Interconnection
De-embedding made easy
By Mathias Leutiger, head of product management broadcasting T&M at Rohde & Schwarz
D
e-embedding, which means eliminating the effects of connectors, cables and adapters in a test setup, is a well-established procedure. It is becoming
increasingly important in the microwave domain, where components that are not part of the DUT have an especially negative impact. Four new algorithms for Rohde & Schwarz network analyzers make life much easier for circuit designers. Next generation RF
communications systems will process data rates in the multiple Gbit/s range and frequencies far above 20 GHz. These signals require tightly toleranced module specifications. In addition, microwave frequencies with wavelengths approaching component dimensions cause significant distortions on test adapters, cables, connectors and PCB traces. How can this be countered with T&M equipment?
De-embedding with software algorithms Vector network analyzers
characteristics of the test fixture, as well as the characteristics of the signal traces to the DUT on the PCB, must be mathematically eliminated from the measurement. This is the job of the new software options. The procedure is as follows: First the test engineer measures the S-parameters of the reference path on a test coupon (Fig. 2, bottom) containing
Fig. 1: Calculating the effects of PCB traces based on reference measurements is one of the functions of the new de-embedding options.
measurement results are input to a software algorithm that calculates the transmission characteristics of the bare DUT from the measured data.
The quality of the de-embedding process is only as good as the quality of the test fixture and the test coupon. The algorithm also assumes that the electrical characteristics of the reference path and the path including DUT are the same. This easy to set up on a PCB (Fig. 2).
Fig. 2: Example test coupon with a 2×thru configuration. The input and output lines of the DUT to be de-embedded are duplicated as reference paths on the same PCB as the DUT so they have the same transmission characteristics as the actual input and output lines, which is a prerequisite for accurate de-embedding.
(VNA) are the preferred tool for precisely measuring the transmission characteristics of RF components and describing them in the form of S-parameters. A prerequisite for precise measurement, however, is characterization of the transmission characteristics of the entire test setup down to the DUT, as otherwise the specific characteristics of the DUT cannot be measured correctly. This is not easy, especially with integrated circuits such as analog components that cannot be probed directly because they are soldered on a PCB. In this case the connection to the VNA is made using a test fixture, unless the PCB has coaxial connectors. The RF transmission
40 April 2021
the components to be de-embedded, such as a coaxial-to-coplanar transition and a length of line. Usually a 2×thru reference is constructed for this, consisting of symmetrical input (fixture A) and output (fixture B) lines to and from the DUT, because this results in more accurate measurements. Then the engineer
measures the S-parameters of the path including the DUT (Fig. 2, top). The two
Components in Electronics Fig. 3: Example screen of the R&SZNx-K220 de-embedding option. The task is performed after a simple three-step process.
www.cieonline.co.uk
Simple workflow fully integrated in the VNA user interface The algorithms used in the software options are widely accepted in the industry The first three options have essentially the same functionality and can be used interchangeably, but the Delta-L method characterises the frequency-dependent attenuation of RF PCBs in dB per inch and, unlike the other options, does not calculate
all the S-parameters of the embedding network. Intel recommends this method for measurements on PCBs for its high-speed components (Fig. 1).
All options use a convenient workflow (Fig. 3) integrated in the VNA user interface. It guides the user quickly and effectively through the required measurement steps. A big advantage of the integrated solution is that the measured de-embedding S-parameters do not have to be imported or exported.
The de-embedding options can run on all top-end and midrange VNAs from Rohde & Schwarz: R&S ZNA, R&S ZNB, R&S ZNBT and R&S ZND. Using these options, it is now possible to eliminate the effects of test fixtures that do not probe the DUT through coaxial connectors. They are suitable for both single-ended and differential DUTs.
rohde-schwarz.com
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