A method, apparatus, and computer program is presented for use in a radio transmitter using a polar transmitter structure in which a transmission signal is separated into an amplitude component and a phase component. The transmission signal includes transmission symbols distributed to a number of transmission resource blocks allocated to the radio transmitter for transmission. The amplitude component of the transmission signal is low-pass filtered in a low-pass filter configured by filtering parameters selected according to the number of transmission resource blocks allocated to the radio transmitter. Then, the low-pass filtered amplitude component is used in power supply of a power amplifier configured to power-amplify a phase component of the transmission signal.