Electromagnetic compatibility EMI receiver and spectrum analyzer difference_Century Wisdom (Suzhou) Testing Technology Co., Ltd

English

Home page

Technical column

CASES

Electromagnetic compatibility EMI receiver and spectrum analyzer difference

Release time:

2010-05-31 00:00

Source:

　　Differences Between Receivers and Spectrum Analyzers

　　——Selection of EMC measuring equipment

　　When selecting EMC test equipment, we often encounter such problems: What is the difference between an EMI receiver and a spectrum analyzer? Why should a receiver be used for EMI testing? This article is based on CISPR16-1 (GB/T6113) and GJB152, for receivers Analyze the test principle, analyze the receiver and provide reference for the selection of spectrum test equipment - the receiver that conforms to the standard is the only choice for EMC conformity assessment test.

　　1. The principle difference between receiver and spectrum analyzer

　　Spectrum analyzer is the main tool for current spectrum analysis, especially the swept frequency heterodyne spectrum analyzer is the mainstream of spectrum analyzers today. It applies sweep frequency measurement technology and obtains heterodyne signal through frequency sweep signal source for frequency domain dynamic analysis.

　　The receiver is the main tool for EMC testing. Based on the point frequency method, the level value of the corresponding frequency point is tested by applying the principle of local oscillator tuning. The scanning mode of the receiver should be obtained by stepping spot frequency tuning.

　　1.1 Basic schematic diagram

　　According to the working principle, spectrum analyzers and receivers can be divided into two categories: analog and digital. Heterodyne analysis is currently the most widely used receiving and analyzing method. The main difference between the heterodyne spectrum analyzer and the receiver is analyzed below.

　　From the schematic diagram, the spectrum analyzer is similar to the receiver, but the spectrum analyzer and the receiver are quite different in the following aspects: front-end preselector; local oscillator signal scanning; intermediate frequency filter; spurious signal and accuracy.

　　1.2 Front-end processing of input RF signal

　　Receivers and spectrum analyzers process signals differently at the input.

　　The signal input of the spectrum analyzer usually has a set of relatively simple low-pass filters, and the receiver should use a preselector with strong anti-interference ability for broadband signals. It usually includes a set of fixed bandpass filters and a set of tracking filters to complete the preselection of the signal.

　　Due to the influence of harmonics, intermodulation and other spurious signals of RF signals, spectrum analyzer and receiver test errors are caused. Compared with the spectrum analyzer, the receiver needs higher precision, which requires an additional preselector at the front end of the receiver than the ordinary spectrum analyzer to improve selectivity.

　　The selectivity of the receiver is clearly specified in GB/T6113 (CISPR16).

　　1.3 Adjustment of local oscillator signal

　　In the current EMC measurement, people not only need to manually tune and search for frequency points, but also need to quickly and intuitively observe the frequency level characteristics of the EUT. This requires that the local oscillator signal can not only test the specified frequency point, but also scan in a certain frequency range.

　　The spectrum analyzer realizes the frequency sweep measurement through the frequency sweep signal source. The frequency sweep signal source is usually controlled by a ramp wave or a sawtooth wave signal, and sweeps within a preset frequency span to obtain the desired mixed frequency output signal.

　　The frequency scanning of the receiver is stepping and discrete, and it is a discrete point frequency test. According to the frequency interval preset by the operator, the receiver performs level measurement at each frequency point through the control of the processor, and the displayed test result curve is actually the result of a single point frequency test.

　　1.4 IF filter

　　The bandwidth of the IF filter of the spectrum analyzer and the receiver is different.

　　It is usually defined that the resolution bandwidth of the spectrum analyzer is the 3dB bandwidth of the amplitude-frequency characteristic, and the IF bandwidth of the receiver is the 6dB bandwidth of the amplitude-frequency characteristic. When the spectrum analyzer and the receiver set the same level of bandwidth, their actual test values for the signal are different. The specific performance is as follows:

　　Spectrum Analyzer RBW Filter Receiver IFBW Filter

　　From the amplitude-frequency characteristics of the spectrum analyzer and the IF filter of the receiver, it can be seen that when the 3dB bandwidth B3 of the spectrum analyzer and the 6dB bandwidth B6 of the receiver are set to be the same, the amplitude-frequency characteristics of the signals actually passing through the two filters are different . According to the EMC standard, whether it is civilian or military standard, the bandwidth should be 6dB.

　　1.5 Geophone

　　According to the EMC standard, the test receiver is required to have peak, quasi-peak and average detectors. General-purpose spectrum analyzers generally have peak and average detectors, but no quasi-peak detectors. Limits in EMC standards usually include quasi-peak limit.

　　1.6 Accuracy

　　From the receiver's signal processing method and EMC test requirements, the receiver has higher precision and lower spurious response than the spectrum analyzer.

　　2 Differences between receivers and spectrum analyzers in EMC test applications

　　In the current market, we can see some receivers transformed from spectrum analyzers. If they are used for testing, they must meet the corresponding standards. For civil EMC testing, the standard basis for measuring equipment is CISPR16-1 (GB/T6113). For military standard testing, the standard basis for measuring equipment is GJB152 (MIL-STD462).

　　According to the principle analysis in the previous chapter, we can summarize the following simple formula:

　　Universal spectrum analyzer + preselector + 6dB IF filter, three detectors + point frequency test function + high precision signal processing = receiver

　　The items on the left side of the formula are not simply listed, and each item has special requirements. At the same time, according to the design principle, it must be operated according to the instructions of the instrument manufacturer in use to meet the corresponding requirements.

　　2.1 Preselector

　　The selection of the frequency band must be based on the manufacturer's instructions. If the sweep span is not set properly, the fixed filter and follower filter in the preselector will not work properly.

　　2.2 Spot Frequency Test and Geophone

　　When testing according to EMC standards, some fixed frequency points need to be tested in real time in many cases. For example, when conducting radiation interference tests, many test engineers need to select an appropriate frequency point according to standard requirements, rotate the turntable and lift the antenna, and quickly observe and record the level value of this point in real time. In this case, the receiver with the point frequency test function can be completed conveniently and accurately, but the general spectrum analyzer cannot accurately test the level change of a single frequency point in real time, and the spectrum analyzer for EMI testing must have additional functions, which can When the scan span (SPAN) is zero, it can quickly and accurately test, not only the peak value display, but also the quasi-peak value and average value.

　　According to the standard CISPR16-1, when testing the impulse response of the peak, quasi-peak and average detectors, the receiver can monitor the point frequency of a single frequency to judge whether it meets the standard, but it is very difficult for a general-purpose spectrum analyzer to complete this measurement of. Impulse response measurement is an important indicator for judging whether the receiver is suitable or not, and those that do not meet the standard can only be used as pre-test equipment.

　　3 Conclusion

　　According to the principle analysis of spectrum analyzer and receiver in this paper, the receiver designed for EMC test is the only choice suitable for judgment and certification test. Many pre-test instruments, such as a spectrum analyzer with built-in 6dB IF bandwidth, quasi-peak and average detectors, or a spectrum analyzer plus a preselector, cannot fully meet the requirements of the receiver and can only be used for factory pre-test.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

www.300.cn nanjing SEO

Business License

全部

全部
产品管理
新闻资讯
介绍内容
企业网点
常见问题
企业视频
企业图册