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Impedance Measurements for Power Delivery Networks

Low Impedance measurements of Power Distribution Networks require maximum precision. Only an appropriate Vector Network Analyzer with an intuitive interface is able to characterize a broad range of impedances with the highest accuracy among its competitors in the same class.

Power Distribution Networks

By- Rohde & Schwarz India

On a printed circuit board (PCB), each conductive trace that connects a voltage regulator module (VRM or DC/DC converter) to the power supply input of one or more circuits is commonly defined as a power rail. The set of all these traces constitutes a PCB’s power delivery network (PDN).

Due to the nature of its purpose, a PDN is expected to have a characteristic impedance in the range of milliohms (mΩ). Moreover, its impedance should ideally not increase or decrease with frequency from its nominal value. An analysis of a PDN frequency response is meaningful because the current flowing from the VRM to the served circuits undergoes transient phases (i.e. during power-on, with dynamic loads, etc.), extending its spectrum up to several hundred megahertz. Similarly, if a device design includes a gigahertz ADC and uses a frontend amplifier, there is a need to be able to measure stability at 1 GHz. If you are building a power supply and need to measure PDN bursts to the speed of your transceivers, that could easily be several GHz.

Motherboard
Fig 1: Motherboard measurements: Power on (red), power off (blue) Flat = resistor | Rising = inductor | Falling = capacitor

At these frequencies, each interconnect of the PDN begins to play an active role in the power transmission, since they behave as coils or capacitors depending on their physical properties. The power rails themselves act as transmission lines, each characterized by their own inductance and capacitance. A current flowing through these resonating structures often represents a problem for the served circuits (i.e. signal integrity issues, electromagnetic field emissions, etc.). The precise characterization of a PDN’s impedance is therefore paramount for example in PCB test and troubleshooting phases.

Measurement Challenge

Not every instrument is able to perform impedance measurements, because some cannot measure low impedances due to the lack of an appropriate dynamic range, some cannot sweep up to the desired frequency and its harmonics, and some do not have the appropriate interface to the PCB. Vector Network Analysers (VNAs) offer all of the above, but the precision of their impedance measurements is proportional to the instrument’s matching and reflection or transmission accuracy.

transmission
Fig 2: R&S®ZNL calibrated specified transmission accuracy (magnitude and phase), valid for the entire spectrum

Notice that for a PDN, an error of 1 mΩ can affect the outcome of a pass/fail test altogether. Therefore, the choice of the appropriate VNA and the correct test setup positively contributes to the production yield by ensuring low measurement uncertainty, thus decreasing the chance of false positives.

accuracy
Fig 3: Impedance accuracy for the low impedance setup calculated from R&S®ZNL calibrated specified transmission accuracy and valid for the entire spectrum. Data between –65 dB and –90 dB is extrapolated
Appropriate solution

It is important to use a Vector Network Analyzer that allows users (even without deep RF knowledge) to set up impedance measurements for a broad range of test scenarios (from very low to very high impedances), and its accuracy makes the results trustworthy where most Vector Network Analysers in the same class fail. For example, an instrument like R&S®ZNL Vector Network Analyzer offers the best matching and accuracy in its class, and its versatility makes the instrument suitable for measurements in the most challenging circumstances, be it on a test bench in a lab or outdoors.

Functionalities such as spectrum analysis and a battery pack can be installed optionally to make the R&S®ZNL the perfect allrounder for every measurement scenario. The interface of the R&S®ZNL is very intuitive and enables even users without deep RF knowledge to set up a measurement easily and visualize the data in all the necessary formats. In order to perform an impedance test, the user simply selects one of the following options in the measurement menu:
► Z ← S11 – Impedance from reflection
► Z ← S21 – Impedance from transmission
► Z ← S21 – Shunt Impedance from shunt-transmission

Each one of these corresponds to a certain measurement setup and is best suitable for a specific range of impedances. One port measurement (Z ← S11), turns out to be a reasonably good measurement for the stability of operational amplifiers and voltage references. The combination of the third option (Z ← S21 Shunt), and a correct calibration with Rohde & Schwarz high-quality calibration kits enables the lowest uncertainty for PDN whose impedance is as low as a fraction of a milliohm.

Correspondence between instrument functions and measurement setups for a correct impedance calculation

measurement

Shunt Impedance Method Example
Screenshot
Fig 4: R&S ZNL Measurement Screenshot with most accurate measurements using Shunt Impedance Method

You can e.g. use PacketMicro probes to make these measurements. To reduce the ground loop error at low frequencies, use a ground isolator or common-mode transformer (e.g., Packet Micro J2102B) or an active isolation device such as the J2113A

ZNL
Fig 5: R&S ZNL Test setup using the Packet Micro Probes (right) and Common mode transformer (left).

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