Digital television transmitter output filter discussion – Digital TV – broadcast
Sunbeam Microwave Engineering Co., Ltd. Chengdu, China's digital TV terrestrial broadcasting standards, the figures TV Transmitter Been rapid development, however, the output of the digital TV transmitter filter, multiple synthesizer requirements, no formal standards, particularly in rural areas now Wireless Coverage Program has been adopted as the national standard DTMB trend, the need for more rural areas covered by television copy number, the larger the radius of the cover system, must be more than one high-power single-channel transmitter for multi synthesis, a pair of antennas to be the best choice , synthesis filter is the core of multiple components of the launch of its technical performance indexes of the system quite a lot, currently the single channels have been launched into the power system, the part of manufacturers to multiple synthesis techniques, but related, uses import multiple synthesizers, time-consuming and expensive and not cost you the output filter, multi synthesizer diverse demands that we find in practice the requirements of some users is not practical. I want this (to discuss the filter-based) published his views a bit, and we discussed together.
Digital TV transmitters actuator indicators should be quite good, after the back of solid state power amplifier, because of the power Amplifiers And a variety of distribution and synthesis of networks will introduce nonlinear distortion and linear distortion. And solid state power amplifier is often Broadband Characteristics, to obtain a standard band features, performance meets the system requirements of the output filter is essential.
Now TV transmitter antenna tower on a variety of TV and FM transmitting antenna packed to the brim, and think of launching channels to increase the number of antennas has been very difficult, so a duplexer, multiplexer so that multiple transmitters share a growing number of users transmitting antenna. Synthesis of these high-power digital network should be asked what is there is no standard to follow. However, these synthetic network performance depends primarily on digital systems use filters, so seriously discuss the requirements of the filter is necessary. (Different from the use of digital technology to stream digital filter, which we tentatively transmitter output linear filter called "digital systems filter" it, this paper called the filter slightly.)
Output filter should be a job within the required frequency band-pass filter, should meet the requirements of the power capacity, in the pass band should meet a certain bandwidth requirements, and should be as small as possible as the amplitude of fluctuations and small delay. The Digital TV also requires a steep transition zone as far as possible, to meet the specific stop band attenuation.
Main properties: 1.0 filter power capacity.
Digital TV transmitter power based on the average band Power Meter Volume, so the output filter also should be able to withstand the number of average power to define its power capacity. However, this "bear" should be the number on the lack of a scientific concept. In practice, it seems there are two ideas. One is the filter itself has better temperature compensation device, it allows greater RF power increases, allowing greater temperature rise, as long as the curve with much drift, even if the shell is hot is also permitted. I have seen some of the import system is like that. The second is the user requirements
insertion loss as small as possible, and requires the rated power, you can not have a hot feeling. Magnitude is not the concept. It seems that only after the provisions of the rated power maximum temperature rise of the number of degrees to a data illustrate the problem. Some users even proposed synthesis system does not allow additional cooling measures, and this is not an overly demanding. I have seen R / S of the 20KW transmitter output filter to add liquid cooling!
Open about the use of one end of the short end of the small and medium-power coaxial resonant cavity filter depends on the capacity of its power outside the cavity of the size of the conductor, while the inner conductor diameter of the Q value selected by the determined. Generally the greater the external cavity for easy insertion loss of smaller, affordable power a little bigger. (This does not involve high-power cylindrical waveguide type filter). Filter file is under the power sub-output power transmitter set. Commonly used are:
20W, 50W, 100W, 300W, 500W, 1KW, 1.5KW, 2KW, etc.. This series should also have standard Caixing.
RF power to bear on the filter definition, I propose to: Canada rated RF power, working more than an hour, the temperature reached homeostasis after the filter, with the temperature measurement of temperature and the highest point, and at room temperature is "The biggest temperature difference t". When the temperature stability of the filter to 2KHz / K ° time, t 15 ° C, filters should be able to steady work. This definition of words with the data than the hand touch should be said that the scientific point, I do not know any objection? Can be released for discussion.
Digital TV with a digital filter in addition to considering the capacity of average power, we should also see the peak of digital television signals? Level ratio, DMB-T our peak? Level ratio of 7dB (5 times). 1KW digital filter that is to be considered to bear the maximum peak power of up to 5KW! Therefore, the selection of connectors and internal lighting as far as the consideration to be carefully designed, or likely to cause high-frequency arcing.
2.0 Insertion Loss Filter insertion loss is defined as the center frequency f. Point of insertion loss. The size and frequency of work, such SPINNER companies working in the UHF filter (Model BN61, 63,67), when in the UHF 470MHz low insertion loss 0.45dB, while the insertion loss can be high-end 860MHz Zuodao 0.65dB.
Insertion loss and power capacity is also related to the greater power capacity outside the conductor the greater the body cavity, such SPINNER companies 2.5KW filters (BN61.63.17) at low frequency can be done 0.45dB, while the 200W filter device (BN61.63.66) can only achieve 0.9dB.
Insertion loss values of good and bad design and manufacturing processes are closely related. Cavity loss including the current flows through the conductive cavity surface of the ohmic loss, connectors and tuning components, contact resistance loss, the loss of the introduction of welding quality. There cavity inappropriate media, will the introduction of dielectric loss. One time we tried to stick to FM PTFE cavity long on support within the conductor, the result clearly increased insertion loss 0.07dB. A design must have the correct filter to ensure correct processing. Plating quality, adjust the quality must be guaranteed to get a lower insertion loss. We have encountered, with a manufacturing design drawings in different factories, insertion loss vary widely. After a long test, that the Division St. filter processing quality of the company more stable.
3.0 degree band of spectrum is not flat
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