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Switchable Directional Coupler IC Lends a Hand to RF Front Ends (.PDF Download)

March 29, 2018
Switchable Directional Coupler IC Lends a Hand to RF Front Ends (.PDF Download)

A directional coupler in cellular RF front ends is located between the power-amplifier/antenna-switch module and the antenna tuner. It’s used as part of the power control and antenna tuning loops.

1. A switchable directional coupler is located in the RF front end of handheld cellular devices.

Infineon’s BGC100GN6 integrated circuit (IC) contains a directional coupler, a direction switch for selecting the coupled or isolated port, and a filter for 5-GHz ISM blockers suppression (Fig. 1). The feedback receiver (FR) sequentially receives forward- and reverse-coupled signals by changing the position of the switch. There are several benefits to integrating the coupling, switching, and filtering functions on a single die:

  • It requires only one RF feedback line from the switchable coupler IC to the FR. Since the RF feedback line must be fully isolated from the main RF path to achieve high directivity, having only one feedback line reduces the complexity and cost of the main phone board.
  • The inactive coupled branch of the coupler is terminated internally via a switch by an accurate 50-Ω resistor. This allows over 20 dB of directivity to be achieved independently of the impedance of the RF feedback path.
  • A filter for 5-GHz Wi-Fi signal suppression prevents the FR from mixing down the Wi-Fi blockers and distorting the cellular feedback signal. An integrated Wi-Fi filter eliminates the need (or significantly relaxes the requirements) for a discrete lowpass filter in the coupled RF feedback path.