Rameswor Shrestha

A Wideband Supply Modulator for 20 MHz RF Bandwidth Polar PAs in 65 nm CMOS

Polar modulated RF amplifiers have the potential to enhance efficiency while achieving sufficient linearity for a signal having non-constant envelope. However, switching modulators used in such architectures to generate the envelope signal are difficult to implement because of the high bandwidth and low switching ripple requirement, while achieving high efficiency even at large power back-off. This paper presents a wideband supply modulator for a 20 MHz RF bandwidth polar modulated PA. Realized in 65 nm CMOS, it consists of a cascoded nested Miller compensated linear amplifier and a class D switching amplifier. The class-AB linear amplifier with cascoded Miller compensation in the modulator reduces the output switching ripple to 4.3 mVrms at a switching frequency of 118 MHz. With an optimized design the modulator achieves a maximum efficiency of 87.5% and a small signal -3 dB bandwidth of 285 MHz. At 10 dB back-off the efficiency is more than 40%. The modulator can deliver 22.7 dBm output power to a 5.3 Omega load. It satisfies the linearity requirement of IEEE 802.11g WLAN signal when tested in a real polar modulated amplifier.

A Polyphase Multipath Technique for Software-Defined Radio Transmitters

Transmitter circuits using large signal swings and hard-switched mixers are power-efficient, but also produce unwanted harmonics and sidebands, which are commonly removed using dedicated filters. This paper presents a polyphase multipath technique to relax or eliminate filters by canceling a multitude of harmonics and sidebands. Using this technique, a wideband and flexible power upconverter with a clean output spectrum is realized in 0.13-mum CMOS, aiming at a software-defined radio application. Prototype chips operate from DC to 2.4 GHz with spurs smaller than -40 dBc up to the 17th harmonic (18-path mode) or 5th harmonic (6-path mode) of the transmit frequency, without tuning or calibration. The transmitter delivers 8 mW of power to a 100-Omega load (2.54 Vpp-diff voltage swing) and the complete chip consumes 228 mW from a 1.2-V supply. It uses no filters, but only digital circuits and mixers

Cognitive radios for dynamic spectrum access - polyphase multipath radio circuits for dynamic spectrum access

Dynamic access of unused spectrum via a cognitive radio asks for flexible radio circuits that can work at an arbitrary radio frequency. This article reviews techniques to realize radios without resorting to frequency selective dedicated filters. In particular, a recently proposed polyphase multipath technique canceling harmonics and sidebands is discussed. Using this technique, a wideband and flexible power upconverter with a clean output spectrum has been realized on a CMOS chip, aiming at flexible radio transmitter application. Prototype chips can transmit at an arbitrary frequency between DC and 2.4 GHz. Unwanted harmonics and sidebands are more than 40 dB lower than the desired signal up to the 17th harmonic of the transmit frequency

Multipath Polyphase Circuits and their Application to RF Transceivers

Nonlinearity and time-variance in radio frequency (RF) circuits leads to unwanted harmonics and intermodulation products, e.g. in power amplifiers and mixers. This paper reviews a recently proposed multipath polyphase circuit technique which can cancel such harmonics and intermodulation products. This will be illustrated using a power upconverter IC as an example. The upconverter works from DC to 2.4 GHz, and the multipath polyphase technique cleans its spectrum up to the 17th harmonic, keeping unwanted spurious responses more than 40dB below the carrier. The technique can also be useful for other applications, and some possible applications will be discussed.