Directional couplers typically create a 90-degree phase difference between outputs, crucial for precise signal processing in applications like 5G networks operating at 28 GHz.
Table of Contents
Fundamental Design
Directional couplers are critical elements in the RF and microwave engineering, which are designed to split the input signal with a defined phase difference, primarily 90 degrees. Such configuration allows for the highly specific and efficient control of the signal inputs and outputs in many applications, such as signal processing units. For example, if a 2 GHz signal is inputted into the coupler, the through port will then have a signal with the opposite direction and 180 degree phase shift relative to the input, whereas the coupled port output signal will have a phase shift of 90 degrees.
State-of-the-art directional couplers use proper arrangement of waveguides or transmission lines in a manner that ensures the required interaction between the electromagnetic waves, which is absolutely critical to guarantee that the coupler provides the needed phase accuracy and does not disperse a significant amount of energy. Direct digital synthesizers, mobile and public networks, such as LTE and GSM, all rely on the use of generating or sampling appropriate phases in order to implement the required signal processing, and a coupler is one of the optimal solution to ensure the coherence between different segments of the setup.
It is also essential for engineers to take into consideration the power rating of the coupler, which defines the effectiveness of the coupler at the operational frequency. Thus, for the example of home Wi-Fi systems, the power of the coupler is usually between 1 and 10 watt. In comparison, a highly effective directional coupler used in cellular radically probably manage roughly several hundred watts for efficient operation.
Naturally, the quality of the coupler directly affects the efficiency of the systems that utilize them and its performance in general, including how accurately the coupler maintains the phase and isolates the coupler arms. Thus, the choice of Teflon as a dielectric is preferable due to its lower insertion loss and better isolation capabilities. The price of the coupler depends on the grade of Teflon and can range between $100 and $200 for a commercial satellite communication system. The longevity of the coupler depends on the conditions of operation, but the standard lifespan is between 10-15 years for most couplers.
Purpose of Phase Shift
The phase shift in directional couplers is widely used in many areas of modern technology including telecommunications and signal processing. A simple example of the application of this phase shift is quadrature amplitude modulation , a method that is extensively used in TV broadcasting as well as LTE communication. Due to the ability of a coupler to adjust the phase of a signal by 90 degrees, it becomes possible to encode the information in both the amplitude and phase characteristics of the carrier wave. That way, the efficiency of data transmission is doubled and it is possible to code data using much more complex methods .
90-degree phase difference plays a particularly important role in the work of mixers and demodulators in various radio frequency applications. In these applications, to mix signals from two frequencies together and to separate them it is possible to use a mixer that is based on a 90-degree hybrid coupler. The advantage of the use of such mixers is the ability to achieve true conversion of frequency without marked degradation of the signal quality of the frequency that is being mixed.
This is possible since the quality of the received signal s strictly limited by the quality of the phase shift. Any shift of the available shift will result in the lost signal quality since the obtained signal will not be the true frequency of two initial waves. In the context of satellite RF applications, the provided frequency can well be prepared for transmission up to several kilometers. The cost of such miscalculations may be quite high as usually, it results in the loss of data for hundreds of thousands of users. The production of a 90-degree coupler that is good enough to be used in such conditions is also quite expensive with its decrease of quality. The cost of a 90-degree hybrid coupler used in satellite communication varies from $500 to $2,000 per piece.
Coupler Types
There are various types of directional couplers, depending on the signal processing and telecommunications they are intended for. A common one is the hybrid coupler, which features a 90 or 180-degree phase shift. These couplers are crucial for applications that require equal division of power on two outputs with a certain phase relationship, such as antennas arrays and beamforming networks. As a result, they help improve the performance of wireless systems by increasing the coverage and quality of signals.
Another widespread alternate design is the branch-line coupler. Not only does it generate a 90-degree phase difference but it also ensures evenly split power with high isolation between the transmission lines. The latter is particularly vital for cases when multiple transmitter or receiver paths have to coexist without affecting one another. In addition, the branch-line coupler is simple in design, which contributes to its low production price. They usually sell for $20-50, depending on the operating frequency and maximum power limit.
The rat-race coupler is a third common type, which produces a 180-degree phase shift. It is indispensable in applications, which require phase inversion or the process of balanced modulation or demodulation. Such couplers are characterized by a circular design that enables them to handle higher levels of power and cover greater frequencies up to several gigahertz. As such, they are optimal for radar systems and phase array antennas, where any signal phases should be balanced in order not to miss any target.
It is important to reference each type of coupler according to the power handling, exploiting frequency, physical attributes, and cost, and, considering the principal difference, explanation, and design. For instance, the hybrid coupler is superior in mobile phones, which require small size and efficient power division. Branch-line and rat-race couplers, conversely, are more common in radars and other aerospace and military case uses, where they are normally bigger and the maximum power and frequency are higher.
The usage of a certain type of coupler directly affects the efficacy and accuracy of the system. A particular wireless system usually has a certain maximum power rating and frequency. For instance, a cell base station often uses a hybrid coupler, which facilitates up to 200 watts and operates at around 2 GHz.
Impact on Signal Processing
The phase difference in directional couplers contributes to a vast proportion of signal processing. It is responsible for absolute control over signal manipulation and distribution. For example, a 90-degree phase difference allows combinations and separations of signals through different applications such as quadrature mixers. These mixers are crucial to frequency conversion processes. In particular, the signals’ frequencies from high to lower intermediate frequencies or vice versa facilitate the use of processing and amplification in radio receivers and transmitters. In a typical case, the communication system might be attributed to 2.4 GHz separated to a 100 MHz IF.
Moreover, this phase difference is necessary to beamforming in phased-array antennas. Notably, changes in the phase difference in the signals’ output direct the radiated signal to the desired direction of objects. This affects radar and communication systems through enhancing an array’s ability to detect and track the objects. Systems may be unable to keep high gain and directivity without accurate phase shifts. Additionally, reducing the destructive interference of signals is to ensure high data rates and low error rates in digital communication systems. Consequently, power dividers and combiners are responsible for dividing or combining these signals at precise phase shifts.
They ensure signal integrity and minimal insertion loss due to phase differences. In these systems, a 50% transmission and reflection are assigned to each port in the two output ports. However, their efficiency is affected by the cost and materials used. On one hand, quality materials and smaller components are used to maintain the phase difference close to 90 degrees. On the other hand, using these materials may result in the increased cost of power dividers and combiners. For instance, the use of high-quality components in a ceramic substrate in a directional coupler increases the power difference to over $100 per unit.
Additionally, the operational lifetime may exceed 10 years if the operational environment is adequately controlled. Notably, these are standard environments without deviations of temperature and humidity. In the field of medical imaging, an MRI machine utilizes the phase difference between signals. In turn, it is used in an imaging process and is vital to the production of a clear image. This difference ensures the images used will be highly accurate, which will affect diagnoses and treatment plans. For instance, if a 90-degree directional coupler is used, the resolution of the images will be high. This is crucial in the anatomical abnormalities found in soft tissue.
Measurement and Calibration
Hence, directions provided by the measurement results are reliable. In designs with significant complexity and heterogeneity such as coupled resonator optical waveguide CROW, the phase measurement may reveal that such phase difference may not looks as 90° and needs compensation.
The most common and primary method is the use of a vector network analyzer VNA which provide realistic and accurate phase-measured results. Phase differentiation between input and output signals of the directional coupler derive to both input and output signals of the VNA can be measured by it. In RF design and testing applications, a small phase difference could potentially alter the function of the coupler. For example, in a 5GHz 802.11a wireless LAN system, a 1-deg.
phase error could result in a considerable loss of throughput or even stop the wireless traffic. Directional coupler can be directly rectified to produce exactly 90° phase shift when differences has been found by calibration. In the high precision radar and radar altimetry applications that used in modern air traffic control, dorrection to a phase difference other than 90° will provide no additional information and could be catastrophe-cal to operators who rely on the radar for accurate altitude information.
In practice, calibration could involve more than tuning the coupler. Real-life adjustment of the physical component of the coupler such as rods and cylinders or change the electrical settings may be done to achieve the desired phase differences. Temperature as factor of influence, also provides field solution of phase drift by the directional coupler and may be also compensated by calibration. At the end this will effect the cost of the calibration service.
In our satellite communication example, phase error in coupler can only compensate through the temperature drift from the earth to the space, which varied from -150°C to 150°C. Thus, setup calibration to ensure the phase error never out of the normal operating range is the only answer. In the test facility, usual schedule with different test at before and after space take off with different range of temperature decided to ensure their setup normal work.
In our lab, we usually have a calibration schedule with fixed intervals to calibrate all our measurements instruments. It often depends on the criticality and use of the setups. For Low cost oscilloscope and spectrum analyzer, it may have a reasonably long 2 years time between calibration. For VNA, we could spend a whopping $1000 every six months for it. Of course the high cost comes since VNA is precision hardware equipment and it’s normal performance and reliable results is very crucial for our work.
Application-Specific Design Choices
The design considerations for application-specific directional couplers are extremely important, as they are used across a wide range of fields, from telecommunications to medical imaging. These choices are related to the phase difference between the outputs. Therefore, each type of coupler is tailored to address the specifics of the application at hand. In telecommunications, and especially in cellular networks, directional couplers must be suited to high-frequency signals. A common kind of coupler designed for a 5G network can usually operate at frequencies up to 28GHz. Moreover, the design accounts for high-field isolation and low insertion loss to minimize signal degradation.
The phase difference in couplers designed for such networks normally amounts to 90° to maintain signal integrity and enhance the efficiency of beamforming. Such couplers are relatively expensive due to the high quality needed for such stringent criteria. They stringently manufactured to ensure that they do no lose phase control or exhibit similar deficiencies. The cost is normally $300 or more because special advanced materials are used, and the design warrants particular precision.
In radar systems, the phase difference between the outputs is just as important but normally has to be 180° to produce the final composite that is used for target detection. The phase is strictly set at this value to ensure that appropriate signal is put into the reference receiver. In the case of an airborne radar, the couplers must be suited to high power, which in this case can get to levels significantly exceeding 1000 watts.
These requirements are embodied in the design by adjustments that account for the potential source of high power and the necessity of phase conservation. Such arrangements need special high-performance materials and can have a cost of $1,000 or more due to the complexity and, therefore, the need for specific expertise, as well as the use of rare materials such as high-grade aluminum, or even some composites.
Directional couplers for medical imaging, and particularly MRI systems, need to produce more precise phase shifts due to the specifics of the process. They normally operate at frequencies around 128MHz. The couplers themselves must not contain any magnetic materials that can obstruct the imaging process. Soft materials and high-quality ultra-low thermal expansion ceramics are used for the same reason, with the costs normally going up to $500 due to medical-grade requirements.
In the aerospace field, high temperatures and vibration are major considerations for couplers. Thus, in satellite communication systems, couplers must be able to operate in hold and cold space, where temperatures can go down to -150°C and up to 150°C. The design therefore calls for a lot of temperature-compensation features and most of the time titanium, or some polymers are used. Such couplers are very expensive, with costs reaching up to $2,000.
