Polar RF Transmitter Research Articles

A Technique to Reduce Phase/Frequency Modulation Bandwidth in a Polar RF Transmitter

A polar modulator for wireless RF transmitters nonlinearly transforms complex-valued Cartesian baseband modulating signal into amplitude and phase components of the polar coordinate representation before they are recombined in a power amplifier. The resulting explosion in the bandwidth requirements of the polar components can so far be only tolerated for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">narrowband</i> transmitters, such as EDGE of the 2G cellular. To enable polar topology for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">wideband</i> transmitters, we propose a technique that alters the signal trajectory such that it avoids crossing (and proximity) of the constellation origin. The resulting substantial decrease of the polar modulator bandwidth is traded off against slight increase of in-band modulation distortion and adjacent channel leakage. We illustrate effectiveness of this method using wideband CDMA (WCDMA) of the 3G cellular. The technique is first mathematically analyzed for various tradeoffs followed by high-level modeling and simulation results. Since the technique is fully contained in the digital domain, its performance effects on the entire RF transmitter can be accurately simulated. A digital architecture to implement the proposed technique is also presented.

Digitised polar transmitter with CMOS switching power amplifier

A delta-sigma (ΔΣ) digitised polar RF transmitter with a CMOS switching power amplifier is presented. The proposed power amplifier consists of a class-E amplifier with a modulating current source which allows high efficiency and high linearity. A current source is introduced in the proposed power amplifier to achieve ΔΣ digitised amplitude modulation. The current mode amplitude modulation is suitable for a high speed modulated signal such as an oversampled ΔΣ signal.

A $\Delta\Sigma$-Digitized Polar RF Transmitter

This paper demonstrates a new polar transmitter architecture, which uses the digitized envelope signal to control the drain voltage of a switching mode power amplifier (PA). It is based on a novel polar modulation using the constant envelope modulated signal. Among the various constant envelope modulators, the DeltaSigma modulator is chosen for its noise-shaping characteristic. It enables the use of a highly efficient switching amplifier with high linearity. For demonstration, the overall transmitter is implemented and tested with a code-division multiple-access IS-95A signal. The class-D and class-F amplifiers are designed and compared for the optimum operation. The experimental results show that the amplifier with small device size is suitable for this application because of the fast switching requirement. For the class-F amplifier, the measured power-added efficiency is 51.7% at 22.1 dBm and the overall efficiency (considering the amplified quantization noise) is 31%. The adjacent channel power ratios at 885 kHz and 1.98 MHz are lower than -44.9 and -55.6 dBc at the output power range from 10.8 to 22.1 dBm without any pre-distortion techniques. The overall efficiency is improved to 48.6% with a three-level quantized DeltaSigma modulator. The results clearly show that the highly efficient switching mode PA can be controlled efficiently using a digital signal from the DeltaSigma envelope modulation technique.

Design of a linearly assisted switcher for a supply modulated RF transmitter

A broadband modulated power supply for envelope tracking or polar RF transmitters has been built. To study the design constraints, an efficient state-based simulation tool was developed. Based on the simulations, a prototype supply was built. It can deliver up to 1 W (6 V, 150 mA) to a resistive load of 30 ohms with spectral purity of ca. ?45 dBc. The power efficiency using a QAM-signal is 49%.