large DC signal. If the AC signal exceeds the tolerance, the design failed to meet its requirements. This illuminated the challenges we needed to overcome. Users needed a very low noise probe & oscilloscope combo so that their ripple and noise were not overshadowed by the noise of the measurement system. They also needed a way to remove the large DC offset so they could put the signal in the center of the screen and zoom-in on it (get down to 1 mv/div if needed). The measurement tools also had to have enough bandwidth to capture the high frequency noise caused by the switching of the digital circuits. Since this is a function of clock speeds many users needed up to 2 GHz of bandwidth (frequencies above this are attenuated quickly by the circuit board fairly close to the noise source). And in addition, they had to have a probe that would not load the supply. For example, some users would connect a 50 Ω cable to the 50 Ω input of the scope to measure ripple and noise—for a 3.3 V supply the scope would sink 66 mA which can change the behavior of the supply.
Here is what we came up with, the Keysight N7020A Power Rail probe. The first and only probe designed specifically for making ripple and noise measurements on supplies. The probe has 1:1 attenuation ratio so that full size signals make it to the oscilloscope. This creates a very favorable signal:noise ratio. There is ±24 V of probe offset. This means the probe can remove up to 24 V of DC content from the signal so that signal can be placed in the center of the screen and be scrutinized at high sensitivity settings.
Desperate users had been making use of DC blocks/AC coupling/DC reject to remove the DC content. They told us they disliked this because if filters the signal. A DC block is a ‘big’ capacitor so it also blocks low frequency supply drift and supply compression from being seen on the scope. Since the probe is active it has a DC input impedance of 50 kΩ which means it won’t change the behavior of the supply when it is attached. Finally the probe has 2 GHz bandwidth so that users can capture high frequency digitally induced noise on their supplies. Not everyone needs this much bandwidth so for those that don’t we recommend using the oscilloscope’s built-in bandwidth limiting capabilities so as to minimize oscilloscope/probe noise. And, I almost forgot, the probe comes with a lot of cool connection accessories so that you can easily probe a variety of locations on your target.
If you are curious to learn more, there is a great teardown video of the probe at The Signal Path: https://www.youtube.com/watch?v=d_Ybe6xnMIg.