An integrated SiGe HBT bandpass-pulse length modulator for switch mode power amplifiers in the SHF range

The need for efficiency improvement of conventional class-AB-amplifiers when used with non-constant envelope modulations has set up interest in introducing switch mode amplifiers in the GHz operating frequency range. In order to be suitable for the amplification of arbitrarily modulated signals, a one-bit modulator is required at the input of the switch mode amplifier. Beside bandpass delta sigma modulation (BDSM), bandpass pulse length modulation (BP-PLM) seems to be a promising approach for this task. Here, the modulator output signal switches with the cycle of the RF-carrier signal frequency itself so that no oversampling is required, thus reducing switching losses in the amplifier. The length of each pulse is varied according to the envelope signal while the phase information is contained in the position of the pulses within the carrier period. We present here the design of a bandpass pulse length modulator circuit, capable of operating at microwave carrier frequencies above 3 GHz. Measurement results for a QPSK-test signal show minimum error vector magnitudes of about 1.4 %. The modulator was manufactured in a 250 nm SiGe:C-BiCMOS technology at IHP GmbH, Frankfurt (Oder), Germany.