4.1.6.2 Antenna Tuning Application Design

To achieve a correct tuned antenna in TXMode and in RXMode, the optimum impedance in both modes must be 50Ω at pin SPDT_ANT. If different optimum impedances are applied in RXMode and TXMode, the antenna tuning does not achieve correct tuning for the RXMode, provided that the C_TUNE is optimized in TXMode. It is, therefore, necessary to optimize the RXMode and TXMode performance by cutting the loop antenna and connecting a power meter in TXMode and a network analyzer in RXMode directly to SPDT_ANT with a 50Ω connection to optimize the matching networks before the rest of the antenna tuning is optimized.

In addition, a 1 nH series inductance (L_Bond) from pin 5/ANT_TUNE to the internal circuit also has to be taken into account for the application design, especially for High-Band applications, because this inductance changes the resonance frequency.

Because pin 5/ANT_TUNE is internally biased to VS_PA, the series capacitor C₂₁, as shown in Figure 4-20, is required to prevent direct DC coupling to the SPDT_ANT pin, which is biased to VS_PA/2 from the power amplifier.

The purpose of the antenna tuning is to reach the maximum current through the magnetic loop antenna, which results in maximum output power. In the case of a maximum current, the voltage at the ANT_TUNE pin is also at maximum. The automatic antenna tuning varies all four available bits of C_TUNE controlled by the RF front-end register FEAT.ANTN[3:0]. The internal state machine for the automated antenna tuning is finished when the maximum amplitude at ANT_TUNE is achieved.

Because the quality factor of the integrated capacitor is limited (see parameter no. 12.20 in RF Transmit Characteristics), the antenna tuning range must be limited with a capacitor C₂₂ to the application needs because this reduces the losses caused by the finite quality factor of C_TUNE. Capacitor C₂₂ can also be used to limit the amplitude at the ANT_TUNE pin to amplitudes below 3V peak (see parameter no. 12.30 in RF Transmit Characteristics). If only end-of-line production tuning is done, the tuning range must not be wider than needed to tune out the tolerances of the components. If automatic tuning is used, ±3% center frequency tuning is a usual compromise. The following table summarizes typical application examples.

If antenna tuning is not used, the 5/ANT_TUNE pin must be left open and not connected to ground. Otherwise, additional current flows from VS_PA to ground, which can be a disadvantage in 3V applications because this current may discharge the lithium cell.