A new predistorter for RF power amplifiers

　　Among various RF power amplifier linearization technologies, feedforward technology has high linearity and bandwidth, but its circuit structure is complex, expensive, and low-efficiency. It is mainly used in high-power amplifiers and in repeaters. , pre-distortion technology has certain advantages. It has low cost, low power consumption and simple circuit structure. In a mechanically distorted RF power amplifier, the performance of the amplifier mainly depends on the characteristics of the predistorter. A good predistorter can greatly improve the linearity of the power amplifier and better suppress spectrum regeneration. This article studies a predistorter that can generate IM3 and IM5 respectively, which can improve the 3rd-order and 5th-order intermodulation components well.

1 Circuit structure of predistorter

　　The basic harmonic generator circuit is shown in Figure 1.

　　This harmonic generator consists of two Schottky diodes with the same characteristics (Skyworks' SMS3922 is used in this article), capacitors, resistors and a 180° 3dB bridge coupler. The two diodes have different bias voltages, making them work in different nonlinear regions to generate distorted signals. The capacitor and resistor absorb signals within a certain band, and an equal two-tone signal Vi is added to the input end of the harmonic generator:

　　Vi＝Acosω1t＋Acosω2t (1)

　　Among them: A is the amplitude of the two-tone signal; ω1, ω2 are the frequencies of the two-tone signal and ω1≠ω2. And assume that the nonlinear transfer function of the harmonic generator is y=f(x), and the output is V0. That is: V0=f(Vi). Expanding it into a power series is:

　　By controlling the diode bias voltage, the coefficient bi (i=1, 2, 3...) in equation (2) can be appropriately adjusted, so that equation (2) can independently generate IM3 and IM5 signals. In the predistortion linear In the circuit, two harmonic generators are used, one is used to generate IM3 signals, and the other is used to generate IM5 signals. In this way, the 3rd-order intermodulation and 5th-order intermodulation components generated by the RF power amplifier can be controlled separately.

　　Perform a two-tone test simulation on the circuit in Figure 1 in ADS. By adjusting the bias voltage of each diode, the desired IM3 and IM5 can be generated at the output. The simulation results are shown in Figure 2(a) and (b) respectively. It can be seen from the simulation results that the output terminal also contains a fundamental frequency component. In order to cancel the fundamental frequency component, the circuit in Figure 1 must be improved and a cancellation circuit added. Add a power divider to the input end, connect one channel to the basic harmonic generator, and adjust the other channel through an attenuator and phase shifter, and then combine it with the output of the basic harmonic generator through a combiner, as shown in Figure 3 Show.

　　Appropriately adjust the attenuator and phase shifter so that the fundamental frequency signal of the previous branch and the fundamental frequency signal at the output end of the harmonic generator are equal in amplitude and 180° out of phase, so that the fundamental frequency signal can be canceled . The simulation results are shown in Figure 4(a) and (b).

2 test results

　　In order to verify the effectiveness of the predistorter studied in this article, the predistorter was applied to a 5W CDMA radio frequency power amplifier, and a two-tone test simulation was performed in ADS. The simulation circuit block diagram is shown in Figure 5.

　　In this simulation test, the main power amplifier uses Freescale's MRF9045 radio frequency device. The vector regulator consists of an attenuator and a phase shifter, which is used to adjust the amplitude and phase of the distorted signal so that it is consistent with the third-order and fifth-order signals generated by the main power amplifier. The first-order intermodulation components are equal in amplitude and 180° out of phase to maximize the cancellation of third- and fifth-order intermodulation components. Two-tone harmonic balance simulation (2-Tone HB) is performed in ADS to test the linearity of the power amplifier. Without predistortion, when the output power of the amplifier is 5W, the distortion is large, and the two-tone intermodulation IMD3 and IMD5 are shown in Figure 6(a) respectively. When the pre-distortion technology in this article is adopted, IMD3 and IMD5 are significantly improved, as shown in Figure 6(b).

　　It can be seen from the simulation results that after using the pre-distortion technology in this article, IMD3 and IMD5 have improved by 14dBc and 9dBc respectively.

3 Conclusion

　　The effectiveness of the predistorter in this paper was confirmed in the simulation test. Using it in a 5W CDMA RF power amplifier, the two-tone test shows that IMD3 and IMD5 are significantly improved, improving by 14dBc and 9dBc respectively.

　　The linearization technology of using harmonic generator to achieve predistortion introduced in this article relies on adjusting the bias voltage of two diodes to generate IM3 and IM5 respectively. Therefore, it can easily be used as an adaptive control end and use an adaptive algorithm. More accurate adjustment leads to better improvements in IM3 and IM5.