Abstract: In view of the phenomenon that electric bicycle charger interferes with cable TV network, the characteristics of such charger and the harm of interference to cable broadband service are expounded. The causes of interference are analyzed. Based on this, the troubleshooting of such interference is introduced. And the treatment method, further put forward measures to prevent and suppress interference.
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1 Introduction
At present, electric bicycles have become a means of transportation for more and more citizens because of their advantages of ease of use, labor saving, and speed. The number of local electric bicycles is very large. I found in the work that most electric bicycles often interfere with the uplink channel of the HFC network when charging, which seriously affects the stability of the network. The user reflects strongly. I used to be in a Four kinds of such interferences were investigated under the optical node. It can be seen that the electric bicycle charger has become a serious and common interference source for interfering with the cable television network. This paper analyzes and discusses this problem, and proposes solutions and defense measures.
2 Interference phenomena and hazards
When the charger interferes with the cable TV network, the noise of the uplink channel of the broadband service is significantly improved, and the carrier-to-noise ratio of the uplink port of the CMTS where the spot is located is reduced to different degrees (up to a dozen dB), thereby causing the uplink port. The user can't access the Internet normally, and the impact is wide. The specific performance is that the CM ( Cab leModem) packet is seriously damaged or even dropped. The user feels that the network speed is obviously slow, and the network is intermittent. This type of interference seriously affects the interference. The stability of the cable TV network and the quality of the user's Internet.
3 Analysis of the cause of interference
Electric bicycle chargers mostly use switching power supplies. Although there are many models, the circuit structure is similar. Figure 1 is a block diagram of its working principle. It is mainly composed of rectification and filtering, high voltage switch, voltage conversion, constant current and constant voltage and charging control. The rectifying and filtering circuit is used to convert the AC 220 V voltage into a DC voltage of about 300 V. Through the high voltage switching circuit and voltage conversion, the low voltage DC voltage required for charging is generated, and then the battery is controlled by the charging control circuit. Charging.
Figure 1 is a block diagram of the working principle of the electric bicycle charger
Such chargers generate strong electromagnetic interference due to their working characteristics. The high-order harmonic interference generated by the basic rectifier circuit and the peak voltage interference generated by the voltage conversion circuit are the main factors, especially the higher harmonics. Conducted interference and radiated interference along the transmission line, on the one hand, the circuit wave on the power line connected to the front end is distorted, and on the other hand, the radio frequency interference is generated through the power line, and the interference signal occupies a wide frequency range, and There is a certain amount, so it will cause interference to surrounding communication and electronic equipment. Therefore, the charger usually adds a high-frequency filter circuit and a magnetic ring to the circuit to suppress conducted interference and radiated interference, but more and more manufacturers save the cost, and save the filter circuit in the production process. Moreover, most of them are made of plastic casing, and they are not shielded by a suitable metal material casing, so that the generated high-order harmonics radiate into the air in the form of electromagnetic waves, which becomes a serious interference source, and the frequency of the interference signal can be extended to 30 MHz. Above, it can cause interference to HFC networks in the range of approximately 5 m.
In the same way as the antenna receiving principle, the coaxial cable also “receives†these interferences, that is, the interference electromagnetic field generates an interference induced current on the cable, and the induced current generates an interference induced voltage on the longitudinal resistance of the outer conductor (woven mesh) of the cable, and the induced voltage Just in series in the long ground wire of the cable signal transmission loop, the interference is formed. The frequency of the interference signal falls within the range of the reverse channel (5~65 MH z), which will increase the channel error rate significantly. The utilization rate is reduced, causing network congestion.
4 Interference troubleshooting
Such interference signals are strong and have a long duration. As long as they respond in time, the interference source can usually be found according to the general noise troubleshooting method.
The following is an example of the 860 DSP analyzer equipped with this unit (used with the front-end Trilithic 9581SST) to explain the troubleshooting steps:
(1) Determine the optical node under the CMTS port affected by the interference; (2) Test the 5~65MHz return waveform of each optical node reverse light receiving test port with the SSR of the 860 DSP analyzer; (3) According to the test Compare the spectrum of the return channel (the noise floor of the normal return channel is usually lower than - 30dB), determine the optical node with interference (see Figure 2, Figure 3); (4) to the optical machine, use the same method Determine which port from the optical machine is interfered, and then check it down step by step until the interference source is found; (5) Whether the noise source can be completely solved according to the actual situation, if not, use a high-pass filter to isolate it.
Figure 2 Normal back channel spectrum 
Figure 3: Reverse channel spectrum with interference
5 interference treatment
In theory, the modification of the charger to comply with electromagnetic compatibility standards or shielding it with a metal casing can eliminate or suppress interference, but the reality is obviously not feasible, so only measures are taken on the interfered network, Interference is blocked and the network's anti-interference ability is improved.
In the past, when I found noise interference in my work, I usually used a high isolation terminal box and a brancher, or replaced the user line with a high-shielded cable, or connected the air-load port to a 75Ω load resistor. It effectively isolates the noise, but finds that the above method can not completely eliminate such interference caused by the charger, and can only weaken the intensity of the interference to a certain extent. Therefore, after determining which branch of the noise is concentrated, it is only necessary to adopt The high pass filter isolates the interference.
6 Measures to prevent and suppress charger interference
In the local area, such noise interference has become more and more intense. At present, we can only find a single processing order through user fault repair, and can only passively check. In order to prevent problems before they occur, it is the best strategy to improve the anti-jamming capability of the network. Based on experience and exploration, the author proposes the following measures to prevent and mitigate the interference of the charger to the cable TV network for reference and selection.
(1) Do a good job of grounding the cable TV network and choose the location where the cables are laid. The author found in practice that the interference of the charger usually occurs in the garage or shop on the ground floor of the residential building, because such places are places where electric bicycles are concentrated. The room is relatively small, and the charger is usually charged next to the set-top box. The amplifiers, branch distributors and cables installed in these places are relatively low in position, so they are very susceptible to interference. Therefore, the amplifiers and strands in these places must be grounded reliably to enhance their shielding effect. Four shielded cables are used. Try to lay the cable in a relatively high position and replace the broken cable as soon as possible so that the cable is less exposed to radiation.
(2) Take measures to ensure the connector of the cable is reliable. The joint process is strictly required, and the joints that are unqualified, have a transition, and are not crimped completely are very easy to be connected to the noise. The connection of the amplifier should be as straight as possible. The joint should pay special attention to waterproofing, so as to avoid the mold from entering the water and causing the outer conductor to woven the mesh, resulting in poor shielding and interference signals.
(3) It is expected that the amplifiers in places where electric bicycles are concentrated can be connected to high-pass filters first, so as to prevent problems before they occur, and prevent the intrusion of noise and affect large users.
(4) It is recommended that the user purchase and use a charger that meets the electromagnetic radiation standard. When charging, the charger should be as far away as possible from the TV terminal.
7 Conclusion
With the completion of the HFC two-way network transformation and the rapid development of the two-way service, the accompanying noise poses a serious threat to the uplink data channel of the cable TV network. Therefore, the noise detection and processing methods must be mastered by the first-line technicians. A work skill, the above analysis of the interference and solution to the electric bicycle charger, I hope to be able to learn from other peers to check out other types of noise.
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