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.
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
The superheterodyne, or more generally heterodyne
1
, receiver
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.
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
Application Note
RF band-select
filter
Image-reject
filter
LNA
Channel-select
filter
RF
IF
RF
IF
f
101735A 1_071801
f
Figure 1. The Superheterodyne Receiver
Superheterodyning entails several trade-offs. Image rejection is
a prevailing concern in this architecture. During the first
Homo: Greek from “homos” - same; Hetero: Greek from
“heteros” – other; Synchro: Greek from “sunkhronos” – same
time; Dyne: Greek from “dunamis” – power.
1
Skyworks Solutions, Inc., Proprietary and Confidential
Preliminary Data Subject to Change
101735A
July 20, 2001
SKYWORKS [ SKYWORKS SOLUTIONS INC. ]
