Choosing Power Splitters to meet Diverse Wireless Applications
Originally appearing in Microwave Product Digest, May 1999
The ever-changing wireless industry has
rigorous electrical and mechanical demands
on its suppliers of Power Splitters or
Dividers. Microlab/FXR, one of the leading
suppliers to the wireless industry, has a wide
variety of catalog, such as the dual band two
way (Figure 1) and application-specific
power splitters. Each is designed to satisfy
those specific demands.
The most common applications of power
splitters are in signal distribution either to
multiple antennas at the wireless base station or within a building to multiple transmission sources. The demands of the industry are high and can be summarized as follows: Electrical Criteria
Loss: Power splitters are used in the RF signal path so lowest loss is most critical.
Components using an air dielectric therefore usually have a significant
advantage over PTFE or similar higher dielectric constant materials.
Power:
The power splitters must be able to handle momentary severe power peaks
without damage, as well as the continuous power expected from the
transmission signal. Components that incorporate any low wattage
components or loads may have a greater risk of failure. Bandwidth: Basic splitters are naturally octave band devices, so traditionally two series
are available to satisfy either the cellular/GSM band between 806 and 960
MHz or the PCS/DCS-1800 & 1900 band between 1710 and 1990 MHz.
Then there is the NMT-450 band which operates from 453 to 468 MHz, and
the Japanese PDC band from 1429 to 1501 MHz. Obviously, the splitter
chosen must cover the band or bands in use, which may require splitters
with bandwidths extended beyond an octave. These are generally available
for either 460 – 960 MHz and 806 – 1990 MHz, and occasionally for all
three bands.
VSWR: Minimizing reflections back to the transmitter protects the final output
amplifier, but equally important is the need to maximize the transmission
efficiency. Therefore a low input VSWR on all splitters is very desirable.
Ways:
The most common splitter is an equal two way, meaning that the input
signal is split equally between two outputs. There are other choices of two
way splitters which divide the signal in a defined ratio such as 80%:20% or
90%:10%, plus equal splitters that divide the input signal into 3, 4, 5, 6 and
more ways. (Figure 2).Figure 1. 2-way Reactive Power Splitter D2-14FN, covering dual bands from 800 to 2,200 MHz.
PIM: Passive Intermodulation (PIM) or Intermodulation Distortion (IMD) has
become one of the most critical parameters of all components used in the
high power transmission path, particularly filters, cable, splitters, couplers
and antennas. PIM is caused by a number of factors including the material
properties of ferromagnetic metals, non-linear dielectrics, thermal
conductivity variations and contact between dissimilar metals.
Then the contact between materials may be non-linear because of micro
arcing, poor contact pressure, electron tunneling through an insulator or
simply uneven surface properties. Poor PIM performance is held
responsible for dropped calls especially in digital systems.
The PIM performance demanded by the industry is lower than –140 dBc
performed with two +43 dBm tones. The typical performances of two
Figure 2. Six way splitter for the NMT-450 and cellular
900 MHz bands. File: D6-1TN-1.jpg
Figure 3. PIM performance of Dual-band 2-
way Power Splitter showing better than –
150dBc at +46 dBm input power. (File: PIM
D2-14.jpg) Figure 4. PIM Performance of Dual Band Unequal Splitter DG-54FN measured over the PCS Band, showing better than –160dBc at
+46 dBm input power.(File PIM DG-54-1.jpg)
Microlab/FXR splitters are shown across the PCS/DCS-1800 frequency band. (Figures 3 and 4). Components that do not specify PIM performance may be unsuitable for any RF transmission path, particularly if power levels are high.
Connectors: High power transmission signals
usually require heavy duty, low
loss, large diameter coaxial cable,
and the connector type which best
shares these characteristics is
generally agreed to be the 7/16
DIN. In addition they have
excellent PIM performance.
(Figures 5a and 5b)
On the other hand, in lower power
situations, found especially in
indoor signal distribution systems
do not require the sophistication
nor the cost penalty of these
connectors. The connector of
choice here is generally the N-
connector. (Figure 6)
Figure 5b. PCS band 3-way Power Splitter in 'Umbrella' style with 7-16 mm DIN connectors. (File: D3-9TD-3.jpg)
Figure 5a. Dual Band (800 – 2,200 MHz) 'Block' style Splitter, D3-24FD, for high
power base station applications.
Mechanical Criteria
Environment: Power splitters are used in many diverse environments and the
construction and mechanical configuration has to change to meet the
special needs. Locations include:
Antenna Towers: Keeping out the effects of rain, etc., requires
weatherproofing the splitter and using appropriate moisture proof male
connectors. Depending on the location, either an 'block' or a 'umbrella'
style splitter with DIN connectors is usually used. (Figures 5a and 5b).
Walls Inside Buildings: For this location the splitter needs to be flat in
one plane for simple attachment. The 'cross' style, attached with simple
clips is frequently employed. (Figure 6).
Figure 7. Three way Cellular Band Splitter
designed for use in conduit. File: D3-A60-4.jpg
In Conduit: Here the connectors must be all in line, so a special
'flashlight' mechanical construction is desirable. (Figure 7).
In Underground Tunnels: Subway wireless
systems often require the leaky antennas or
micro-base stations to carry other than
cellular/PCS traffic, which operates at lower
frequencies. This requires ultra wide band
splitters such as junctions, which give up
impedance match for small size and broad
bandwidth. (Figure 8).
On Board Ships: Space is generally at a
premium aboard ships, so PTFE loaded
splitters, which are smaller size, have an
advantage.
In an Equipment Rack: Space is generally
flexible, so the standard 'cross' style catalog
part is generally the choice. (Figure 6).
Splitter Techniques and their Suitability for Wireless Applications
There are five basic types of signal splitters:
1. The Wilkinson Power Divider provides equal in-phase division of a
signal. Its principal attribute is that it provides high isolation between
outputs, limiting the effects of reflected signals. Its limitation in wireless
is insertion loss, which is only reasonably low (~0.2 dB) and the
isolation resistor(s), which are usually low power and are placed
between the output ports. Unfortunately, they can easily be destroyed by
a power surge returned on one of the divided feeds so they must be used
with special care.
2. The Hybrid Junction , or Magic Tee, is a four port device which can be
used in many different ways. As a divider it can divide a signal equally
with outputs in anti-phase (0° and 180°), or operate as a Wilkinson as an
in-phase divider. It has the same shortcomings as the Wilkinson and to
date it has found little use in wireless applications. Figure 8. The Junction provides 100 – 2,200 MHz bandwidth at the cost of impedance matching. File: J4-1TN-4.jpg
3. The 3 dB Hybrid Coupler , or Quadrature Hybrid, (Figure 9), is another
four port device, but now divides or combines a signal equally with a
90° phase difference. As a hybrid combiner it is frequently used as an
efficient way to combine transmitter signals on to one common antenna.
It also can combine
two signals and then
split them equally
between two
outputs. This feature
makes the 3 dB
reactive toHybrid Coupler
attractive to in-
building installers of
wireless distribution
systems, since it
allows the use of the
same antenna system for two service providers. This unit can be used in
either leaky-line or micro base station installations, and circuitry can be
expanded to accommodate more than two service providers.
4. The Reactive Splitter has a single input and two or more outputs. They
contain a broadband impedance transformer to convert the 50Ω input
impedance to the lower impedance presented by the junction of the
multiple outputs. For a two-way splitter this would mean a 50Ω to 25Ω
transmission line transformer. In wireless applications the reactive
splitter has some very significant advantages – it has exceptionally low
loss (of the order of 0.05dB), has excellent PIM performance and is
virtually indestructible. (Figures 1,
2, 6 and 7).
5. The Directional Coupler is usually
thought of as a monitoring
component, and because it has
directivity, it is able to separate
transmit signals from their
reflections. This makes the
directional coupler very useful in
monitoring the VSWR (forward and reflected power) on the line between base station and antenna. However, the directional coupler
is also a kind of splitter, except that the split is unequal. This can be
useful when distributing a signal around a building using micro-base
stations or leaky line antennas. A 6 dB directional coupler effectively
divides the signal in a 1:3 ratio or as installers prefer to define as a
25%:75% power split. The problem with directional couplers is their
fairly high cost. (Figure 10).
Figure 10. 6 dB Directional Coupler covering dual bands from 800 to 2,200 MHz. File: CK-
46N-1.jpg
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