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Solution of high frequency interference problem
From: Dongguan brothers high frequency Machinery Co., Ltd. Post date: 2017-07-06


High frequency plastic welding machine (referred to as high frequency machine, high frequency English Taiwan said, Hing freguency plastic welder) users belong to the vast majority of small factories or family factories scattered in residential, often affect the residents in the vicinity of cable TV, therefore, engineers are discussed specially, basically clarified on high frequency machine the characteristics of cable TV interference, and found out some high-frequency machine to prevent and reduce the interference in CATV way.
1 、 frequency spectrum analysis of high frequency jammer
Double headed Zhou Bo
There are two types of high-frequency machines used in the local area, 1 of which are mostly GP2.5 - J ordinary type high frequency machines (hereinafter referred to as ordinary machines) produced by Hangzhou and Ling'an, with power of 2.5kW. High frequency machine radiated interference signal has 3 main parts: one is the electron tube and the oscillation circuit below the working table side iron plate, although there are two directions on sealing sheet iron, but the other two directions is detachable door; two is the high frequency feeder, mold and fixture above the table, this is radiation the most important part; three is a high frequency switch and the output table below the variable capacitors, because of its close to the iron table and chassis shell and iron wire is very short, the estimated radiation is weak. 2) anti interference type high frequency machine, only the high frequency oscillation tube and the circuit are arranged in a circular shield metal drum, while other parts are without any shielding, although its anti jamming effect, but the effect is limited, we measured the level of interference intensity is about 75% of the ordinary machine, can still form the cable television signal interference.
High frequency machine above two types belong to the self-excited high frequency oscillator, its characteristic is the power output tube itself is oscillating tube, so that the output frequency is not single, but a band, the oscillation output of a frequency of the strongest known as the fundamental, the rest are called harmonic. The fundamental frequencies of 3 ordinary machines have been measured to be 32.50MHz, 32.80MHz and 33.65MHz respectively. The harmonic is distributed on both sides of the fundamental wave, and the total frequency band of the ordinary machine is about 20~38MHz.
Use the DS1240 field strength meter used for spectrum analysis can analyze the lowest frequency of 46MHz, not the fundamental harmonic signal of high frequency machine group direct spectrum analysis, we had to deal with the two harmonic spectrum analysis. Because of the harmonic intensity is too large, a scan can not take into account all the Swiss branch, high level and low level two scan, scan results showed that the two harmonic level near 120dB fundamental horn antenna with ordinary TV supporting the use of high frequency in locomotive indirect received, and the two harmonic harmonic level in most 60dB the difference of more than 60dB.
In addition to high frequency machine harmonic output high strength wave outside, also has considerable strength of each harmonic, harmonic wave may interfere the corresponding TV channel, the author observed in the adjacent high-frequency machine several TV, TV or sound proof of the corresponding image layer by different degree of interference. Through test and observation, it is considered that the fundamental wave and harmonic frequency of high frequency machine are lower than 45MHz, and they will not interfere with cable TV outside the cable frequency band, and their higher order harmonics are interfered by cable tv.
Due to the much lower strength than that of harmonic wave, but the frequency bandwidth of the harmonic components is very complex, through field observation that it is likely to within high frequency 10m TV many (all) channel image and sound form of interference; long distance television, may interference on several television channels into a heavy sound no one, and what specific interference channel is also difficult to predict.
Power output tube self oscillation and the output signal of the high frequency machine, so its fundamental frequency stability is poor, with the cabinet temperature and working conditions (output power adjustment and the workpiece size, thickness etc.) varies, though only a few tenths MHz, there may be some interference signal transfer from a channel to the adjacent channel.
The output power of the high frequency machine, high voltage high current DC power supply (3kV) is relatively large (about 1A), and the high voltage rectifier power it is very simple, the filter capacitor 4 1A/10kV high-voltage silicon stack and a 0.1 F/8kV. Because of the capacity of the filter capacitor is too small, high voltage DC power supply is actually a pulsating DC power supply of 100Hz, the high frequency signal of high frequency output in the modulation of AC signal modulation is close to 100% 100Hz, the high frequency machine work performance is not much adverse effects, but may be due to non oscillation signal and reduce the power consumption of electronic tube, prolong the service life of the electron tubes. So the essence of high-frequency machine interference TV is hum interference modulation of a high-frequency signal, when the interference signal of high frequency machine on TV, will produce two wide stripes on the TV screen 100MHz communication system like tone interference, interference intensity, two stripes all become white, the interference intensity hours. Two horizontal distribution of mesh corrugated (unlike the general AC noise interference), and sound is 100MHz hum.
2, the high frequency interference signal propagation and intrusive machine special cable trunk cable and cable distribution line is the user is along the wall layout consumer housing, only in the neutral section of Calgary cement pole, not hang in the power house and the bar users, many users and trunk cable distribution line cable through the high-frequency machine at a distance only a few meters away, the nearest is less than 1m (houses length 10~20m, general room 2 Building 2 floor facility machine, pull steel wire hanging wall

After careful analysis of all the measured data and the actual investigation, it is considered that there are 3 main ways to interfere with the signal intrusion into the television set:
After a cable shielding failure or weakened into receiving antenna frequency interference signal into the TV, the TV is disturbed, the fault is more than 80% plug external conductor plug wire connection box and TV users weld sealing off (pressure in the plastic, from the outside can not see, at this time) as long as the redo plug. There is indoor since then the branch line is not used when the branch is not connected or shielded wire, cable or shielding performance is poor even double core cable, or outer conductor rat gnawed by.
Two is a small number of TV itself shielding performance is bad, high-frequency machine interference signal directly into the TV set, when the TV from the high-frequency machine is near, shielding performance of the TV is the most vulnerable to this situation. The inspection method is: in the TV channel high frequency interference signal, and then unplug the plug line TV, the screen of the best interference disappeared, indicating the shielding performance of this TV itself is quite good; conversely, if the TV picture is interference, even worse, that the shielding performance the TV is bad, its interference signal will receive high frequency machine.
Three is the home line and the user plug line, generally use -5 cable, its shielding loss than four shielding cable is small, in the distance from the high frequency machine within tens of meters, invasion of interference signal will affect the TV signal. If the cable quality is no problem, this effect is slight, but the individual channel in two place fine textured stripes.
The above shows that in the rich stem (main) TV cable shielding line under normal conditions, the high frequency interference signal machine is not first and then spread to the distance through the cable through the cable, but directly to the distance from the space radiation transmission, but transmission distance is limited, usually no more than 500m.
3, to prevent and mitigate high-frequency interference measures
(1) building well system shield, galvanized plate 0.3 or small wire to shield around the 50mm pipe into the underground 2-3 meters, every two meters will play a shielding mesh is connected with the pipe, so the high frequency signal is not easy to radiate out from the factory to the overall shielding effect. ? (2) high frequency machine frame and the shell is well grounded, strengthen body shield, reduce body radiation with a width of 50mm * 50mm or above, the length of not less than 2m angle iron into the ground as the grounding, and more than 10mm2 copper wire will high frequency machine frame and the casing and the grounding body is connected. The connection should be as short as possible, not ring, as far as possible to avoid turning, so that you can strengthen the high frequency machine parts of the screen, to reduce the intensity of interference signal outward radiation. If there are many high frequency machines in the car (room), it is better for each high-frequency machine (or two adjacent units) to close a ground wire near the machine.
(3) adjust the position and direction of high frequency machine
If the wall of the house is hung with cables, the high frequency machine is best located at one end away from the cable. In addition, two directions of ordinary type high frequency machine tube chassis has a right angle is galvanized iron plate is fixed on the frame of the zinc plating iron shielding, radiation is weak, should be to the two direction at or near the cable TV users.
(4) it is recommended to buy and use the anti-interference high-frequency machine
(5) do the grounding of the cable TV network and select the best position of the cable
Amplifiers and steel strands near the high frequency machine must be grounded reliably so as to enhance their shielding capability. Cable hanging in the wall with high frequency machine housing, to be elected in the reinforced concrete member (such as beam, cement floor also) outside don't choose the brick wall outside, so the cable radiation weak.
(6) adjust the position of an amplifier or cable
When the distance of the entrance cable amplifier high frequency machine is very near (within 10m), can be moved to export amplifier, cable near the high frequency machine, because the TV signal level is high, is conducive to the increase of television signal and interference level intrusion, or adjust the main cable to a little far away from the high frequency machine.
(7) ensure the reliability of the cable joint
The connection between the amplifier as a straight line type joint, joint general to do standardized, reliable, special attention should be paid to the joint waterproof, so as to avoid the water caused by joint outer conductor braid was broken, resulting in poor shielding on the interference signal.
(8) and high frequency machine close only twenty or thirty meters within the user, the measures can be taken: 1 guarantee or improve the user signal level, user level entrance of each channel to ensure not less than 65dB, to increase to 75~80dB. 2 indoor not user box, home line directly plug the TV, if there are many TV sets, then use sealed branch (such as Z106) or distributor directly connected to the tv. 3, the user indoor layout of the cable must adopt qualified products or four shielded cable, disable inferior cable, from the high frequency machine 10m within the user, it is best to use aluminum pipe cable home. 4 Indoor TV as far as possible without plastic injection molding plug line. 5 good plug line, try not to use plastic injection pressure type plug line (which is not easy to find and easy internal desoldering), using four shielded cable plug line Handmade. 6 received user fault complaints, first for a new plug line try, if the problem is resolved, users will plug wire plu