Real-time spectrum analysis tools for complex signal measurement needs
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The characteristic of digital RF equipment is that the RF signal no longer uses simple AM or FM modulation, but uses highly complex time-varying modulation, and the signal changes greatly over time. This article will discuss the relationship between time and frequency in the carrier signal and explain: In order to truly measure the characteristics of the signal changing over time, engineers need to use real-time instruments to trigger unexpected events, seamlessly capture and analyze the accumulated data over a period of time. With the rapid popularization of wireless devices and systems, people are beginning to pay more attention to "ubiquitous wireless signals." The number of cellular phone users has grown rapidly in the past decade, but it is not only the growth of cellular phone users that drives the future development of wireless, but also many other types of wireless devices, systems and applications. Laptops with built-in wireless networking modules are now common, and various game devices are also equipped with wireless controllers. Consumer devices of various models and configurations make people full of longing for the beautiful wireless world of the future. Whether it is today's inventory tracking or tomorrow's wireless digital wallets, RFID technology will be widely used. It seems that there is news about new wireless devices or services every day. Some business publications also report on the latest wireless technologies in the process of standardization. Wireless innovation is accelerating at an unprecedented rate. 对无线设备和系统的潜在性需求已经不是什么新鲜事了。最终用户都将是渴望彼此间交流的移动用户。自从Chester Gould于1946年首次提出内置双向无线链路的腕表并藉他的Dick Tracy漫画闻名于世后，这一概念至今仍未实现。 Removing the various cables around us has always been our wish. Signal cables are very expensive, and most people want to remove the messy bundles of cables behind their computers or home entertainment systems. Therefore, people's motivation for wireless demand is old. Now we have entered a new era that can meet these requirements faster and better. The foundation of this new era is digital radio frequency (digital RF) technology. In this emerging digital RF era, wireless devices and systems that can bring huge benefits to end users are becoming more and more popular. The digital RF era creates excellent development opportunities for latecomers with strong digital technology capabilities that can accelerate innovation in the wireless field. Digital RF technology will be an important booster to accelerate innovation in the wireless field. Digital RF Technology Digital RF technology allows us to achieve the "wireless everywhere" dream faster than in the past. Although there are already wireless systems using digital technology, for systems that already use traditional analog technology architecture, digital functions can only be added gradually. Future digital RF devices and systems will be optimized around the benefits brought by digital technology. All the benefits brought to people by the digital world can be reproduced in the wireless field. The development and change of the digital world is very fast, especially the price/performance improvement that people expect is very fast. Rapidly reducing prices and increasing flexibility are the driving forces of rapid innovation. We can now see all these benefits appearing in the wireless field.
Figure 2: Two instrument screens showing the results of an in-depth
RTSA analysis at two slightly different time points
.
Digital RF functions are implemented by digital RF chips. Instead of relying on complex mixed signals or special semiconductor processes, digital RF chips use the same technology used in computers, PDAs, and other digital devices (Figure 1). As the RF field evolves toward digital RF chips, we will see similar phenomena as in the development of the digital chip business. As traditional digital semiconductor companies apply their core technologies to these new application areas, these companies will play an important role in the new areas. The price/performance of digital RF is improving at a rate comparable to that of digital technologies in the past. As costs drop dramatically, wireless capabilities will be applied to more areas than ever before. A look at the development trajectory of RFID technology will give you an idea of how cost reductions are enabling new applications. These cost/benefit trends will further enhance the capabilities of future devices, while also increasing device complexity accordingly. Digital intelligence can easily address the traditional challenges of wireless communications in three major areas: enhanced spectrum utilization/interference mitigation, improved information security, and reduced power consumption. Although many of the technologies that meet these challenges in digital RF have been around for a long time, they can now be integrated into ultra-low-cost, increasingly popular end-user devices and systems. Since RF spectrum is a scarce resource, it is very important to improve spectrum utilization. The cellular telecommunications industry has demonstrated this well. Although simple time division multiplexing systems have been used for a long time, the intelligence contained in digital RF technology can enable system/network designers to embrace the concept of better sharing time with other spectrum users. For example, time-based spectrum resource sharing technology has been widely used to allow WLAN and Bluetooth systems to coexist peacefully in the same combined device, by coordinating the time of transmission. WLAN and Bluetooth can use the same frequency band. More complex systems can use adaptive wireless technology to detect environmental conditions, understand user needs and control wireless system operations, and decide when, how and where to send wireless signals. Security is very important because the wireless environment is shared with other users. This is where the intelligence of digital RF technology comes in handy. Transmission occurs only when needed, and the burstiness of the actual bit transmission in a bursty communication system greatly reduces the chances of detection and interception. Signal characteristics can change at any time, so unauthorized receivers will not be able to follow and decipher the signal. Extremely robust security strategies are now available at a fraction of the cost. Since end users are generally mobile, power consumption will be a critical factor in many wireless systems. Digital RF technology provides the necessary intelligence to meet this requirement. By measuring the ideal power required to reach the target receiver, the device only sends the power necessary to reach the target receiver. When necessary, the system remains in a "sleep" state, running only the wireless radio, thereby achieving further power savings. A matter of time Today’s RF signals no longer use simple AM or FM modulation, but rather highly complex modulation methods that vary greatly over time. Digital RF technology brings with it RF signals that vary over time. This article will not only discuss the inverse relationship between time and frequency in the carrier signal, but will also discuss in more detail the time changes that are not at the same level as the intelligence in the signal. These changes can reflect different types of modulation, such as the signal jumping to a new frequency or sending a signal in short pulses. In order for the system to work properly, the time points at which these changes occur must be very precise. In addition, because the current instantaneous RF signals use complex modulation methods, they often have some side effects that are in line with their own characteristics, such as random transient phenomena, interference, abnormal switching, etc. One important property that all of these phenomena have in common is time. Changes that occur in real time (sometimes microseconds, sometimes seconds, minutes, or even longer) can all be represented as changes in the frequency domain. This was not possible with previous generations of RF technology, but it is now a reality. Timing can no longer be ignored. RF design engineers are under tremendous pressure Engineers who design, inspect, manufacture, and maintain RF equipment are finding it increasingly difficult to meet new challenges. Shorter design cycles and the increased signal complexity caused by digital RF technology have brought new problems to the design environment. Digital technology is based on the premise that events must occur at precise points in time. The basic principles of RF systems also believe that the main problem of RF systems is the control of frequency occupancy. Digital RF technology uniquely combines these two points. Frequency occupancy must still be controlled, and the events that affect the performance of signal characteristics must occur at precise points in time. Traditional measurement instruments like spectrum analyzers were developed in another era. Like the famous superheterodyne analog radios of history, they tune narrowband receivers in the band of interest to detect the ratio of signal power to frequency. Their internal scanning architecture means that they cannot characterize how the signal frequency changes over time. As a result, the complexity and difficulty of testing increases as designers assemble various instruments and design specialized test modes into the equipment to try to understand the signal behavior. To better characterize the modulation quality, the industry uses vector signal analyzers (VSAs) to complement the shortcomings of traditional spectrum analyzers, but VSAs do not have enough dynamic range to fully characterize the signal changes over time. Test environments can also include custom designed test equipment to meet special test challenges. These test setups are time-consuming and labor-intensive to set up. In addition, some transient states may be missed, causing eventual system or network failures. If these errors are not detected before the system is sent to the user, they will be very expensive to correct. 随着技术的不断发展，迫切需要一款能够反映目前信号随时间变化这种本质特性的工具。工程师们需要能够对未知事件进行触发、无缝捕捉以及分析过去一段时间内累积数据的实时RF仪器。这样的仪器可以显著缩短设计时间、节省最终设备中测试建立和特殊模式的成本。而且这些仪器还能帮助工程师尽早捕捉到问题，避免潜在性故障发生在实际使用中而损害用户利益，甚至某些情况下造成网络瘫痪。 Fortunately, a new generation of real-time spectrum analyzers (RTSA) can help engineers deal with a variety of RF signals. Unlike traditional swept spectrum analyzers, advanced RTSAs can use extremely robust real-time bandwidth and high-speed digital signal processing to fully analyze target signals. Because these signals are more complex and more difficult to predict, only the trigger, capture, and analysis functions provided by RTSA can help designers understand the various signal behaviors that change over time, from frequency hopping to EMI transients. Trigger, capture, analyze Today's advanced RTSAs have powerful triggering capabilities that can capture RF events in both the frequency and time domains. These triggers can store the captured RF signal in a continuous time record in memory for further analysis. Some RTSAs also provide power triggering, which will be triggered when the total power of all signals in the analysis span exceeds a user-defined threshold. Other analyzers can provide frequency mask triggering, which will be triggered when the signal frequency, amplitude or bandwidth changes discretely, or when a signal appears or disappears at a certain frequency point. Since the fault signal may only occur once an hour or even once a day, some advanced RTSAs provide a continuous trigger mode to continuously monitor the spectrum of interest, but only capture when the user-defined trigger conditions are met. Once triggered, the analyzer will capture the spectrum activity content and record the time, and store the relevant information in the memory. If the event that meets the conditions occurs again, it will return to the trigger state. This can make efficient use of the capture memory and ensure that the content in the memory is useful information, so that users can capture instantaneous events at any time even if they are away from the instrument. 由于RTSA能够非常方便地根据时域或频域中的特殊事件对动态和瞬态信号进行触发，因此工程师能够凭藉RTSA可靠地识别和捕捉单个事件或复杂的序列事件，并将它们记录于分析仪的存储器中。 By seamlessly capturing real-time signal behavior, these analyzers support a wide range of powerful analysis tools. RTSAs can display spectrograms that record frequency and power versus time. Frequency, time, and modulation domains are all visible in the time-correlated display, and the spectrogram itself summarizes long-term observations, allowing for an intuitive three-dimensional visual of signal behavior over time that is not visible in traditional frequency domain views. The following example illustrates the power of real-time analysis. Figure 2 is a snapshot of a real-time spectrum analyzer screen showing a situation when the WLAN link performance is poor. The RTSA allows the user to move the cursor between capture time records to control the analysis point. The horizontal axis of the spectrum graph represents frequency and the vertical axis represents time, thus providing a global visual effect. The graph uses color to indicate the magnitude of the signal power, with brighter colors indicating greater power. On the left side of the screenshot, you can see a frequency hopping Bluetooth signal next to the target WLAN signal. You can also see the energy leaking from a nearby microwave oven. When the cursor is placed on this point in time, the WLAN link is functioning properly. The spectrum and constellation of the WLAN signal both indicate that the signal is properly synchronized and the EVM performance is good. On the right side of the screenshot, the cursor has moved forward slightly. At this point in time, the Bluetooth signal jumps right on top of the WLAN signal. This is reflected as a signal spike in the spectrum of the WLAN signal. This Bluetooth signal interferes with the useful WLAN signal, so that the WLAN signal cannot be decoded correctly. The synchronization loss can also be seen in the corresponding constellation diagram. The end user device will experience a loss of WLAN signal and even a loss of Bluetooth signal. As shown in the example above, a simple spectrum plot that represents an average over a period of time cannot fully describe the current RF signal environment. A sufficient time scale is required to accurately describe what is happening in the signal environment. In addition, cordless phones, wireless game controllers, and HVAC control systems will all compete with these devices for spectrum resources. As wireless devices become more popular, this situation will become more and more complicated. RTSA is an excellent instrument for testing, measuring and checking broadband, dynamic RF signals. It can observe the frequency domain, time domain and modulation domain from a time-related perspective, and has comprehensive triggering and analysis functions. Therefore, engineers can use RTSA to fully observe RF signals, complete signal characterization, and quickly solve problems. Today's complex RF signals and RTSAs are important for RF spectrum monitoring, interference tracking, signal characterization, and EMI diagnostics. They can easily characterize, troubleshoot, and debug RF devices, and help ensure that potential system instabilities are eliminated in the design, effectively preventing faulty systems from entering the market and disrupting the already crowded and disordered RF spectrum. As technology continues to evolve and change in the wireless field, time analysis will prove to be a major strength of real-time spectrum analyzers. By: Rick King,
Vice President and General Manager,
Real-Time Spectrum Analyzer Product Line,
Tektronix
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