Two common PDN measurement questions
I receive 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 1 shunt resistor, and, why do I use DC blockers on both ports? In this article Ill provide responses to both of these questions. The measurement setup in Figure 1 is an example where I used both the 1 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 1 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 lets 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 50) 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 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.
Since the calibration is performed with 1 calibration resistor the result is also direct reading in Ohms. The calibration factor is:
This simplification isnt 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:
The error is shown graphically in Figure 3 indicating that an error of +4% exists for resistance values below about 25m and a -5% error is exists for resistance values above 2.37. Despite the greatly reduced measurement range, the resulting range is quite usable in many applications. The resistance can be increased using a modified 2-port measurement. 
Figure 3: Measurement error as a function of the resistance being measured. The maximum error due to calibration is +4% and -5% corresponds to a resistance value of 2.37.
In order to verify the measurement, several 2512 size resistors are mounted to circuit boards along with SMA connectors (see Figure 4). The 1 calibration resistor is connected in this setup picture. Three low value resistors, selected from the same vendor, are used to verify the measurement calibration.
Figure 4: The VNA and DC blockers are shown connected to a 2512 size 1Ohm resistor. Similar resistance values of 1m, 2m and 5m are shown in the foreground.
The pre- and post- calibration sweeps along with the measurement sweeps for 1m, 2m, and 5m resistance values are shown in Figure 5.
Figure 5: VNA measurement sweeps prior to calibration, after calibration and for the three low value resistors.
The post-calibration sweep is flat a 1 confirming the THRU calibration was performed and applied. The low frequency resistance values are reported as 1.2m, 2.3m, and 5.6m, while the corresponding inductance values are 420pH, 500pH, and 660pH respectively.
For comparison purposes, the resistors are measured using a 1A current source and a precision voltmeter (see Figure 6). This figure shows the results for the 1 calibration resistor.
Figure 6: A power supply with current limit set to 1.00A is connected to one side of the resistor and a precision voltmeter is connected to the other side of the resistor. These connections replicate the connections used for the VNA measurements.
The DC measurements of the three resistors are 1.08m, 2.14m, and 5.26m, respectively. The VNA measurements are within 10% (1 dB) for all three low value resistors using this simple 1 shunt calibration technique.
About the author
Steve Sandler is the founder and former CEO of Analytical Engineering, Inc., the predecessor of AEi Systems. He has over 30 years experience in the design and analysis of power conversion equipment for military and space applications. Mr. Sandler is also the CEO of Picotest.com, a company that distributes test equipment including the Signal Injector product line designed for testing linear and switching power supplies, a Test & Measurement World "Best in Test" Finalist.