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Addressing common PDN measurement questions

28 May 2015  | Steve Sandler

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I get many emails with questions about measuring power distribution networks (PDNs), but these two are very common. Why do I calibrate the 2-port measurement with a 1Ω shunt resistor, and, why do I use DC blockers on both ports? In this article I'll provide responses to both of these questions. The measurement setup in figure 1 is an example where I used both the 1Ω calibration and the inclusion of the DC blockers.

Figure 1: Voltage regulator test board with a pair of SMA connectors connected to the regulator output and a pair of SMA connectors connected to a 1Ω calibration resistor (R3) . (Image courtesy AEI Systems).

Why are DC blockers used on both ports?
The second question is easier to answer than the first, so let's answer that one first. There are two reasons for the use of the DC blockers. First, the standard 2-port measurement connects both instrument ports (typically 50Ω) to the voltage regulator module (VRM) being tested. These ports result in a load current of Vout/25 and this can be significant in comparison to the device loading. For example, measuring a 2.5V low power voltage regulator the port loading would add 100mA of output current. This 100mA can be more than the maximum load of the device, can overload the regulator, and can greatly alter the regulator performance, which is load current dependent.

The DC blockers isolate the instrument from the VRM to eliminate such DC loading. A second reason for the inclusion of the DC blockers is to protect the instrument inputs from over-voltage. With that said, there are two points to keep in mind.

Wideband low-frequency DC blockers are generally constructed using ceramic dielectric capacitors, which are DC voltage bias sensitive. The low frequency limit will increase with DC voltage applied. For example, the graph in figure 2 shows the typical impact of DC bias on Picotest P2130A wideband DC blockers along with a curve-fitted equation that can be used to estimate the low frequency limit. Always calibrate the setup with the DC blockers installed and beware of this DC bias effect on the low frequency limit.

A low-frequency DC blocker can still allow a significant voltage transient when the regulator is powered up, so be certain that your instruments offer transient protection and power the regulator gradually if that is possible to minimise the transient energy.

Figure 2: DC Blocker typical low frequency limit vs DC Bias voltage applied to the DC blocker (using Picotest P2130A 500Hz—6GHz DC blocker).

Why use a 1Ω shunt calibration?
Many ask me why I calibrate using this unusual method. The test board shown in figure 1 also includes two SMA connectors connected to a 1Ω shunt resistor (R3) for calibration of the 2-port measurement. There are several reasons I use this method.

For one, not every vector network analyser (VNA) includes the 2-port impedance transformation, and this includes the OMICRON Lab Bode 100 we use for our low-frequency measurements. We could export the data and perform the transformation outside of the analyser, or we could use the automation interface to apply the transformation, but I like the simplicity of this method. I also like the minimal calibration hardware required. Rather than having to perform short-open-load (SOL) on each port and a THRU calibration between ports, it is reduced to a single calibration.

Since the calibration is performed with 1Ω calibration resistor the result is also direct reading in Ohms. The calibration factor is

This simplification isn't without consequence as there is an error term that greatly reduces the measurement range compared with the standard 2-port transformation. The error can be calculated as

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