Dielectric Concerns For Directional Couplers
Directional couplers are vital components for sampling signal power in an RF/microwave system without necessarily disturbing the signal path. Such couplers come in many forms, including in metal housings with coaxial connectors. A typical coaxial directional coupler has four connectors, for input, output, coupled, and isolated ports. The coupled port provides a small amount of power taken from the input port, defined by the coupling factor, such as a 20-dB coupler. The isolated port offers power flowing in reverse from the output, although this port is typically not used and terminated in 50 Ω. In fact, a directional coupler is a linear device, and can be used in reverse, with the output as the input and the isolated port as the coupled port.
The signal path from a directional coupler’s input port to its output port is usually referred to as the main line path, while the path from the input port to the coupled port is the coupled path. As much or as little power can be coupled from the main line path, to a limit of about 3 dB power. In a 3-dB coupler, one-half of the power at the input port is coupled from the main line path. In a 20-dB coupler, one-hundredth of the input power is coupled from the main line path. Such small samples of signal power are typically used for power measurements, for example, to calibrate a receiver’s sensitivity or to monitor transmitted power at an antenna for a cellular tower or broadcast facility.
Directional couplers are characterized and compared by a handful of parameters, including the coupling factor, directivity, isolation between ports, and insertion loss, all with reference to a specified operating frequency range. The coupling factor (in dB), for example, is based on the relationship of the input power to the coupled power. It tends to vary with frequency and is described by coupling flatness with frequency.
For most passive components, insertion loss describes the difference in power between the output port and the input port. In a directional coupler, some of this loss is due to the coupled power. Ideally, a directional coupler’s insertion loss would just be the coupling loss. But other losses come into play, due to conductor losses and dielectric losses from the printed-circuit-board (PCB) material. Thus, the choice of circuit material can have an impact on the performance that can be expected from a directional coupler design.
The simplest directional coupler is formed by a pair of transmission lines close enough together that energy from the main line passes to the coupled line. Directional couplers can be designed and fabricated with numerous transmission-line technologies, including microstrip, stripline, and coplanar-waveguide (CPW) technologies. Couplers have also been designed with more-exotic circuit techniques, including substrate-integrated-waveguide (SIW) structures and microstrip coupled lines using a dielectric material overlay. This latter approach creates a circuit akin to a stripline configuration, with dielectric materials beneath and on top of the circuit conductors. Microstrip transmission lines are popular for directional couplers for their ease of integration with other microstrip components, such as amplifiers and filters.
In all transmission-line cases, the quality of the PCB material is critical to directional-coupler performance. Characteristics such as coupling factor and directivity assume a homogeneous dielectric material as the base for the coupler’s transmission lines. The coupling accuracy depends on the dimensional tolerances of the transmission lines and the precision of the spacing between them. For accurate coupling, a directional coupler must maintain 50-Ω impedance from input to output and coupled ports. In the coupled region, there are also critical concerns with even- and odd-mode impedances. The width of the transmission lines as well as the dielectric constant of the PCB material determine the characteristic impedance of the transmission lines, calling for tight tolerances for the lines and consistent dielectric properties for the material. The thickness of the dielectric material can also have an impact on impedance and coupling performance.
When directional couplers have been designed on PCB materials with less consistent behavior, such as FR-4, compensation techniques are typically used to adjust for variations in the dielectric constant of the material with frequency and temperature. These techniques can include shaping the transmission lines to correct for impedance variations resulting from the shifts in PCB dielectric constant. Unfortunately, whatever cost savings realized by the use of the FR-4 material are offset by the higher fabrication costs needed to form the specially shaped transmission lines.
For these reasons, PCB material for a directional coupler should be chosen carefully. The material should exhibit its advertised permittivity values at their specified frequencies so that design calculations and simulations performed with a commercial computer-aided-engineering (CAE) software program will provide valid and usable results.
One PCB material that has proven its value for directional couplers is RO4350B™ laminate, a reinforced hydrocarbon/ceramic laminate material from Rogers Corp. (www.rogerscorp.com). RO4350B laminate, which is also ideal for cellular-radio antennas and amplifiers, boasts stable electrical properties as a function of frequency, simplifying the fabrication of controlled-impedance transmission lines needed for high-performance directional couplers. It has a dielectric constant of 3.48 at 10 GHz. In addition, its manufacturer provides a value of design dielectric constant of 3.66 for accurate results when used with commercial CAE simulators.
As evidence of the suitability of RO4350B laminate for directional couplers, modeling/design/test specialist Modelithics (www.modelithics.com) designed and fabricated a trio of 10-dB couplers on the PCB material, and compared their simulated performance with actual measurements. They used single-layer, 20-mil-thick RO4350B material, designing the couplers at center frequencies of 850, 2050, and 2550 MHz and evaluating them for frequency bands of 0.7 to 1.0 GHz, 1.8 to 2.3 GHz, and 2.3 to 2.8 GHz, respectively. They actually built four couplers, designing a serpentine version of the 850-MHz coupler for reduced size. Simulations of scattering (S) parameters were performed from 40 MHz to 10 GHz based on microstrip models within the Advanced Design System (ADS) simulation software from Agilent Technologies (www.agilent.com). Measurements were made on a commercial vector network analyzer (VNA), calibrated using a custom calibration substrate designed by Modelthics on 20-mil-thick RO4350B material.
The results of the measurements reinforce the rock-solid consistency of the RO4350B PCB material. Performance design goals for the 10-dB directional couplers across their evaluation frequency bands included reasonable minimum values of 20-dB directivity and 14-dB return loss, with a somewhat optimistically low figure of 0.25 dB for insertion loss. The actual performance as provided by the VNA measurements included coupling of 9.5 dB for all designs, with directivity from 25 to 27 dB and return loss of 16 to 20 dB exceeding expectations. The only “disappointment” in terms of the initial design goals was for insertion-loss performance, which was measured from 0.8 to 0.9 dB—representing somewhat more realistic values than the low 0.25 dB design goal.
The Modelithics study points out the consistency of the RO4350B material for such material-sensitive components as directional couplers. Working with the “design” dielectric-constant value for the material, the simulated results closely matched the measured results through 10 GHz, indicating the stability of the PCB material and its transmission lines.
Next month, the ROG blog series will take a closer look at transmission lines, in the first of a two-part series on impedance matching.
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