Distortion cancellation by polyphase multipath circuits

Abstract

It is well known that in balanced (or differential) circuits, all even harmonics are canceled. This cancellation is achieved by using two paths and exploiting phase differences of 180 between the paths. The question addressed in this paper is: what distortion products (harmonics and intermodulation products) are canceled if more than two paths (and phases) are used? These circuits are called polyphase multipath circuits. It turns out that the more paths (and phases) are used, the more distortion products are canceled. Unfortunately, some intermodulation products cannot be canceled without also canceling the desired signal. An analysis of the impact of mismatch between the paths shows that the suppression of distortion products will be larger if more paths are used. As an application example, the design of an upconversion mixer with a clean output spectrum is presented.

Multipath Polyphase Circuits and their Application to RF Transceivers

“…The signals at the output of these phase shifters can be written as: In equation (2), the phase of the fundamental component is identical for all the paths, but the phases of the harmonics are different for each path. If the phase φ is chosen such that φ=360 o /n, then all the higher harmonics are cancelled [2], except for the k th harmonics for which k equals j×n+1 (j=0, 1, 2, 3, ..).…”

“…Due to the nonlinearity, the phase rotation for the k th harmonic is k times the input phase. Thus the respective phases at the output of the nonlinear block for path [1,2,3] are [0°, 120°, 240°] for ω, [0°, 240°, 120°] for 2ω and [0°, 0°, 0°] for 3ω products. Fig.2 also shows how the phases of the harmonics at the output of each path combine.…”

“…to realize a software defined radio platform, such filters are problematic as LC band-pass filters work at a fixed frequency related to the LC-resonance frequency, which limits the flexibility in choosing a TXfrequency. The next section reviews a polyphase multi-path technique [2][3] to eliminate these filters or relax their requirements significantly. In section III we will discuss a way to implement the different phases using mixers, and in section IV an example of a power up-converter exploiting the technique.…”

“…Section V discusses alternative application directions and section VI draws conclusions. Fig.1 shows a nonlinear circuit excited by a single sine wave at ω, producing a wanted output signal at ω but also unwanted harmonic distortion at 2ω, 3ω, 4ω, etc.. Fig.2 shows a polyphase 3-path circuit, cancelling many harmonics of ω [2]. The basic idea is to divide the nonlinear circuit of Fig.1 into 'n' equal smaller pieces, and apply an equal but opposite phase shift before and after each nonlinear circuit.…”

Flip-Flops for Accurate Multiphase Clocking: Transmission Gate Versus Current Mode Logic

“…Phase errors degrade performance, e.g. by generating spurious tones [3] or limiting the achievable image and harmonic rejection [4].…”