This important first step is often ignored, resulting in meaningless measurements and wasted time. During the pretest, measurements of the device‐under‐test (DUT) are performed to coarsely determine some of its attributes. During pretest, it is also determined if the DUT is plugged in, turned on, and operating as expected. Many times the gain, match, or power handling is discovered to be different than expected, and much time and effort can be saved by finding this out early.
Optimize: Once the coarse attributes of the device have been determined, the measurement parameters and measurement system can be optimized to give the best results for that particular device. This might include adding an attenuator to the measurement receivers, adding booster amplifiers to the source, or just changing the number of points in a measurement to capture the true response of the DUT. Depending upon the device's particular characteristic response relative to the system errors, different choices for calibration methods or calibration standards might be required.
Calibrate: Many users will skip to this step, only to find that something in the setup does not provide the needed conditions and they must go back to the first step, retest, and optimize before recalibration. Calibration is the process of characterizing the measurement system so that systematic errors can be removed from the measurement result. This is not the same as obtaining a calibration sticker for an instrument but really is the first step, the acquisition step of the error correction process that allows improved measurement results.
Measure: Finally, some stimulus is applied to the DUT, and its response to the stimulus is measured. During the measurement, many aspects of the stimulus must be considered, as well as the order of testing and other testing conditions. These include not only the specific test conditions but also pre‐conditions such as previous power states to account for non‐linear responses of the DUT.
Analyze: Once the raw data is taken, error correction factors (the application step of error correction) are applied to produce a corrected result. Further mathematical manipulations on the measurement result can be performed to create more useful figures of merit, and the data from one set of conditions can be correlated with other conditions to provide useful insight into the DUT.
Save data: The final step is saving the results in a useful form. Sometimes this can be as simple as capturing a screen dump, but often it means saving results in such a way that they can be used in follow‐up simulations and analysis.
1.2 A Practical Measurement Focus
The techniques used for component measurements in the microware world change dramatically depending upon the attributes of the components; thus, the first step in describing the optimum measurement methods is understanding the expected behavior of the DUT. In describing the attributes and measurements of microwave components it is tempting to go back to first principles and derive all the underlying mathematics for each component and measurement described, but such an endeavor would require several volumes to complete. One could literally write a book on the all the attributes of almost any single component, so for this book the focus will be on only those final results useful for describing practical attributes of the components to be characterized, with quotes and references of many results without the underlying derivation.
There have been examples of books on microwave measurements that focus on the metrology kind of measurements (Collier and Skinner 2007) made in national laboratories such as the National Institute for Standards and Technology (NIST, USA) or the National Physical Laboratory (NPL, UK), but the methods used there don't transfer well or at all to the commercial market. For the most part, the focus of this book will be on practical measurement examples of components found in commercial and aerospace/defense industries. The measurements focus will be commercial characterization rather than the kinds of metrology found in standards labs.
Also, while there has been a great deal written about components in general or ideal terms, as well as much academic analysis of these idealized components, in practice these components contain significant parasitic effects that cause their behavior to differ dramatically from that described in many textbooks. Unfortunately, these effects are often not well understood, or difficult to consider in an analytic sense, and so are revealed only during an actual measurement of a physical devices. In this chapter, the idealized analysis of many components is described, but the descriptions are extended to some of the real‐world detriments that cause these components' behavior to vary from the expected analytical response.
1.3 Definition of Microwave Parameters
In this section, many of the relevant parameters used in microwave components are derived from the fundamental measurements of voltage and current on the ports. For simplicity, the derivations will focus on measurements made under the conditions of termination in real valued impedances, with the goal of providing mathematical derivations that are straightforward to follow and readily applicable to practical cases.
In microwave measurements, the fundamental parameter of measurement is power. One of the key goals of microwave circuit design is to optimize the power transfer from one circuit to another such as from an amplifier to an antenna. In the microwave world, power is almost always referred to as either an incident power or a reflected power, in the context of power traveling along a transmission structure. The concept of traveling waves is of fundamental importance to understanding microwave measurements, and to engineers who haven't had a course on transmission lines and traveling waves, and even some who have, the concept of power flow and traveling waves can be confusing.
1.3.1 S‐Parameter Primer
S‐parameters have been developed in the context of microwave measurements but have a clear relationship to voltages and currents that are the common reference for most electrical engineers. This section will develop the definition of traveling waves and from that the definition of S‐parameters, in a way that is both rigorous and ideally intuitive; the development will be incremental, rather than just quoting results, in hopes of engendering an intuitive understanding.
This signal traveling along a transmission line is known as a traveling wave (Marks and Williams 1992) and has a forward component and a reverse component. Figure 1.1 shows the schematic of a two‐wire transmission structure with a source and a load.
Figure 1.1 Voltage source and two‐wire system.
If the voltage from the source is sinusoidal, it is represented by the phasor notation
(1.1)
The voltage and current at the load are
(1.2)
The voltage along the line is defined as V(z), and the current at each point is I(z). The impedance of the transmission line provides for a relationship between the voltage and the current. At the reference point, the total voltage is V(0) and is equal to V1; the total current is I(0). The power delivered to the load can be described as
(1.3)
