The vast majority of users of high-frequency plastic welding machines (referred to as high-frequency machines, called high frequency in Taiwan, or Hing freguency plastic welder in English) belong to small factories or family factories, scattered in residential areas, and often used to varying degrees. It affected nearby residents to watch cable TV. To this end, engineers conducted special research and basically clarified the interference characteristics of high-frequency machines to cable TV. At the same time, they found some ways to prevent and reduce the interference of high-frequency machines to cable TV.
1. Spectrum analysis of high-frequency machine interference signals
Double-headed high-frequency wave
There are two major types of high-frequency machines used locally, 1) Most of them are The GP2.5-J ordinary high-frequency machine (hereinafter referred to as "ordinary machine") produced by Hangzhou Lin'an has a power of 2.5kW. There are three main parts where the high-frequency machine radiates interference signals: first, the electron tube and oscillator circuit under one side of the iron flat workbench. Although two directions are sealed with thin iron sheets, the other two directions are removable doors; The second is the high-frequency feeders, processing molds and fixtures above the workbench, which are the most important radiation parts; the third is the high-frequency switch and output adjustment variable capacitor below the workbench, because they are close to the iron workbench and chassis. The casing is made of iron and the leads are extremely short, so the radiation is probably weak. 2) The anti-interference high-frequency machine only installs the high-frequency oscillating tube and circuit in a round iron barrel for shielding, and no other parts are shielded. Although it has an anti-interference effect, its effect is limited. We tested The intensity of the interference level is about 75% of that of ordinary machines, and it will still cause interference to cable TV signals.
The above two types of high-frequency machines are self-excited high-frequency oscillators. Their characteristic is that the power output tube itself is an oscillating tube, so the output frequency is not a single one, but a frequency band. The strongest oscillation output of one frequency is called the fundamental wave, and the rest are called harmonics. The author has measured the fundamental frequencies of three ordinary machines as 32.50MHz, 32.80MHz and 33.65MHz respectively. Harmonics are distributed on both sides of the fundamental wave, and the total frequency band of ordinary machines is approximately between 20 and 38MHz.
The lowest analyzable frequency of the DS1240 field strength meter used for spectrum analysis is 46MHz. It is unable to directly perform spectrum analysis on the fundamental wave and harmonic signal group of the high-frequency machine. Now we have to deal with its secondary Harmonics perform spectrum analysis. Since the intensity of the fundamental wave and harmonics are very different, one scan cannot take into account everything. Ruifen scanned twice with high level and low level. The scan results showed that the horn antenna used with ordinary TVs was received between high-frequency locomotives. The second harmonic level of the fundamental wave is close to 120dB, while the second harmonic level of the harmonic wave is mostly below 60dB, and the difference between the two is more than 60dB.
In addition to outputting high-intensity fundamental waves and harmonics, high-frequency machines also output high-order harmonics with considerable intensity. Each high-order harmonic of the fundamental wave may interfere with the TV of the corresponding channel. program, the author made experimental observations on several TV sets adjacent to the high-frequency machine, and proved that the TV image or sound layer of the corresponding channel was interfered with to varying degrees. Through testing and observation, it is believed that the fundamental and harmonic frequencies of high-frequency machines are lower than 45MHz and will not interfere with cable TV outside the cable TV band. What interferes with cable TV is their high-order harmonics.
Since the intensity of harmonics is much lower than the fundamental wave, but the frequency is wide, the high-order harmonic components are very complex. After field observation, the author believes that it may affect many TV sets within 10m of the high-frequency machine. The images and sounds of (or even all) channels will cause interference; for TVs far away, it may cause varying degrees of interference to the sounds of a few TV channels, and it is difficult to predict which channels will be interfered with.
The power output tube of the high-frequency machine oscillates by itself and outputs a signal, so its fundamental frequency stability is poor and will change with the temperature and working conditions in the cabinet (output power adjustment and workpiece size, thickness, etc. ) fluctuates with changes in the frequency, although the change is only a few tenths of a MHz, there may be individual interference signals transferred from a certain channel to its adjacent channels.
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 high-voltage rectified power supply is very simple, using four 1A/10kV high-voltage power supplies. It is composed of silicon stack and a 0.1μF/8kV filter capacitor.
Because the filter capacitor capacity 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%, which is very important for the operation of the high-frequency machine. There is not much adverse effect on the performance, but it can reduce the power consumption of the electronic tube and extend the service life of the electronic tube due to non-constant amplitude oscillation signals. Therefore, the essence of the high-frequency machine interfering with the TV is a hum interference modulated with a high-frequency signal. When the interference signal of the high-frequency machine is connected to the TV, it will produce two wide horizontal bars of 100MHz hum on the TV screen. Modulated interference. When the interference intensity is high, both horizontal bars turn white. When the interference intensity is small, mesh ripples are distributed within the two horizontal bars (different from general hum interference), and the accompanying sound is 100MHz hum.
2. High-frequency machines interfere with signal propagation and intrude into special cable TV trunk cables and user distribution main lines, which are civilian cables. They are laid along the outer walls of users’ houses. Cement poles are only erected in vacant areas. There are no Hanging on the power poles in front and behind the user's house, many trunk cables and user distribution main cables pass only a few meters away from the high-frequency machine, and the nearest one is less than 1m (the length of a private house is 10~20m, and the facility machine on the second floor of a general house , the steel stranded cable is hung outside the wall on the second floor, and the cable is approximately horizontally parallel to the high-frequency machine), and several levels of trunk amplifiers or first-level user amplifiers are connected below, or user distribution is performed directly. If the interference signal from the high-frequency machine is first received by the dry (main) cable and inserted into it, then (amplified by the amplifier) ??and then distributed through the cable into the user's TV, then all the TVs below it will be affected by the high-frequency machine interference. But the actual situation is not like this. It is usually just sporadic and "wandering" interference. Users who are close to the high-frequency machine have a larger proportion of complaints, while users who are farther away have a smaller proportion. Users who are more than 500m away from the high-frequency machine have no complaints. Pass. This proves that the interference signal from the high-frequency machine does not first invade the trunk (main) cable and then (amplified by the amplifier) ??spread to each TV set through the cable. Although the interference signal radiated by the high-frequency machine is very strong, the shielding loss of the trunk (main) cable (-12 or -9 physical foam cable) can reach about 80dB, and the interference level that intrudes into the cable will not be Very high, much less than the TV signal level. The author used spectrum analysis to measure the interference level of a high-frequency machine at the end of an 80m-long -9 physical bubble cable with a copper braided outer conductor. This is a user distribution main line. The high-frequency machine is 18m away from the user amplifier and 3m away from the cable. (There is a wall in the middle). During the measurement, the amplifier was powered off and the entrance cable was removed (replaced with a 75Ω load resistor). The measured interference level of the intruding high-frequency machine was: second harmonic <20dB, and the TV signal level was It is 42~60dB higher than that.
A large number of data tests have been conducted on the radiation conditions of high-frequency machines, as listed in Table 1 and Table 2. Both tables are test records of the interference signal propagation of high-frequency machines, using Deli DS1240 field strength meter spectrum analysis. Measured in this way, the high-frequency machine works continuously for an interference wave scanning time during measurement. There are two types of signal receiving antennas. A is a 3m ordinary -5 cable. There is no resistor at the end, and the outer conductor is stripped for a week near the field strength meter to simulate shielding and damaged cables.
Because the high-voltage selector switch of the high-frequency machine is placed in different positions (there are 5 gears, and the middle gear is set during measurement), the output adjustment capacitor is adjusted in different positions, and there are false artifacts during testing (the high-frequency machine works continuously The difference in thickness (cannot reproduce the real workpiece) is related to the radiation intensity. In addition, the test method used cannot accurately measure the actual field strength radiated by the high-frequency machine. Therefore, the data in the two tables can only be used as a reference when analyzing related characteristics.
Table 1 High-frequency machine signal propagation test record unit in the wilderness: (dB)
10m 30m 70m 100m 200m 300m 400m from the high-frequency machine
Receive 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 High-frequency signal propagation test record unit through wall: (dB)
House number 4 District 27 4 District 28 4 District 29 4 District 30 4 District 31 4 District 32 Zone 4 33
Distance from frequency transmitter 2m 6m 9m 13m 17m 20m 28m
Number of partition walls 1 2 3 4 5 6 8
Reception method A B A B A B A B A B A B A B< /p>
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 a normal -5 cable when more than ten meters away from the high-frequency machine is within 20dB, which is more than 45dB different from the TV signal level (65dB) entering the home, and will not interfere with cable TV. signal; and the interference signal received by a -5 cable with damaged shielding will still interfere with the cable TV signal even if it is 300m away. Therefore, the main reason why wealthy TV systems are interfered by high-frequency machines is that there is a problem with its own shielding.
After careful analysis and study of various measurement data and actual investigations, it is believed that there are three main ways for interference signals to invade the TV:
First, the shielding effect of the outer conductor of the cable fails or weakens After that, it becomes a receiving antenna to receive interference signals from high-frequency machines and enters the TV, causing interference to the TV. More than 80% of the faults are caused by the desoldering of the outer conductor of the plug cable connecting the user box and the TV (pressed inside the plastic). , not visible from the outside), at this time just redo the plug. There are also cases where the user did not use a brancher or did not connect the shielded wire when he or she connected the branch line indoors, or used cables with extremely poor shielding performance or even two-core sheathed wires, or the outer conductor was bitten off by mice.
Secondly, a small number of TVs have poor shielding performance, and the interference signal from the high-frequency machine is directly connected to the TV. When the TV is close to the high-frequency machine, TVs with poor shielding performance are most likely to have this problem. situation.
The test method is: select the TV channel with interference from high-frequency machines, and then unplug the signal plug cable of the TV. If the interference on the screen disappears, it means that the shielding performance of the TV itself is better; otherwise , if the TV picture still has interference at this time, or even becomes more serious, it means that the shielding performance of this TV is poor, and it will receive interference signals from high-frequency machines.
Third, -5 cables are commonly used for home entrance lines and user plug lines. Its shielding loss is smaller than that of four-shielded cables. The intruding interference signal will affect the TV when it is within tens of meters of the high-frequency machine. Signal. If there is no problem with the quality of the cable, this effect is slight, with only a slight mesh appearing on the two horizontal bars on individual channels.
The above situation shows that when the TV trunk (main) cable is normally shielded, the interference signal from the high-frequency machine does not first invade the cable and then spread to a distant place through the cable, but directly passes through the space. Radiation propagates to distant places, but the propagation distance is limited, usually no more than 500m.
3. Measures to prevent and reduce interference from high-frequency machines
(1) When building a factory, shield the system well and use 0.3 galvanized sheet or small-purpose wire mesh to shield the surrounding area , drive 50mm iron pipes 2-3 meters into the ground, drill one every two meters, and connect the shielding network to the iron pipes, so that high-frequency signals are not easily radiated from the factory, achieving an overall shielding effect. ? (2) Ground the frame and shell of the high-frequency machine well, strengthen the shielding of the machine, and reduce the radiation of the machine. It is recommended to drive an angle iron with a width of 50mm×50mm or more and a length of not less than 2m into the ground as the grounding body, and then use copper with a thickness of 10mm2 or more The wire connects the frame and shell of the high-frequency machine to the ground body. The connection should be as short as possible, without looping, and avoid turning as much as possible. This can strengthen the shielding of the high-frequency machine body and reduce the intensity of the interference signal radiated outward. If there are multiple high-frequency machines in the garage (room), it is best to bury a ground wire next to each high-frequency machine (or two adjacent machines).
(3) Adjust the position and direction of the high-frequency machine
If there are cables hanging on the exterior wall of the house, it is best to set the high-frequency machine at the end far away from the cable. In addition, the two directions at right angles to the electronic tube chassis of ordinary high-frequency machines are fixed on the shield on the galvanized angle iron frame with galvanized iron plates. The radiation is weak. These two directions should be facing the cables or nearby TVs. user.
(4) It is recommended to purchase and use anti-interference high-frequency machines
(5) Ground the cable TV network and choose the best location for the cable
< p>Amplifiers and steel strands near high-frequency machines must be reliably grounded to enhance their own shielding capabilities. When the cable is hung on the outer wall of a building with a high-frequency machine, it should be installed outside the reinforced concrete components (such as ring beams, concrete floor slabs), not outside the brick wall, so that the cable will be less radiated.(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 changed to the outlet cable Close to the high-frequency machine, because the TV signal level here is high, which is helpful to increase the level difference between the TV signal and the intrusive interference, or adjust the trunk cable so that it is slightly farther away from the high-frequency machine.
(7) Ensure that the cable joints are reliable
Try to use straight-through joints for connections between amplifiers. The joints of ordinary lines must be standardized and reliable. Special attention should be paid to the waterproofness of the joints to avoid Water entering the connector causes the outer conductor braid to break, resulting in poor shielding and interference signals.
(8) For users who are only twenty or thirty meters away from the high-frequency machine, the measures that can be taken are: 1. Ensure or improve the user signal level. The user entrance level of each channel must not be less than 65dB, try to increase it to 75~80dB. 2. There is no user box indoors. The household line is directly connected to the TV with a plug. If there are multiple TVs, a sealed branch (such as Z106) or distributor is used to directly connect to the TV. 3. The cables laid indoors by users must use qualified products or four-shielded cables. Low-quality cables are prohibited. Users within 10m from the high-frequency machine are best to use aluminum tube cables to enter the home. 4. Try not to use plastic injection plugs on indoor TV sets. 5. Make good plug wires. Try not to use plastic injection-type plug wires (the inside is easy to desolder and difficult to detect), and use handmade four-shielded cable plug wires. 6. After receiving a fault complaint from the user, first try a new plug cord. If the problem is solved, cut off the plug of the user's plug cord and redo it. Otherwise, check the indoor cables and connectors, and even the outdoor cable connectors.