Analysis and Research on Shield Attenuation of four Shielded Cables

In a two-way cable TV system, to make reverse transmission work normally, the confluence noise must be reduced. Most of the confluence noise is formed by a space electromagnetic interference string into the reverse system. In a Bidirectional CATV system, cables are one of the most vulnerable components to interference. In order to make the reverse system work normally, four Shielded Cables with High attenuation Shielding must be used. This paper introduces two methods for measuring the four shielded cables, the principle of measuring the four shielded cables, and the problems to be paid attention to during the measurement. The measurement results of shielded attenuation of domestic and imported four shielded cables are analyzed.
1 two methods for measuring shielding attenuation of RF coaxial cable
There are two methods to measure the shielding attenuation of RF coaxial cables: leakage method and penetration method. The leakage method is to generate a strong electromagnetic field in the coaxial cable. Due to the poor shielding of the cable, the strong electromagnetic field between the cable and the outer conductor may leak out through the gap of the outer conductor. The leakage field is measured to indicate the Shield Attenuation of the cable. The absorption clamp method uses the leakage method. The penetration method is to place the tested cable in a uniform electromagnetic field. Because there is a gap in the outer conductor of the cable, the electromagnetic field passes through the gap to penetrate into the inside of the cable. The measurement of the penetration intensity can also characterize the Shield Attenuation of the cable. With the development of Electromagnetic Compatibility measuring equipment, the shielding attenuation of cables is measured using a GTEM chamber that can generate a uniform transverse electromagnetic resonance. This measurement method is called the GTEM chamber method and is more complex than the absorption clamp method. However, the measurement range of GTEM chamber method is DC-1 GHz, Which is wider than that of absorption clamp. Theoretically, the electromagnetic field strength produced by the GTEM chamber is only related to the input signal power. As long as the power amplifier is large enough, a high field strength can be generated. This greatly improves the sensitivity of GTEM Chamber shielding attenuation Measurement and is a method suitable for measuring high shielding attenuation of four shielded cables.
2. Why should we use the GTEM chamber method instead of the absorption clamp method to measure the four shielded cables?
In the measurement of shielding attenuation of RF coaxial cables, the attraction clamp method is the most traditional measurement method due to the limitations of measurement equipment. This measurement method is simple and easy to use, but has low sensitivity. Generally, the measurement range is below dB. In addition, the measurement frequency range is limited to over 30 MHz.
With the development of two-way transmission and Multi-Function Application of CATV system, four shielded cables have emerged, and the shielding attenuation of the four shielded cables is higher. Generally, they can reach more than dB, and are measured by means of absorption clamp, the shielded attenuation of the four Shielded Cables cannot be accurately measured because it has exceeded the maximum measurement range of the general absorption clamp ). In particular, for Bidirectional HFCs, the reverse transmission frequency ranges from 5 to 65 MHz, and the shielded attenuation of the 5 MHz frequency point must be measured. This is beyond the Working Scope of the absorption clamp. Obviously, the absorption clamp method can no longer meet the needs of the four shielded cable shielding attenuation Measurement, and is replaced by the GTEM chamber method.
3 GTEM chamber method for measuring the shielding attenuation of four Shielded Cables
The GTEM cell is designed based on the principle of coaxial and asymmetric rectangular transmission lines. In order to avoid reflection of Internal Electromagnetic Waves and generate high-order mode harmonious vibration, design them into sharp split. The input end adopts the N-type coaxial joint, and then gradually changes to the asymmetric rectangular transmission line to reduce the radio wave reflection caused by structural mutation. In order to achieve good impedance matching and obtain a large uniform field area, select and adjust the appropriate angle, height and width. In order to make the spherical TEM wave transmit well from the input end to the load, and have good high and low frequency characteristics, the terminal uses a resistance Matching Network and the absorption material to form a composite load. The appearance of the GTEM chamber is a four-pyramid shielding box. The leftmost is the flange with N-type sockets, And the RF signal is fed in from here. There is a door on the side that can be placed or taken out of the tested part. There is a cable outlet at the bottom of the shielding box. The electromagnetic signals infiltrated into the tested parts can be drawn out for measurement. The bottom of the shielding box also has a filtered v ac power supply to power the tested active device.
GTEM small indoor electromagnetic field distribution. The solid line is the same as the measured electric field intensity changes (1 dB, 2 dB, and 3 dB ). The dotted line indicates the uniform zone of 1 dB and 2 dB in which the electric field intensity changes.
1) Basic Principle of Shield Attenuation measured in GTEM Chamber
After the RF signal generator is amplified by power, it is fed to the input port of the GTEM chamber, and the Uniform Electromagnetic Field is stimulated in the GTEM chamber. Cut off the four Shielded Cables for a long period of 30cm and end the F-type cable plug on both sides. End one end of the cable with a well-shielded 75Ω load resistance, and the other end is connected to the cable plug in the GTEM chamber to keep the cable perpendicular to the electric field ). Due to the small gap still exists in the outer conductor of the four shielded cables, the strong field of the GTEM chamber will penetrate into the cable.
The weak signal penetrating into the cable is amplified by the Front amplifier and displayed by the spectrum analyzer.
The basic principle of GTEM chamber for measuring Shield Attenuation of four shielded cables is as follows:
After the four Shielded Cables with a length of 30cm are used as the fheader, the valid length of the cables is only 25 cm. The inductive level produced by a 25cm long conductor in a uniform electromagnetic field in the GTEM chamber is the electric field strength multiplied by the conductor length. The four shielded cables have poor shielding with small gaps), and the strong electromagnetic field in the GTEM room will penetrate into the cable, which will also generate a penetration level, this level is very weak and can only be displayed on the spectrum analyzer after being amplified by a low-noise, high-gain pre-amplifier. The level displayed on the spectrum analyzer minus the gain of the front amplifier gets the size of the intrinsic penetration level of the four shielded cables. The shielding attenuation of the 25cm long conductor is obtained after the inductive level generated in the Uniform Electromagnetic Field of the GTEM Chamber minus the penetration level of the four shielded cables.
2) GTEM chamber method for measuring the shielding attenuation of four Shielded Cables
1) In the GTEM room, four shielded cables are connected to the GTEM room outlet. It is best to use 75 Ω cables with metal outer conductors. To avoid the penetration of strong electromagnetic fields in the GTEM room into the connected cables, affecting the measurement accuracy of Shield Attenuation.
2) The cable connector and the 75Ω ending resistance must be well shielded) be tightened. When necessary, use a copper wire cloth to compress the joint and tie it together to prevent the strong electromagnetic field in GTEM from penetrating into the system under test.
3) shield the self-check of the attenuation test system. In the GTEM room, the 75Ω ending resistance is directly connected to the cable head of the metal outer conductor and tightened and fastened. Shielding attenuation should be at least 130dB to prove the reliability of the system, without affecting the measurement accuracy of shielding attenuation of the four shielded cables around 120dB.
4) The spectrum analyzer and the front amplifier should be separately placed in the shielding room. It should not be in the same room as the RF signal source or power amplifier, so as to avoid mixing the electromagnetic field leaked from the RF signal source or power amplifier into the analyzer or front amplifier, affecting the measurement accuracy.
4 can the absorption clamp method increase the sensitivity and reduce interference? Can the four shielded cables be measured?
The following strict measures can be taken to increase the sensitivity and reduce external intrusion interference, and the shielding attenuation can be measured to more than DB. Two methods are used to measure the shielding attenuation of the four Shielded Cables simultaneously by means of absorption clamp and GTEM chamber. However, because of the high requirements, non-general laboratory conditions can be met by the absorption clamp method with strict measures. It is not suitable for measuring the shielded attenuation of the generally four shielded cable.
In order to measure the shielding attenuation of the four shielded cables, the following measures are taken to improve the sensitivity and reduce the interference caused by external intrusion.
1) Add a front amplifier between the output end of the absorption clamp signal and the spectrum analyzer.
We choose high-gain, low-noise front-amplifier from Hewlett-Packard. Gain G = 32dB, noise coefficient NF = 2.5dB, band width 9 KHz-1 GHz. The RF signal output by the absorption clamp can be increased by 32 dB, thus increasing the sensitivity of the shielded attenuation Measurement.
2) reduce the intermediate frequency bandwidth of the spectrum analyzer to 30 KHz, and increase the measurement sensitivity of the spectrum analyzer to 0 dBpV. Reduces the intermediate frequency bandwidth of the spectrum analyzer, reduces the inherent low noise level of the spectrum analyzer, and improves the measurement sensitivity.
The RF signal generator with a higher output level can output up to 127 dBpV.
In this way, the shielded attenuation of the four shielded cables can be measured by the absorption clamp method up to 130dB. Because the RF signal generator output level is 127 dBpV, coupled with the front amplifier 32 dB, this is equivalent to the signal level up to 159dB. Minus the insertion attenuation of the absorption clamp to around 20 dB and the maximum sensitivity of the spectrum analyzer to 0 dBpV, the maximum shielding attenuation of the four Shielded Cables measured by the absorption clamp can reach dB.
(3) In the shielded attenuation system for measuring the four shielded cables by means of the absorption clamp method, all the cable joints and the load resistance joints are tightened and fastened with copper wires, in addition, the test range of Shield Attenuation is expanded from 90dB to 120dB. In particular, the absorption clamp RF output port is a BNC head, And the shielding attenuation is not high enough. Only the head is covered with copper wires, so that the system's measurement shielding attenuation range is increased by 20 dB.
4) the output level of the RF signal generator is as high as 127 dBpV, the signal source itself is not shielded enough, there is a weak electromagnetic field leaked to the space, and then stringed into the front amplifier or spectrum analyzer input, this affects the accuracy of high Shield Attenuation measurement. The RF signal source can be placed outside the shielding area, and the RF signal is introduced into the shielding room through a cable to measure the external interference of shielding and attenuation by means of the absorption clamp, which can extend the measurement range of shielding and attenuation to DB.
The shielding attenuation of the same four shielded cable was measured by the GTEM chamber method and the absorption clamp method. The result is that the average value of shielding attenuation measured by the GTEM method is at most 10 dB different from that measured by the absorption clamp method. Why? Careful analysis shows that the shielding attenuation on a four shielded cable is partial. The cable length is 6 m for measuring Shield Attenuation by means of absorption clamp. The absorption clamp moves forward and backward of the cable to find the minimum point of Shield Attenuation. The length of the cable used to test the Shield Attenuation by GTEM is 30 cm, which is equivalent to 20 points of the length of the cable by the absorption clamp method. Therefore, the minimum probability of shielding attenuation is much lower. The GTEM method measures that the shielding attenuation is usually higher than the absorption clamp method. To solve this problem, we select a 6 m long and four shielded cable, and use the absorption clamp method to measure the shielding attenuation. Then, we use the absorption clamp to find the worst section of the cable shield and cut it down by 30 cm, use the GTEM chamber method to measure the shielded attenuation of the four shielded cables. In this way, the shielding attenuation of the same cable is basically the same by using the absorption clamp and GTEM chamber.
The measurement of Shield Attenuation also found that the worst Shield Attenuation measured at various frequencies of 30MHz-1GHz was not overlapped on the cable when the four shielded cables were measured by means of an absorption clamp method, and the maximum difference was about 1 MB. Why? It is common sense that the shielded attenuation of the four shielded cables must almost overlap at each frequency in a small segment. Taking into account another factor, RF signals cannot be transmitted over cables as full traveling waves. Due to insufficient matching, a standing wave is inevitable. The above phenomenon is caused by the Standing Wave formed by the signal of each frequency on the 6 m long cable. To confirm this problem, we have used a network analyzer to measure the signal supply system of the 6 m cable by means of absorption clamp: this includes the 50Ω cable from the signal source, 50Ω dual-pass and 50Ω-75Ω impedance converter for the shielding room, and the reflection loss of the 6 m long quad-shielded cable and 75Ω ending resistance. The system has a reflection loss of only 8 dB, which is equivalent to 40% of the standing wave. This is because the reflection loss of domestic devices is not high enough. After all devices are switched to the imported device, the system reflection loss reaches 16 dB, and the standing wave is only 16%, the above problems are successfully solved.
5 results and comparison of four shielded cable shielding attenuation measured by absorption clamp and GTEM Chamber
1) results of four shielded cable shielding attenuation measured by absorption clamp and GTEM Chamber
Because of the uneven shielding attenuation of the four Shielded Cables in China, it indicates the range of shielding attenuation of the coaxial cable in China, while the second is the imported four shielded cable.
2) Comparison of measurement results of shielded attenuation of four Shielded Cables Using absorption clamp and GTEM Chamber
As follows:
1) the best four shielded cable in China, the average value of shielded attenuation GTEM chamber is 2.8dB higher than the absorption clamp, and the minimum GTEM value of shielded attenuation is 4dB higher than the absorption clamp.
2) the average GTEM chamber for shielded attenuation of domestic four shielded cables is 2.6dB higher than the absorption clamp, while the minimum GTEM chamber for shielded attenuation is 3 dB higher than the absorption clamp.
3) The average GTEM chamber for shielded attenuation of the imported four shielded cables is dB higher than the absorption clamp, while the GTEM chamber for shielded attenuation is 3 dB higher than the absorption clamp.
Four shielded cables are tested using two methods: absorption clamp and GTEM Chamber, with a difference of about 3 dB, whether the average value of Shield Attenuation or the minimum value of Shield Attenuation. The two methods for measuring Shield Attenuation are basically the same.
6 Analysis of measurement results of Shield Attenuation of four Shielded Cables
There are many differences in the shielded attenuation of domestic four shielded cables. Generally, Chinese cables can be divided into three categories: Best, better, and average. The shielding attenuation of the best four Shielded Cables in China is higher than that of the imported cables. The difference between the four Shielded Cables in China is slightly lower than that of the imported cables. The shielding attenuation of the domestic four shielded cables is more than 6 dB lower than that of the imported cables. The causes are as follows:
1) generally, the best four Shielded Cables made in China are produced using imported equipment. The raw materials are of good quality, the process is strict, and the product quality is of course good. In terms of appearance, these cables are closely structured, solid, and the outer sheath is smooth and flexible. Its features are as follows:
1) woven wire coarse and confidential
2) The Knitting angle standard is not greater than 45 °, and the best four Shielded Cables in China are less than 45 °.
3) Aluminum Foil of the four shielded outer conductor is bonded and overlapped. Lap is to wrap the aluminum foil on the physical foam Insulator of the cable, and the joint overlaps a part, generally 3mm. The aluminum foil is attached together with the physical foam insulator. The bonding performance is better than that of lap shielding. The best domestic four shielded cables are bonded.
2) domestic equipment is used to produce four Shielded Cables with poor raw material quality. From the cable appearance, this type of cable is relatively soft, not quite solid, the outer jacket is hard and less glossy.
1) fine woven wire with Low Density
2) The braid angle is large, usually greater than 45 °, and some even greater than 70 °.
3) the aluminum foil of this type of four shielded cables is mostly overlapped.
3) Summary
The best domestic four shielded cable shielding attenuation is more than 6 dB higher than the imported. Compared with the imported coaxial cable, the domestic coaxial cable is usually about 3 dB lower.
The shielded attenuation of the domestic four shielded cables is much different from that of the imported cables. Generally, it is 6 dB or more lower.