While most RF and microwave test systems are designed to cover a broad range of categories—such as amplifiers, transmitters, and receivers—each system operates under unique environmental conditions, requirements, and challenges. Although no two situations are exactly the same, when designing an RF or microwave test system, three key factors always play a critical role: performance, speed, and repeatability. The optimal balance between these elements depends on whether the system can meet the required level of measurement integrity.
From the Device Under Test (DUT) to the signal path between measuring instruments (as shown in Figure 1), there are numerous points where timing and trade-offs occur. This paper presents an architecture that takes these considerations into account and offers six practical tips for addressing common RF signal path issues.
Figure 1: No matter the test system architecture, there are multiple opportunities to strike the best balance between performance, speed, and stability in order to ensure accurate and reliable measurements.
Tip 1: Prioritize Efficiency, Speed, and Stability
To fully understand the six secrets discussed in this paper, it’s essential to clearly define what we mean by efficiency, speed, and stability. In most cases, only one or two of these factors will be the primary focus, which ultimately shapes your testing needs and equipment selection. Regardless of the priority, carefully reviewing the interactions and trade-offs between these three elements (summarized in Tables 1 through 3) will help you better manage the specific demands of your system.
1.1 Basic Definitions
In RF and microwave test equipment, Agilent defines "performance" primarily as the accuracy, measurement range, and bandwidth of the instrument. Accuracy includes the absolute precision of amplitude and frequency measurements. Measurement range refers to dynamic range, distortion, noise level, and phase noise—factors that affect the accuracy of signal level measurements. Bandwidth relates to the frequency range or data rate that the instrument can process and analyze.
Speed, or throughput, in a test system is influenced by hardware, input/output (I/O) interfaces, and software. Our focus will be on the hardware and four main factors that impact speed: measurement setup time, measurement execution time, data processing time, and data transmission time. At RF and microwave frequencies, settling time is particularly important, as it refers to the time the DUT or test system requires after each change—such as switching on or off, or adjusting power levels.
Stability refers to the consistency of results across tests and over time. Good stability doesn’t necessarily mean high accuracy, as accuracy is determined by the instrument's performance, while stability ensures consistent results regardless of accuracy. For each instrument, stability may vary depending on the measurement or mode, so it’s important to review product specifications or consult the manufacturer. In some cases, stability can be improved by averaging more measurements or using algorithms that closely approximate standard measurement results. Minimizing changes in measurement settings—like center frequency, span, or attenuation level—can also enhance stability.
1.2 Overview of the Relationship Between the Three
Understanding the testing requirements of the DUT and commercial constraints can help you assess the relative importance of performance, speed, and stability. Once you’ve identified the key priorities and their level of importance, it becomes easier to determine how they interact and influence the overall system. Tables 1, 2, and 3 summarize these relationships under different scenarios—when performance, speed, or stability is either high or low.
In Tables 1 and 3, there is an important secondary relationship between stability and performance, which stems from measurement uncertainty. When dealing with uncertainty, some system developers create an “error budget,†whose size depends on the difference between the test requirements and the system's uncertainty. The two main contributors to uncertainty are absolute accuracy (related to performance) and measurement consistency (related to stability). If an instrument has high absolute accuracy, it can tolerate lower stability. Conversely, if the instrument provides consistent results, it can afford lower accuracy.
1.4 Many Requirements Are “Highâ€
Meeting multiple “high†requirements—such as high speed and high stability, or high performance and high speed—often requires sophisticated instruments, which tend to be more expensive than simpler alternatives. However, many high-performance instruments come with built-in hardware accelerators that speed up tasks like averaging or calibration. Some models even include multiple algorithms for calculating parameters such as adjacent channel power (ACP).
If all three factors—performance, speed, and stability—are considered “high,†then every component of the system must be carefully evaluated: test equipment, switching subsystems, cables, connectors, and more. While the best solution might not be cheap, it often comes with additional features and long-term benefits that justify the investment.
Tip 2: Examine the Nature and Characteristics of the DUT
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