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FAQ

Why is it important to equalize different inputs power levels?

DAS fiber systems accept usually 0dBm input levels but there are variations depending on the manufacturer. A multi-carrier scenario may show different input signals levels. If one carrier delivers a significantly higher signal level, it will occupy most of the available RF energy. This leads to insufficient RF levels of the remaining carriers. Equalizing will eliminate this problem, all carriers are set to an even distribution scheme.

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

Choosing the right splitter.

There are basically 3 types of broad-band microwave splitter/combiners: Wilkinson, Reactive (Air-Line) and Hybrid.
  • Wilkinson: Generally low-cost micro-strip devices which offer good performance characteristics for both splitting and combining at lower power levels. Wilkinson splitters typically provide 20 dB of isolation at the output ports (as splitters, or input ports as combiners.)  The insertion loss is higher than that for reactive splitters or hybrid couplers and they are not highly rated for PIM performance. A popular series is the Dx-77FN.
  • Reactive (Airline): Low-cost, high performance splitters.  Lowest loss and superior PIM performance as a splitter used in the most loss sensitive locations.  Not recommended for power combining applications due to the lack of internal resistor elements and poor isolation characteristics.  The Dx-55FN and Dx-88FD are popular reactive models.
  • Hybrid: Highest cost but best overall performance.  Superior isolation characteristics are especially desirable in power combining applications. Also suitable for high powers where the external termination keeps the dissipated heat away from the circuitry.  The CA-84FN is frequently used in such situations.

Can I use a splitter as a combiner?

In general the answer is yes. Wilkinson, Reactive (Airline) and Hybrid circuits are all reciprocal devices, meaning that the insertion loss from port A to port B is the same as the insertion loss measured from port B to port A. The difference is in the level of performance that your system requires.

  • Reactive splitters offer higher power handling capability, lowest dissipative loss and superior PIM performance.  However, they do not have good input return loss or isolation when used as combiners.
  • Wilkinson devices offer good performance as both splitters and combiners, but they have higher dissipative loss, poor PIM performance, and they are typically not suitable for higher power combining applications since their isolation resistors are embedded in the circuit. The Dx-64FN is one of many such series of Wilkinson Splitters.
  • Hybrid combiners are usually the superior performers as both splitters and combiners. They offer lower dissipative loss and higher isolation, than the Wilkinson combiners/splitters.  When used as combiners, hybrid couplers can achieve higher power and low PIM performance by simply terminating the unused port with high power, low PIM 50W load. CA-84N is a typical 2 x 2 Hybrid Combiner.

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.

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.

What is the difference between signal combining and signal mixing?

  • Signal combining refers to the addition of multiple signals (from one or more operators) to a single output feed leading to a single or multiple antennas. The most important specification is the isolation of the inputs from each other, or non-interference.   Combining is typically accomplished through the use of hybrid circuits which enable system designers to distribute signals from multiple carriers with only minimal insertion loss. CA-84N is a typical hybrid ombiner.
  • Signal mixing refers to the interference of signals as one would desire in a modulation system, usually using a non-linear device. Obviously this is most undesirable in wireless signal distribution systems.

How is a directional coupler bi-directional?

Directional couplers are directional in the sense that the coupled port only couples the signal going from the input port to the output port. This is defined as "coupling." When the return signal goes from output port to input port, the coupled port sees a very low level of that signal. This is defined as "isolation." The directivity is defined as the "isolation" minus "coupling." However, designers should note that directional couplers are bi-directional in the sense that the signal loss from the input port to the coupled port is the same as signal loss from the coupled port to the input port, under the law of reciprocity.

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.

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.

What are the tradeoffs between cavity filters and strip-line filters?

Each type of filter design has its strengths and weaknesses, so here is a brief overview:

  • Cavity filters are typically higher Q, higher power, narrower band (per section) and higher cost.  They are preferred for duplex applications where high isolation is required between bands with close proximity.  The BK-31N is a typical cavity filter.
  • Stripline filters are typically lower Q, broader band and lower cost.  They are typically preferred for most diplex applications.  The BK-20 series from Microlab is an example of a diplexer that combines or separates the cellular bands between 700 and 960 MHz with those between 1710 and 2170 MHz.

What is meant by an IP Rating?

The IP (Ingress Protection) Rating is a measure of a device’s ability to withstand solid objects and liquids.  In-door applications typically call for a modest rating of IP64.  Outdoor ratings are typically IP65 to IP68.  For a more in depth look at what these ratings actually mean, view the following website:   http://www.aquatext.com/tables/ip_ratings.htm

What is PIM, and why should I be concerned about it in my system design?

PIM (passive intermodulation) occurs when 2 or more signals are present in a passive device that exhibits a non-linear response. The nonlinearity is caused by dissimilar metals, anodic effects or loose connections. This nonlinearity does not manifest itself at low input signal levels, and is therefore also a function of the high power level transmit signals in the system. The resultant products are referred to as 3rd, 5th, order intermodulation products, etc. The 3rd order products are typically the focus in most system design. The real problem occurs if the 3rd order PIM products generated by the transmitter channels fall within adjacent receiver channels. This upsets the adaptive settings of the receiver and desensitization will occur. The result is typically a dropped call.

What is meant by RoHS? Are Microlab's products RoHS compliant?

RoHS means: Restriction of Hazardous Substances. The RoHS standards have been established to control/eliminate the use of identified hazardous substances in the manufacture of products used in our market. All of Microlab's Wireless products are RoHS compliant.

Why would I want to use a T-configuration or opposed connector splitter?

T-configuration splitters offer several advantages over in-line splitters in certain applications.  They are particularly useful in leaky (radiating) cable DAS were it is necessary to separate the cables as quickly as possible in order to prevent coupling between the cables.  Additionally, many installers find it easier to secure their connections when there is more space between connectors, and a torque wrench can be applied. The Dx-85FD and Dx-77FN are examples of these splitters.