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Frequency Tuning Architecture for an OC192 Quadrature LC Voltage-Controlled Oscillator in 0.13um CMOS

IP.com Disclosure Number: IPCOM000022671D
Publication Date: 2004-Mar-25
Document File: 4 page(s) / 260K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for the coarse and fine tuning of a quadrature LC voltage-controlled oscillator (VCO) using MOS switches in a differential configuration, along with fine tuning through the controlled coupling of two LC oscillators. Benefits include immunity to process induced variations, and multi-rate operation by allowing a large total tunable range.

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Frequency Tuning Architecture for an OC192 Quadrature LC Voltage-Controlled Oscillator in 0.13um CMOS

Disclosed is a method for the coarse and fine tuning of a quadrature LC voltage-controlled oscillator (VCO) using MOS switches in a differential configuration, along with fine tuning through the controlled coupling of two LC oscillators. Benefits include immunity to process induced variations, and multi-rate operation by allowing a large total tunable range.

Background

A VCO is a key component in phase-locked loop (PLL) circuits for clock and data recovery (CDR), and transmission for high speed optical communication systems. Fiber communication standards such as SONET have stringent requirements on jitter transfer and generation by the PLL circuitry. In addition to low noise requirements, these standards also have stringent requirements on CDR robustness. Since the VCO is a critical circuit block in determining overall CDR performance, these specifications require a high-quality VCO design.

The LC VCO offers superior noise performance over more conventional ring oscillators used in optical CDR and transmission circuits. However, it lacks the advantage of the wide tuning range of a ring oscillator, which is essential for robust PLL design. In addition, most LC oscillators use a single-ended voltage control, which has inferior noise immunity, as compared to differential voltage control. Also, some CDR architectures require multiple clock phases from the VCO, which is easily available from any ring oscillator but is lacking in LC VCOs.

General Description

The disclosed method uses a differentially tuned 10GHz LC VCO with quadrature phases and switched capacitor tuning that addresses the shortcomings of current LC VCOs. The schematic of the quadrature VCO is shown in Figure2(b). The VCO is composed of two coupled LC oscillators. Each oscillator has a differential negative gm structure with an on-chip LC tank, as shown in Figure 2(a). The coupling between these stages is obtained by devices M3-4, and the coupling factor can be varied by controlling the current through these devices. The coupled VCO stages oscillate in quadrature under certain conditions, and the resulting oscillation frequency is a function of the coupling factor between the two stages. Referring to figure 2(a), the coupling factor is gm3/gm1 (gm4/gm2), where gmi is the transconductance of device Mi. The transconductance of devices M3-4 is well controlled by changing the bias current. The resulting VCO has a fairly linear frequency vs. the tuning voltage transfer function.

The...