In the EMC certification test of wireless products, there are two items related to radiated emission: Radiated Emission (RE) and Radiated Spurious Emission (RSE).
RE and RSE, there is only one word difference in the name, and the test results look like twin brothers. Below are two screenshots of the test results, can you tell which one belongs to the RE test and which one is the RSE?
For EMC engineers who are familiar with traditional ITE equipment, they rarely have the opportunity to come into contact with RSE; for comrades engaged in radio frequency transceiver links, radiation spurious is easy to understand, and RE is relatively unfamiliar-drinking the same river, looking at each other Two do not know. So, why do wireless products have two radiated emission test items? Where are their differences and connections?
On this topic, we might as well start with the history of RE testing.
one,
Frequency is a key factor in the evolution of RE testing .
When engineers began developing radiated emission measurement standards for consumer electronics in the 1970s, they first considered the following characteristics:
The operating frequency of these products is about tens of megahertz to hundreds of megahertz, and most of the electromagnetic disturbance energy generated is distributed below hundreds of megahertz.
Most products are placed at a height close to the ground, and the noise signal received by the interfered equipment comes from the superposition of direct waves and space reflected waves
The external electromagnetic disturbance of these products is unintentional emission, and the antennas are all "temporary workers" - such as cables, "floating" metal parts, wiring forming loops, etc.
For equipment with a working frequency lower than 108MHz, people simplify its radiated emission to the situation that the disturbance signal radiated from the equipment under test (EUT) has controlled reflection in the surrounding space, and only the ground Apart from this reflective surface (semi-free space), there is no other reflection; what is received at the measurement antenna is the vector sum of the direct wave and the ground reflected wave. This is the two-line model for radiated emission measurements . The frequency upper limit of the measurement is 1GHz.
(two-line model for radiated emission measurements)
There are many organizations engaged in the establishment of EMC basic standards, the most famous of which are Europe's CISPR (International Special Committee on Radio Interference) and the United States' ANSI (American National Standards Institute). Around the consensus of the two-line model, the two organizations successively defined the test site, test antenna, measuring instrument, test method and limit value, with some differences in details.
open test field
As an approximation of the semi-free space model, the Open AreaTest Site (OATS) came into being. Let's look at two photos first.
(Vision)
(medium shot)
This is the metrology-grade OATS of the China Institute of Metrology in Ming Tombs Town, Changping, Beijing (special thanks to Dr. Meng Donglin for the high-resolution photos).
Since the site selection of the open field requires absolutely clean environment noise (modern metrology-grade OATS can eliminate the influence of background noise through vector-network phase-locked loop technology), it can only be built in the suburbs that are inaccessible and far away from the city center. The open field is surrounded by open space without any reflections. The rectangular white area is the paved metal ground, surrounded by triangular metal grids as the impedance gradient from the metal plate to the ground, thus approaching the ideal mirror reflection.
With the rise of civilian wireless communication services and the increasing severity of industrial radio noise, it is becoming more and more difficult to select an open field site. So, by the 1980s, testing grounds for alternative OATS began to emerge. This is the Semi-Anechoic Chamber (SAC).
semi-anechoic chamber
The main frame of the semi-anechoic chamber is a metal shielding chamber. Since it is a replacement for OATS, the ground is still an electrically continuous metal reflective ground, and in order to get rid of the reflection of radio waves on the four walls and the ceiling, these five surfaces are pasted with wave-absorbing materials to absorb the incoming electromagnetic waves. The ground is not covered with absorbing material - this is also the source of the so-called "half".
After the test site is built, how to check the performance? This involves site performance testing methods. ANSI and CISPR successively formulated NSA (Normalized Site Attenuation) inspection methods for radiated emission test sites in the early 1990s. In China, Professor Zhang Linchang of Northern Jiaotong University was the first to introduce this theory, and researched and expanded the theoretical data of NSA not defined in foreign standards. He is a veritable pioneer in the field of domestic anechoic chamber theory research.
The photo above was taken around 2000. It was the semi-anechoic chamber of the EMC Laboratory of Northern Jiaotong University at that time. The SAC, which looks a little old in the photo, was the most leading frontier in the field of EMC research in China and even in the world at that time.
measuring instrument
In the early noise measurement, the instrument used was a superheterodyne frequency-selective voltmeter, and the principle was similar to that of a radio. Later, a measuring receiver (EMI Receiver) appeared on the basis of a frequency-selective voltmeter.
There is a key component in the receiver called the detector. The circuit composition of the detector is different, and the noise detected by the receiver is also different. An important purpose of early RE testing is to protect broadcast communication from interference. In the measurement frequency band below 1GHz, quasi-peak (QP) detection can better simulate the response of the human ear to impulse noise, so when defining the detection method of radiated emission , the earliest used is QP detection.
Unit of measure
After the measuring antenna receives the noise signal, it converts the electromagnetic field into an induced current. Engineers place the measurement reference point for radiated emission at the phase center point of the measurement antenna, and the unit of measurement is electric field strength (dB(µV/m)). The ability of the antenna to transform the space field into the circuit guided wave is reflected in the antenna coefficient in the RE test.
Measuring distances and limits
Since the measurement reference point is at the measurement antenna, the measurement result of RE is related to the test distance. The predecessors defined different test distances, which are commonly referred to as 3m method, 5m method, 10m method and 30m method. What kind of test distance to use is related to the size of the test equipment and the measurement frequency. The ultimate goal is to reduce the coupling between the EUT and the measurement antenna and ensure that the measurement meets the far-field conditions. Different test distances have different limit values because of different test results.
Traditional ITE equipment and mobile phones belong to Class B products used in civilian environments. According to FCC regulations, the radiation emission test distance of Class B products is 3m; according to the CISPR22 standard, the 10m method should be used for testing. Due to the small size of the mobile phone, the distance of 3m basically meets the far-field conditions. Considering the construction cost of the test site, it is usually tested according to the 3m method, but the limit value of the CISPR 10m method is converted into the limit value of the 3m method. Since the field strength of the far field is inversely proportional to the distance, it is easy to derive the conversion relationship between the two limits:
E(10m, dBuv/m)=E(3m, dBuv/m)-10.5(dB)
antenna scan
In the two-line model, due to the different path lengths of the two waves, the phases at the final arrival at the receiving antenna are also different, and the amplitude may increase or decrease after superposition. In order to capture the maximum value of the noise emitted by the EUT radiation, the measuring antenna needs to scan back and forth at a certain height. For a test distance of 3m, the scanning height of the antenna is 1~4m.
All the above provisions together realize the RadiatedEmission test, namely the RE test.
two,
With that said, let's pause for a moment to catch our breath.
The measurement method of RE is a slow evolution process from conception to finalization. Summarizing the early RE tests discussed above, let's draw the key points:
The measurement site is OATS or SAC
The measuring instrument is a receiver
The measurement reference point is at the receiving antenna, and the measurement result is the electric field strength (dB(µV/m))
To specify the test distance
The antenna should scan up and down to catch the maximum value
Measurement frequency band below 1GHz
QP detection
Site performance test adopts NSA method
In the process of actual measurement, many disadvantages are gradually exposed. For example:
The antenna needs to scan up and down, the test time is long, the stepping accuracy of the antenna has a great influence on the test results, the requirement for darkroom space is high, and the construction cost is high
QP detection, long test time
The NSA method test is more cumbersome, and the result accuracy is not high
。。。
At this time, the clock frequency of ITE products has been getting higher and higher (far exceeding 108MHz), and the originally set 1GHz frequency measurement upper limit is not enough. When EMC engineers turned their attention to the RE measurement of the frequency band above 1GHz, they suddenly found that this is a good opportunity to restart.
After long-term research, CISPR has proposed a completely different method for RE measurement above 1GHz:
On the measurement site, use a full anechoic chamber instead of a half anechoic chamber (the radiation emission above 1GHz frequency band mainly arrives at the direct wave)
In the detection method, AV and PK replace QP
In site inspection, voltage standing wave ratio method replaces NSA
。。。
Therefore, when the RE test limits for the full frequency band are put together, the following picture will be obtained:
(CISPR limit value, 30MHz~6GHz RE test, the limit value below 1GHz is converted to 3m method according to the 10m method)
This picture is very interesting. With 1GHz as the dividing line, the left and right sides are clearly separated. What can be seen on the paper is the change of the limit value and detection method; what is behind the paper is the semi-anechoic chamber and the full radio-anechoic chamber, the vertical scanning of the antenna and the fixed height, and the comparison between test time and investment cost... When I When I saw it for the first time, what I thought of in my mind was, "Abandoned well fields, open fields"~
three,
While the RE test standards are gradually evolving, in the 1990s, wireless products represented by mobile phones also ushered in a new era of personal communication. Engineers are faced with new questions: Are existing radiated emissions tests appropriate for wireless products?
Wireless products work at high frequencies. Take the early GSM as an example, the working frequency band is 850/900/1800/1900MHz, and these working frequencies are either close to or much higher than 1GHz. As we mentioned above, the RE test uses 1GHz as the boundary, and the gameplay is completely different. Wireless products are like standing at a crossroads on a street corner, go left, or go right?
In addition, wireless products have transceivers for the purpose of transmitting and receiving electromagnetic waves that carry useful information. When there is no transceiver in the electronic product, noise radiation is an unintentional emission. When there is a transceiver, the transmitter will cause additional noise, such as harmonics, various intermodulation, intermodulation products, etc., This is an intentional emission, and one of the main sources of noise radiation is the product's own antenna.
The amplitude of noise generated by intentional emission is far greater than that of unintentional emission. So, should the original RE test limit be changed? how to change?
The above articles are from the Internet, if there is any infringement, please contact to delete! We have been paying attention to the development in this area, and welcome to discuss together if you have different opinions on the topic.
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