Application Note

Skyworks Solutions, Inc., Proprietary and Confidential

101735A

Preliminary Data Subject to Change

July 20, 2001

CX74017

On the Direct Conversion Receiver

Abstract

Increased pressure for low power, small form factor, low cost,

and reduced bill of materials in such radio applications as mobile

communications has driven academia and industry to resurrect

the Direct Conversion Receiver (DCR). Long abandoned in favor

of the mature superheterodyne receiver, direct conversion has

emerged over the last decade or so thanks to improved

semiconductor process technologies and astute design

techniques. This paper describes the characteristics of the DCR

and the issues it raises.

Introduction

Very much like its well-established superheterodyne receiver

counterpart, introduced in 1918 by Armstrong [1], the origins of

the DCR date back to the first half of last century when a single

down-conversion receiver was first described by F.M. Colebrook

in 1924 [2], and the term homodyne was applied. Additional

developments led to the publication in 1947 of an article by

D. G. Tucker [3], which first coined the term synchrodyne, in a

receiver, which was designed as a precision demodulator for

measurement equipment rather than a radio. Another paper by

the latter in 1954 [4] reports the various single down-conversion

receivers published at the time, and clarifies the difference

between the homodyne (sometimes referred to as coherent

detector) and the synchrodyne receivers: the former obtains the

Local Oscillator (LO) directly, for example, from the transmitter,

whereas the latter synchronizes a free-running LO to the

incoming carrier.

Over the last decade or so, the drive of the wireless market and

enabling monolithic integration technology have triggered

research activities on DCRs, which integrated with the remaining

analog and digital sections of the transceiver, has the potential

to reach the “one-chip radio”. Besides, it favors multi-mode,

multi-standard applications and constitutes thereby another step

towards software radio.

The present article often refers to several recent publications

[5-6], providing a thorough survey and insight, and displaying

the renewed interest for DCRs.

Overcoming some of the problems associated with the

traditional superheterodyne and being more prone to integration,

DCR has nevertheless an array of inherent challenges. After a

brief description of alternative and well-established receiver

architectures, this paper presents the direct conversion

reception technique and highlights some of the system-level

issues associated with DCR.

Traditional Reception Techniques

The Superheterodyne

is the most widely used reception technique. This technique

finds numerous applications from personal communication

devices to radio and TV tuners, and has been tried inside out

and is therefore well understood. It comes in a variety of

combinations [7-9], but essentially relies on the same idea: the

RF signal is first amplified in a frequency selective low-noise

stage, then translated to a lower intermediate frequency (IF),

with significant amplification and additional filtering, and finally

downconverted to baseband either with a phase discriminatory

or straight mixer, depending on the modulation format. This is

illustrated in the generic line-up of Figure 1.

RF band-select

filter

LNA

Channel-select

filter

RF

IF

RF

IF

f

f

Image-reject

filter

101735A 1_071801

Figure 1. The Superheterodyne Receiver

Superheterodyning entails several trade-offs. Image rejection is

a prevailing concern in this architecture. During the first

1

Homo: Greek from “homos” - same; Hetero: Greek from

“heteros” – other; Synchro: Greek from “sunkhronos” – same

time; Dyne: Greek from “dunamis” – power.