Use Surface-Mount Directional Couplers to Shrink RF Power-Monitoring Devices

For engineers involved in non-RF circuit analysis or hands-on board and bench work, the primary signal parameters of interest are voltage and current at specific points in a design. These can be measured using a voltmeter, oscilloscope, or a current-sense resistor.

In contrast, those working in the wired and wireless RF domain focus on power, measured in watts or milliwatts (mW), or decibels (dB) referenced to 1 mW (dBm). However, measuring RF power is not easy, as there is no equivalent to a simple voltage or current signal-pickoff point that won’t also interfere with the power flow. Instead, unique signal transducers and schemes are used to assess RF power levels.

One of the most common approaches uses a directional coupler, a passive arrangement that “picks off” RF signals with a defined degree of coupling while offering high isolation between the signal and sampled ports.

A well-proven technology, let’s look at how directional couplers work. We’ll then examine how they have benefited from advances in materials to shrink them to miniature surface-mount technology (SMT) devices suitable for low-power circuits.

How directional couplers work

A general four-port coupler is a passive RF function that includes coupled (forward) and isolated (reverse or reflected) ports (Figure 1, top). A directional coupler is a three-port structure that eliminates the use of the isolated port; this configuration is used in applications that only need a single forward-coupled (directional) output (Figure 1, bottom).

The role of the directional coupler is to sample power from a signal transmission line without altering line characteristics. It is somewhat analogous to using a high-impedance voltmeter so as not to load down the source on the line being measured.

This directional coupling enables signal power measurement using simple, low-level detectors or field strength meters and power measuring equipment. A small, fixed fraction of the power incident on the input port P1 appears at the coupled port P3 for measurement use. The remainder of the input power is delivered to the transmitted (called through or output) port P2.

An important benefit of a directional coupler is that it only couples power flowing in one direction; any power inadvertently entering the output port is coupled to the unused, terminated isolated port P4 and not to P3, but this is not an issue for the directional flow of the coupler.

Figure 1 : A directional coupler is a three-port passive RF function that diverts part of the incident power on P1 to coupled port P3, where it can be measured without affecting the primary single path from input port P1 to transmitted (output) port P2; it is a unidirectional subset of the four-port bidirectional coupler. (Image source: Wikipedia)

These top-level parameters are used to specify a directional coupler:

• Coupling factor: The fraction of the input power (at P1) delivered to the coupled port (P3).
• Directivity: A measure of the coupler’s ability to separate forward and reverse-propagating waves, observed at the coupled (P3) and isolated (P4) ports.
• Isolation: The amount of power delivered to the uncoupled load (P4).
• Insertion loss: The reduction in input power delivered to the transmitted port, accounting for power diverted to the coupled and isolated ports.
• Return loss: A measure of how much power is reflected back to P1 due to an impedance mismatch.

Advanced materials shrink directional couplers

There are different ways to build a directional coupler. Historically, it was done with waveguides or coaxial cables, which are still required for higher-power applications. However, modern lower-level RF circuits, such as those in base stations, need a much smaller coupler. This can be achieved using stripline or microstrip processes on high-permittivity ceramic substrates.

Microstrip is a planar transmission-line technology that uses a conducting strip separated from a ground plane by a dielectric substrate. Entire components such as antennas, couplers, filters, and power dividers are formed from metalized patterns on the substrate with high-precision dimensional accuracy. Tiny components built using microstrip techniques are lighter, more compact, and typically less expensive than alternative transmission-line technologies. They can handle modest amounts of power on the order of ten watts.

The availability of high-K materials as substrates results in a shorter RF signal wavelength and an overall reduction in component size. Note that academic literature sometimes uses a lowercase k, more formally referred to as κ (Greek kappa).

Using directional couplers fabricated with high-K materials and high-precision thin-film microstrip-process technology from Knowles, RF designers can reduce RF circuits' size, weight, and power (SWaP) while maintaining tight performance tolerances.

The beneficial impact of these high-K materials is dramatic, as seen in a comparison (Figure 2) showing the dielectric constant and wavelength at 25 gigahertz (GHz) for three common dielectric materials (PTFE, FR-4, and alumina), as well as three custom substrates developed by Knowles (PG, CF, and CG). Their CF substrate has a dielectric constant of 25 compared to a dielectric constant of 4.8 for FR-4 material. As a result, the wavelength for a device using CF material is 2.5 times smaller than that for a device using FR-4, leading to a dramatic shrinkage in device size.

Figure 2 : Thin-film microstrip directional couplers (left) leverage very high-K dielectric substrates to enable devices with significantly reduced size and weight (right). (Image source: Knowles)

Examples of SMT directional coupler performance

The performance and size of directional couplers based on microstrip technology and high-K dielectric substrates are seen in the Knowles FPC06073 and FPC07182 couplers, each supporting different ranges and bandwidths within the gigahertz spectrum (Figure 3, top and bottom, respectively).

Figure 3 : The FPC06073 (top) and FPC07182 (bottom) couplers perform well across their respective bands for four top-tier parameters: return loss, insertion loss, coupling factor, and isolation. (Image source: Knowles Precision Devices)

The FPC06073 50 ohm (Ω) SMT directional coupler covers 4 to 8 GHz with a 10 dB coupling factor and a directivity of 20 dB. Its diminutive size of approximately 4.3 × 2.0 × 0.38 millimeters (mm) (0.170 × 0.080 × 0.015 inches (in.)) makes it a good fit for compact designs. It is rated for 25 watts (continuous). Performance for the four metrics shown in Figure 3, especially coupling and insertion loss, is relatively flat over the entire band, with operating and storage temperatures both specified at -55˚C to 125˚C.

Moving higher in frequency, the FPC07182 SMT coupler is designed for 20 to 40 GHz. Like the FPC060073, it has a 10 dB coupling but a directivity of 10 dB. Even smaller at just 1.65 × 1.270 × 0.254 mm (0.065 × 0.050 × 0.010 in.), this 50 Ω device handles up to 14 watts and shows very flat coupling and insertion loss across the entire 20 GHz bandwidth.

Conclusion

Directional couplers based on high-permittivity ceramic substrates and microstrip techniques now provide this RF function in nearly invisible SMT devices, with outstanding performance and power handling across their designated gigahertz bands.

Related Content

1: Knowles Precision Devices, “Reduce RF Circuit SWaP with High K Materials and Precision Thin-Film Microstrip Technology”

https://info.knowlescapacitors.com/hubfs/White%20Papers/Device_Minaturization_WP_V7.pdf

2: DigiKey, “The Fundamentals of RF Directional Couplers and How to Use Them Effectively”

https://www.digikey.com/en/articles/the-fundamentals-of-rf-directional-couplers-and-how-to-use-them-effectively

3: DigiKey, “Solving the RF Power-Detection Challenge” (Cites Analog Devices)

https://www.digikey.com/en/articles/solving-the-rf-power-detection-challenge

4: DigiKey, “Tiny Directional Couplers Meet Demands of Compact RF Applications”

https://www.digikey.com/en/articles/tiny-directional-couplers-meet-demands-of-compact-rf-applications