VCO Design Tutorial: Part 1

Introduction

A voltage controlled oscillator or VCO is an oscillator that can be frequency controlled by an external voltage. The external voltage is called a tuning voltage and it is typically connected to a voltage variable capacitor (varactor) within the resonant oscillator circuit. As the voltage changes, the capacitance of the varactor changes which, in turn, dithers the resonant frequency of the oscillator.

This may sound simple but there are many constraints to good VCO design including spectral purity, output power, temperature behavior, tuning sensitivity and noise immunity. All of these constraints depend on the customer's specifications dictated by the application or end-use of the product. For instance, high spectral purity is important in high data rate systems or communication systems where reciprocal mixing may be a problem. Reciprocal mixing occurs when the phase noise skirt of the local oscillator VCO down-converts environmental interferers which may desensitize a receiver. Temperature behavior is usually critical for airborne or remote applications such as radar or point-to-point radio base stations. Tuning sensitivity is usually important when the VCO is part of a phase-locked system in a synthesizer or coherent receiver; which depends on the frequency range and the customer's tuning voltage limits. Noise immunity can be a problem when the VCO is used in an environment where noise or unwanted signals can leak into the VCO either via the power supply or the output load. Output power level and flatness are typically critical when the VCO is used as a local oscillator to drive a mixer circuit. The designer's job is to understand all of the design constraints and optimize the VCO accordingly. All of these constraints will be described in this paper and the reader will be enabled to learn how to incorporate them into a design process. First, we will discuss basic VCO theory and then we will treat the constraints one-by-one.