Split-band supply modulator for OFDM polar transmitter: Filter design

Split-band modulator can be used to implement wide band and high efficiency supply modulator needed in polar transmitters. This paper describes design process of filters that splits and combines signal in split-band modulator. It involves selecting appropriate filter structures and component values. Design process leads to unstable synthesis filters with right hand side poles. It is shown that these filters can be approximated with stable digital filters in the desired frequency band. Design of modulator with 35 MHz bandwidth, 10 MHz splitting frequency and 160 MHz sample rate is used as an example. Modulator has 0.2 dB variation in amplitude response and group delay variation of 0.3 samples in the pass band. Power dissipated in opposite side signal paths source impedances is -30 dB compared to power driven to the load impedance, when IEEE 802.11a signal was used.