In the International Electrotechnical Commission standard IEC defines electromagnetic compatibility as: the system or equipment can work normally in the electromagnetic environment, and will not cause interference to other systems and equipment. EMC includes EMI (Electromagnetic Interference) and EMS (Electromagnetic Resistance). The so-called EMI electromagnetic interference is electromagnetic noise that is not harmful to other systems in the process of performing the function. The EMS refers to the machine. The ability to be unaffected by the surrounding electromagnetic environment during the performance of the intended function.

Divided into two kinds of natural and artificial. Natural sources of interference mainly include various phenomena occurring in the atmosphere, such as lightning, snow, rain, hail, sandstorms and the like. Natural sources of interference also include cosmic noise from the sun and outer space, such as solar noise, interstellar noise, and galaxy noise. The sources of human interference are various, such as various signal transmitters, oscillators, motors, switches, relays, xenon lamps, fluorescent lamps, engine ignition systems, electric bells, electric heaters, arc welding machines, high-speed logic circuits, gate circuits, SCR inverters, gas rectifiers, corona discharges, various industrial, scientific and medical high-frequency equipment, urban noise, noise caused by electric railways, and nuclear electromagnetic pulses generated by nuclear explosions.

Difference between receiver and spectrum analyzer

——Selection of EMC measuring equipment

When selecting EMC test equipment, it often encounters the problem: What is the difference between EMI receiver and spectrum analyzer? Why should EMI test use receiver? Based on CISPR16-1 (GB/T6113) and GJB152, this paper analyzes the test principle of the receiver and analyzes the choice of receiver and spectrum test equipment. The receiver that meets the standard is the only choice for EMC conformity test.

1. Differences in principle between receiver and spectrum analyzer

The spectrum analyzer is the main tool of current spectrum analysis. Especially the sweeping heterodyne spectrum analyzer is the mainstream of today's spectrum analyzers. The sweep frequency measurement technology is applied to obtain the heterodyne signal through the 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 local oscillator tuning principle is applied to test the level value of the corresponding frequency point. The scanning mode of the receiver should be obtained by step frequency tuning.

1.1 Basic schematic

According to the working principle, the spectrum analyzer and receiver can be divided into two categories: analog and digital. Heterodyne analysis is currently the most widely used method of reception and analysis. The following is an analysis of the main differences between the heterodyne spectrum analyzer and the receiver.

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

1.2 Input RF signal front-end processing

The processing of the signal at the input of the receiver and the spectrum analyzer is different.

The signal input of the spectrum analyzer usually has a relatively simple set of low-pass filters, and the receiver uses a preselector that has strong immunity to wideband signals. A set of fixed bandpass filters and a set of tracking filters are typically included to complete the preselection of the signals.

Due to the effects of harmonics, intermodulation and other spurious signals of the RF signal, the analyzer and receiver test errors are caused. The receiver requires higher accuracy than the spectrum analyzer, which requires an additional preselector at the front end of the receiver to increase selectivity.

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

1.3 Adjustment of local oscillator signal

In today's EMC measurement, people not only need to be able to manually tune the search frequency point, but also need to quickly and intuitively observe the frequency level characteristics of the EUT. This requires that the local oscillator signal can test both the specified frequency point and the frequency range.

The spectrum analyzer performs sweep measurement by sweeping the signal source. The sweep source is typically controlled by a ramp or sawtooth signal and scanned over a preset frequency span to obtain the desired mixed output signal.

The frequency sweep of the receiver is stepped, discrete, and is a discrete point frequency test. The receiver performs level measurement at each frequency point according to the operator's preset frequency interval, 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 receiver is different.

The resolution bandwidth of the spectrum analyzer is usually defined as 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

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 is set to the same as the receiver 6dB bandwidth B6 value, the amplitude and frequency characteristics of the signals actually passing through the two filters are different. . According to the EMC standard, the bandwidth should be 6dB for both civilian and military standards.

1.5 detector

According to the EMC standard, the test receiver is required to have peak, quasi-peak and average detectors. The general spectrum analyzer generally has peak and average detectors, no quasi-peak detector, and the EMC standard mid-range usually includes quasi-peak. Limit.

1.6 accuracy

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

2 Differences between receiver and spectrum analyzer in EMC test applications

In the current market, we can see some receivers that have been modified by spectrum analyzers. If they are used for testing, they must meet the corresponding standards. For civil EMC testing, the standard 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 of 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. 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 order to meet the corresponding requirements.

2.1 Preselector

The choice of frequency band must be based on the manufacturer's instructions. If the scan span setting is not appropriate, the fixed and follower filters in the preselector will not function properly.

2.2 point frequency test and detector

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

According to the standard CISPR16-1, when the peak, quasi-peak and average detectors are tested for impulse response, the receiver can perform point frequency monitoring on a single frequency to determine whether it meets the standard, and it is difficult for the universal spectrum analyzer to perform such measurement. of. Impulse response measurement is an important indicator to determine whether a receiver is suitable or not. Non-compliant standards can only be used as pre-test equipment.

3 Conclusion

Based on the principle analysis of the spectrum analyzer and receiver, the receiver designed for EMC testing is the only choice for decision and certification testing. Many pre-tested instruments, such as the spectrum analyzer's built-in 6dB IF bandwidth, quasi-peak and average detectors, or spectrum analyzer plus preselector, do not fully meet the receiver's requirements and can only be used for factory pre-testing.