Functions, technical characteristics and development trends of modern spectrum analyzers

The spectrum analyzer is known as an oscilloscope in the RF field. Modern spectrum analyzers not only have traditional spectrum analysis functions, but also can integrate power meters, frequency meters, scalar / vector network analyzers, signal analysis, communication testers, etc. through expansion options. The main function of the instrument can be called the master of RF testing. With a high-performance spectrum analyzer, it can complete most of the RF testing and signal analysis functions. The emergence of modern real-time spectrum analyzers further expands the application field of spectrum analyzers to fast-change transient signal testing and wide real-time bandwidth signal analysis.

The basic function of the spectrum analyzer is to measure the amplitude / frequency response of the signal, such as spectrum component analysis, distortion measurement, modulation signal spectrum analysis, signal attenuation measurement, and electronic component gain measurement. The basic working principle is that the frequency of the swept local oscillator is scanned within a certain range with the output of the sawtooth wave generator, so that the input signals of different frequencies are mixed with the local oscillator and then sequentially fall into the passband of the resolution bandwidth filter. After amplification and detection, it is added to the Y amplifier. The vertical offset of the bright spot on the screen is proportional to the amplitude of the frequency component. As the scanning voltage modulates the oscillator, it also drives the X amplifier, so that the spectrum of the measured signal can be displayed on the screen.

This is the oldest working principle of the spectrum analyzer. With the development of modern science and technology, especially the continuous improvement of digital signal processing technology and AD sampling technology, modern spectrum analyzers are mostly designed using software radio ideas: build a versatile hardware platform, The realization of software functions makes modern spectrum analyzers have the characteristics of "software-defined instruments". By maintaining the hardware platform basically unchanged, by updating the software, the spectrum analyzer can be integrated with many instruments, such as receivers and power meters. , Most of the functions of frequency meters and network analyzers have greatly expanded the measurement capabilities and application fields of spectrum analyzers. Modern spectrum analyzers are developing rapidly. A variety of spectrum analyzers have appeared in response to different application requirements. Different types of spectrum analyzers have very different performance indicators and functional configurations. However, starting from the difference in working mechanism and main application fields, modern spectrum analyzers can be divided into two types: swept frequency spectrum analyzers and real-time spectrum analyzers.

Compared with the working principle of the ancient spectrum analyzer, the most significant change of the basic working principle of the modern frequency sweeping spectrum analyzer is that the intermediate frequency filter is AD sampled, and the resolution bandwidth filtering, detection, and video filtering all use digital signal processing. achieve. Because the hardware structure before AD sampling is a general superheterodyne receiver structure, and the specific function of the instrument after AD sampling depends entirely on the software program, so modern spectrum analyzers have the characteristics of "software-defined instruments". The spectrum analyzer can have vector signal analysis, signal demodulation of various modulation systems, modulation degree analysis, communication measurement and other functions. At the same time, as long as it pays a small price, purchase some auxiliary measurement hardware, such as standing wave bridge, tracking source, receiving antenna, etc., the spectrum analyzer can complete standing wave ratio test, component transmission characteristic test, field strength test, transmission line test, antenna Test and other functions.

Sweep frequency spectrum analyzer is still the mainstream of current spectrum analyzer. According to different application fields, it can be further divided into desktop spectrum analyzer, handheld spectrum analyzer, VXI bus spectrum analyzer, PXI bus spectrum analyzer, LXI bus spectrum analyzer, etc.

The desktop spectrum analyzer has the characteristics of high performance indicators and powerful functions. It has a wide range of application fields, such as measurement and inspection laboratory, EMC / EMI laboratory, satellite reception system testing, radar system testing, radio communication system testing, and base station maintenance. Their figure.

The typical characteristics of portable spectrum analyzers and handheld spectrum analyzers are low price, small size, light weight, and easy to carry. Its performance index is close to the level of a medium desktop spectrum analyzer, and its functional configuration is also very rich. Taking RS FSH3 as an example, the measurement sensitivity of -120dBm and the phase noise of -90dBc / Hz are two orders of magnitude worse than those of desktop instruments. However, in terms of functional configuration, in addition to basic spectrum analysis functions, FSH3 provides power meter options, standing wave ratio bridges, high gain directional antennas, vector transmission and reflection measurement options, receiver mode options, etc. With the support of the software, FSH3 can be used for power test, standing wave ratio test, field strength test and electromagnetic interference location, transmission line test and fault location, electromagnetic compatibility diagnosis, etc.

Portable spectrum analyzers and handheld spectrum analyzers are particularly suitable for field use. When the functions and performance indicators meet the requirements, due to the advantages of price and volume, it can completely replace the use of desktop spectrum analyzers. Therefore, it has developed rapidly in recent years and has become an important direction for the development of swept frequency spectrum analyzers.

VXI bus spectrometer, PXI bus spectrometer, LXI bus spectrometer, etc. belong to virtual instrument spectrum analyzer products, and have an important position in the construction of various comprehensive test platforms. VXI / PXI bus spectrum analyzers are chassis-inserted card-type instruments. Due to the large size of broadband spectrum analyzers, broadband spectrum analyzers above 3 GHz are currently rare. Relatively speaking, the LXI protocol does not strictly control the size and appearance of the instrument, and it is easy to expand on the basis of desktop spectrum analyzers and handheld spectrum analyzers. At the same time, LXI network-based distributed test structure is more suitable for modern test systems. Development direction.

The real-time spectrum analyzer is a new type of spectrum analyzer developed in recent years. Compared with the swept frequency spectrum, the significant advantage of the real-time spectrum analyzer is that it has higher data processing speed and signal analysis bandwidth, and various triggering methods. It is suitable for instantaneous rapid frequency changes. In the state-of-the-art system test, the hardware structure of the real-time spectrum analyzer and the modern swept-frequency spectrum analyzer are almost the same.

1. The intermediate frequency processing of the real-time spectrum analyzer has the characteristics of "real-time processing". The frequency spectrum analyzer adopts the "spectral scanning" method to obtain the signal spectrum. One spectrum scan can only obtain part of the spectrum data in the entire frequency sweep width. A certain time interval is allowed between two scans. For two scans, The change of the signal cannot be detected by the swept spectrum analyzer. The real-time spectrum analyzer adopts the "real-time signal processing" method to obtain the signal spectrum. One spectrum scan can obtain the spectrum data in the entire sweep width. Its excellent data processing ability combined with a flexible trigger mode can continuously capture the instantaneous input signal. Change information.

2. The "real-time processing bandwidth" of the real-time spectrum analyzer is wider. In order to adapt to the capture and analysis of broadband fast-changing signals, the "real-time processing bandwidth" of the real-time spectrum analyzer reaches 80MHz ~ 110HMz, and the signals within the entire bandwidth can be analyzed in real time. The processing bandwidth (intermediate frequency bandwidth) of a swept spectrum analyzer is generally below 40MHz, and allows a certain time interval between two spectrum scans. Therefore, the real spectrum analyzer requires much higher hardware performance than the swept spectrum analyzer. .

Typical applications of real-time spectrum analyzers are rapid burst interference signal testing, frequency hopping system testing, frequency-agile radar system testing, electronic countermeasure system testing, etc.

Beijing Aerospace Measurement and Control Technology Development Company is located in Shijingshan Industrial Park, Beijing. It is the China Aerospace Science and Technology Measurement and Control Center, Weapon Equipment Testing and Comprehensive Support Technology Center, and the National Defense Science and Technology Industrial Automation Test Technology Research and Application Center. R & D and production of chemical equipment, general test platform for missiles, system integration, industrial control, basic test and measurement instrument products and software and information products. The company has made outstanding achievements in the field of measurement and control represented by weapon equipment testing and comprehensive support technology. Its products and services cover military fields such as troops, aerospace, aviation, weapons, nuclear industry, ships, electronics, and civil aviation aircraft, oil, automobiles, Chemical, electric power, railway and other civil fields.

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