Interference problem and solution of high frequency machine (high frequency)The majority of users of high-frequency plastic welding machines (referred to as high-frequency machines, referred to as high frequency in Taiwan, Hing freguency plastic welder in English) belong to small factories or home factories. They are scattered in residential areas and often affect the nearby residents to watch cable TV to varying degrees. To this end, engineers of Hongyouxing Machinery (Suzhou) Co., Ltd. conducted a special discussion and research, basically clarified the characteristics of high-frequency machine interference to cable TV, and at the same time explored some methods to prevent and mitigate high-frequency machine interference with cable TV. Method.
1. Spectrum analysis of high-frequency interference signals
There are two main types of local high-frequency machines:
1. Most of them are GP2.5-J ordinary high-frequency machines (hereinafter referred to as "ordinary machines") produced in Hangzhou, Xiaoshan, Lin'an, Ruian, etc., with a power of 2.5kW.
There are three main parts of the high-frequency machine radiating interference signals:
① It is the electron tube and the oscillating circuit on the lower side of the iron flat workbench. Although there are two directions to seal the thin iron sheet, the other two directions are removable doors;
② It is the high-frequency feeder on the worktable, the processing mold and its fixture, which is the most important radiation part;
③ It is a high-frequency switch and output adjustment variable capacitor under the workbench. Because it is close to the iron workbench and the iron plate of the case shell and the lead is extremely short, it is estimated that its radiation is weak.
2. Anti-interference high-frequency machine-only the high-frequency oscillating tube and circuit are installed in the round iron barrel for shielding, and the other parts are not shielded. Although it has anti-interference effect, but its function is limited, we measured The intensity of its interference level is about 75% of that of ordinary machines, and it will still interfere with the cable TV signal.
The above two types of high-frequency machines are self-excited high-frequency oscillators, which are characterized by the power output tube itself is an oscillation tube, so the output frequency is not a single, but a frequency band, and the oscillation output of one frequency is the most The strong ones are called fundamentals, and the rest are called harmonics. The author has measured the fundamental frequency of three ordinary machines as 32.50MHz, 32.80MHz and 33.65MHz, respectively. The harmonics are distributed on both sides of the fundamental wave, and the total frequency band of the ordinary machine is about 20~38MHz.
When using the DS1240 field strength meter for spectrum analysis, the lowest analyzable frequency is 46MHz. It is impossible to directly perform spectrum analysis on the fundamental wave and harmonic signal group of the high-frequency machine. Now we have to perform spectrum analysis on its second harmonic. Due to the great difference between the fundamental and harmonic intensity, one scan cannot take all of them into consideration. The Swiss high-level and low-level two scans, the scan results show that the claw antenna used with the ordinary TV set is received in the workshop of the high-frequency machine. The second harmonic level of the fundamental wave is close to 120dB, and the second harmonic level of the harmonic is mostly below 60dB, and the difference between the two is more than 60dB.
In addition to outputting high-intensity fundamental waves and harmonics, the high-frequency machine also has relatively high-intensity harmonics. Each higher-order harmonic of the fundamental wave may interfere with the TV program of the corresponding channel. Experiments were conducted on several TV sets adjacent to the radio to prove that the TV image or sound layer of the corresponding channel was disturbed to varying degrees. Through testing and observation, it is believed that the fundamental and harmonic frequencies of the high-frequency machine are lower than 45MHz. Outside the cable TV band, it will not interfere with the cable TV. It is their higher harmonics that interfere with the cable TV.
Because the intensity of the harmonics is much lower than the fundamental wave, but the frequency bandwidth and high-order harmonic components are very complicated, the author believes that it is possible to image many (or even all) channels of the TV within 10m from the high-frequency machine after field observation. Interference with sounds; for TVs at a long distance, the sounds of a few TV channels may be disturbed in varying degrees, and it is difficult to predict which channels are specifically interfered.
The power output tube of the high-frequency machine oscillates itself and outputs a signal, so its fundamental frequency stability is poor, and it will fluctuate with changes in the temperature and working conditions of the cabinet (output power adjustment and workpiece size, thickness, etc.), although With only a few MHz of change, there may be individual interference signals transferred from a certain channel to its adjacent channel.
The output power of the high-frequency machine is large, and the current of the large high-voltage (3kV) DC power supply is also relatively large (about 1A), but its large high-voltage rectifier power supply is very simple, using four 1A/10kV high-voltage silicon stacks plus a 0.1μF /8kV filter capacitor. Because the capacity of the filter capacitor is too small, the high-voltage DC power supply is actually a 100Hz pulsating DC power supply. The high-frequency signal output by the high-frequency machine is actually modulated with a 100Hz AC signal with a modulation degree close to 100%. The performance does not have much adverse effects, but it can reduce the power consumption of the tube due to the non-equal amplitude oscillation signal and extend the service life of the tube. Therefore, the essence of the high-frequency machine interfering with the TV is a kind of hum interference modulated with high-frequency signals. When the interference signal of the high-frequency machine is stringed into the TV, it will produce two wide horizontal stripes of 100MHz hum on the TV screen. Modulation-like interference. When the interference intensity is large, the two bars are all white. When the interference intensity is small, mesh ripples are distributed in the two bars (different from general hum interference), while the accompanying sound is 100MHz hum.
2. Interference signal propagation of high-frequency machine and intrusion of special cable TV trunk cable and user distribution main line are civilian cables that are laid along the outer wall of the user's house, only cement poles are added in the vacant section, and are not hung on the front and back of the user's house On the power pole, many mainline cables and user-distributed mainline cables pass only a few meters away from the high-frequency machine, and the nearest one is less than 1m (the length of the private house is 10~20m, the general machine is installed on the second floor of the room, and the steel is drawn on the second floor. The stranded cable is hung, and the cable is approximately horizontally parallel to the high-frequency machine), then connect a few levels of trunk amplifiers or a level of user amplifiers, or directly assign users. If the interference signal of the high-frequency machine is first received by the main (main) cable and connected into it, then (amplified by the amplifier) and then distributed into the user's TV through the cable, then all the TVs below it will be subjected to high frequency Machine interference. But the actual situation is not the case, usually only sporadic, "walking" interference, users who are close to the high-frequency machine have a larger proportion of complaints, users who are farther away have a smaller proportion, and users who are 500m away from the high-frequency machine have no complaints. Ever. This proves that the interference signal of the high-frequency machine does not first invade the trunk (main) line cable, and then (amplified by the amplifier) propagates through the cable to each TV. Although the intensity of the interference signal radiated by the high-frequency machine is very large, the shielding loss of the dry (main) line cable (-12 or -9 physical foam cable) can reach about 80dB, and the interference level intruding into the cable will not be Very high, much lower than the TV signal level. I have used the method of spectrum analysis to measure the interference level of the high-frequency machine at the end of a -9 physical bubble cable with an 80m long outer conductor of copper braided net. This is a user distribution main line. The high frequency machine is 18m away from the user amplifier and 3m away from the cable. (A wall in the middle interval), the amplifier is powered off and the home cable is removed during the measurement (the 75Ω load resistance is changed). The measured interference level of the high-frequency machine is: second harmonic <20dB, the TV signal level must be 42~60dB higher than it.
A large number of data tests have been carried out on the radio frequency of the high-frequency machine, which are listed in Table 1 and Table 2. Both tables are the test records of the interference signal propagation of the high-frequency machine. They are measured by the Deli DS1240 field strength meter spectrum analysis method. The frequency machine works continuously for one interference wave scanning time. There are two types of signal receiving antennas. A is a 3m ordinary -5 cable. No resistance is connected at the end, and the outer conductor ring is stripped around the field strength meter for a period of time to simulate shielding and damage the cable.
Because the high-frequency machine high-voltage selector switch is placed in different positions (there are 5 gears, the middle gear is set during measurement), the output adjustment capacitor is adjusted at different positions and the thickness of the fake workpiece (the high-frequency machine can not put the real workpiece in continuous operation) during the test The differences are all related to the radiation intensity. In addition, the test method used cannot accurately determine the actual field strength of the high-frequency machine radiation. Therefore, the data in the two tables can only be used as a reference when analyzing the relevant characteristics.
Table 1 Test record unit of high-frequency machine signal propagation in the wilderness: (dB)
10m 30m 70m 100m 200m 300m 400m
Receiving method A B A B A B A B A B A B A B
Second harmonic <20 90 <20 70 <20 65 <20 45 <20 30 <20 <20 <20 <20
Third harmonic <20 72 <20 63 <20 53 <20 48 <20 50 <20 42 <20 <20
Ninth harmonic <20 65 38 65 <20 55 <20 52 <20 46 <20 30 <20 <20
Sixteenth harmonic 35 65 45 73 <20 53 <20 60 <20 43 <20 33 <20 <20
Average level 73.5 67.5 56.6 51.3 42.3 31 <20
Table 2 Test record unit of high-frequency machine signal transmission through the wall: (dB)
House number 4 Zone 27 4 Zone 28 4 Zone 29 4 Zone 30 4 Zone 31 4 Zone 32 4 Zone 33
Distance frequency machine 2m 6m 9m 13m 17m 20m 28m
Number of partitions 1 2 3 4 5 6 8
Receiving method A B A B A B A B A B A B A B
Second harmonic 50 105 47 100 25 90 22 83 <20 67 <20 65 <20 72
Third harmonic 45 95 40 86 <20 67 <20 60 <20 62 <20 58 <20 60
Ninth harmonic 55 100 42 81 40 78 32 70 <20 70 <20 70 <20 52
Sixteenth harmonic 58 90 35 78 <20 60 <20 60 <20 52 <20 52 <20 50
Average level 48.3 95 41 86.3 <26 73.8 <24 68.3 <20 66.8 <20 61.3 <20 58.5
It can be seen from the measurement results that the interference level received by the normal -5 cable at more than ten meters away from the high-frequency machine is within 20dB, which is more than 45dB different from the household TV signal level (65dB), and will not interfere with the cable TV signal. ; The shielded damaged -5 cable, even if the interference signal received beyond 300m will still interfere with the cable TV signal, so the main reason for the rich TV system to be interfered by the high-frequency machine is its own shielding problem.
After careful analysis and research of various measurement data and actual investigations, it is believed that there are three main ways for interference signals to invade TV sets:
First, after the shielding effect of the outer conductor of the cable fails or weakens, it becomes the receiving antenna to receive the high-frequency machine interference signal and enter the TV, causing the TV to be interfered. More than 80% of the faults are outside the plug line connecting the user box and the TV. The soldering point of the conductor is de-soldered (pressed into the plastic and cannot be seen from the outside), at this time, only the plug can be redone. There is also the reason that the branch wire is not used or the shielded wire is not well connected, or the cable with extremely poor shielding performance or even the double-core sheathed wire is used, or the outer conductor is bitten by the mouse.
The second is that a few TVs have poor shielding performance, and the interference signal of the high-frequency machine is directly connected to the TV. When the TV is close to the high-frequency machine, the TV with poor shielding performance is most prone to this situation. The test method is: select the TV on the channel with high-frequency interference, and then unplug the signal plug of the TV. If the interference on the screen disappears, the shielding performance of this TV is better; otherwise If the TV picture still has interference at this time, or even becomes more serious, it means that this TV has poor shielding performance, and it will receive the interference signal of the high-frequency machine itself.
Thirdly, the -5 cable is generally used for the incoming line and the user plug line. Its shielding loss is smaller than that of the four-shielded cable. The interference signal that invades when it is within tens of meters from the high-frequency machine will affect the TV signal. If there is no problem with the quality of the cable, this effect is slight, except that individual channels appear fine-grained on the two horizontal bars.
The above situation shows that under the condition that the shield of the rich (main) cable of the rich TV is normal, the interference signal of the high-frequency machine does not invade the cable and then propagate to the distance through the cable, but radiates directly from the space to the distance, but The propagation distance is limited, usually no more than 500m.
3. Measures to prevent and mitigate high-frequency interference
(1) Shield the system well when building the plant, use 0.3 galvanized sheet or small mesh wire mesh to shield the surrounding area, drive 50mm iron pipe into the ground 2-3 meters, hit one every two meters to shield the net It is connected with iron pipes, so that high-frequency signals are not easily radiated from the factory, and the overall shielding effect is achieved.
(2) Ground the high-frequency machine frame and the shell well, strengthen the body shielding, and reduce the body radiation
It is recommended to use an angle iron with a width of 50mm×50mm or more and a length of not less than 2m to penetrate the ground as a grounding body, and then use a copper wire of 10mm2 or more to connect the frame and casing of the high-frequency machine to the grounding body. The connection should be as short as possible, and no loops should be made. Turns should be avoided as much as possible, which can strengthen the shielding of the body of the high-frequency machine and reduce the intensity of interference signals radiating outward. If there are multiple high-frequency machines in the car (room), it is best to bury a ground wire near the machine for each high-frequency machine (or two adjacent ones).
(3) Adjust the position and direction of the high-frequency machine
If cables are hung on the outer wall of the house, the high-frequency machine is best installed at the end away from the cable. In addition, the ordinary high-frequency machine tube chassis has two directions at right angles, which are fixed on the shield on the galvanized iron frame with galvanized iron plates. The radiation is weak, and these two directions should be directed to the cable or the nearby TV. user.
(4) It is recommended to buy and use anti-interference high-frequency machine
(5) Do a good job of grounding the cable TV network and choose the best location of the cable
The amplifiers and steel strands near the high-frequency machine must be reliably grounded to strengthen their shielding capabilities. When the cable is hung on the outer wall of the house with a high-frequency machine, it should be tested outside the reinforced concrete components (such as ring beams, concrete and floor slabs), and not outside the brick wall, so the cable is weaker.
(6) Adjust the position of the amplifier or cable
When the inlet cable of the amplifier is very close to the high-frequency machine (within 10m), the amplifier can be moved forward and the outlet cable can be changed to close to the high-frequency machine, because the TV signal level is high here, which is conducive to increasing the interference between the TV signal and intrusive interference. Level difference, or adjust the trunk cable so that it is slightly away from the high-frequency machine.
(7) Ensure reliable cable joints
The connection between the amplifiers should use straight-through connectors as much as possible. The joints of ordinary lines should be standardized and reliable. Special attention should be paid to the waterproofing of the connectors to avoid the intrusion of the outer conductor braided network due to water ingress of the connectors, resulting in poor shielding and interference signals.
(8) For users close to the high-frequency machine within 20 to 30 meters, the measures that can be taken are: 1. Guarantee or increase the user signal level, and ensure that the user entrance level of each channel is not less than 65dB, and try to increase it to 75~ 80dB. 2 There is no user box in the house, and the incoming line is directly connected to the TV with a plug. If there are multiple TVs, use a sealed branch (such as Z106) or a distributor to directly connect to the TV. 3 The cables laid indoors must use qualified products or four-shielded cables. Inferior cables are forbidden. Users within 10m from the high-frequency machine are best to use aluminum tube cables to enter the house. 4 As far as possible, indoor TV sets do not need plastic injection plugs. 5 Do a good job of plug wires, try not to use plastic pressure injection plug wires (the inside of which is easy to be de-soldered and not easy to detect), and use hand-made four shielded cable plug wires. 6 After receiving the user's complaint, first try a new plug cable. If the problem is solved, cut the plug of the user plug cable and redo it, otherwise check the indoor cable and connector, and even the outdoor cable connector.