FAQs Archive

Q: What is a cross-band coupler?

A cross band coupler refers to the same type of device as a diplexer or filter combiner. These devices allow the combination and separation of signals in different wireless bands with higher isolation (to minimize band interaction) and without the combining loss incurred in other types of combining technologies. The BK-21N is an example of a cross-band coupler.

Q: Should I use tappers, directional couplers or unequal splitters for my DAS applications?

System designers have a choice in the use of directional couplers, tappers or unequal splitters in the "main spine" of a DAS. The directivity of the directional coupler in theory reduces interference between signals on the return path, however since the signals are so small on the return path the effect is generally small. The return loss of the coupled port is generally good which simplifies system modeling. Microlab has a wide range of directional couplers from the low cost CK-66 series to the high performance CK-10 series. Tappers are generally less expensive, are very rugged and have broad bandwidths. The negative of the tapper is the lack of isolation and poor VSWR on the coupled port, but once again these factors may not be critical to the overall system performance. The DK-x8 series are typical tappers.

Q: What is meant by combiner loss?

Combiner loss is a term that refers to the theoretical loss that is incurred in a power combining situation. In a 2:1 combine, the loss is 3dB or 50% of the total input power. In a 3:1 combine, the loss is 4.8dB or ~67%. In a 4:1 combine, the loss is 6dB or 75%, and so on. Insertion loss is the theoretical combiner loss plus the dissipative loss.

Q: What is the difference between a hybrid combiner and a filter combiner?

Both combine signals to a common path, but do so in different ways. Filter combiners are band specific devices used to combine signals from different frequency bands. In a 2:1 combining situation, there is no 3dB combining loss, but there may be a higher insertion loss than that found in a hybrid device. Microlab has a range of standard diplexers corresponding to the common wireless bands with typical >50dB isolation between input ports. The BK-26N is such a filter combiner. The basic 2 input, 2 output hybrid combiner can be used to combine any two signals in the band without interaction of the signals. The two signals, A and B, can be far apart or close together, and each of the outputs will have an output of (A+B)/2. In a 2-to-1 combine, only one output is used, so one of the outputs must be terminated in 50W. These devices typically offer 20 dB minimum of isolation between input ports. Microlab/FXR hybrids, such as the CA-84N, offer 25 – 35 dB isolation across the band of 700-2700 MHz for superior performance in broad-band wireless applications. Microlab also offers broadband 3 x 3 and 4 x 4 hybrids for multi-input combining applications as may be required in neutral host in-building systems. These can be found as CM-81 and CM-84 series.

Q: Is it necessary to equalize the Rx signals as well?

Rx signals, delivered from the fiber system to the RF network, may be too high for the network receivers. This scenario would distort the signal and reduce overall receiver sensitivity. It is not necessary to equalize Rx signals, but signal levels need to be set to a range that meets the actual network receiver requirements