April 1, 2015
Traditional de-embedding techniques are oftentimes insufficient for today’s high-speed component designs utilized in high data rate applications. As frequencies approach 70 GHz or even 110 GHz, errors associated with fixturing can mask the behavior of the device under test (DUT). Poor de-embedding can lead to both passivity and causality errors. In addition, high fixture loss may affect the accuracy and repeatability of de-embedding.
Signal integrity engineers given the task of overcoming these challenges have found that a new generation of Vector Network Analyzers (VNAs) can help them gain greater confidence in their designs. Some modern VNAs, such as the Anritsu VectorStar™ MS4640B (figure 1), utilize a unique instrument architecture, as well as special software tools to help make embedding/de-embedding straightforward and simple.
New de-embedding capabilities offered in VNAs can be used to remove test fixture contributions, modeled networks and other networks described by S-parameters (S2P files) from the measurement results for greater accuracy and repeatability. Similarly, enhanced embedding functionality can simulate matching circuits to optimize DUT performance, which is helpful in amplifier design applications. The function can also be used to add effects of a known structure to a measurement. With the new capabilities, engineers have a greater ability to locate design weaknesses, such as discontinuities, impedance changes, and crosstalk issues in high-speed DUTs.
Let’s continue to use VectorStar as an example. With the VNA, engineers can combine a wide range of calibration capabilities with up to seven de-embedding techniques to address the various situations found in many high-frequency component designs. By matching the calibration and de-embedding method to specific DUT and fixture structures, measurement accuracy and repeatability are improved. In addition, external software is no longer necessary to de-embed differential devices, for faster and more accurate differential device analysis.
Better Device Modeling
With the enhanced embedding/de-embedding capabilities, engineers are able to acquire better device modeling data. This gives them greater design confidence and results in fewer design turns. Other elements of VNAs for improved device models include broad frequency coverage, say from 70 kHz to 70/110/145 GHz in a single connection. Achieving highly accurate low-end frequency coverage, which is enabled by a hybrid bridge-coupler VNA architecture, reduces the risk of DC extrapolation errors in device models. Additionally, wide frequency coverage eliminates the time-consuming, error-prone, concatenation process across the RF, microwave, and millimeter bands.
High data resolution is also important for engineers to experience improved device modeling. For example, data resolution of 100,000 points and 700 kHz frequency step size provide highly accurate, highly resolved, low pass mode measurements. Plus, the high resolution gives engineers the ability to zoom in on narrow band responses without re-calibrating. If calibration is necessary, precise auto calibration techniques are employed with a push of a button, for simplicity as well as higher accuracy compared to the traditional SOLT technique.
Software applications are also beneficial in embedding and de-embedding environments. A tool such as PulseView™ (figure 2), allows DUTs to be tested and characterized under pulse conditions. The software can take advantage of the improved resolution and dynamic range of VNAs, as well as independent measurement gates, for detailed results. Ultra-high resolution enables engineers to see performance perturbations not only within a pulse, but on the rising/ trailing edges, as well. With a capture time of 500 ms, engineers can measure amplifiers under long pulse repetition interval conditions. The right software can also permit pulse-to-pulse measurements to be conducted over an extended number of pulses.
Evaluating differential amplifier designs under true mode stimulus (TMS) conditions can also be achieved with software, such as DifferentialView™. When used with certain graphical user interfaces (GUI), engineers can change parameters and see the results immediately without switching screen displays, for time savings and deeper analysis.
To learn more about why high-frequency designs require new de-embedding techniques, download a free white paper on the subject.