| The repeater business in China started to bloom as early as 1998～1999. Because the construction speed of mobile communication networks fell behind the increasing speed of cell phone subscriber, there have been many areas unreachable for cellular signals. Ever since 2000, cellular communication operators put forward the slogan "No aperture coverage" for the reason to provide the better services to users. The requirement of repeaters, as the companies of base station, became ever increasingly dramatic.The functions of repeater in the early stages are very limit. Receiving the signal from base stations and resending with amplifications are the only way to process cellular signals. The wideband amplifiers are used for both uplink and downlink in these kinds of repeaters. These solutions have many drawbacks, as both desirable and unwanted signals will be amplified simultaneously. As a result, very strong interferences will appear for the adjacent channels. Therefore the concept of "channel selector" amplifications became the new solution. The channel selector can only "select" the desired channels and reject all the unwanted ones. Basically, channel selector is a kind of filters working in RF bands. But the center frequency can be tuned and good rejections are required for out band signals. Besides, the band selector is not the normal electronic components, and no vendor in the world is selling it. In the paper, full design solution is introduced based on active RF circuit design techniques.The "band selector" repeaters are the new requirement from cellular operators in China. Most frequently, for a given area, there is unnecessary to use all the cellular bands. The redundant signals actually are interferences for the desirable bands. Consequently, the repeater is supposed to set the operating bands followed by actual requirements. GSM system, for instance, band selector repeaters normally requested to tune within 2MHz～19MHz. Like channel selector, band selector also is a kind of RF filters. Bandwidth should be tuned as required and out side band rejections are also very critical. Again, in this paper, full solution is introduced by active RF circuit design techniques.Most channel and band selector designs are relied on working experience of designers. But in real RF circuit designs, diversity and rationality of solutions must be very careful. The mixer mentioned in this paper is a good example. Up side band and low side band are symmetrical technically. Once a side band is changed in RF designs, the LO frequencies must be recalculated correspond. In most of the designs, LO is usually controlled by programs. So the RF designers must tell the programmer the control range avoiding the invalid orders. These topics have been included in this paper clearly enough.The rationality of active RF circuit designs can be largely depended on if real RF components in the market can be found to achieve the designs. In this paper, there are two examples to illustrate how to design the real channel and band selector modules, step by step from component selections to RF link simulations. The design procedures and methods can also be very helpful for other RF circuit designs. |
