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1. Basic knowledge of EMI receivers
1. EMI receiver, also known as electromagnetic interference measuring instrument, is the most widely used and most basic measuring instrument in electromagnetic compatibility testing.
2. An EMI receiver is a primary instrument for measuring interference emissions. It is essentially a frequency-selective measuring instrument, which can select the preset frequency component of the interference signal input by the sensor with a certain passband, display and record it, and continuously change the set frequency to obtain the frequency spectrum of the signal .
3. The EMI receiver can be regarded as a voltmeter that can be tuned, can change the frequency, and can measure the amplitude precisely.
EMI conducted radiated interference test system EMI-9KB
2. EMI receiver working principle
When the EMI receiver measures the signal, the instrument is first tuned to a certain measurement frequency fi, which enters the mixer after passing through the high-frequency attenuator and high-frequency amplifier, and is mixed with the frequency f1 of the local oscillator to generate many mixed frequency signals . After the intermediate frequency filter, only the intermediate frequency fo=f1-fi is obtained. The intermediate frequency signal is detected by the envelope detector after passing through the intermediate frequency attenuator and intermediate frequency amplifier, and the intermediate frequency is filtered to obtain the low frequency signal A(t). Further carry out weighted detection on A(t), select the detector according to the needs, and obtain the peak value, effective value, average value or quasi-peak value of A(t). After these values are amplified by low frequency, they can be pushed to the meter indication or displayed on the digital tube screen come out.
EMI receiver measures the signal voltage input to its port. To measure field strength or interference current, a transducer is needed. With the help of its conversion coefficient, the measured The port voltage is converted into field strength (unit u V/m or dB V/m), current (unit A, dB u A) or power (unit w, dBmW). Depending on the measurement object, the transducer can be an antenna, a current probe, a power absorbing clamp or a power supply impedance stabilization network, etc.
3. Detection method of EMI receiver
1. Average value detection: Its biggest feature is that the charge and discharge time constants of the detectors are the same, especially suitable for the measurement of continuous waves. The integral time constant is very long and can reach the second level.
2. Peak detection: Its charging time constant is very small (100ns), and even a very narrow pulse can be charged to a stable value quickly. After the intermediate frequency signal disappears, due to the large discharge time constant of the circuit (up to 100s), the output voltage of the detector can be kept at the peak value for a long time. It is preferred, because many military equipment can cause explosion or malfunction of digital equipment as long as a single pulse excitation, without the need to pay attention to the accumulation of time like audio equipment.
3. Quasi-peak detection: The charging time constant of this kind of detector is between the average value and the peak value (the charging time constant is about 1ms, and the discharging time constant is about 160ms). It is related to the pulse amplitude and pulse repetition frequency, and its output is consistent with the effect of interference on hearing. Because the interference in the early CISPR research is the interference in the broadcasting system, since the quasi-peak detector is very suitable for describing the characteristics of radio interference noise, So CISPR publications recommend the use of quasi-peak detectors.
4. Effective value detection: Random noise refers to the noise emitted by some electronic components when they are working, and the noise caused by crosstalk during information transmission. It is characterized by disorder, and some random noise (such as thermal noise, shot noise, etc.) obeys the normal distribution law. For them, the peak value is worthless. Therefore, effective value and average value detection are usually used. In EMI testing, effective value detection is used the least.
4. Comparison of EMI Receiver and Spectrum Analyzer
Through the introduction to the EMI receiver, we know that it has similarities with the spectrum analyzer, that is, both adopt the superheterodyne structure and display the amplitude of each frequency component. But they are different, mainly in the following aspects:
1. The processing of the signal at the input end of the receiver and the spectrum analyzer is different
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 anti-interference ability for broadband signals, usually including a set of fixed band-pass filters and a set of tracking filters to complete the pre-selection of the signal.
2. The frequency sweep signal is different
The frequency sweeping signal source of the spectrum analyzer is usually realized by controlling the frequency sweeping signal source through a ramp wave or a sawtooth wave signal. The frequency change is continuous, and the desired mixing output signal is obtained by scanning within a preset frequency span.
EMI receiver frequency sweep is a discrete point frequency test. The receiver operates according toThe frequency interval preset by the author is controlled by the processor to measure the level at each frequency point, and the displayed test result curve is actually the result of a single point frequency test. 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 is to require that the local oscillator signal can not only test the specified frequency point, but also scan in a certain frequency range. This is something a spectrum analyzer cannot do.
3. Different definition of IF filter bandwidth
It is usually defined that the resolution bandwidth of a 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.
4. Different detectors
Spectrum analyzers are typically peak and average detectors. In addition to this, the receiver also has a quasi-peak detector and an effective value detector.
5. The test accuracy is different
From the receiver's signal processing method and EMC test requirements, the receiver has higher precision and lower spurious response than the spectrum analyzer. According to the above comparative analysis, we can conclude the following simple formula:
Universal spectrum analyzer + preselector + 6dB IF filter + three kinds of detectors + point frequency test function + high precision signal processing = EMI receiver
This formula vividly illustrates the similarities and differences between the spectrum analyzer and the EMI receiver, but it should be noted that the items on the left side of the formula are not simply listed, and each item has special requirements. That is to say, the receiver transformed from the spectrum analyzer, If they are used for testing, the corresponding standards must be met. Those that do not meet the standard can only be used as pre-test equipment. In the current market, we can see some modified spectrum analyzers.
EMI receivers, such as spectrum analyzers with built-in 6dB IF bandwidth, quasi-peak and average oscilloscopes, or spectrum analyzers plus preselectors, etc., cannot fully meet the requirements of the receiver, and can only be used for factory pre-testing. Receivers designed for EMC testing are the only option suitable for qualification and certification testing.
5. Basic operation of EMI receiver
1. Test frequency range setting: press the sweep key, then press USE Scan Table
2. Peak/QP/AV measurement setting: first press Meas, then press the detector key to select Peak/QP/AV. Max Hold setting, press DISP.
3. After setting the parameters, press sweep, and then press run scan to test.
