Power dividers (also known as power splitters, or inversely, power combiners) and directional couplers are the most commonly used passive components in radio technology. They couple a certain amount of electromagnetic power from a transmission line to a port, allowing the signal to be used in another circuit. A power splitter is a microwave passive device that distributes the input signal energy to multiple output ports in a specific ratio. Its opposite function is a power combiner. Power dividers play a vital role in RF and microwave systems, and their performance directly affects the overall performance of the system.
I. Basic Introduction and Core Specifications
For an ideal N-way power divider, a signal at its input port (usually defined as Port 1) produces signals of equal amplitude and phase at its output ports (Port 2, Port 3, ..., Port N+1), and all ports are impedance-matched.
Key technical specifications for evaluating a power divider include:
Operating frequency range: The frequency bandwidth over which the device operates normally.
Insertion Loss: This refers to the power loss caused by a signal traveling from an input port to a specific output port. It consists of two components:
Theoretical Division Loss: This refers to the loss inherent in dividing energy into N paths, calculated as 10log10(N) dB. For example, the division loss of an ideal two-way power divider is 3 dB.
Excess Loss: This refers to the additional loss caused by conductor loss, dielectric loss, and radiation loss. This is a key indicator of device design and manufacturing quality.
Amplitude Unbalance: This refers to the maximum difference in output power between output ports across the entire operating frequency band. A smaller value indicates better power division consistency.
Phase Unbalance: This refers to the maximum difference in phase between output ports across the entire operating frequency band.
Isolation: This refers to the degree of signal isolation between any two output ports when each output port is connected to a matched load. High isolation means that signal changes at one output port do not significantly affect other output ports. Return Loss/Voltage Standing Wave Ratio (VSWR): A measure of the impedance matching between the input port and each output port. The greater the return loss (the closer the VSWR is to 1), the better the port matching and the less reflected energy.
Power Handling: The maximum input power a device can stably handle over a long period of time.
II. Operating Principle and Common Types
The design of power dividers is based on microwave network theory. Its core goal is to achieve power distribution while ensuring impedance matching at all ports. The following are several classic structures:
1. Resistive Power Divider
This is the simplest implementation, consisting of multiple resistors forming a T- or star-shaped network. Its advantages are simple structure, low cost, and an extremely wide operating frequency band (ranging from DC to tens of GHz). However, its disadvantages are significant:
There is no isolation between ports (i.e., poor isolation).
Resistors dissipate energy, resulting in insertion loss greater than the ideal divider loss, resulting in low efficiency.
Therefore, it is typically used in broadband measurement or signal sampling applications where performance and isolation are not critical.
2. Wilkinson Power Divider
The Wilkinson power divider is the most classic and widely used structure in the RF microwave field, particularly renowned for its high isolation and low loss.
Basic Structure (using bisection as an example): The input port and two output ports are connected via a quarter-wavelength (λ/4) microstrip or stripline line. A resistor (usually 2Z0, where Z0 is the system characteristic impedance, usually 50Ω) is connected in parallel between the two output ports.
Operating Principle:
Power Division: After the input signal reaches a node, it is transmitted to the output ports via two λ/4 transmission lines. The λ/4 lines perform impedance transformation, resulting in matched impedance as seen from the input ports.
High Isolation: When the two output ports are connected to matched loads, no current flows through the isolation resistor, dissipating no power. If a reflected signal is detected at one of the output ports, it is split into two paths and transmitted to the isolation resistor. Due to the λ/2 path length difference, the two signals are superimposed and dissipated at the resistor, preventing them from affecting the other output port, thus achieving isolation between the two ports. The disadvantage of the Wilkinson power divider is its relatively narrow bandwidth. By cascading multiple λ/4 lines, the operating bandwidth can be significantly widened. The Wilkinson structure can also be extended to N outputs.
3. Hybrid Couplers (such as 90° quadrature hybrids and 180° ring bridges)
These devices can function as both power dividers and combiners and have inherently high isolation and specific phase relationships.
90° Hybrid Coupler (Branch-line Hybrid): The output signals are equal in amplitude but 90° out of phase. They are commonly used in balanced amplifiers and phase detection circuits.
180° Hybrid Coupler (Rat-race Hybrid): The output signals are equal in amplitude, with the two output ports having a 180° phase difference. They are commonly used in mixers and power amplifier push-pull configurations.
III. Key Usage Scenarios
Power dividers are fundamental components of modern wireless systems:
Phased array radar systems: In the transmit channel, they distribute signals to hundreds or thousands of antenna elements; in the receive channel, they combine the signals from each element. The amplitude and phase consistency of the power divider directly impacts beam shape and pointing accuracy.
Multi-channel test systems: In RF automated test equipment (ATE), a single signal source must simultaneously drive multiple devices under test (DUTs) through a power divider to improve test throughput.
Indoor distribution systems: Base station signals are evenly distributed to antennas within a building, ensuring comprehensive coverage.
Power amplifier combining: Power dividers are used to distribute input signals to multiple power amplifiers, and power combiners are used to combine the outputs to achieve higher total output power. This places extremely high demands on the power divider's power handling and linearity.
Balanced mixers and amplifiers: 90° or 180° hybrid couplers are used to provide phase-specific input signals to mixers or amplifiers to suppress local oscillator leakage, improve linearity, or increase output power.
IV. Power Divider Selection and Design Considerations
In practical applications, selecting or designing a power divider requires comprehensive trade-offs:
Number of channels and division ratio: Is the power divider equal (e.g., 1:1) or unequal (e.g., 1:2, 1:4)? How many output channels are required?
Frequency and bandwidth: For narrowband applications, a single-section Wilkinson divider may be suitable, while ultra-wideband applications may require a resistive divider or a multi-section/tapered-line structure.
Performance priority: Is low insertion loss, high isolation, or excellent amplitude/phase consistency more important?
Power handling: Both average and peak power must be below the device rating, with sufficient margin.
Packaging and connectors: Depending on the system integration method, choose a surface-mount (SMD) package, a package with connectors (e.g., SMA, N-type), or a cavity package.
Cost and size: While meeting performance requirements, select the most economical and compact solution.
In short, a power divider is far more than a simple "signal splitter." The electromagnetic field and circuit theories behind it are profound, and their performance parameters are mutually constrained. A thorough understanding of its principles and characteristics is essential for the proper selection and design of this device, thereby ensuring the stable and efficient operation of the entire RF system.
