A study of digital and analog automatic-amplitude control circuitry for voltage-controlled oscillators

Abstract

This paper presents both analog and digital automatic-amplitude control techniques for voltage-controlled oscillators (VCOs). These feedback mechanisms help to keep the VCOs at optimum amplitude over temperature, process, and voltage variations. The VCOs were fabricated in a 50-GHz SiGe BiCMOS process. They use MOS varactors and achieve a 600-MHz tuning range in the 2-GHz band. The phase noise of the VCO with analog control was measured to be 99 dBc/Hz at 100-kHz offset from the carrier. The digital loop allows for a more optimized VCO core that achieves a phase noise of 108.5 dBc/Hz at 100-kHz offset in a low-gain mode. Techniques for suppressing the phase noise in regions of high gain are also presented. The VCOs draw between 4 and 8 mA from a 3.3-V supply. Index Terms-Broad-band communications, phase noise, radiofrequency integrated circuits, SiGe, voltage-controlled oscillators (VCO). I. INTRODUCTIONR ECENTLY, there has been much interest in the prospect of combining voice-over-IP (VOIP) services with cable television and internet access. This means that consistent operation meeting Data-Over-Cable Service Interface Specification (DOCSIS) over a wide range of operating temperatures ( 40 C to 85 C) will be the driving specification for any telecommunications chips that will be used in these systems.This has led to much interest in doing highly integrated cable tuner receivers, capable of delivering high-quality 256 QAM demodulated signals to the baseband processor. Target specifications for phase noise in these systems is usually better than 100 dBc/Hz at 100-kHz offset. Thus, the voltage-controlled oscillator (VCO) in the system must be of good quality. Also, since cable is broadband (47-870 MHz), these VCOs must be tunable over a broad range (1.897-2.72 GHz for an intermediate frequency of 1.85 GHz) and, even so, several VCOs may be necessary to cover the whole range. Automatic-amplitude control is desired because without an adaptive loop it would be impossible for the designer to set the VCO current at an optimal level over all conditions [1]. In this brief, the design of two VCOs with two different approaches to this will be presented. II. GENERAL VCO DESIGN CONSIDERATIONSThe purpose of this design was to create a VCO with a wide tuning range, good phase noise, and very robust performance over process and temperature variations. The requirement for as Manuscript

A 1.8-GHz LC VCO with 1.3-GHz tuning range and digital amplitude calibration

“…The VCO performance in terms of phase noise and tuning range determines basic performance characteristics of a transceiver. The current trend toward multiband multistandard transceivers and broadband systems has generated interest in VCOs that simultaneously achieve very wide tuning range and low phase noise performance [1]- [9]. Whereas relaxation oscillators easily achieve very wide tuning range (i.e., 100% or more), their poor phase noise performance disqualifies them in many of today's wireless and wireline applications.…”

“…Furthermore since the tank amplitude of most LC VCOs to first order changes with the square of frequency, practical implementations must provide some way to stabilize this parameter. Conventional amplitude control schemes use continuous feedback methods and have been successfully demonstrated [9]- [11]. Their crucial and effective role in stabilizing the oscillation amplitude comes at the cost of added complexity and a noise penalty due to the presence of additional noise contributors that feed back to the oscillator [9]- [11].…”

“…Conventional amplitude control schemes use continuous feedback methods and have been successfully demonstrated [9]- [11]. Their crucial and effective role in stabilizing the oscillation amplitude comes at the cost of added complexity and a noise penalty due to the presence of additional noise contributors that feed back to the oscillator [9]- [11]. Section II discusses basic aspects of wideband LC VCO design, drawing attention to the frequency dependence of well-known parameters.…”

A linearized, low-phase-noise VCO-based 25-GHz PLL with autonomic biasing

“…This range covers operation over 22. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. 4 GHz, with margin, meeting requirements for a 60-GHz superheterodyne radio [2].…”

Low voltage low noise open loop automatic amplitude control for voltage-controlled oscillators

“…In recent years a number of works have been devoted to the automatic amplitude control circuits [2][3][4][5][6][7]. The main drawback of these circuits is the noise introduced by the AAC loop itself which increases the phase noise of the VCO.…”