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TWG Phase Interpolator

IP.com Disclosure Number: IPCOM000179711D
Publication Date: 2009-Feb-23
Document File: 2 page(s) / 653K

Publishing Venue

The IP.com Prior Art Database

Abstract

A novel current-mode analog phase interpolator (TWG PI) was designed. The circuit integrated a novel triangular waveform generator (TWG) circuit, which can transfer the pulse waveform input signals into triangular waveform signals with quite constant amplitude over wide range of frequencies and PVT. This feature makes the TWG PI has very good linearity over wide range of frequencies.

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TWG Phase Interpolator

Jun Xia, Charles Everett Moore, Avago Technologies Pte. Ltd.


Abstract

A novel current-mode analog phase interpolator (TWG PI) was designed. The circuit integrated a novel triangular waveform generator (TWG) circuit, which can transfer the pulse waveform input signals into triangular waveform signals with quite constant amplitude over wide range of frequencies and PVT. This feature makes the TWG PI has very good linearity over wide range of frequencies.

Introduction

Current-mode analog phase interpolators have become an important part of high-speed timing circuits due to its high speed, low noise coupling, and ease of the weighting summation implementation. One of their applications is in the PLL and DLL [1], and another is in the clock and data recovery (CDR) loop in SerDes [2]. The analog phase interpolation consists of combining two signals having the same waveform shape but offset in phase to obtain an output signal with a similar shape and a phase that is between those of the two starting signals. Figure 1 shows a schematic diagram of mixer core circuit of a typical current-mode analog phase interpolator. The phase interpolator receives two clocks,  and generates the main clock  whose phase is the weighted sum of the two input phases. The current sources of the two differential pairs are current DACs.

Figure 1

Linearity is very important for many applications of phase interpolators. For proper functionality with good linearity, the used phase interpolator needs input signals with a sufficiently low slew rate. The conventional solution is to place a low-pass filter at its input [2]. However, the disadvantage of this solution is that it is narrow band, and it is not suitable for wideband applications.

A novel idea is proposed to create a current-mode analog phase interpolator with a triangular waveform generator (TWG) instead of a low pass filter. The new design is based on the assumption which was gotten from our research, that is, if the following two critical conditions are met, the current-mode analog phase interpolator will have good linearity: firstly, the currents waveforms of the two differential pairs which are controlled by the current DAC are triangular waveform; secondly, the input signal waveforms of the two differential pairs are also triangular waveform, and the amplitudes of the triangular waveform input signals keep a quite constant and appropriate value which makes the differential pairs working at saturation over PVT. To realize the conditions, a novel triangular waveform generator (TWG) circuit was designed, and it was integrated into the current-mode analog phase interpolator which is called TWG PI.

Theory

As shown in Figure 2, a voltage-to-current converter is constructed, where Vref is one reference voltage. Because

The bias current is given by

Figure 2

If a p...