Analog Circuit Design Volume 2

Chapter 28

Low noise varactor biasing with switching regulators

Vanquishing villainous vitiators vis-à-vis vital varactors

Jim Williams and David Beebe

Introduction

Telecommunication, satellite links and set-top boxes all require tuning a high frequency oscillator. The actual tuning element is a varactor diode, a 2-terminal device that changes capacitance as a function of reverse bias voltage.¹ The oscillator is part of a frequency synthesizing loop, as detailed in Figure 28.1. A phase locked loop (PLL) compares a divided down representation of the oscillator with a frequency reference. The PLL’s output is level shifted to provide the high voltage necessary to bias the varactor, which closes a feedback loop by voltage tuning the oscillator. This ...