Differences between receivers and spectrum analyzers in EMC testing applications

When selecting EMC test equipment, we often encounter this question: What is the difference between an EMI receiver and a spectrum analyzer? Why should we choose a receiver for EMI testing?

Based on CISPR16-1 (GB/T6113) and GJB152, this article analyzes the testing principle of the receiver and provides a reference for the selection of receiver and spectrum test equipment - a receiver that meets the standard is the correct choice for EMC conformity assessment testing. 

1. Principle differences between receivers and spectrum analyzers 

Spectrum analyzer is the main tool for current spectrum analysis. In particular, swept-frequency heterodyne spectrum analyzer is the mainstream of today's spectrum analyzers. Frequency-sweep measurement technology is used to obtain heterodyne signals through frequency-swept signal sources for frequency domain dynamic analysis. The receiver is the main tool for EMC testing. It is based on the point frequency method and applies the principle of local oscillator tuning to test the level value of the corresponding frequency point. The scanning pattern of the receiver should be obtained by step 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 reception and analysis method. The following is an analysis of the main differences between heterodyne spectrum analyzers and receivers. 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; IF filter; spurious signal and accuracy.

1.2 Front-end processing of input RF signals 

Receivers and spectrum analyzers process signals differently at the input end. The signal input end of the spectrum analyzer usually has a relatively simple set of low-pass filters, and the receiver needs to 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 signals. Due to the influence of harmonics, intermodulation and other spurious signals of RF signals, spectrum analyzer and receiver test errors are caused. Compared with a spectrum analyzer, a receiver requires higher accuracy, which requires an additional preselector at the front end of the receiver to improve selectivity. The selectivity of the receiver is clearly specified in GB/T6113 (CISPR16). 

1.3 Adjustment of local oscillator signal 

In today's EMC measurements, people not only require the ability to manually tune and search frequency points, but also 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 within a certain frequency range. The spectrum analyzer implements frequency sweep measurement through a frequency sweep signal source. The frequency sweep signal source is usually controlled by a ramp or sawtooth wave signal, and the desired mixing output signal is obtained by scanning within a preset frequency span. The frequency scan of the receiver is step-by-step, discrete, and a discrete point frequency test.

The receiver performs level measurement at each frequency point under the control of the processor according to the frequency interval preset by the operator. 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 receiver is different. It is usually defined that the resolution bandwidth of the spectrum analyzer is the 3dB bandwidth of the amplitude-frequency characteristics, while the IF bandwidth of the receiver is the 6dB bandwidth of the amplitude-frequency characteristics. When the spectrum analyzer and receiver are set to 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 It can be seen from the amplitude-frequency characteristics of the spectrum analyzer and the receiver IF filter that when the spectrum analyzer 3dB bandwidth B3 and the receiver 6dB bandwidth B6 values ​​are set to the same , the actual amplitude-frequency characteristics of the signals passing through the two filters are different. According to EMC standards, whether it is civilian or military standards, the bandwidth should be 6dB. 

1.5 Detector 

According to the EMC standard, test receivers are required to have peak, quasi-peak and average detectors. General-purpose spectrum analyzers generally have peak and average detectors but no quasi-peak detectors. The limits in the EMC standard usually include quasi-peak. limit. 

1.6 Accuracy 

Judging from the signal processing method of the receiver and the EMC test requirements, the receiver has higher accuracy and lower spurious response than the spectrum analyzer. 

2. Differences between receivers and spectrum analyzers in EMC testing applications

In the current market, we can see some receivers modified from spectrum analyzers. If they are used for testing, they must comply with the corresponding standards. For civil EMC testing, the measurement equipment standard is CISPR16-1 (GB/T6113). For military standard testing, the standard basis for measuring equipment is GJB152 (MIL-STD462). Based on the principle analysis in the previous chapter, we can summarize the following simple formula: universal spectrum analyzer + preselector + 6dB intermediate frequency filter, three types of detectors + point frequency test function + high-precision signal processing = receiver on the left side of the formula. It is not a simple list. Each item has special requirements. At the same time, according to the design principle, the instrument manufacturer's instructions must be followed during use to meet the corresponding requirements. 

2.1 Preselector 

The frequency band must be selected according to the manufacturer's instructions. If the sweep span setting is not appropriate, the fixed filter and the following filter in the preselector will not work properly. 

2.2 Point frequency test and detector 

When testing according to EMC standards, in many cases it is necessary to conduct real-time testing on certain fixed frequency points. 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 raise and lower the antenna, and quickly observe and record the level value at that point in real time. In this case, a receiver with a point frequency test function can complete it conveniently and accurately, but a general spectrum analyzer cannot accurately test the level change of a single frequency point in real time. The spectrum analyzer for EMI testing must have added functions and can When the scan span (SPAN) is zero, the test can be carried out quickly and accurately, not only the peak value is displayed, but also the quasi-peak value and average value must be displayed. According to the standard CISPR16-1, when performing impulse response tests on peak, quasi-peak and average detectors, the receiver can perform point frequency monitoring on a single frequency to determine whether it meets the standard. However, it is very difficult to complete this measurement with a general-purpose spectrum analyzer. of. Impulse response measurement is an important indicator to judge whether the receiver is suitable or not. Those that do not meet the standards can only be used as pre-test equipment.

3. Conclusion

According to the principle analysis of spectrum analyzers and receivers made in this article, the receiver designed for EMC testing is the right choice for determination and certification testing. Many pre-test instruments, such as spectrum analyzers with built-in 6dB IF bandwidth, quasi-peak and average detectors, or spectrum analyzers with preselectors, cannot meet the requirements of the receiver.