26-11-2012, 12:41 PM
Direct-Conversion Radio Transceivers for Digital Communications
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Abstract
Direct-conversion is an alternative wireless receiver
architecture to the well-established superheterodyne, particularly
for highly integrated, low-power terminals. Its fundamental advantage
is that the received signal is amplified and filtered at
baseband rather than at some high intermediate frequency. This
means lower current drain in the amplifiers and active filters
and a simpler task of image-rejection. There is considerable
interest to use it in digital cellular telephones and miniature
radio messaging systems. This paper briefly covers case studies
in the use of direct-conversion receivers and transmitters and
summarizes some of the key problems in their implementations.
Solutions to these problems arise not only from more appropriate
circuit design but also from exploiting system characteristics,
such as the modulation format in the system. Baseband digital
signal processing must be coupled to the analog front-end to make
direct-conversion transceivers a practical reality.
INTRODUCTION
TH E CURRENT interest in portable wireless communica- tions devices is prompting research into new IC technologies,
circuit configurations and transceiver architectures. Lowpower
miniature radio transceivers are sought to communicate
digital data in cellular telephones, wireless networks, and
radio messaging systems. While transistor technology scaling
and improved circuit techniques will contribute evolutionary
advances towards this goal, architectural innovations in the
transceiver may lead to revolutionary improvements [ 11. It
is in this context that there is a resurgence of interest in
direct-conversion.
The superheterodyne receiver, which Armstrong introduced
in 1918 [2], is generally thought to be the receiver of choice
owing to its high selectivity and sensitivity. Something like
98% of radio receivers use this architecture. In a superheterodyne
receiver, the input signal is first amplified at RF in a tuned
stage, then converted by an offset-frequency local oscillator
to a lower intermediate frequency (IF), and substantially
amplified in a tuned IF “strip” containing highly-selective
passive bandpass filters. The role of the various filters is
illustrated by the typical frequency plan of a superheterodyne
receiver (Fig. 1). The IF must be sufficiently high so that the
image channel lies in the stopband of the RF preselection filter
or the antenna, otherwise the IF filter will pass this channel
unattenuated in its own image passband. These considerations
determine the familiar intermediate frequencies used in radio
and TV receivers.
THE DIRECT-CONVERSIOANR CHITECTURE
Suppose that the IF in a superheterodyne is reduced to
zero. The LO will then translate the center of the desired
channel to 0 Hz, and the portion of the channel translated
to the negative frequency half-axis becomes the image to the
other half of the same channel translated to the positive frequency
half-axis (Fig. 3). The downconverted signal must be
reconstituted by a phasing method of the type described above,
otherwise the negative-frequency half-channel will fold over
and superpose on to the positive-frequency half-channel. Zero-
IF, therefore, mandates quadrature downconversion into two
arms and a vector-detection scheme. However, this scheme
does not suffer from the strong-image problem when the
image-reject downconverter is used in a nonzero IF heterodyne
receiver, and the typical gain mismatches and phase errors in
the two branches cause only a small loss in detected SNR.
A lowpass filter, which is in effect a bandpass centered at
dc when the negative frequency axis is included, may be
used to select the desired channel and to reject all adjacent
channels.
DIRECT-CONVERSIFOSNK RECEIVERS
Digital data in broadcast paging modulates the carrier by
frequency-shift keying (FSK). The carrier frequencies may lie
in the 400 MHz or the 900 MHz bands and binary data at 512
b/s or 1.2 kb/s rates shifts the carrier frequency by f4.5 kHz.
This large modulation index results in a spectrum with two
lobes symmetrically offset around the carrier (Fig. 3). Vance
at ITT Standard Telecommunications Labs was the first to
apply direct-conversion to this signal spectrum with a singlechip
paging receiver [8], thus establishing a key concept for
small, light paging receivers. Not all pagers today, though, use
direct-conversion; some continue to use the superheterodyne
implementation for higher performance [9].
DIRECT-CONVERSIROENC EIVERS
FOR DIGITAL CELLULAR TELEPHONES
Designers of portable digital cellular telephones are very
interested in low-power radio architectures. Several integrated
receiver and transmitter IC's conforking to established standards
such as GSM and DECT have been developed in the
past few years. This section summarizes some of their main
features.
All transmitters in these portable phones use direct upconversion
to produce a single-sideband output. In receivers,
however, the superheterodyne architecture is more common.
For instance, a 900 MHz bipolar IC GSM receiver from
Siemens 1171 downconverts the amplified RF signal from an
off-chip low-noise amplifier to an IF of 45-90 MHz (Fig. 6).
At this IF, the image lies in the stopband of the fixed RF
preselect SAW filter. The amplified IF signal is sent to another
off-chip SAW filter to reject adjacent channels. A quadrature
mixer then downconverts the signal to baseband, and the
vector baseband signal is finally detected. This architecture
is preserved in later generations of this transceiver operating
up to 2 GHz for DECT use [IS], [19]. Other recent GSM
transceivers build on a similar single superheterodyne architecture
[20], in one case with a very high IF of 400 MHz
[21]. Alcatel has publicized its use of direct-conversion in
GSM and DECT receivers [22]-[24], although others [25]
are exploring its possible use, and not all companies using
direct conversion have published their experience.
PROBLEMS IN DIRECT-CONVERSIROENC EIVERS
Among the problems in direct-conversion receivers, spurious
LO leakage is probably best known. This arises because
the LO in a direct-conversion receiver is tuned exactly to the
center of the LNA and antenna passbands. In receive mode,
a small fraction of the LO energy may make its way back
to the antenna through the mixer and LNA, owing to their
finite reverse isolation, or couple into the antenna through
external leads, and then radiate out [27]. This becomes an
in-band interferer to other nearby receivers tuned to the same
band, and for some of them it may even be stronger than
the desired signal. Regulatory bodies such as the FCC strictly
limit the magnitude of this type of spurious LO emission. The
problem is much less severe in a superheterodyne whose LO
frequency usually lies outside the antenna passband. However,
experimental studies [28] suggest that standard shielding in the
receiver may control LO leakage to the point that it does not
seriously handicap the use of direct-conversion.
CONCLUSIONS
The direct-conversion receiver eliminates many off-chip
components and may offer significant power savings by amplifying
a received signal mostly at dc rather than at an
IF of tens or even hundreds of MHz. Direct-conversion is
already widely used in single-sideband transmitters. Several
problems in direct-conversion receivers have been identified,
of which static and dynamic dc offset are probably the
most important. In a wideband FSK receiver, this offset is
removed very simply. Otherwise, DSP-based offset-removal
must supplement good F G and baseband analog design in the
receiver. Inspired by the simplicity of the paging receiver, our
research group at UCLA is developing a single-chip spreadspectrum
transceiver that uses binary-FSK modulation on a
frequency-hopped carrier [48], This transceiver uses directconversion
in the transmit and receive paths.