The New RF Modulator Schemes for Wide Band Signals

摘要

This paper presents a new RF modulation schema for wideband signals with the constant envelope. The proposed modulation schema is fitting well for the LINC types of high power amplifiers (HPA) or some other advanced modulations where high modulation linearity over a wide frequency range is required. The LINC amplifier takes an envelope modulated bandpass waveform and resolves it into two phase modulated (PM) constant envelope RF signals, which are applied to highly efficient and highly nonlinear power amplifiers, whose outputs are summed. The bandwidth for such PM constant envelope signals significantly exceeds the bandwidth of the original baseband signal and therefore requires speedy and costly high-end digital hardware. Alternatively, any restrictions for the bandwidth in the LINC arms result in intermodulation distortions in the outputs of the highly nonlinear high power amplifiers. In the proposed modulation scheme, the bandwidth for the baseband signals in the LINC arms is reduced by removing exceed PM, i.e. the PM with high modulation indexes. Thus, the baseband signals in the arms have a compact narrowband spectrum. Such narrowband signals can be generated with low-cost commercial speed grade hardware. Late, during quadrature modulation, the exceed PM return into LINC arms RF signals through LO path of quadrature modulators. If so, the original signal bandwidth for RF signal in the HP As inputs is restored. Therefore the linear high-efficient signal amplification with a non-linear HPA can be realized; hence no intermodulation distortion is produced in the LINC output signal.