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COMPENSATION OF FREQUENCY DEPENDENT QUADRATURE IMBALANCE IN A ZERO-IF DOWNCONVERTER

IP.com Disclosure Number: IPCOM000009312D
Original Publication Date: 1999-Jun-01
Included in the Prior Art Database: 2002-Aug-15
Document File: 4 page(s) / 179K

Publishing Venue

Motorola

Related People

Jim Michels: AUTHOR [+2]

Abstract

Radio receivers traditionally employ an interme- diate frequency (IF) section to perform the major portion of the radio's selectivity. The process of translating a signal down to a zero Hertz intermedi- ate frequency is known as zero-IF downconversion. One of the problems associated with a zero-IF downconverter is amplitude and phase imbalance in the quadrature oscillator and mixers along with amplitude and phase mismatch between the low pass filters in each quadrature path. Prior inventions have compensated for the imbalance due to the oscillator and mixers which is constant across the frequency bandwidth of the received signal [l]. This invention is novel in that it also.compensates for the phase imbalance due to the mismatch between the low pass filters which varies across the bandwidth of the received signal.

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0 MOTOROLA Technical Developments

COMPENSATION OF FREQUENCY DEPENDENT QUADRATURE IMBALANCE IN A ZERO-IF DOWNCONVERTER

by Jim Michels and Steve Jasper

INTRODUCTION

  Radio receivers traditionally employ an interme- diate frequency (IF) section to perform the major portion of the radio's selectivity. The process of translating a signal down to a zero Hertz intermedi- ate frequency is known as zero-IF downconversion. One of the problems associated with a zero-IF downconverter is amplitude and phase imbalance in the quadrature oscillator and mixers along with amplitude and phase mismatch between the low pass filters in each quadrature path. Prior inventions have compensated for the imbalance due to the oscillator and mixers which is constant across the frequency bandwidth of the received signal [l]. This invention is novel in that it also.compensates for the phase imbalance due to the mismatch between the low pass filters which varies across the bandwidth of the received signal.

  The invention provides a DSP-based solution to compensate for the imbalance after the downconver- sion. It consists of two parts: 1) the means for com- pensating the imbalance, and 2) the means for con- trolling the compensation. The remainder of this article will describe each of these in detail. In sec- tion II, a model of the quadrature imbalance process in the zero-IF downconverter will be provided. In section III, the structure used to compensate the imbalance will be given. Finally, section IV will provide the method used to control the compensa- tion structure.

QUADRATURE IMBALANCE MODEL

  A zero-IF downconverter consists of an oscilla- tor, mixers, and low pass filters. A quadrature imbalance results when a mismatch exists between the components in the in-phase (I) and quadrature
(Q) signal paths. A model of a zero-IF downcon- verter with an imbalance is given in Figure 1. The

  Zero-IF Downconver -------_----_--

I

2cos(wt) AnEma I I

1-2sin(&t+$) L--_---__--__--

Fig. 1 Quadrature Imbalance

downconverter translates the real signal at the anten- na. X{(R,(t) + jR,$t))@>, from the frequency o to zero Hertz, producing the complex signal (rt(t) + jrr$t)) where Jj = (-1). The oscillator and mixer gam and phase imbalance is given by a and + respectively. These imbalances are constant across frequency. The model also includes a frequency dependent imbalance. This imbalance results from a mismatch between the two low pass filters, LPF I and LPF Q. The downconverter is balanced when a = 1, I$ = 0, and the low pass filters match. The combined constant and frequency dependent imbal- ance can be represented in a simpler form as seen in Figure 2.

  As in Figure 1, this model separates the zero-IF downconverter quadrature imbalance into a constant and frequency dependent component. It shows how RI and 1+2 are each affected by the imbalance. The simplification reduces the constant phase and ampli- tude imbalance into two scale factors, kl and k2, with a cross coupling of the...