Abstract
A low-noise phase modulator, using finite-impulse-response (FIR) filtering embedded delta-sigma (ΔΣ) fractional-N phase-locked loop (PLL), is fabricated in 0.18 μm CMOS for GSM/EDGE polar transmitters. A simplified digital compensation filter with inverse-FIR and -PLL features is proposed to trade off the transmitter noise and linearity. Experimental results show that the presented architecture performs RF phase modulation well with 20 mW power dissipation from 1.6 V supply and achieves the root-mean-square (rms) and peak phase errors of 4° and 8.5°, respectively. The measured and simulated phase noises of −104 dBc/Hz and −120 dBc/Hz at 400-kHz offset from 1.8-GHz carrier frequency are observed, respectively.
Highlights
Polar transmitters can achieve both high power efficiency and good linearity and become growing popular in modern wireless systems [1, 2]
The high power efficiency is achieved by using a nonlinear switched-mode power amplifier (PA) to handle the constant-envelope phase-modulated RF signal, and the good linear transmission is accomplished by modulating the signal envelope through the supply voltage of the switched-mode PA [3]
The proposed digital compensation filters and FIR-embedded fractional-N phase-locked loop (PLL) are fabricated in 0.18 μm CMOS, and 16-bit baseband components are generated in a field programmable gate array (FPGA)
Summary
Polar transmitters can achieve both high power efficiency and good linearity and become growing popular in modern wireless systems [1, 2]. The delta-sigma (ΔΣ) fractional-N phase-locked loop (PLL) [4] enables phase modulation function when the fractional division ratio is modulated by the baseband signal, simplifying the overall transmitter architecture without requiring digital-to-analog converters (DACs) and RF upconverters [5]. With simplified inverse-FIR and -PLL digital filters proposed to compensate for the signal attenuation, an FIR-embedded ΔΣ fractional-N PLL is presented to perform RF phase modulation for GSM/EDGE polar transmitters, achieving good trade-off between transmitter noise and linearity.
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