Multiplexers are combinations of filters connected in such a way as to provide access to the passband and stopband characteristics of each filter from a common connection. Such a device permits the use of a single antenna with several receivers or transmitters, for example. The common port must display a low VSWR and isolation must be maintained between each of the component filters. A two channel version is called a diplexer, a three channel- triplexer, etc. (click to see Fig. M-1). If the adjacent passbands of each channel "cross-over" at a level of about -3db the device is called a "contiguous" multiplexer. The frequency separation of channels is called the "guardband".
A multiplexer is normally used if a wide spectrum must be accessed equally and instantaneously. Conventionally, multiplexers have had the disadvantage of requiring at least 3 dB excess loss (”crossover” loss) at frequencies common to two channels. Thus, the passband characteristics for contiguous structures always showed an insertion loss variation over the passband of at least 3 dB.
To construct any multiplexer, it is necessary to connect networks to the constituent filters such that each filter appears as an open circuit to each other filter. While this is simple for narrow band channels, it is difficult for broadband or contiguous filters. Normally, the filters and the multiplexing network are synthesized as a set, with computer optimization being used to simulate the results before construction begins. Some of the more common multiplexing techniques include line lengths, circulators, hybrids, and transformers. More recently, the multiplexer filter channels have been combined using power dividers (click to see Fig. M-2). This recent adaptation of always-available technology is due to newly-available cheap and compact amplifier stages. Such gain blocks provide flat gain and low noise over wide bandwidths.
In the case of two-way combining, conservation of energy means that the 3 dB insertion loss is still experienced...but on a flat-loss basis. Although each channel is subject to the additional 3 dB loss, it is essentially constant loss over each channel and thus the excess passband loss variation is less than 1 dB. Excess loss is defined as that loss not attributable to the individual channel filter roll-off. This power divider based combining can be extended to triplexers (4.7 dB flat loss), quadruplexers (6 dB flat loss), etc. Because the loss variation is minimized, the overall insertion loss can frequently be made up using amplifiers, which display flat gain versus frequency.
Filters can be multiplexed by parallel combination at both ends. For example, if two bandpass filters are multiplexed at both input and output, a network results which provides one input and one output, with two passbands, essentially attenuating everything else. Such assemblies are useful in systems such as GPS which have two or more operating frequencies, with the requirement for isolation between the operating channels and adjacent, cluttered regions of the spectrum (click to see Fig. M-3). Another approach employs switched selection of filters (see section on switched filters). Hybrid combinations using multiplexers with power dividers, switches and amplifiers are now possible. The interactions of these essentially reactive components can cause undesirable degradation of stopbands or passbands, if precautions or not taken.
At RS Microwave, available computer simulation techniques are sufficiently sophisticated that accurate prediction of performance and dimensions minimizes the time required to develop and deliver such complex assemblies. Interconnection of sub-components or sub-modules within multiplexers is sometimes difficult, with parasitic lengths causing degradation of performance. Although the computer can predict these problems, sometimes the parasitics reach levels for which compensation cannot be effected. At RS Microwave, proprietary blind-mate interconnection of submodules is used to minimize both parasitic interconnections and spurious crosstalk. Thus, the physical structure, including all interactions, can be predicted accurately and the unacceptable interactions and crosstalk eliminated using the mechanical elegance and electrical isolation of blind-mate internal connections.
Multiplexer development is impacted heavily by network synthesis and computer simulation techniques. As it becomes possible to synthesize combinations of lumped, distributed and evanescent elements as well as predict and compensate their interactions, multiplexers will shrink in size, increase in order (number of channels) and display improved performance in insertion loss, isolation and bandwidth.
