Diplexers and Triplexers
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Diplexers and Triplexers |
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The basic diplexer consists of two filters in parallel, a high pass/low pass or a band pass/band stop, that are driven from the same source and usually contains LC series tank elements in parallel as well. The LC tank compensates for undesired reflections.
The two legs of the diplexer are synthesized using normal low pass and high pass, or band pass and band stop design parameters except that the reactive elements are calculated assuming a zero ohm source resistance. This is done to achieve a desirable reflection response, but slightly degrades the pass band frequency response. When an even order diplexer is specified for the stop band filters of Chebyshev II, Hourglass, or Elliptic, the stop band is automatically modified to map transmission zeros to zero and infinity. The reflection zeros are left alone. See more on this at Even Order Mods. The parallel compensator calculation is derived from the impedances of the low pass and high pass legs at their respective cut off frequencies and the frequency width of the diplexer. Band pass/band stop filters have two compensators, one for the low break frequency and another for the high. The first element of a diplexer must always be a series element.
Diplexers may be composed from lumped or continuous passive elements. For the stop band filters of Chebyshev II, Hourglass, or Elliptic, even orders are preferable to odd order in that they minimize the inductor count.
Triplexers are three way splitters consisting of a low pass, band pass, and high pass legs. They are loosely referred to as "Lo/Hi/Mid Diplexers" in the Filter Solutions control panel.
An example of a diplexer is shown below. As you can see, it is composed of a 30MHz low pass filter in parallel with a 40Mhz high pass filter and has a parallel compensator. The rule of thumb is to keep the reflections outside the middle frequencies (30 to 40 MHz in this case) at least -20dB down.
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Without the parallel compensator, this circuit would contain undesirable reflections that do not meet the -20dB rule as shown here.
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With the parallel compensator, this circuit contain much more desirable reflections that are well below -20dB outside the middle frequency area.
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Below is an example of a band pass/band stop diplexer. The diplexer legs in this example are balanced filters, but could have been constructed from unbalanced filters. Notice that there are two reflection compensators, one for each break frequency.
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The parallel compensators reduce the reflections in each pass band below -20 dB.
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Triplexers
Below is an example of a low/high/mid diplexer or triplexer. The triplexer legs in this example are
unbalanced filters, but could have been constructed from balanced filters. Notice that there are two reflection compensators, one for each break frequency.
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The parallel compensators reduce the reflections in each pass band below -20 dB.
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Below is an example of a band pass diplexer. The compensating elements compensate for reflections between the legs. The first shunt elements in each leg of the diplexer serve as reflection compensators on the outer edges of the diplexer band pass frequencies.
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The parallel compensators almost reduce the reflections in each pass band below -20 dB.
Some manual adjustments to the compensators may reduce the reflections further.
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