Pulse-Width Modulated RF Transmitters

Abstract

To my mother, father, and uncle (mamoo), AbstractThe market for wireless portable devices has grown significantly over the recent years. Wireless devices with ever-increased functionality require high rate data transmission and reduced costs. High data rate is achieved through communication standards such as LTE and WLAN, which generate signals with high peak-to-average-power ratio (PAPR), hence requiring a power amplifier (PA) that can handle a large dynamic range signal. To keep the costs low, modern CMOS processes allow the integration of the digital, analog and radio functions on to a single chip. However, the design of PAs with large dynamic range and high efficiency is challenging due to the low voltage headroom.To prolong the battery life, the PAs have to be power-efficient as they consume a sizable percentage of the total power. For LTE and WLAN, traditional transmitters operate the PA at back-off power, below their peak efficiency, whereas pulse-width modulation (PWM) transmitters use the PA at their peak power, resulting in a higher efficiency. PWM transmitters can use both linear and SMPAs where the latter are more power efficient and easy to implement in nanometer CMOS. The PWM transmitters have a higher efficiency but suffer from image and aliasing distortion, resulting in a lower dynamic range, amplitude and phase resolution.This thesis studies several new transmitter architectures to improve the dynamic range, amplitude and phase resolution of PWM transmitters with relaxed filtering requirements. The architectures are suited for fully integrated CMOS solutions, in particular for portable applications.The first transmitter (MAF-PWMT) eliminates aliasing and image distortions while allowing the use of SMPAs by combining RF-PWM and band-limited PWM. The transmitter can be implemented using all-digital techniques and exhibits an improved linearity and spectral performance. The approach is validated using a Class-D PA based transmitter where an improvement of 10.2 dB in the dynamic range compared to a PWM transmitter for a 1.4 MHz of LTE signal is achieved.The second transmitter (AC-PWMT) compensates for aliasing distortion by combining PWM and outphasing. It can be used with switch-mode PAs (SMPAs) or linear PAs at peak power. The proposed transmitter shows better linearity, improved spectral performance and increased dynamic range as it does not suffer from AM-AM distortion of the PAs and aliasing distortion due to digital PWM. The idea is validated using push-pull PAs and the proposed transmitter shows an improvement of 9 dB in the dynamic range as compared v vi Abstract to a PWM transmitter using digital pulse-width modulation for a 1.4 MHz LTE signal.The third transmitter (MD-PWMT) is an all-digital implementation of the second transmitter. The PWM is implemented using a Field Programmable Gate Array (FPGA) core, and outphasing is implemented as pulse-position modulation using FPGA transceivers, which drive two class-D PAs. The digital implementation offers the flexibility to adapt the transmitter...

Power‐efficient aliasing‐free PWM transmitter

“…An analogue PWM signal contains an infinite number of spectral components, and as a result, an image distortion is introduced when the PWM signal is modulated to the carrier frequency. Image distortion can be reduced by either exploiting the image gap in the PWM signal, or by quantising the amplitude information such that the image component distorting the transmit signal is eliminated [10], or by low-pass filtering the PWM signal [17]. However, all of these techniques have their limitations.…”