An Ultra-low Power CMOS RF Transceiver for Medical Implants

RF PA Linearization: Open-Loop Digital Predistortion Using Cartesian Feedback for Adaptation

Nested Chopper Stabilization in Analog Multipliers and Down-conversion Mixers

System Design Using Convex Optimization from Circuit Level to System Level

Wideband Two-point Modulators for Multi-standard Transceivers

Chopper Stabilization in Analog Multipliers

A DC Stabilized Fully Differential Amplifier

Design of a High Efficiency RF Power Amplifier for an MCM Process

An Ultra-low Power CMOS RF Transceiver for Medical Implants

Jose Bohorquez

Until recently, very few medical implantable devices existed and fewer still provided the capability of wireless transmission of information. Of those devices that were capable of data transmission, almost all did so through inductive coupling which requires physical contact with the "base-station" and only allows for low data rates. In 1999, the FCC commissioned a frequency band specifically for medical telemetry which allows for RF communications between a medical implant and a base-station that is up to two meters away. Although this standard has been available for almost a decade, very few publications have been produced to address the challenges associated with it, and only a few companies have designed compliant transceivers. This research looks to explore unconventional approaches to radio communications specifically geared towards low power, short distance data transmission in a temperature regulated environment (i.e., the human body).

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RF PA Linearization: Open-Loop Digital Predistortion Using Cartesian Feedback for Adaptation

Sungwon Chung, Jack Holloway, Jeffrey Huang

RF power amplifiers are commonly linearized using a symbol predistortion scheme in the digital domain. This approach benefits from high symbol rates inherent from it's open-loop nature, however the technique relies on a detailed PA model -- one that is often difficult to formulate and cumbersome in implementation.

Cartesian feedback, by comparison, operates in continous time with little knowledge of the PA's dynamics or time-dependent behavior. This technology makes use of analog feedback, and thus sacrifices symbol rate for loop stability.

This work focuses on implementing a digital predistortion technique for RF PA linearization in which no PA model is needed a priori. Instead, cartesian feedback is used to train a digital predistorter over an application's symbol constellation. During transmitter usage, the digital predistorter is operated in an open-loop configuration. In this way, the system offers high symbol bandwidths while not relying on complicated PA models.

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Nested Chopper Stabilization in Analog Multipliers and Down-conversion Mixers

Philip Godoy

Analog multipliers are an important building block in many electronic systems which require analog signal processing.  Examples include phase alignment systems, neural networks, and automatic gain control circuits.  A persistent problem with analog multipliers is DC offset, which limits the precision of these systems.  With an important modification, chopper stabilization, a technique long used to achieve low-offset DC amplification, can be applied to DC multipliers to continuously reject DC offset.

This project implements a DC analog multiplier which uses two levels of chopper stabilization.  The inner choppers greatly suppress the inherent offset of the multiplier, and the outer choppers reduce any residual offset of the inner chopped multiplier system.  The nested chopper stabilized multiplier provides continuous, high-precision DC multiplication which is insensitive to drift.

Chopper stabilization can also be applied to RF direct down-conversion mixers for improved receiver sensitivity.  Two well-known problems with the direct-conversion receiver architecture are (1) dynamic DC offsets generated primarily from the self-mixing of RF or LO signals through leakage paths within the RF front-end; and (2) 2nd-order inter-modulation which introduces undesirable spectral components at base-band.  This work demonstrates the effectiveness of chopper stabilization in solving both of these problems.

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System Design Using Convex Optimization from Circuit Level to System Level

Tania Khanna

Within a system, input and output specifications for individual circuit blocks are commonly assigned with little knowledge of optimality. These specifications allow for almost optimal design of individual circuit blocks, but it remains questionable whether the pre-defined specifications are optimal over the entire system.

Equation-based and simulation-based optimization techniques have already been used to optimize circuit blocks. By creating signomial models for transistors and formulating a program describing the circuit specifications, we result in an optimal circuit design.

We extend the equation-based technique to the system level by treating the problem hierarchically. Specification trade-off curves from the circuit block topologies are quantified and passed up to the system level in order to optimize each circuit block's input and output specifications simultaneously. This technique will provide system designers with a better system starting point as opposed to several starting points — one for each circuit block within the system. Furthermore, the generated trade-off curves are well behaved and modeled easily with simple monomial functions allowing for fast optimization at the system level, making a case for equation-based design over simulation-based design.

I am applying this new technique to the design of 10-bit pipeline analog-to-digital converter in a 0.18u process with sampling speed of 100 MHz.

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Wideband Two-point Modulators for Multi-standard Transceivers
Surapap Rayanakorn

Two-point modulators are a fundamental building block in polar transmitters, which have the potential to accommodate multiple wireless standards. A primary challenge for polar transmitters, however, is that they demand large baseband bandwidths compared to their Cartesian counterparts. To put polar transmitters into use, the separate amplitude and phase paths have to be extremely broadband. This project addresses the need on the phase path.

The two-point modulator, used to perform phase modulation, is a phase-locked loop (PLL) with two inputs. The input data through the first path is low-pass filtered to the output by the closed-loop transfer function of the PLL. If this terminal were the only input, the speed of the PLL would therefore limit the achievable data rate. However, in a two-point modulator, data injected into the second path is high-pass filtered to the output. The corner frequency of this high-pass filter is exactly equal to the low-pass corner of the PLLs closed-loop transfer function. In theory, the bandwidth of a two-point modulator is therefore unbounded. However, nonlinearity in the voltage-controlled oscillator (VCO) is a barrier to realizing this potential of two-point modulators. The high-pass second path does not benefit from the linearized VCO tuning characteristic that the PLL provides for the first path. If this linearity goes uncorrected, a wideband two-point modulator can introduce significant phase error.

Adaptive digital predistortion, a linearization technique commonly applied to RF power amplifiers, is a promising solution. Recent work using analog feedback to train a predistorter has been shown to enable dramatic bandwidth extensions for Cartesian feedback power amplifiers. With this same principle and the observation that the PLL continuously performs VCO linearization, a predistortion block is added in the second data path. The introduction of this predistortion circuit will eliminate the phase error, and it therefore enables the two-point modulator to function as a truly broadband phase path in polar transmitters.

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