The landscape of power splitters, combiners, couplers and hybrides can be daunting at first glance. However, when we classify these devices according to their functions and take the different kinds of operating principles into account, the pictures becomes clear. To assist the RF engineer in choosing the right device for the job, we constructed a taxonomy of these devices.
In this article, we choose to look at all devices as three-port models. Because, in most cases, that is what they will physically look like. However, some of them really are three-port devices while others are implemented as four-port devices with one port internally terminated at 50 Ohms. The latter will look from the outside like a three-port device too.
All of these devices are passive and reciprocal. Passive means that there is no power generated inside the device, in a reciprocal device there is no difference in behaviour, dependent on the direction of propagation (Sxy = Syx).
These are the device model we will use in the taxonomy:
Figure 1 RF power device models used in the taxonomy
The proposed taxonomy is organized along several dimensions. The first starts by looking at the use case. What is the main function of the device? Next we take a closer look at the general properties of the devices described by its S-parameters. However, we will not only talk about S-parameters but also about their physical, meaningful names. These are referred to as their properties. All devices have similar properties but their (desired) values differ a lot according to the function of the desired qualities of the device. Function and property values will define out desired device.
Next we take a look at the operating principle of the device and the technology used to implement it. These will fix the function and its property's values.
Finally, we will bring all elements together within the taxonomy.
The following functions are recognized.
|Splitter / combiner||Splits power from an input port between to output ports or (vice versa) combines two signals.|
|Coupler||Takes a sample of the signal power flow|
|Phase shift||Shifts the phase between the two output ports by 90 or 180 degrees|
Table 1. Overview of device functions
Note that real life devices can often be used for more then one function. However, it is almost always optimized for one function over another.
2.2.1 Device Properties
Here we list all relevant device properties. As stated earlier, these all correspond with their different S-parameters. However, we choose to use their more meaningful (property) names which (in most cases) are self-explanatory in the particular application context. We don't go into a detailed treatment of each function / property set here. Since each function would deserve an article of its own.
|Bandwidth||fLow - FUp||fLow and FUp correspond with the lower and upper -3dB points in the graph of interest.||Talking about bandwidth makes sense not only for insertion loss but also for all the other properties as well.|
|Insertion loss||-20Log(S21)||Insertion loss (in dB) of the forward propagating signal from P1 to P2. For the power splitter / combiner, the insertion loss from port 1 to 3 is equally important (S31).||For the coupler, S31 should be as small as possible and is now called isolation. A good example that the names of S-parameters are dependent upon the application context.|
|Return loss||-20Log(Sxx)||Amount of reflected power at port X.|
|Isolation||-20Log(Sxy)||The isolation between port X and Y. For the splitter / combiner that is the isolation between port 2 and 3. For a coupler, the isolation is created between the ports 1 and 3.|
|Phase shift||arg(S21) - arg(S31)||Difference in phase between the output ports 2 and 3.|
|Quadrature||arg(S21) - arg(S31) = 90||Difference in phase between the output ports 2 and 3 is 90 degrees.|
|Coupling||20Log(S32)||Amount of 'sampled' power from port2 to port3|
|Directivity||Isolation (dB) - Coupling(dB)||
This value equals the (highest) return loss that can be measured at which there will be 100% interference with the power wave that is transmitted from port 1 to 3.
|If you want to know how well the coupler is suited for return loss measurements then this is the most important parameter.|
|Hybrid||S21 = S31||This is not really a separate property but it denotes the situation when the power is split between the ports in two equal parts (hence the name).||Note that this can occur in both a device designed as a power splitter or as a coupler. That is why the adjective hybrid pops up so regularly.|
Table 2 Overview if device properties
Note that both quadrature and hybrid are not really properties of a device, but a constraint put on some property values as already seen in table 2. So a quadrature hybrid is a device that will split input power into two output signals of the same power. Note that the insertion loss therefore will be at least 3 dB (power split by factor of 2).
If both output signals differ 90 degrees in phase, its called a quadrature device.
2.2.2 Property Values
Now that we have explained what the properties are, it is time to take a closer look at what property values would be ideal for our application. It will give us a reference point and a direction when it comes to evaluating operating principles and the technologies used to realize a device in practice. The values are listed in the next table.
Bandwidth requirements differ a lot between applications. However, there are four classes of bandwidth requirement. The first class is the use of broadband devices that start at DC and go up to 10s of GHz. These are mostly used in test setups. The second class are broadband devices that span more than a decade. The third class spans one or several octaves. Finally, there are the narrow-band devices that only cover half of an octave or less.
Both operating principles and the technology used in the device determine its bandwidth to a large degree.
Note that, in our taxonomy, the bandwidth requirements defines the region for which all device properties are as specified. If the device is operated outside its bandwidth specification, its behaviour is no longer defined.
For two-port hybrid power splitter and combiners the insertion loss values are either 3 dB for a lossless design and 6 dB for devices that dissipate power (by using resistors). These are the theoretical values, which can be approached fairly well.
Insertion loss of a coupler typically ranges between 0 and 6 dB
|Return loss||In general lower is better here. Minus 20 dB can be considered a really good value. The lower the return loss, the less signal amplitude variation will be created by standing waves due to reflections generated at the port under consideration.|
Of course, the higher the better. However, the required minimum isolation figures vary a lot depending on the application.
For the application of combining two signal together, the goal is to decouple the signal sources so that they will not influence each other. An isolation of 20 dB or better will do this job quite sufficiently.
But if we want to make a return loss measurement with a coupler, the situation quickly becomes much more demanding. If we want to measure a D.U.T. with a return loss of -20dB with an accuracy of 3 dB using a 6dB coupler then that device would need an isolation of 41 dB or better!
|Phase shift||This really depends on the application.|
|Quadrature||Nothing to evaluate here. It is set to 90 degrees per definition.|
The amount of coupling varies from around -3 dB (which leads to an equal power split between port 2 and 3) up to -20 dB. The lower the coupling, the less power is being taken from the forward propagating signal which results in a lower insertion loss.
|Directivity||This is the figure of merit for a coupler. If you want to measure a D.U.T. with a return loss of 20dB with an maximum measurement error of 3 dB, you will need a directivity of at least 35 dB.|
|Hybrid||A hybrid splits power in equal parts between its ports. 3dB and 6 dB are quite common values. In power amplifier applications, a 6 dB insertion loss is unacceptable so there you will find the 3 dB version. 6 dB splitters are used in R&D environments or at the testbench because of their very broad bandwidth specifications.|
Table 3. Ideal property values
In this section we will summarize common operating principles that are applied in splitters, combiners and couplers. We can't treat them in any sort of detail here. There wouldn't be space enough to give them the treatment they deserve. But we will point out how they impact the device properties. So you'll get a feeling of what operating principles will match well with a specific function.
|Operating principle||Circuit diagram
||Important design parameters||Properties|
|Branchline||Center frequency, length and impedance of the segments, symmetry of the layout. Port four is terminated 'internally'.||
This is a narrow band device. However, performance can be improved by adding an extra section. The phase shift of port 2 is -90 and for port 3 equals -180 degrees. Power is split in equal parts (hybrid). The operating principle is lossless (there is no power dissipated), so insertion loss for S21 and S31 are both 3 dB due to the power split.
This circuit can't be used as a coupler, because it doesn't posses the isolation property.
|Rat-race||Center frequency, length and impedance of the ring. Port four is terminated 'internally'.||
Narrow band design, phase shift depends on design used: or -90 at both output ports (rr-1) or -90 at one port and -270 at the other one (rr-2). Power is split in equal parts (hybrid). The operating principle is lossless (there is no power dissipated), so insertion loss for S21 and S31 are both 3 dB due to power split.
Ports 2 and 3 are isolated at the design frequency.
Configuration RR3 shows the use of the rat-race circuit as a coupler (note the reassignment of port numbers in order to stay consistent with our model in figure 1).
|Coupled transmission lines||Geometry of the transmission lines. Port four is terminated 'internally'.||Insertion loss for S21 is very low. This design isolates port1 from port 3, which makes it suitable as a coupler. The use of the transmission lines make it a rather narrow band device (one to two octaves at best). Phase shift is 0 degrees at port 2 and 90 degrees at port 3. So, it is a quadrature device too.|
Transmission line based design: center frequency, length and impedance of the two segments. Impedance-matching network.
Transformer-based design. Bandwidth requirements of transformer, impedance matching network.
The Wilkinson splitter splits power in equal parts. The circuit is lossless, so this is a 3 dB hybrid. What this design adds is isolation between port 2 and 3. There is no phase shift between the output ports. The input port (port 1) should have its impedance matched by some circuit.
Bandwidth of the transmission line based designs is approximately one octave. However, it can be improved by cascading multiple sections.
Bandwidth of the transformer based design (lower image) is determined by the transformer used. Typically, it can spend one or more decades.
|Resistive bridge||Insertion loss / coupling ratio by setting the appropriate resistor values.||
Due to the use of resistors, this is a true broadband circuit. In practice, its frequency range is limited mainly by the technology used to connect the bridge to the coupled port 3.
The design creates a fair amount of isolation between port 1 and 3. It can be used as a coupler (upper image) or a power splitter (lower image) as illustrated.
In itself, the design does not introduce a phase shift. However, a phase shift of 180 degrees can easily be introduced if a transformer is used to connect the bridge network to port 3.
|D / Y -resistive network||No parameters to tune.||Insertion loss and isolation between all ports is 6 dB. This is not a lossless splitter as half of the power (3 dB) is dissipated in the device. Hence the insertion loss of 6 dB (3dB due to loss and 3 dB caused by the power being split in two equal parts). This is a true broadband design as there are no frequency-dependent components used.|
|Divider network (two resistors)||No parameters to tune.||
Insertion loss (S21 and S31) are both 6 dB. Internal power dissipation is 3 dB. Its a true broadband design because there are no frequency-dependent components used.
Although the device might look rather similar to the D/Y splitter configuration, they both are actually quite different. First, the isolation in the two resistor network between ports 2 and 3 is 12 dB instead of 6dB.
But the biggest difference comes from the fact that the impedances at port 2 and 3 are not matched. This becomes clear immediately when you look at the S-matrix of the circuit as S22 and S33 are equal to 0.25 instead of (the probably expected) 0.
The circuit also possesses an equivalent return loss which can approaches 0. Which makes this circuit of special interest in applications that demand a return loss as low as possible. But this is a whole new topic in itself.
This power splitter is used in specialized application only like power leveling that uses a feedback loop and in applications where power ratio's are measured.
Table 4. Overview of operating principles
The last dimension to consider in the taxonomy is the technology used to implement the operating principles and their influence on the device properties.
Usable from DC up to 20 GHz, if special care is taken in resistor and substrate selection.
|Lumped RLC components||
Usable from DC up to several GHz.
The main limiting factors are the component's parasitic capacitance and (lead)inductances. In order to reach the GHz range, the components must be small, so the use of SMD components is common practice.
Usable from the KHz range up to 3 GHz.
RF transformers are much used devices that easily can span several decades of bandwidth.
Usable from DC up to 10s of GHz.
Although transmission lines operate down to DC, they aren't usable at these low frequencies because they would be impractically long (a quarter wave length at 1MHz is still 47.4 meter using an Er of 2.5). Therefore, transmission lines are used starting at the GHz range only.
Table 5. Overview of RF technologies
In this section, all dimensions are brought together in one overview. It gives you an overview of power splitters, combiners and couplers according to their function with its optimum property values (in green) on one hand and the influence of operating principle and technology on the actually realized device properties (in orange and magenta) on the other.
Figure 2 A taxonomy of RF splitters, combiners and couplers
This concludes the taxonomy of splitters, combiners, couplers, hybrids and quadrature devices. If you have any suggestions to help improve this taxonomy, then please leave a reply.