Splitter Types

Introduction

Power splitters (a.k.a. dividers) are important building blocks in RF/microwave systems.  They come in a variety of types, each with their own advantages and limitations.  This application note serves to help achieve a better understanding of the various types of splitters, and why the systemn engineer may choose one type over another.

Key Characteristics

Theoretical Splitting Loss.  By the fact that the signal is being split between any number of outputs, the resultant signal strength at each output is divided by the number of outputs.  This is typically expressed in decibels (dB) and is useful in calculating the resultant power at each ouytput.

The formula is dB = 10*log (1/n), where n = the number of outputs.

The result will always be a negative number, because the splitter is a passive device, and thus signal will be reduced at each output.  It should also be fairly obvious that in a closed system, dividing a signal will result in reduced power at the outputs.

Please note that this is theoretical or ideal loss, and does not take into account other factors such as material losses.  This will be explained below.

Insertion Loss.  The relative strength of the signal that is lost due to factors inherent in the practical device itself.  This is typically expressed in addition to theoretical/ideal loss.  This loss may come from a variety of sources:

The circuit board substrate material has some degree of inherent loss.  Air or vacuum is said to be lossless.  Thus it follows that anything that takes up volume will cause some of the energy to dissipate into it.

Surface finish and roughness of the copper artwork contributes to signal loss, particularly at high frequencies.  Copper is one of the lowest loss materials when it is fresh.  However it will oxidize over time, which adds considerable loss.  In high power applications this can result in heat build up and thermal runaway.  To counteract the oxidation of copper, circuit boards are often coated with a layer of tin.

Amplitude Balance.  Think the strength or volume of the signal.  As a simple alalogy, consider a home stereo system with a left and right speaker.  If the volume of music coming out of each speaker is equal, the system is said to have equal amplitude balance.  If the left speaker is noticable louder than the right, then the ampltiude balance is unequal.

Phase Balance.  Think timing or delay of the signal.  Consider we are sending news information two four cities (ignoring time zones) from our location in Whippany, New Jersey:

  • Newark, NJ.
  • Manhattan, NY.
  • Philadephia, PA.
  • San Frnacisco, CA.

The first three cities are relatively close to each other, and should receive the signal at about the same time.  In their case, we could say they are phase balanced.

However, San Francisco is 3,000 miles away.  So there may be a delay of several minutes.  To avoid this, we would balance the system by introducing delay to the other cities, such that all four cities receive the message at the same time.  This may seem wasteful, but in most systems, you are only as strong as your weakest link.

Isolation.  Isolation is the performance of how well the outputs are independant from each other.  More is ususally better, and is expressed in decibels (dB). 

What this means from a practical viewpoint is minimizing interference.  I will attempt to make an analogy to illustrate this.  Consider a highway with 3 lanes of traffic.  The lanes are not isolated, because cars may freely cross into each others’ lanes.  A traffic accident may easily block all lanes, rendering them impassable.

Conversely, consider three separate, parallel roads, each with a single lane.  Same traffic capacity as in the previous scenario.  However, an accident on one road will not affect the other paths.  I realize it is not the best real-world example, but hopefully it illustrates the point.

Wilkinson Splitter, Divider, Combiner

Wilkinson splitters are named for their inventor, Ernest Wilkinson, who developed the topology in the 1960s.  Since then, they are the go-to method for splitting and combining signals.

As a building block, this type of splitters is used to split RF signals into a certain number of output paths.  They key point is the output signals are equal in amplitude (think strength or volume) and phase (think timing).

Wilkinson splitters feature a “dump” resistor (or series of resistors, to increase bandwidth) between the output paths.  This serves to balance the ouputs, maintaining good isolation and output VSWR performance, by absorbing mismatched or reflected signals.

These types of splitters are typically available in configurations from 2-32 outputs, and most often in binary-splits (2-way, 4-way, 8-way, etc.) due to their symmetry and ease of fabrication.

While it is in fact possible to create splitters using odd numbers of outputs, such as 3-way and 9-way, but this is very difficult due to the high impedance of the lines necessary.  Amplitude balance is also a challenge in non-binary configurations.  I will write an application note about this issue in the future. 

The inherent limitation to this configuration is in the power handling capability of the internal resistors, particularly at higher frequencies (in the GHz range), where the physical size of the resistors must be small to maintain acceptable RF performance.  This is often not a concern when using the device as a splitter, but can be disastrous if care is not taken when using as a combiner.

Technically, the device is reversible, and may be safely used as a combiner when the power is low (tyically under 1W per port, but always consult the datasheet or applications engineer).  When the power is high, half of the power is absorbed into the dump resistors.  If they are not rated to handle such power levels, they will burn out.

Pros: Wide bandwidth, low loss, high isolation.

Cons: Combining power requires special design considerations.

Werbel Microwave designs and manufactures Wilkinson splitters, and has an in-house PCB production line ready to take on high volumes.

Resistive Splitter

Resistive splitters are unique in they provide exceptionally wide, flat bandwidth, at the cost of higher signal loss and minimal isolation.

Schematically, they are often represented as a star of resistors.  Each resistor connects to the input and any number outputs.

Because of this unique structure, they can rather easily be made to have any number of outputs, including odd numbers such as 3-way and 9-way, with relative ease.  They also can operate from DC, with very wide bandwidth.

The tradeoff however, is twice the theoretical insertion loss versus an equivalent Wilkisnon of same number of outputs.  For example, a 2-way Wilkinson loses -3.01dB from splitting, while a 2-way resistive splitter will lose -6.02dB. 

Unlike the Wilkinson divider however, the resistors are placed in the direct RF signal path.  Hence, their power handling capability is inherently limited.  This fact, along with the higher insertion loss mentioned above, makes them mainly suitable for low power signal applications (typically 1W max.), such as synchronizing measurement equipment with common clock signals.

Pros: Wide bandwdith, compact size, any number of outputs.

Cons: Higher loss and lower isolation compared to other types.  Low power handling.

They are unique enough to warrant their own product line, which you can find here.

Reactive (Resistor-less)

Reactive splitters are often used for splitting high power signals. 

The absence of a dump resistor allows them to handle extremely high power levels if properly designed. 

The tradeoff is no isolation between outputs and poor output return loss.  Hence, if used for combining, care must be taken to ensure very well matched amplitude and phase between the combining signals. 

Pros: High power handling.

Cons: No isolation.  Poor otuput return loss.  Requires precise matching of external components.

Unequal Power Division using Couplers

In certan situations, such as distributed antenna systems (DAS), it may be desirable to split signals unequally.  This is best achieved using directional couplers in the forward direction. 

For example, a 6dB coupler may be used to provide a 75/25% split ratio, while a 10dB coupler will provide 90/10% splitting.

The directivity of the coupler provides the isolation.

It should be noted that there is no control of the phase relationship between outputs using this method.  This may not be an issue depending on the application, but is worth mentioning.

Pros: Practical method using readily available components.

Cons: No control of phase balance.

Quadrature (Hybrid) Splitters

These products are unique enough to warrant their own section, however they are indeed related to splitters. 

From a block diagram perspective, they function as a 2-way splitter, but the key difference is a 90-degree phase shift between the outputs.

The resistor is external, which allows for wideband operation (if the circuit is designed for it) and high power handling.

Werbel Microwave designs and manufactures hybrids with precision phase control.