image rejection measure.pdf

University of Toronto Department of Electrical and Computer Engineering Prof. K. Phang ECE 1352 Reading Assignment On-Chip Image Reject Techniques for Wireless Receivers By: Nora Yan Nov 12, 2001 1 Introduction Wireless receivers can generally be divided into two categories according to their architectures. The two categories are commonly known as homodyne receivers and heterodyne receivers. In a homodyne receiver, the desired signal is first selected by a bandselect filter and then amplified by a LNA. It is then frequency translated by a mixer to DC before other baseband operations are performed on the signal. In the more popular heterodyne architecture, the signal goes through a similar receiver chain except that it is frequency translated to a lower but non-zero intermediate frequency (IF) where signal processing operations are performed. Each type of architectures has its own characteristics and presents different challenges to the designers. This paper is a study on a number of on-chip techniques for resolving an inherent problem in heterodyne architectures. This inherent problem is referred to as image. To understand how the problem of image arises, consider the operation of a mixer as illustrated in Figure 1. A mixer can be viewed as a simple analog multiplier. ωLPF cos(ωLOt) ω ωLO ωIFωIF ωsignal ωimage ωIF ω ωLO Figure 1: Operation of a mixer in a heterodyne receiver. As shown in Figure 1, the desired band and the image band, which are located at an equal distance from either side of the local oscillator (LO) frequency, are translated to the same IF frequency. This is because the mixing operation does not preserve the polarity of the difference between its two input frequencies. In other words, 2 tt )cos()cos( 1221 ωωωω ?=? Equation 1-1 The problem of image could cause serious distortions to the desired signal since the power of the image signal may not be regulated and can be much higher than that of t