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Switch-mode amplifiers have gained interest in the RF-frequency range, due to their promising features of high linearity and especially high efficiency, compared to conventional class-AB amplifiers. For the operation of a class-S amplifier, it is necessary to convert the arbitrarily modulated RF signal into a pulselength modulated signal. This can be accomplished by delta-sigma modulation or by pulselength modulation. Here, the classical design of a pulselength modulator circuit is presented and applied to the UHF frequency range, which converts digitally modulated 450 MHz RF-signals, with a clock rate of 1.8 GHz. The developed integrated circuit features a forward structure without any feedback, which makes the modulator inherently stable and offers a high degree of frequency flexibility. The modulator was manufactured in a 0.25 μm SiGe-BiCMOS technology (IHP, Frankfurt/Oder, Germany). Measurement results for a 64-QAM test signal show promising results like minimum error vector magnitudes of about 1%rms and a carrier-to-noise ratio of up to 60 dB.