Othello™: A New Direct-
`Conversion Radio Chip
`Set Eliminates IF Stages
`
`by Dan Fague
`INTRODUCTION
`Analog Devices recently announced the revolutionary Othello
`direct-conversion radio for mobile applications. By eliminating
`intermediate-frequency (IF) stages, this chip set will permit the
`mobile electronics industry to reduce the size and cost of radio
`sections and enableflexible, multistandard, multimode operation.
`The radio consists of two integrated circuits, the AD6523 Zero-
`IF Transceiver and the AD6524 Multiband Synthesizer. The
`AD6523 contains the main functions necessary for both a direct-
`conversion receiver and a direct VCO transmitter, known as the
`Virtual-IF™ transmitter. It also includes the local-oscillator
`
`generation block and a complete on-chip regulator that supplies
`powerto all active circuitry for the radio. The AD6524 is a
`fractional-N synthesizer that features extremely fast lock times to
`enable advanced data services over cellular telephones—suchas
`high-speed circuit-switched data (HSCSD) and general packet
`radio services (GPRS).
`
`Together, the two ICs supply the main functions necessary for
`implementing dual- or triple-band radios for GSM cellular phones.
`Thedirect conversion technology, combined with a new twist on
`the translation loop (or direct VCO) modulator, reduces the
`amount of external filtering needed in the radio to an absolute
`minimum.
`
`THE GSM STANDARD
`
`The Global system for mobiles (GSM) wasofficially launched in
`1992, after over five years of standards writing by the European
`Telecommunications StandardsInstitute (ETSI).Thegoal of GSM
`wasto unite a Babel of European communications under onedigital
`cellular standard. Before GSM, Europe maintained in effect one
`separate cellular network for each country, making international
`roaming on the continentvirtually impossible. With GSM,a citizen
`of any of the original seventeen countries could roam to any other
`country using a single cellular handset. The standard, which was
`written with future expansion to data services and other
`applications in mind, soon became popular aroundthe world.It is
`nowaccepted in more than 140 countries, with over 200 networks
`running.
`
`The frequency bandsoriginally allocated to GSM were 890 to
`915 MHzfor mobile transmitting and 935 to 960 MHzfor mobile
`receiving. That band was expandedtothe so-called E-GSM bands
`of 880 to 915 MHz and 925 to 960 MHz. Another frequency
`allocation was made to further expand GSM capacity. This band,
`allocated to digital communicationsservices (DCS), was 1710 to
`1785 MHz and 1805 to 1880 MHz.All countries adopting GSM
`use oneof these two pairs of frequency bands, except the United
`States, where both bands werealready allocated by the FCC. The
`Personal Communications Services (PCS) frequency auctions in
`the mid-1990s madeavailable a set of bands for GSM in the U.S.—
`1850 to 1910 MHz and 1930 to 1990 MHz.
`
`Othello, Superhomodyneand Virtual-IF are trademarks ofAnalog Devices,Inc.
`
`Analog Dialogue 33-10 (© 1999 Analog Devices)
`
`Today’s typical GSM handset (or handy) will have 2-W output
`powerandis required to receive signals as low as -102 dBm (less
`than 1/10 of a picowatt). The handy includes a powerful digital
`signal processor (DSP) core (equivalent to an ADSP-218x) to
`encode, encrypt, interleave, packetize, transmit, receive, de-
`packetize, de-interleave, de-encrypt, and de-encodethe data going
`to and coming from the voiceband A/D and D/A converters. An
`equally powerful microcontroller (ARM or Hitachi H8), combined
`with a hardware burst processor, controls the timing necessary to
`implement the time-division multiple-access (TDMA) and
`frequency hopping functions to keep the phonecall on a specific
`time and frequency channel. The microcontroller also implements
`the man-machineinterface, and operatesall the necessary protocols
`for communicationto the base stations.
`
`RADIO ARCHITECTURE DESIGN
`
`Most digital cellular phones today include at least one
`“downconversion”in their signal chain. This frequency conversion
`shifts the desired signal from the allocated RF bandfor the standard
`(say, at 900 MHz) to some lower intermediate frequency (IF),
`where channelselection is performed with a narrow channel-select
`filter (usually a surface acoustic-wave (SAW) or a ceramic type).
`The now-filtered signal is then further down-converted to either a
`second IF or directly to baseband, where it is digitized and
`demodulated in a digital signal processor (DSP).
`
`Theidea of using direct-conversion for receivers has long been of
`interest in RF design. The reason is obvious: in consumer
`equipment conversion stages add cost, bulk, and weight. Each
`conversion stage requires a local oscillator, (often including a
`frequency synthesizer to lock the LO onto a given frequency), a
`mixer, a filter, and (possibly) an amplifier. No wonder, then,that
`direct conversion receivers would be attractive. All intermediate
`
`stages are climinated, reducing the cost, volume, and weight of
`the receiver.
`
`Thefirst Othello radio reduces the component count even more
`by integrating the front-end GSMlow-noise amplifier (LNA).This
`eliminates an RF filter (the “image”filter) that is necessary to
`eliminate the image, or unwanted mixing product of a mixer and
`the off chip LNA.This stage, normally implemented with a discrete
`transistor, plus biasing and matching networks, accounts for a total
`of about 12 components.Integrating the LNAsavesa total of about
`15 to 17 components, depending on the amount ofmatchingcalled
`for by the (now-climinated)filter.
`
`SUPERHOMODYNE™DIRECT-CONVERSION RECEIVER
`
`A functional block diagram of the Othello dual band GSM radio’s
`architecture is shown in Figure 1. Thereceive section is at the top
`of the figure. From the antenna connector, the desired signal enters
`the transmit/receive switch and exits on the appropriate path,either
`925-960 MHz for the GSM band or 1805-1880 MHz for DCS.
`
`The signal then passes through an RF bandfilter (a so-called
`“roofing filter”) that serves to pass the entire desired frequency
`band while attenuating all other out-of-band frequencies
`(blockers—including frequencies in the transmission band) to
`prevent them from saturating the active componentsin the radio
`front end. Theroofingfilter is followed by the low-noise amplifier
`(LNA). This is the first gain element in the system, effectively
`reducing the contribution ofall following stages to system noise.
`After the LNA,the direct-conversion mixertranslates the desired
`signal from radio frequency (RF) all the way to baseband by
`
`1
`SAMSUNG 1050
`SAMSUNG 1050
`SAMSUNG v. SMART MOBILE
`SAMSUNGv. SMART MOBILE
`IPR2022-01004
`IPR2022-01004
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`1
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`multiplying the desired signal with a local oscillator (LO) output
`
`at the samefrequency.
`
`Figure 1. Block diagram of the Othello dual-band radio.
`
`The outputof the mixer stage is then sent in quadrature (I and Q
`channels) to the variable-gain baseband amplifier stage. The VGA
`also provides somefiltering of adjacent channels, and attenuation
`of in-band blockers. These blockingsignals are other GSM channels
`that are some distance from the desired channel, say 3 MHz and
`beyond. The baseband amplifiers filter these signals so that they
`will not saturate the Receive ADCs. After the amplifier stage, the
`desired signal is digitized by the Receive ADCs.
`
`VIRTUAL-IF™ TRANSMITTER
`
`schemeis Gaussian-filtered minimum-shift keying (GMSK).This
`type of modulation does notaffect the envelope amplitude, which
`meansthat a power amplifier can be saturated andstill not distort
`the GMSKsignal sent throughit.
`
`GMSKcan be generated in several different ways. In another
`European standard (for cordless telephones), GMSKis created
`by directly modulating a free running-VCO with the Gaussian
`filtered data stream. In GSM, the method of choice has been
`quadrature modulation. Quadrature modulation creates accurate
`phase GMSK,but imperfections in the modulatorcircuit (or up-
`conversion stages) can produce envelopefluctuations, which can
`in turn degradethe phase trajectory when amplified by a saturated
`poweramplifier. To avoid such degradations, GSM phone makers
`have been forced to use amplifiers with somewhathigherlinearity,
`at the cost of reduced efficiency and talk time per battery charge
`cycle.
`
`Thetranslation loop modulator combinesthe advantagesofdirectly
`modulating the VCO andthe inherently more accurate quadrature
`modulation. In effect, the scheme creates a phase locked loop
`(PLL), comprising the modulator, the LO signal, and the VCO
`output and feedback mixer. Theresult is a directly modulatedVCO
`outputwith a perfectly constant envelope and almost perfect phase
`trajectory. Phase trajectory errors as low as 1.5 degrees have been
`measured in the AD6523transceiver IC, using a signal generator
`as the LOsignalto provide a referencefor the loop.
`
`FREQUENCY PLANNING
`An important aspect of the Othello radio design is the frequency
`plan. The GSM standardhasstrict requirements with regard to
`The Transmit section begins ontheright, at the multiplexed I and
`in- and out-of-band spurious emissions. A GSM cellular phone
`Q inputs/outputs. Because the GSM system is a time division
`must be able to withstand blockers at extremely high levels (0 dBm)
`duplex (TDD) system,the transmitter and receiver are never on
`while continuing to receive normally. The phone must also not
`at the same time. The Othello radio architecture takes advantage
`emit spurioussignals into other bands aboveacertain level (in the
`of this fact to save four pins on the transceiver IC’s package. The
`GSMreceive band, -112 dBrelative to the transmitted signal!).
`quadrature transmit signals enter the transmitter through the
`multiplexed I/Os. These I and Q signals are then modulated onto
`a carrier at an intermediate frequency greater than 100 MHz.
`
`The Othello radio architecture was designed with the entire system
`in mind. The frequency plan was carefully crafted to satisfy three
`equally importantcriteria:
`
`The output of the modulator goes to a phase-frequency detector
`(PFD), where it is compared to a reference frequency that is
`generated from the external channel selecting LO. The outputof
`the PFDis a charge pump,operating at above 100 MHz, whose
`outputis filtered by a fairly wide (1 MHz)loopfilter. The output
`of the loop filter drives the tuning port of a voltage-controlled
`oscillator (VCO), with frequency ranges that cover the GSM and
`DCS transmit bands.
`
`The output of the transmit VCO is sent to two places. The main
`path is to the transmit power amplifier (PA), which amplifies the
`transmit signal from about +3 dBm to +35 dBm,sendingit to the
`transmit/receive switch and low-passfilter (which attenuates power-
`amplifier harmonics). The power amplifiers are dual band, with a
`simple CMOScontrolvoltage for the band switch.TheVCO output
`also goes to the transmit feedback mixer by means of a coupler,
`which is either a printed circuit, built with discrete inductors and
`capacitors, or a monolithic (normally ceramic) coupling device.
`The feedback mixer downconverts the transmit signal to the
`transmit IF, and usesit as the local oscillator signal for the transmit
`modulator.
`
`This type ofmodulator has several names, but the mostdescriptive
`is probably “translation loop.”The translation loop modulatortakes
`advantage of one key aspect of the GSM standard: the modulation
`
`2
`
`1) Reduce spurious emissions from theradio.
`
`2) Minimize bandwidth ofthe dual bandlocal oscillator (LO) VCO.
`
`3) Eliminate as many potential blockers as possible.
`
`Bysatisfying all of these criteria, major radio problems have been
`solved, always keeping the end-user and the application in mind.
`Thefinal solution turned out to be both elegant and practical.
`
`Reducing Spurious Emissions from the Radio
`Spurious emissions from the radio can cause problems in both
`Transmit and Receive modes. A wayward LOsignal canfindits
`way to the antenna and “self block” a direct-conversion receiver,
`reducing sensitivity. The LO signal can also radiate from the
`antenna and degrade the performanceofotherreceivers.
`
`In the Othello frequencyplan,thelocal oscillator’s center frequency
`was chosen to be about 1350 MHz.This placed the LOstrategically
`between the GSM and DCSfrequency bands, enabling a single
`LO to be used for both GSM and DCS,saving components. Since
`that frequencyis distant from either of the bands, the radio’s front
`endfilters will attenuate any radiated LOsignal, and so it doesn’t
`pose a problem as a radiated spurious emission. Evenif the signal
`is coupled directly from pin to pin ontheIC,its powerlevel would
`
`Analog Dialogue 33-10 (© 1999 Analog Devices)
`
`2
`
`

`

`be lower than the GSM requirements for in- or out-of-band
`blockers received at the antenna.
`
`In the Transmit section, spurious signals can also pose a problem.
`Though the transmitter is a direct VCO modulator, the feedback
`mixer will introduce spurious signals at its output that must be
`filtered before entering the phase detector. Otherwise, they could
`appear at the output themselves or causestill other spurious signals
`to appear by mixing with the desired modulation signal due to the
`non-linear operation of the phase detector input stage. This is a
`problem inherent in any translation-loop modulator. By using a
`widely separated LO frequency, the Othello architecture simplifies
`filtering of these products.
`Minimize bandwidth of the dual bandlocal-oscillator VCO
`The Othello architecture was designed to minimize the number
`of external components needed to build a complete dual-band
`radio. The frequency plan was specifically chosen to make it
`possible for a single LO VCO to cover both GSM and DCS
`frequency bands while still meeting the necessarily stringent phase-
`noise specifications at the 3-MHz offset demandedof all GSM
`LOVCOs. By keeping the bandwidth requirements of theVCO to
`a minimum, the VCO can be designed with a maximum supply
`voltage of 2.7 V. This allows the entire dual-band radio to run at
`2.7V, reducing power consumption and enablingthe useofnickel-
`cadmium (NiCd), nickel metal-hydride (NiMH), or lithium-ion
`(Li-ion) battery types.
`
`Eliminate as Many Potential Blockers as Possible
`As a result of the direct-conversion receiver architecture, the
`Othello radio has fewer “trouble” channels for the blocking tests
`required by GSM.Superheterodynereceivers must always contend
`with half-IF responses that are difficult to filter with RF filters
`due to the shape factors required. By going to direct conversion
`Othello eliminates the half-IF response.
`
`PERFORMANCE
`
`Oneofthe key advantagesofthe Othello radiois that the reduction
`in the number of components needed to implement it does not
`engender performancesacrifices. In both the GSM and DCS
`bands, the Othello system noise figure allows for a production
`margin of about 6 dB from the required receiver sensitivity of
`—102 dBm. The transmitter provides a similar production margin,
`with phase trajectory errors of 2.5° rms, compared with the
`requirement of 5° rms.
`
`FUTURE BENEFITS
`
`Another importantfeature ofthe Othello radio is that the AD6524’s
`fractional-N synthesizer has a lock time short enough to enable
`GPRSoperation. [GPRS, an extension to the GSM network
`coming in year 2000,will allow very high data rates to be used by
`a compliant GSM handset.] A requirement of GPRS operation is
`that the LO synthesizer must lock in less than half a GSM time
`slot (lock timesless than 250 ps). The AD 6524,with its fractional-
`N synthesizer, is able to reach lock faster than conventional
`synthesizers, because fractional-N types operate at reference
`frequencies that are higher than the channelspacing, thus jumping
`more than one channel per reference cycle. In the case of the
`AD6524,the 26-MHzreference frequency, twice the system crystal
`frequency (compared to a channel spacing of 200 kHz), ensures
`that the Othello radio will meet the required lock time for GPRS.
`Fast lock timealso helps to reduce power consumption by allowing
`the basebandsectionto keep the radiooff for longer timeintervals.
`
`Analog Dialogue 33-10 (© 1999 Analog Devices)
`
`The Othello radio has opened new doors of opportunity for the
`future. Today, a complete dual-band Othello radio, includingall
`power-managementfunctions, can be implemented with only 90
`components. With so few components, the radio can be
`implemented in less than 10 cm? of board space. Figure 2 is a
`photographof a prototype radio design for Othello, implemented
`on a four layer PCB. Comparethis to a superheterodynereceiver
`that today uses about 225 components crowded into somewhat
`less than 15 cm? of board space for the same functionality. (Even
`that is an improvementover radios of just two years ago, which
`used the same numberof components to implementa single band
`GSMradio!) The advantages of direct conversion translate directly
`to lower costs in many ways: fewer components means that an
`original-equipment manufacturer (OEM) spendsless on bill of
`materials (BOM)andless for inserting the components (about a
`pennyperinsertion). The time to assemble a phoneis reduced,
`increasing factory throughput; and the improved manufacturability
`of the phone (less to go wrong with fewer solder joints, etc.)
`
`increasesreliability.
`
`Figure 2. Photograph of a prototype Othello radio printed
`circuit board.
`
`Because Othello radios can be so compact, they enable GSM radio
`technology to be incorporated in many products from whichit
`has been excluded, such as very compact phones or PCMCIA
`cards. However, the real power of direct conversion will be seen
`when versatile third-generation phones are designed to handle
`multiple standards. With direct-conversion, hardware channel-
`selection filters will be unnecessary, because channelselection is
`performedin the digital signal-processing section, which can be
`programmedto handle multiple standards. Contrast this with the
`superheterodyne architecture; the multiple radio circuits required
`to handle the different standards (because cach will require
`different channel-selectionfilters) will all have to be crowdedinto
`a small space. With direct conversion, the same radio chain could
`in concept be used for several different standards, bandwidths,
`and modulation types. Thus, Web-browsing and voice services
`could, in concept, occur over the GSM network using the same
`radio in the handset.
`
`ANALOG DEVICES AND GSM
`
`For the future, the Othello radio is only the first of a family of
`direct-conversion receiver solutions from Analog Devices. More
`are in the works. But this technology is well-groundedin nearly a
`decade of product designs for the GSM industry. ADI chips can
`be found in millions of GSM handsets in use worldwide.
`
`3
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