Predistortion of Digital RF PWM Signals Considering Conditional Memory

The trend in transmitter systems is to move the digital domain closer towards the antenna using digital modulators and drivers to reduce circuit complexity and to save power. A common assumption made is that they are capable of generating ideal pulses and thus do not suffer from analog imperfections. But the output signals of real drivers for high frequency operation are not perfectly rectangular anymore, which leads to distortion lowering the signal quality. In this paper the general properties of high frequency digital driver circuits operating at 2.6 GHz are analyzed and the impact of the different effects is presented. The predistortion of such drivers in the context of digital discrete time RF PWM modulators is studied. It has been found that conventional sample based predistortion can only correct the driver nonlinearity from -29 dBc to -49 dBc for the example considered using a 40 MHz bandwidth signal at 2.6 GHz. Therefore a special predistortion scheme considering the impact of pulses adjacent to the other samples is proposed. The mitigation of effects due to the discrete time nature of the signal is considered and discussed in detail. The capabilities of the proposed predistortion scheme are verified by extensive simulations as well as by measurements. By applying the proposed predistortion concept the spectral quality can be further improved to -66 dBc. In addition different scenarios with limited resolution and a carrier frequency offset are analyzed.