Due to increased wireless connection requirements for Internet of Things (IoT), cellular communication, automotive electronics, various systems are increasingly employed RF signals, components and subsystems. Designers often need to direct these signals to multiple destinations, or combine multiple signals. However, the combination or separation signal may be more difficult because the designer needs to ensure that the signal routing is not reduced due to impedance mismatch or load, and it is always necessary to meet key sizes and cost requirements.
The RF power splitter and the combiner can satisfy such a need to separate or combine signals between multiple inputs or outputs. While these utilities are executing these tasks, you need to keep all signal sources to properly load impedance, and provide isolation.
This article will introduce the basics of the following three common RF power distributor / combiners: resistance, hybrid and Wilkinson, and use product instances from Susumu, Anaren, Macom, and Analog Devices. It will discuss the specifications and common applications of these devices, helping designers make sensible devices, including implementation considerations.
Power distributor
The power distributor has a single input signal and two or more output signals. The power level of the output signal is 1 / n of the input power level, where n is the number of outputs of the dispenser. In the most common power distributor, the signal of the output is in the same phase. Some special power distributors can provide controlled phase shifts between the outputs. As mentioned above, the common radio frequency application of the power splitter is to direct a common radio source to a plurality of devices (Fig. 1).
The first example is the phased array antenna, where the radio frequency source is separated between the two antenna elements. This type of antenna typically has two to eight or more elements, and each element is driven by the power splitter output port. The phase shifter is typically outside the power splitter to achieve electronic control, the direction of the guide field.
The second example is an orthogonal demodulator. The local oscillator needs to provide signals for two mixers, and the mixer further dispenses the frequency carrier demodulation as the same phase (I) and orthogonal (q) modulated components. The 90 ° phase shift required to demodulate Q signals can be implemented outside the power splitter (as shown in the figure), or within the power splitter. In both cases, the signal power level is equal.
If the power splitter "reverse" runs, the multiple inputs will be combined into a single output, and the shake becomes the power combiner. In the combiner mode, these devices can be subjected to the vector addition of the signal based on the amplitude and phase value of the signal.
Power distributor topology
When attempting to separate a signal into two attenuated amplitude components, the designer may be relatively simple, i.e., using a "T" shape to place two loads on a common source (Fig. 2).
This configuration seems to work, but it is limited. The most obvious problem is that the impedance does not match. If two outputs (port 2 and port 3) are fed in 50 ohms (W), the load of the input port (port 1) will be 25 W. If the input source is a 50 W device, the load problem will be brought. The second problem is lack of isolation. For example, if one of them outputs a short circuit, another port is shorted.
Three main circuit topologies of the power distributor can eliminate the limit of T-shaped connections. These three topologies are: resistance, hybrid and Wilkinson (Figure 3). Wilkinson Power Distributor and Hybrid Power Distributor belongs to the same class distributor called passive distributor.
Resistance power distributor
As the most common power splitter, the resistive power distributor uses three equivalents of resistors, most common in the star configuration. Due to the symmetry of the device, no port is used as an input port. The value of the resistor is one-third of the feature impedance used by the power distributor. For 50 W systems, this value is 16.67 W; for the 75 W system, this value is 25 W. As a group, since there is no passive component associated with the frequency in the design, the resistive power distributor typically has a widest frequency band width.
The main advantage of the resistive power distributor is simple; it can be easily implemented using the minimum cost. It is still the smallest device. The main disadvantage is the power loss of the series resistor between the output port. These devices have rated power specifications. Most applications of the resistive power distributor use relatively low power consumption. Compared to T-shaped configuration, the isolation effect provided by the resistors between ports is enhanced.
The signal amplitude of the resistive power distributor output port is half the level of the input signal (Fig. 4).
The trace of the upper left corner represents the input signal, a 50 MHz sinusoidal pulse, the root mean square (RMS) amplitude is 179.5 mV. The traces of the upper left and left corner mesh indicate the output signal, and the thial amplitude is 91.7 mV and 88.7 mV, which is lower than the input signal, which is -5.8 dB and -6.1 dB. The three traces on the right are horizontal, showing detailed information. Please note that all signals are in concert, as we expect.
An example of a resistive power distributor is a SUSUMU's PS2012GT2-R50-T1. This 50 W, two-port power distributors provide 20 GHz bandwidth, rated power dissipation of 125 mW, insertion loss is 6 ± 0.5 dB, where 3 dB is caused by power dissipation in the internal resistor. The device adopts a surface mount package with a size of 2 x 1.25 x 0.4 mm.
Wilkinson Power Distributor
The Wilkinson power distributor is a passive dispenser that uses two parallel and non-coupled quarter-wavelength transmission line transformers. Due to the use of transmission lines, the function of Wilkinson distributors can be easily implemented using standard printed circuit transmission lines. The length of the transmission line typically defines the frequency range of the Wilkinson distributor to be 500 MHz. The resistors between the output ports allow these distributors not only with a matching impedance, but also provide isolation. Since the output port includes a signal having the same amplitude and phase, there is no voltage across the resistor, so there is no current, and the resistor does not generate any power dissipation.
Anaren's PD3150J5050S2HF is two ports, 50 W Wilkinson-type power distributors, frequency ranges from 3.1 GHz to 5 GHz, maximum power rated value of 2 W. The dispenser has a 1 dB (typical value) insertion loss (excluding 3 dB power reduction), and isolates greater than 15 dB (typical). The size is 2.0 x 1.29 x 0.53 mm.
Hybrid power distributor
The hybrid power distributor shown in Figure 3 is based on the transformer. The transformer T2 is a central tap-centered, which constitutes an autotransformer having a turns ratio of 2: 1. The impedance of the entire output is four times the impedance of the center tap to the ground. If each output port (port 2 and port 3) is 50 W, the total load impedance is 100 W. Reflect by the transformer backwards, the impedance of the T2 center tap is 25 W. To match this load with the input (port 1), a transformer T1 is required, which is an impedance matching transformer of 25 w to 50 W.
When the input is applied to the port 1, the port 2 and port 3 use 50 W load termination, the current will result in current, and the phase shift is 180 °. The impedance of the resistor R is equal to the total impedance sum of the port 2 and the port 3 (in this case 100 W), and the phase is equal to the opposite of the current, and therefore, it will serve. There is no voltage from the signal from the port 3 on the port 2, and vice versa. In theory, Isolation has reached infinity. The power of each output port is half of the input power.
Macom's MAPD-009278-5T1000 is a hybrid power distributor with a frequency range of 5 MHz to 1 GHz. It is configured as a two-port zero distributor. Its insertion loss (excluding 3 dB is reduced) less than 1.4 dB. Isolation specifications are typical 20 dB. The dispenser can handle the power level of up to 250 mW, with a physical size of 4.45 x 4.22 x 3 mm.
Source power distributor
Applications that require lossless signal allocation can use active power distributors, such as ADA4304-3ACPZ-R7 of Analog Devices. It is a 3: 1 power distributor with a built-in amplifier, which provides 3 dB gain. The device has a bandwidth of 2400 MHz and can be used within a frequency range of 54 to 865 MHz. The input and output are preferably 25 dB. The 75 W impedance and frequency range determine this dispenser for television applications, including multi-tuner set-top boxes and pre-tier TVs.
In the device described above, the resistance power distributor is the most simple, the bandwidth may be available, and the size is minimal, but their insertion loss is high and the isolation is low. The insertion loss of the Wilkinson power distributor is low and has higher isolation, but the bandwidth is limited. The physical dimensions varies with the required specific frequency range. The insertion loss of the hybrid power distributor is low, providing good isolation, but the physical dimensions are large. The active power splitter eliminates insertion loss, but it is often more expensive.
Implementation consideration
Although the power distributor is very simple, if it is incorrect, they will still lead to problems. For example, pay attention to the DC offset of the input. A mixed combiner using a transformer does not generate direct current.
In the resistive power distributor, the presence of direct current reduces their power rating. All passive power combines use symmetrical topologies, and designers must maintain this symmetry when applying. Must match and balance the load. The use of mismatched load impedance will result in the output level is not equal.
In applications where fixed phase differences are required, for example, feed local oscillators to the orthogonal modulator or demodulator, the output path length must be equal to preventing the phase of the mixer from doing.
Summarize
In modern RF design, various applications require a combination or separation signal, such as in various applications such as Internet of Things, digital communication, and automotive driver assistance. The power distributor / combiner can provide such a function. Designers who need to use the power splitter can select one from the above three power distributor topologies, each topology has its own advantages and disadvantages. Understand the basics associated with the characteristics of each topology, help designers choose the appropriate power distributor.
Our other product:
