1. Field of the Invention
The present invention relates generally to transmission of radio frequency (RF) signal waves and specifically to the combination of different RF signals into a common transmission line, such as a waveguide, for application to a common propagating device such as an antenna for further transmission of the electromagnetic wave.
2. Background and Related Art
In electronic apparatus concerned with amplification and transmission of electromagnetic energy, the energy is usually transferred from one component of the system to another in the form of current through electrical conductors such as wire or cable. At very high frequencies and high power, the transfer of electrical energy through current carrying conductors becomes inefficient and impractical. Instead of transferring electrical energy as current through conductors, the high frequency high power energy can be transmitted by electromagnetic waves constrained within a guiding structure or waveguide. The transmission of energy by waveguide is carried out by radiating the energy, in the form of electromagnetic waves, into the inside of the waveguide structure through various coupling mechanisms such as probes, loops, crossbars and the like. This same energy may be extracted from the waveguide by a similar coupling mechanism at another location on the waveguide. Where the energy is in the form of current, conveyed in multiple conductors, devices such as coaxial cross-coupled hybrids may be used.
Many applications exist where it is desired to combine signals of various frequencies into a common transmission line structure. One example of such an application is in the field of microwave communications. One requirement of such combination is that the different signals must not improperly interfere with each other.
In this regard, various tuned filtering devices have been used in the prior art. However, due to the non-ideal nature of some of such devices, unwanted distortions and losses are inevitably introduced into the frequency bands of the different signals, causing the signals to be degraded in quality.
One particular application for combining electromagnetic signal waves has arisen with the advent of digital television (DTV). Broadcasters who wish to provide DTV service still must provide broadcast signals in the conventional NTSC (National Television Systems Committee) format to accommodate viewers with conventional NTSC television receivers. Absent a feasible and efficient way to combine the DTV signal and the NTSC signal for transmission by a common broadcast antenna (possibly on adjacent frequency channels) without causing unacceptable interference, interaction and distortion of the signals, this will require some broadcasters to build additional antennae and towers to carry their DTV signals. This represents a very expensive and undesirable proposition.
Under present NTSC broadcast allocations, each active channel of 6 MHz bandwidth is separated from other channels by one or more 6 MHz wide inactive channels, in order to prevent interchannel interference between signals. Because of the limited nature of the electromagnetic spectrum and the requirement for broadcasters to continue to provide conventional NTSC signals to serve viewers who do not own DTV receivers, DTV signals will, in some cases, be broadcast on currently inactive 6 MHz wide channels adjacent to the upper side of current NTSC broadcasting channels.
Various techniques have been proposed to provide an acceptable combination of analog (i.e., NTSC) and digital DTV signals on adjacent broadcast channel allocations to meet this requirement. Proposed combiners utilizing wideband and other LC filters unavoidably introduce large signal phase delay and amplitude artifacts which are at best difficult to correct in order to reestablish the original signal purity specifications of the transmitter. The difficulty arises from the fact that a broadband filter is required for each signal having a "cliff" response directly at the common band edge between the combined signals, because there is virtually no guard band between the two signals. Such LC filters have much difficulty achieving good performance characteristics under such constraints, and the need for stability over varying environmental conditions (mainly temperature) adds further to the difficulty in designing acceptable combiners using these LC filters.
Accordingly, there exists a need in the art to enable the combination of signals of different frequencies on adjacent broadcast channels without mutual interference and distortion.