Two common PDN measurement questions: Page 2 of 5

May 20, 2015 //By Steve Sandler
Two common PDN measurement questions Steve Sandler provides answers two common PDN measurement questions.
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 overvoltage. 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 minimize the transient energy.

Figure 2: DC blocker typical low frequency limit versus DC bias voltage applied to the DC blocker (using Picotest P2130A 500 Hz - 6 GHz DC blocker).

Why use a 1 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 1 shunt resistor (R3) for calibration of the 2-port measurement. There are several reasons I use this method.

For one, not every vector network analyzer (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 analyzer, 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.

Design category: