by Masashi Nogawa and Steve Sandler

Published by Signal Integrity Journal, June 14, 2022

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New electronic applications, such as electric vehicles, 48 V telecom/data centers, and 240 W USB power delivery, demand higher voltage power supplies. These higher voltage power rails are no different from traditional 3.3 V, 5 V, and/or 12 V rails. Having a noisy power supply significantly degrades the performance of the load circuit, hence the need for very low output ripple and noise voltage. Since many of these application circuits convert power supply noise into output signal noise of amplitude or phase, it is most useful to measure the power supply noise using a spectral noise density plot.

In this power-electronics article, we focus on noise measurement of 48 V range voltage regulators (VRs), up to a maximum of 65 V. We propose a noise measurement setup and examine this method.

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Regulator Noise Impact

As an example, we know that an RF power amplifier (PA) modulates its DC bias noise on the RF output. This modulation is known as the power supply modulation ratio (PSMR), and the measurement of the DC bias supply spectrum is the input of this ratio characteristic. Figure 1 illustrates the concept of PSMR, the input and output signals.


The base plot, in blue, is an output signal spectrum of an RF PA, where we superimpose a DC bias VR noise curve obtained by the method of this article, in light red color.

In this example, we use a switch mode power supply (SMPS) as the VR, so we can see some spurs from the switching operation and its harmonics. This VR is operating at 500 kHz, and 0.5 MHz spurs are the main switching noise in both the light red and blue plots. Then, we observe second and third harmonics at 1 MHz and 1.5 MHz.