For the PDF version of this article, click here.

Designed for RF and microwave signal routing, directional couplers are general-purpose devices used for isolating, separating or combining signals. Consequently, they find use in a variety of power measurement and monitoring applications. Such couplers separate signals based on the direction of propagation (Figure 1). The wave induced in the auxiliary arm (coupling line) is proportional to the forward wave traveling in the transmission system (main line). The ratio of the induced power to the input power is known as the coupling factor.

Hence, coupling factor = 10 log P₂/P₁, where P₁ is the input power, and P₂ is the coupled power.

Directivity is another important parameter. It is defined as the difference in dB of the power output at a coupled port, when power is transmitted in the desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction.

Directivity = 10 log P₂/P_2R, where P₂ is the coupled power for the forward signal, and P_2R is the coupled power for the reverse signal.

Directivity (dB) = Isolation (dB) — Coupling (dB)

Finite isolation is the reason for limited directivity.

Power meters measure power in the transmission line by measuring the output of a directional coupler at the coupling port. This output is affected by the coupling factor.

The power reflected in the main line will produce a measurement error (directivity error).

Power measurement accuracy can be improved by eliminating the reflected signal or by using a high directivity coupler.

Mobile phones receive and transmit signals through an antenna connected to RF circuitry.

In the 3G band (Figure 2), WCDMA modulation results in a variable signal amplitude typically delivering a peak to average output power in excess of 3 dB, spread over a channel bandwidth of 5 MHz shared among up to 256 users. The output power level of the mobile handset is tuned by the base station, which requires that uplink signals from all mobiles transmitting in the same channel be received at a power level within a 1 dB accuracy. Failure to meet this requirement results in interrupted calls for the weakest signals, hence a poor level of service perception from the users standpoint.

By contrast, the GSM band uses the TDMA standard with GMSK-type signal modulation where the mobile station is the only user in a given time slot and channel (Figure 2). This means that the impact of poor power control on the other users of the network is negligible.

Any changes in the close environment of the antenna may result in mismatch and under certain circumstances increased reflected power. The maximum VSWR, which we will consider is 6:1, is the worst case when the antenna is not extended.

In order to transmit the forward signal and reject any reverse signal coming from the antenna because of mismatch, isolators are inserted between the power amplifier (PA) output and the antenna. Normally, input and output impedances are 50 Ω.

However, there is always a strong incentive to eliminate the isolator for the following reasons:

• Isolators are thick, bulky and take up valuable PCB area.
• Isolators are expensive.
• Isolators introduce a high insertion loss (typically above 0.7 dB).
• Isolators are temperature sensitive and exhibit aging.

Directional couplers have been developed to monitor the PA output signal in power control applications and they can be adapted to monitor the transmitted power and the reflected power resulting from the antenna mismatch (Figure 3).

The “Vi/Vr” method consists in monitoring both the incident and the reflected power. When excessive VSWR is detected at the antenna, the output power is reduced to a value as close as possible to the target power.

Implementation of the Vi/Vr concept with standard directivity couplers (below 10 dB) is impractical because 1) It requires one coupler for each direction; 2) control accuracy is poor; and 3) cost and complexity are high.

Couplers with high directivity (above 15 dB) and symmetrical performance are critical for the precision monitoring of incident and reflected power signals They increase the accuracy of the power control circuit and allow the use of a single coupler.

A single miniature AVX wideband high directivity coupler detects Vi and Vr allowing the measured ratio between incident and reflected power to be used for precisely controlling the output power.

It is important to note that this ratio is independent of the coupling factor accuracy (as long as the signal to the detector is sufficient). It is, however, strongly dependent on directivity as illustrated in Table 1.

To meet the stringent requirements of this application, AVX has developed high directivity couplers. High directivity in a small physical size is difficult to achieve. The coupler directivity can be optimized by balancing contributions from electric and magnetic fields, as well as accurately applying phase calculations. Plus, to achieve precise geometry of metal conductors (Figure 4), the company has exploited its semi-conductor manufacturing techniques such as metal sputtering, lithography and plating.

Consequently, using its thin-film multiplayer technology, the company has readied high directivity couplers like the CP0402 and CP0603. These couplers provide a miniature part with excellent high-frequency performance and rugged construction for reliable automatic assembly (Figure 5). They are offered in a wide range of frequency bands and intermediate coupling values.

Some key characteristics of these couplers include a frequency range of 400 MHz to more than 6 GHz, a coupling factor range from 8.5 dB to 27.5 dB at 1950 MHz, and low insertion loss (0.2 dB). Directivity is specified at more than 20 dB. They come in 0402 and 0603 package sizes.

Implementing the output power control technique described offers the following benefits:

• Cellular handset talk time is increased.
• PA output power is accurately controlled.
• An isolator is no longer required.
• Cost is reduced.
• Insertion loss between the PA and the antenna is minimized.
• PCB height and size are reduced.

Leonid Talalaevsky is the marketing manager for AVX Thin Film Products, AVX Corporation.