RF directional couplers shrink in size, not performance: part 2

This essential measurement accessory for microwaves has morphed from waveguides down to tiny surface-mount components.

Part 1 looks at the need for and the basic principles of directional couplers. This part explores some of the construction techniques and implementations.

The basic arrangement of a single directional coupler is shown in Figure 1. There are two parallel transmission lines with a length of one-quarter wavelength, corresponding to the center frequency of operation. The main and secondary lines are separated by a calculated physical distance, which determines the device’s coupling factor.

Figure 1. The conceptual physical principle of the directional coupler uses two parallel lines, with energy of one coupling into the other. (Image: MECA Electronics, Inc.)

In simple terms, the closer the lines are to each other, the more RF or magnetic energy they have, and the more power will be introduced onto the secondary line.

Q: Who are the vendors that offer directional couplers?
A: As they are so widely used, they are offered as standard products by dozens of sources. With few exceptions, every component vendor serving microwave and millimeter-wave system designers offers them.

Some vendors specialize in a single fabrication technology, while others support multiple technologies. Also, some vendors specialize in a limited frequency band, such as 10-to-20 GHz, while others offer a list of dozens of wideband and narrowband models for different slices of the spectrum. Choosing a vendor can be a daunting decision. It is analogous to choosing the “right” op amp among the dozens of vendors and hundreds of models, which differ in both major and minor specifications.

Q: What are some of the construction techniques used?
A: Among the possibilities are directional couplers based on waveguides, coaxial cables, transformers, monolithic microwave integrated circuits (MMIC), and stripline/microstrip. We will look at waveguide, transformer, and microstrip implementations.

Waveguide directional couplers

Q: What is the principle of the waveguide-based directional coupler?
A: As the name indicates, it begins with a primary waveguide that is sized for the frequency range in use. Anther smaller pickoff waveguide is attached alongside, as seen in Figure 2. A pair of matching holes between the two allows some of the power to “leak” from the primary waveguide to the pickoff waveguide.

Figure 2. The basic waveguide-based directional coupler shows the physical arrangement and flange connections for its three ports. (Image: Narda-MITEQ)

The amount of coupling and the operating range are determined by the size and location of the matching holes. In some cases, multiple sets of matching holes are used to widen the operating span or adjust other parameters. Every dimension is critical to performance, so high-accuracy machining is needed.

Note that waveguides, originally built from solid pipes, were the first RF-transmission media and predate our now-common coaxial cable. They are still used for the highest power levels (kilowatts and above) or when the lowest-loss performance is needed. The industry has standardized different flange sizes for waveguides called WR sizes so they can be easily connected to other waveguides. There are also coaxial cable-to-waveguide interface assemblies so that a waveguide can be “mixed” with a coaxial RF transmission line.

Q: What are the dimensions of a representative waveguide-based directional coupler?
A: For the coupler units above, Table 1 shows the key dimensions for the vendor’s lowest- and highest-frequency models:

Table 1. These are the primary linear dimensions for the two frequency extremes of this waveguide-based family of directional couplers. (Image: Narda-MITEQ)

Q: Are these the only waveguide-based directional couplers?
A: Not at all. There are many variations, including 4-port couplers, crossguide directional couplers, crossguide directional dual-arm couplers, broadwall multi-hole dual-arm couplers, loop couplers, and double-ridge loop couplers. Each serves a specific need and solves a well-defined problem, but the details are beyond the scope of this article.

Transformer directional couplers

Q: What is a transformer-based directional coupler?
A: As the name indicates, these units use coupled coils to achieve the coupling performance and have a simple schematic diagram, seen in Figure 3.

Figure 3. The transformer-based directional coupler uses pairs of windings with appropriate phasing, indicated by the dots. (Image: Michael Gellis)

Two coupled transformers usually implement the generic transformer directional coupler. These split the signal flowing in the primary line unequally. Ideally, some portion of the signal flowing into input port A will appear at coupled Port C. Similarly, any signal flowing into port C will be coupled fully to port A. However, output ports B and C are isolated, so any signal flowing into port B will not appear at port C but will feed through to port A.

Q: Sounds easy, but what are the issues?
A: In RF designs, things may appear simple, but they always have many subtleties. One way – and not the only way – to build this coupler is to use a ferrite “binocular” for both the T1 and T2 transformers on a single twin-hole ferrite core, shown in Figure 4.

Figure 4. One widely used transformer-based construction approach uses a binocular ferrite core for controlled coupling. (Image: Michael Gellis)

The characteristics of the ferrite material determine the low-frequency response. High-frequency response is partially governed by total wire length, since the core effects are no longer dominant near the high-frequency end. Interwinding capacitance, leakage inductance, copper losses, and transformer coupling with a coupling factor below unity (k < 1) also degrade high-end performance.

Small shunt capacitances to ground at the coupler ports can be used to improve match and directivity at the expense of bandwidth. At higher frequencies, lead length must be kept to a minimum to limit parasitic inductance. The entire design must be carefully modeled to account for these effects. The construction must follow the modeled design, but with additional corrective tweaks as needed.

Q: What does a transformer-based coupler look like, and what are its ratings?
A: These couplers can be designed to operate up to about 1 GHz. For example, the Mini-Circuits RDC20-92DC-5W+ is a directional coupler that provides 20 dB of coupling from 150 MHz to 900 MHz. It is a tiny PCB with an on-top transformer and is designed for surface mounting, as shown in Figure 5.

Figure 5. This transformer-based directional coupler mounts the transformer and windings on top of a tiny surface-mount-compatible carrier. (Image: Mini-Circuits)

Microstrip directional couplers

Q: How does the microstrip directional coupler design work?
A: Microstrip refers to a type of planar transmission line technology that consists of a conducting strip separated from a ground plane by a dielectric substrate. Entire components are formed from metallized patterns on the substrate. Modern dimensional precision, including accuracy and repeatability, is key to making this a viable technique.

Q: Why are microstrip and other MMIC or monolithic approaches seeing increased use and acceptance?
A: First, it can be done using today’s technology. Second, these couplers are lighter, more compact, and typically less expensive than alternative technologies such as waveguides, presuming the microstrip can handle the power. They are especially attractive for situations such as base stations and even satellites, where power levels are low (under 10 W) and space, weight, and cost are critical factors.

Q: In addition to the microstrip process technology, what factors are critical to making this a viable technique?
A: A high dielectric constant (also called permittivity; they are different but very closely related) is critical for the ceramic substate and overall performance. For example, Knowles Precision Devices uses a proprietary CF material with a dielectric constant of 25, compared to a dielectric constant of 4.8 for FR-4 and 9.9 for Alumina (99.6%). As a result, the wavelength for a comparable device using CF material is 2.5× smaller than the wavelength for the device using FR-4, with some of the final units shown in Figure 6.

Figure 6. These directional couplers utilize microstrip technology and a high-permittivity ceramic substrate, resulting in a small size, low profile, low weight, and low cost. The coin shown is a US dime (diameter: 0.7 inches, or 18 millimeters). (Image: Knowles Precision Devices)

Q: What is an example of a microstrip-based, high-dielectric substrate directional coupler?
A: The FPC06078 from Knowles Capacitors is a good example. It is a 12-to-18 GHz, 20 dB, 50-ohm directional coupler that measures just 0.1 × 0.08 × 0.015 inches (2.54 × 2.0 × 0.38 millimeters) in height, yet it can handle up to 25 watts of input power. Its performance is comparable to that of directional couplers using other technologies, except for waveguides, as seen in Figure 7.

Figure 7. This single graph displays the four key parameters for a directional coupler using microstrip technology on a proprietary substrate. (Image: Knowles Precision Devices)

Q: What packaging is used for directional couplers?
A: Obviously, waveguide and surface-mount devices don’t have distinct enclosures in the conventional meaning. However, for many couplers based on other technologies, along with the practicality that users need or prefer, these units often come in small, shielded enclosures.

Depending on the frequency and version chosen, almost any standard microwave connector is offered in these packages. Many vendors offer the same coupler core but with a user choice of connector. As these enclosed coupler boxes have a meaningful size, the vendors can also put their name, model number, and connector callouts on the housing, as seen in Figure 8.

Figure 8. Many directional couplers are packaged in a shielded enclosure with easy-to-access connectors and clear labeling; this unit has basic identification labeling as an added benefit. (Image: KRYTAR, Inc.)

This Krytar directional coupler is part of a broad family of units that cover 0.3 GHz to 125 GHz in various overlapping ranges, handles up to 20 watts, offers a choice of 6, 10, 13, 16, 20, 30 dB coupling, and options for female connectors of 1.0, 1.85, and 2.4 mm. It measures 1.3 x 0.50 x 0.62 inches (L × W × H), not including connectors; other units in the series are about 50% larger.

Q: What sort of analysis is done when designing a directional coupler?
A: As with all microwave/millimeter wave components, the principle may appear simple, but the execution due to details is not. Vendors of these devices use extensive modeling, beginning with electromagnetic field theory and principles. They must then account for and adjust for parasitics, tolerances, temperature effects, and more. The analysis ranges from intense, equation-laden academic analysis, featuring wave equations and vectors, to mechanical arrangements, electrical parameters, and mass-production considerations.

That’s the bad news. The good news is that users, or buyers, of these components don’t need to worry about these details. What they require and get instead is a detailed data sheet, including graphs, tables, S-parameters, dimensions, and temperature performance, so that they can choose and design effectively with the coupler.

Summary

Directional couplers are among the many underappreciated yet essential and widely used passive components in microwave and millimeter-wave design. They come in many forms based on vastly different technologies and implementations, offering a range of specifications for operating band, bandwidth, flatness, top- and second-tier parameters, size, power handling, connectors, packaging, and more. They are offered by a large number of credible vendors, so there are many variations to choose from.