Analyzing Power Integrity on a Power Distribution Network

Power Integrity on a Power Distribution Network

Analyzing Power Integrity on a
Power Distribution Network (PDN)

Power Distribution Networks (PDNs) must provide many low-noise DC power rails for sensitive loads such as microprocessors, DSPs, FPGAs and ASICs. The quest for more speed and higher density means faster edge rates, higher frequencies and more rails, with lower voltage levels and higher currents. This places pressure on design for both signal integrity and power integrity.

The goal of making power integrity measurements is to validate that the voltage and current reaching the Point of Load (POL) meet the load’s power rail specifications under all expected operating conditions. Special attention is required to accurately measure millivolts of power rail noise at GHz frequencies.

Measuring high-frequency ripple without blocking DC or loading your power rails

Measuring high-frequency ripple without blocking DC or loading your power rails

Power rail probes offer extreme clarity into the power distribution network compared to traditional passive probes.

Noise specifications on power rails can go up into MHz or GHz frequency ranges with amplitudes in millivolts.

An oscilloscope with low noise contribution and high bandwidth can make these measurements but getting the signal into the instrument is challenging.

The high-impedance 10X passive probes that come with oscilloscopes may have enough bandwidth, but they attenuate the very noise signal you’re trying to measure. The scope then amplifies both the signal and measurement system noise making them impossible to tell apart.

1X probes pass the noise signal without attenuation, but they are limited to several MHz bandwidth.

Transmission line probes or cable using a 50 Ω scope input offer great high-frequency performance but present significant loading at DC.

The ideal probe for making power rail measurements offers high Impedance at DC and act as a 50 Ω transmission line for high-frequency AC. Power rail probes such as TPR1000 and TPR4000 are designed to meet these challenges with high bandwidth, no attenuation and minimal loading.

Dealing with supplies from 1 V to 48 V and above

Although the main supply voltages for many FPGAs and SoCs have dropped dramatically, these are not the only supplies to consider. On-chip I/O supplies can cover a much wider range than the main logic supplies. Bulk supplies feeding POL regulators or Voltage Regulator Modules (VRMs) are often much higher voltage.

Although many scopes and probes can provide some DC offset, it may not be enough to deal with all the power rails in your system. And at lower volts per division (higher sensitivity) settings instrument systems tend to afford less offset. While blocking DC is an option, it is often undesirable (see above).

In addition to addressing the high-frequency needs outlined above, power rail probes such as TPR1000 and TPR4000 offer high offset range to address a wide range of voltage levels.

Dealing with supplies from 1 V to 48 V and above

Minimizing measurement system noise contribution

Minimizing measurement system noise contribution

Channel 1 (yellow trace) is an oscilloscope channel with no input while channel 2 (blue trace) is a TPR1000 with its input shorted. Notice that at 1 GHz bandwidth the probe is only adding 17 µV of noise to the oscilloscope input.

Measuring noise on the order of 10 mV requires careful attention to measurement system noise. As noted above, using non-attenuating, or 1X, probes reduces the burden on your oscilloscope’s amplifiers. The scope’s internal noise and measurement resolution also play critical roles.

The 6 Series MSO includes a new front end with industry-leading noise performance. The scope offers open channel noise as low as 50μVRMS and 466μVpeak. When paired with TPR1000/4000 power rail probes, the system noise can be as low as 70μVRMS.

The 6 Series offers 12-bit resolution at 12.5 GS/s. A High Res function boosts resolution to 16 bits at 625 MS/s and below. 4 and 5 Series MSOs also offer 12-bit resolution with up to 16 bits using High Res mode.

Measuring power distribution network impedance

For PDNs powering FPGAs, processors and other complex IC’s, power rail impedances must be low in order to deliver high current in response to rapidly changing demand. However, the network is made up of many impedances including the voltage regulator, decoupling capacitors, and PCB traces. High-speed switching involves broadband frequencies and unexpected variations in impedance can result in excessive transients or noise. Measuring the impedance of a network design over a wide frequency range provides confidence that the network won’t produce unwanted surprises.

Network impedance measurements are traditionally performed using VNAs, such as the two-port TTR500 which can measure from 100 kHz to 6 GHz.

5 and 6 Series MSO oscilloscopes can measure power rail impedance down to 10 Hz using analysis software, a signal generator (built-in or AFG31000 Series), and an isolation transformer.

Measuring power distribution network impedance

Characterizing noise using simultaneous spectrum and waveform analysis

Let’s say you’ve measured the power rail noise and it’s out of spec. Is it coming from a DC-DC converter? The bulk supply? A PLL? A clock? Crosstalk? Spectrum analysis can provide clues to noise sources – helping to correlate noise frequencies to switching frequencies and harmonics.

A spectrum analyzer such as an RSA306 connected to your power rail with a DC block can help provide insight into your noise.
The FFT function on your scope is also useful, but these functions use the sample clock on your scope, making it difficult or impossible to look at the spectrum and voltage waveforms at the same time. The unique spectrum view on 4, 5 and 6 Series MSOs provide independent spectrum analyzer controls so you can see synchronized time domain spectrums and frequency domain waveforms at the same time.

Characterizing noise using simultaneous spectrum and waveform analysis

Faster power rail measurements with automated analysis software

Making even simple measurements such as ripple, overshoot, and undershoot on dozens of power rails requires significant time and attention to detail.

The 5 and 6 Series MSOs are available with Digital Power Management software to automate these repetitive measurements and generate in-depth reports. The software also includes jitter analysis (TIE, RJ, DJ and Eye measurements) to check for excessive jitter on clocks and communications signals powered by your PDN.

Faster power rail measurements with automated analysis software
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