Low Noise Power Integrity Measurements - A Guide to Effective Probing with the MXO58 Oscilloscope

Updated: Apr 10

In this blog, we will examine the noise performance of various probing solutions with a Rohde & Schwarz (R&S) MXO 5 oscilloscope. The goal is to understand how to achieve the best low-noise solution all the way from the device under test (DUT) to the scope.

When it comes to acquiring a signal, traditionally, we consider the oscilloscope's bandwidth, dynamic range, and some other features necessary to make a low noise measurement of a signal.

However, it is easy to forget the importance of looking at the signal chain between the scope and DUT. A high bandwidth oscilloscope is worthless if your probing solution does not have the dynamic range or low enough noise performance necessary for today's power integrity measurements.

With voltage compliance requirements getting lower and tighter, understanding the noise impact from your probing solution is critical to making good voltage ripple measurements at your point-of-load or anywhere in your system to assess performance, including compliance.

Signal Edge Solutions is a company that takes power integrity measurements very seriously, which is why we partnered with Picotest to sell their products. So, we wanted to explore the impact of noise during measurement using various browser probe solutions with the new MXO 5 oscilloscope. The MXO 5 oscilloscope is the latest scope platform from Rohde & Schwarz. This mighty scope has many exciting new features and capabilities and is available as a 2GHz bandwidth option.

Figure 1 shows a TPS7H4003 evaluation board from Texas Instruments as the DUT to explore this topic. The TPS7H4003 is a radiation-tolerant buck converter configured for a 1V output. All measurements shown will be with 5Vin and a 2A load.

Figure 1 - Texas Instruments TPS7H4003 EVM [1]

Figure 2 depicts the measurement setup used, where only the probing solution is swapped. The MXO58 oscilloscope is set to 350 MHz bandwidth, and the probe termination is also set as appropriate for each probe under evaluation.

Figure 2 - Voltage Ripple Measurement Setup on TPS7H4003 EVM with 2A DC Load

Since we are using an MXO 5, we will evaluate two R&S probing solutions and a Picotest probe solution for these measurements. The three probes evaluated were an R&S RT-ZP11, an R&S ZPR20 power rail probe, and a Picotest P2104A-1X probe. As shown in Figure 3, all three probes are placed on the same measurement point, which is an unpopulated capacitor (C23).

Figures 4, 5, and 6 depict the voltage response at the TPS7H4003's output for the RT-ZP11, ZPR20, and P2104A-1X probes, respectively. It is important to note that the high probe impedance (10 MOhm) of the RT-ZP11 significantly limits the dynamic range of the measurement setup. As noted in Figure 4, the vertical range is set to absolute max MXO 5; again, this is because the RT-ZP11 is a 10X probe. This means that due to the 10 MOhm RT-ZP11 probe impedance, the signal-to-noise ratio is limited, which limits the max signal level to the MXO 5 and hinders our measurement from taking full advantage of the scope's dynamic range.

Figure 3 - Voltage Ripple Measurement Setup at C23 with each Probing Solution

Figure 4 - Voltage Ripple Measurement Result R&S RT-ZP11 Probe

Figure 5 - Voltage Ripple Measurement Result R&S ZPR20 Power Rail Probe

Figure 6 - Voltage Ripple Measurement Result Picotest P2104A Probe

After initial analysis, the Picotest P2104A-1X probe solution provides the lowest noise measurement solution with the MXO 5. These results are also summarized in Table 1 below. As a final thought of exploration, we wanted to combine the P2104A-1X probe with the R&S RPZ20 to assess the performance of this measurement solution. This measurement setup is shown in Figure 7, and the measurement result is shown in Figure 8.

Figure 7 - Voltage Ripple Measurement Setup R&S ZPR20 with P2104A Probe

Figure 8 - Voltage Ripple Measurement Result R&S ZPR20 with P2104A Probe

Even though this guide is focused on probing solutions with the MXO 5, we still want to cover all of our bases and do one last evaluation to explore the noise performance of a soldered COAX solution with the R&S ZPR20. This setup is shown in Figure 9. A close-up view of the soldered COAX solution can be seen in Figure 3. These results summarized in Table 1 are shown in Figure 10.

Figure 9 - Voltage Ripple Measurement Setup R&S ZPR20 with Soldered COAX

Figure 10 - Voltage Ripple Measurement Result R&S ZPR20 with Soldered COAX

As shown in Table 1, it is apparent that the best probing solution evaluated is the P2104A. The P2104A combined with the ZPR20 provides the absolute lowest noise solution that we have evaluated thus far. This low noise is due to higher shielding on the Picotest PDN cable® and the very low probe loading from the P2104A probe solution. If you want to improve your measurement setup further, the Picotest P2105A is the only other browser probe with even lower inductance and probe loading than the P2104A. Since the Picotest P2105A is only available in a 1X attenuation solution, it needs to be combined with a Picotest Port Saver DC block solution (when your voltage is greater than 5Vrms), like the P2131A to protect the front of your MXO 5 scope. The P2105A can also be combined with the ZPR20 to provide an even better lower-noise solution, which is not shown in this blog. The P2104A is available in multiple standard attenuations and can be customized.

Table 1 - Voltage Ripple Measurement Results Summary

Probe	Voltage Ripple Result	Voltage Ripple Delta	% Change
R&S RT-ZP11	19.50 mV-pk-pk	N/A	N/A
R&S ZPR20	12.43 mV-pk-pk	7.07 mV	36.3% to RT-ZP11
Picotest P2104A-1X	10.4 mV-pk-pk	2.03 mV to ZPR20 w/ZA-25/ 9.1 mV to RT-ZP11	16.33% to ZPR20 w/ZA-25 / 46.67% to RT-ZP11
R&S ZPR20 with Soldered COAX	11.4 mV-pk-pk	1.03 mV to ZPR20 w/ZA-25/ 8.1 mV to RT-ZP11 /	8.3% to ZPR20 w/ZA-25 / 41.53 to RT-ZP11
Picotest P2104A-1X with *R&S ZPR20*	10.08 mV-pk-pk	2.35 mV to ZPR20 w/ZA-25 / 9.1 mV to RT-ZP11 / 0.32 mV to P2104A only	18.9% to ZPR20 w/ZA-25 / 48.3% to RT-ZP11 / 3% to P2104A only

Summary

Being right matters! The P2104A-1X probe offers a very high fidelity and reliable method of measuring low noise signals. Results shown here reflect that this browser probe is even a better lower noise solution than the soldered COAX solution. The fact that this is a browser probe makes this an easier measurement solution to implement over a soldered COAX. The P2104A probe combined with the Rohde & Schwarz ZPR20 Power Rail solution provides the absolute best low noise measurement solution for the MXO 5. The benefit of using the ZPR20 with the P2104A-1X probe is that engineers can now take advantage of the highly accurate DC voltmeter features built into the ZPR20. In addition, the ZPR20 has a +/-60V offset compensation, combining this with a 50-ohm probe like the P2104A-1X or P2105A, which offers the highest signal-to-noise ratio (SNR) and dynamic range.

In short, there is no replacement for a single-ended 50-ohm browser probe. A single-end 50-ohm probe is similar to having a direct measurement coax connection from your DUT to your oscilloscope. This also means if more than one measurement is done on the same DUT simultaneously, a ground loop exists, and a coaxial transformer (J2102B) should be added to your measurement setup. To learn more about improving your measurement results with your MXO 5 oscilloscope, check out our store at the link below.

Picotest Probes | Signal Edge Solution (signaledgesolutions.com )

If you have questions about probing solutions for any oscilloscope, then please reach out by sending us an email at info@signaledgesolutions.com

References

Low Noise Power Integrity Measurements - A Guide to Effective Probing with the MXO58 Oscilloscope

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