For engineers involved in non RF circuit analysis or actual circuit board and desktop work, the main signal parameters of interest to them are the voltage and current at specific points in the design. These parameters can be measured using a voltmeter, oscilloscope, or current sensing resistor.
In contrast, workers in the wired and wireless RF fields focus on power in watts or milliwatts (mW), or decibels (dB) based on 1 mW (dBm). However, measuring RF power is not an easy task as there is no such thing as a simple voltage or current that would interfere with the power transmission signal pickup point. On the contrary, unique signal transmitters and schemes should be used to evaluate RF power levels.
Directional coupler is one of the most common methods, which is a passive device that can both "pick up" RF signals with a specified coupling degree and provide high isolation between the signal and the sampling port.
This is a fully validated technology that allows us to understand the working principle of directional couplers. Then, we will explore how advances in materials can drive the development of couplers, reducing them to micro surface mount technology (SMT) devices suitable for low-power circuits.
Working principle of directional coupler
The universal four port coupler has passive RF function, including coupling port (forward) and isolation (reverse or reflection) port (Figure 1, top figure). Directional coupler is a three port structure that does not require the use of isolated ports; This configuration is used for applications that only require a single forward coupling (directional) output (Figure 1, figure below).
The function of a directional coupler is to perform power sampling in the signal transmission line without changing the line characteristics. This is somewhat similar to using a high impedance voltmeter to avoid adding load to the power supply on the test line.
With this directional coupling technology, simple low-level detectors or field strength meters and power measurement devices can be used to measure signal power. A small portion of the fixed input power will be incident from input port P1 to coupling port P3 for measurement purposes. The remaining input power is transmitted (referred to as pass or output) to the transmission port P2.
One important advantage of directional couplers is their unidirectional power coupling characteristics; Only coupling unidirectional transmission power; Any unexpected power entering the output port will be coupled to the unused terminal isolation port P4 instead of port P3, but this situation will not interfere with the directional flow of the directional coupler.
Figure 1: A directional coupler is a three port passive RF functional device that can transfer some of the incident power on P1 to the coupling port P3 for measurement without affecting the main single path from input port P1 to transmission (output) port P2; A directional coupler is a unidirectional sub device of a four port bidirectional coupler. (Image source: Wikipedia)
These top-level parameters are used to specify directional couplers:
Coupling degree: The proportion of input power (at P1) transmitted to the coupling port (P3).
Directionality: This parameter represents the ability of the coupler to distinguish between forward and backward wave propagation, which can be observed from the coupling (P3) port and the isolation (P4) port.
Isolation: The power delivered to non coupled loads (P4).
Insertion loss: refers to the attenuation of input power at the transmission port, including the power component diverted to the coupling port and isolation port.
Return loss: This parameter represents the power reflected back to the P1 port due to impedance mismatch.
The use of advanced materials can reduce the volume of directional couplers
There are many methods to construct directional couplers. From a historical perspective, directional couplers have been achieved through waveguides or coaxial cables, which are still necessary for higher power applications. However, modern low-end RF circuits, such as those in base stations, require much smaller couplers. This can be achieved by using strip lines or microstrip processes on high dielectric constant ceramic substrates.
Microstrip line is a planar transmission line technology that uses a conductive strip isolated from the ground plane by a dielectric substrate. The complete devices, such as antennas, couplers, filters, and power dividers, are formed by metalized pattern structures on the substrate and have high-precision dimensional characteristics. Compared to other transmission line technologies, small devices constructed using microstrip line technology are lighter, more compact, and typically cheaper. This type of device can handle a medium level power of approximately ten watts.
Using high-K materials as substrates can shorten the wavelength of RF signals and reduce the overall size of the device. Please note that academic literature sometimes uses lowercase 'k', which is referred to as' kappa 'in more formal materials.
By utilizing directional couplers made of high-K materials and Knowles' high-precision thin-film microstrip process technology, RF designers can reduce the size, weight, and power (SWaP) of RF circuits while maintaining strict performance tolerances.
The advantages and effects of these high-K materials are very significant, as shown in Figure 2: the dielectric constants and corresponding wavelengths of three common dielectric materials (PTFE, FR-4, and alumina) and three customized substrates developed by Knowles (PG, CF, and CG) at 25 gigahertz (GHz). Their CF substrate has a dielectric constant of 25, while the dielectric constant of FR-4 material is 4.8. Therefore, devices made of CF material have a wavelength shortened to 2/5 of FR-4 material devices, achieving a significant reduction in device size.