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A highly integrated_ dual-band_ multi-mode Wireless LAN Transceiver

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A highly integrated_ dual-band_ multi-mode Wireless LAN Transceiver Powered By Docstoc
					    A highly integrated, dual-band, multi-mode
            Wireless LAN Transceiver
                                Thomas Rühlicke, Markus Zannoth, Bernd-Ulrich Klepser

                    Infineon Technologies AG, Secure Mobile Solutions, Technology and Innovations
                                              81609 Munich, Germany


                         Abstract:                              transceiver, including the major blocks of the transceiver:
                                                                low noise amplifier (LNA), transmitter, synthesizers,
A fully integrated radio transceiver chip for the 2.4 and       voltage regulators and the 3-wire bus interface including
5 GHz WLAN standards 802.11a/b/g is presented in a              the control logic. For the established 802.11b standard a
0.25 µm 40 GHz BiCMOS technology. The chip                      direct conversion architecture is used at the receive path
integrates the LNAs, mixers, channel filters, PGC,              to achieve a low power consumption and a high adjacent
synthesizers with VCOs and reference oscillator,                channel rejection. With this standard the requirements
transmitters, anti-aliasing filters and voltage regulators.     regarding linearity, dc-offset and signal to noise ratio
The presented transceiver exceeds all IEEE                      (S/N) are not as constrained as for the high data rate
requirements for the 802.11a/b/g CCK and OFDM                   standards 802.11a and 802.11g, where 54 MBit/s data
standards. At 2.4 GHz, an operational range of –85 to           rate can be achieved. For the high data rate, a low
+8 dBm and of -74 dBm to +4 dBm have been                       intermediate frequency (LIF) based architecture has been
demonstrated for the 11 Mb/s CCK and 54Mb/s OFDM                chosen. Compared to a standard IF architecture [2], the
modes, respectively. Furthermore a maximum EVM of               LIF architecture provides a low-cost solution and
-32 dB have been measured.                                      overcomes the dc-offset problems of the Zero-IF solution
                                                                with a direct conversion architecture.

1. Introduction                                                            RX 2.4GHz                               RSSI
                                                                                                                                RSSI
  In recent years there has been a increased demand in              RX                                                          2.4/5GHz
wireless local area networks (WLANs). The most                    2.4GHz
                                                                                                             RX Filter / PGC
                                                                                                    CCK Filter

commonly used frequency bands are the 2.4 to 2.5 GHz
                                                                           RX 5.2GHz
ISM band and several frequency bands from 5.15 and                 RX
                                                                                                                                I
                                                                                                                                Q
5.85 GHz, with several communication standards defined            5GHz                       PGC1
                                                                                                                        PGC2
in the IEEE 802.11 standards. Thereby, the standards                                                OFDM Filter
                                                                                                                               Loop Filter
802.11a and 802.11g enable data rates of up to 54 MBit/s                   TX 5.2GHz                  0°
                                                                                                                         PLL
                                                                                                                                5 GHz
                                                                                                                                2.4GHz
using orthogonal frequency division multiplexing
                                                                                                      90°
                                                                   TX
                                                                  5GHz                                                          40 MHz out
(OFDM) [1]. The established standard 802.11b uses the                                                Synthesizer
direct sequence spread spectrum (DSSS) and has a                                                                                 3-wire
                                                                           TX 2.4GHz                        3-wire bus
maximum data rate of 11 MBit/s.                                                                                                  bus
                                                                   TX                                             LDO
This paper presents the circuit design and measurement           2.4GHz                                                         I
results of highly integrated, 2.4 and 5 GHz dual-band,                                                                          Q
802.11a/b/g multi-mode RF transceiver. The circuit is           Fig. 1: Block-diagram of the dual band transceiver
fabricated in a 0.25 um 40 GHz BiCMOS technology.
The chip is assembled in a 48 pin package for small size        The high frequency 5 GHz transmit path for 802.11a uses
and small number of external components.                        a low-IF direct modulator, while for the lower band at
                                                                2.4 GHz a zero-IF direct modulator is implemented.
2. Architecture of the transceiver                              A major advantage of the chosen RF architectures in the
                                                                receive and transmit path of the transceiver is the low
  For the architecture of the presented RF transceiver a
                                                                component count of the system reference design. The
direct conversion architecture with a combination of low-
                                                                external parts are: blocking capacitors at each power
IF and zero-IF interfaces to the baseband IC has been
                                                                supply close to the pins; the external 40 MHz crystal
chosen. In general, the advantage of a zero-IF architecture
                                                                (xtal); one reference resistor to generate an exact
is higher adjacent channel rejection using real channel
                                                                reference current; the matching networks for LNA-inputs
filters while the advantage of a low-IF architecture is a
                                                                and modulator outputs and the external loop filters for
larger robustness of the signal with respect to dc offset and
                                                                both synthesizers. For the complete WLAN system, an
image rejection since these impairments lead to out of-
                                                                additional band-select filter at the LNA inputs, a
band spectral components.
                                                                baseband processor, power amplifiers for 2.4 GHz and
Fig. 1 shows the schematic block diagram of the
                                                                5 GHz, antennas and switches are required. No further
channel filtering at the receive path, anti-aliasing filtering        give sufficient margins for the power amplifier to meet
at the transmit path or voltage is needed.                            the specified EVM value of –25 dB for the 54 MBit/s
                                                                      mode [1].
                                                                                                  11a        11b        11g
3.    Description of the major blocks of the transceiver
                                                                      Frequency                  5.15-       2.4-       2.4-
3.1. Transmit path                                                                            5.85GHz     2.5GHz 2.5GHz
For the transmit path a direct modulator architecture is              Sensitivity (54Mb/s)     -74dBm                -72dBm
used for 2.4 GHz and for 5 GHz modes. As low-IF is                    Sensitivity (11Mb/s)                -85dBm
used at the 5 GHz path a larger bandwidth is                          Max. input power         -10dBm      +4dBm      +4dBm
implemented at the low frequency blocks than at the                   I-Q impairment             0.6%       0.8%       0.8%
blocks for the 2.4 GHz path, where direct modulation is                                            1°       1.3°        1.3°
used (see Fig. 2). The complex analog input signals are               Adjacent channel        0dB/9dB      -28 dB 0dB/9d
fed directly to a integrated low pass filter that is                  suppression                        (7dB marg.)     B
integrated to reduce the harmonic spectra of the digital              Alternate channel         >30dB                 >30dB
analog converter of the baseband chip to the required                 suppression
value of -40 dBc for the OFDM modes and –50 dBc for                   RX EVM limit              -32dB      -32dB      -32dB
the CCK modes, respectively [1].                                      TX EVM                    -32dB      -32dB      -30dB
The filtered signal is then fed to a programmable gain                PLL rms phase error          1°       0.6°        0.6°
amplifier that is used to convert the voltage output signal           Table. 1 : Summary of WLAN transceiver performance
of the active RC filters to a current. With the
                                                                      The 2.4 GHz output spectrum of the transmitter is shown
programmable gain the production tolerances of the
                                                                      in Fig. 4 for the 11 MBit/s CCK mode as well as for the
output power can be canceled out. The modulator core
                                                                      54 MBit/s OFDM mode. The transmission spectrum
itself consists of two bipolar switching pairs where the
                                                                      masks are exceeded with sufficient margins for the
current is directly inserted [3].
                                             LO for I Signal
                                                                      nonlinearities of the power amplifiers.
                                                                      The whole transmit path operates with a supply voltage
                 3rd Order                                            of 2.6 V and has a current consumption of 155 mA at the
                Butterworth
 I
                                   U/I-                               5 GHz mode and 110 mA at the 2.4 GHz modes. These
                3rd Order        Converter
                                                                      measured values demonstrate a state-of-the art direct
 IX             Chebyshev
                                                                      modulation transmitter performance, with the capability
                                                               out
                                                               outx
                                                                      of supporting the 54 MBit/s modes with a very small
                 3rd Order
 Q              Butterworth                                           number of external components, e.g. output matching
                                   U/I-
                3rd Order        Converter
                                                                      network and supply blocking capacitors.
 Q              Chebyshev                                                                           -13

 X
                                                                                                    -23



                                                                                                    -33


                                             LO for Q Signal                                        -43




Fig. 2: 2.4 GHz transmit path of the WLAN transceiver                                               -53



                                                                                                    -63




For OFDM signals with 10 dB back-off, a mean output                                                 -73




power of –4 dBm can be measured with a matched output
                                                                                                    -83



                                                                                                    -93



at the 5 GHz band as well as the 2.4 GHz band. The                                                 -103




constellation diagrams for the CCK 11 MBit/s mode and                                              -113
                                                                                                      2392   2402   2412   2422   2432       2442
                                                                                                                                         Frequency/MHz
                                                                                                                                                       2452   2462   2472   2482   2492




for the 54 MBit/s OFDM mode are shown in Fig 3.
                                                                      Fig. 3: Transmission mask for 802.11g 11Mb/s QPSK
                                                                      CCK (left) and 54Mb/s 64QAM OFDM (right) signals


                                                                      3.2. Receive path
                                                                      The receive path consists of two LNAs with mixers at the
                                                                      input for the 5 GHz band and 2.4 GHz band, a first
                                                                      programmable gain control stage (PGC1), channel filters
                                                                      and second PGC stages (PGC2) as shown in see Fig. 5.
                                                                      The down conversion mixers consist of Gilbert-Cell
Fig. 4: 2.4 GHz transmit constellation diagram for                    mixers. The first PGC optimizes the level of the signal to
802.11g 11Mb/s QPSK CCK (left) and 54Mb/s 64QAM                       reduce the linearity requirements of the channel filter. Both
OFDM (right) signals                                                  OFDM and CCK channel filters have been integrated in
                                                                      order to replace external SAW filters. For the Low-IF
The measured error vector magnitudes are –32 dB and                   OFDM modes (802.11a/g) polyphase bandpass filters are
-30 dB for the 5 GHz and 2.4 GHz band, respectively                   used. For the OFDM signals a Butterworth approximation
                                                                          th
(see Table. 1). These measured values for the transceiver             of 5 order with a center frequency of 10 MHz and a 3 dB-
                                                                          RSSI out
                                                                                                     -20
                                                                 RSSI
                                                                          RSSI outx


  1b/g
       RF in
                         I-Signals
                                                    Channel
                                                     Filter
                                                                                                     -30
                 LO IQ
      RF inx                                       Chebyshev              I out
                                     PGC1                        PGC2
                                                    Channel               IX out
                                                      Filter                                         -40
                                                   Butterworth
                                            Polyphase                   to ADC




                                                                                       Level [dBm]
                                                    Channel
                                                      Filter
                                                   Butterworth            Q out
                                                                                                     -50
                                     PGC1                        PGC2
         RF in                                      Channel               QX out
  11a                        Q-Signals               Filter
        RF inx
                 LO IQ
                                                   Chebyshev                                         -60

                                                                                                     -70
                                                                                                                                           measured ACP
Fig. 5: Receive path of the WLAN tranceiver
                                                                                                     -80                                   Spec limit
bandwidth of 25 MHz is used. For the 802.11b and                                                                                           Channel
802.11g CCK mode a low-pass filter with a bandwidth of                                               -90
8 MHz is used. Both filters are implemented with active                                                2397      2417         2437         2457             2477
                                                                                                                         frequency [MHz]
RC circuits in leap-frog structures. In addition to the signal
path, a received signal strength indicator (RSSI) has been                                 Fig. 7: Measured ACP in 11 MBit/s CCK mode
implemented to detect any signal to wake up the baseband
                                                                                      For the 11 MBit/s 2.4 GHz CCK mode, a sensitivity of
chip and to measure the signal level.
                                                                                      -85dBm has been measured, i.e. 9 dB better than the
The LNA has a gain of 30 dB which can be reduced by
                                                                                      IEEE specification of -76dBm. The high sensitivity and
20 dB at the 5 GHz mode and by 27 dB at the 2.4 GHz
                                                                                      low noise floor measurements demonstrate the excellent
mode, respectively. Fig. 6 shows the measured
                                                                                      low noise performance of the presented receiver, which
constellation diagram for a 11 MBit/s CCK signal as well
                                                                                      will enable a large operational range and a high
as for a 54 MBit/s OFDM signal for received 2.4 GHz
                                                                                      robustness against multi-path disturbances.
signals.

                                                                                                      15
                                                                                                                                             EVM
                                                                                                      10
                                                                                                                                             Specs limit
                                                                                                       5
                                                                                                       0
                                                                                                      -5
                                                                                       EVM [dB]




                                                                                                     -10
                                                                                                     -15
                                                                                                     -20
Fig. 6: Receive constellation diagram for 11Mb/s QPSK                                                -25
CCK (left) and 54Mb/s 64QAM OFDM (right) signals
                                                                                                     -30
An EVM of –30 dB has been measured, which is 12 dB                                                   -35
better than the minimum requirement for a 54 Mb/s                                                       -100   -80      -60      -40       -20          0
AWGN channel [1]. The adjacent and alternate channel                                                                 OFDM input power [dBm]
suppression has been determined by measuring required
ACP (adjacent/alternate channel power) for a EVM                                           Fig. 8: Measured sensitivity for the 54 MBit/s 802.11g
degradation to -18dB of a 54 Mb/s OFDM channel with                                        OFDM mode
–62 dBm [1].
An adjacent and alternate channel suppression of 0dB
and 30dB has been measured, respectively, which                                       3.3. Frequency synthesizer
demonstrated the feasibility of a Low-IF receiver                                     To achieve minimum power consumption at each band
architecture for 802.11a/g OFDM receiver. At the                                      (2.4 GHz and 5 GHz), two different frequency
11 MBit/s CCK mode an adjacent channel power of                                       synthesizer blocks are used (cf. [3]).
-28 dBm has been measured which shows a margin of                                     The reference oscillator running at 40 MHz is a partly
7 dB to the specified value of –35 dBm, cf. Fig. 7.                                   integrated Colpitts oscillator. Only the xtal and a
The sensitivity for an EVM value of –18 dB, which                                     capacitor has to be connected externally (cf. [3]).
corresponds to a 54 MBit/s PER (package error rate) of                                The synthesizer for the 5 GHz band consists of an
10% as required by the IEEE standard [1], has been                                    integer-N offset phase locked loop (PLL) with a voltage
measured to be -73 dBm, cf. Fig. 8. The measured value                                controlled oscillator (VCO) at 4/3 of the desired
is 8 dB better than the IEEE requirement. For higher                                  transmitted frequency. With using this architecture (cf.
inputs powers the specification is met up to an input level                           [4]) a quadrature phase error of only 1° has been
of +4 dBm (see Fig. 8).                                                               measured, which is a state-of-the-art value for a 5 GHz
synthesizer and demonstrates the advantages of the            receivers a maximum EVM of -32 dB has been measured,
presented architecture.                                       which enables the design of highly robust WLAN systems
In order to combine a large tuning range with low tuning      regarding multi-path distortion. Using a reference design
sensitivities the VCOs feature a band selection algorithm     board, a large operational range from -85 dBm to +8 dBm
to set one out of 128 VCO frequency bands followed by         in 11 MBit/s CCK mode and -74 dBm to +4 dBm in
an analogue settling process controlled by the PLL [5].       54MBit/s OFDM mode has been measured with a 2.4 GHz
The bands of the VCO can be selected by switching             input signals.
capacitances in the oscillator tank (see Fig. 9) [6].

                              VCC
                                                              5.      Acknowledgement
                                                              The presented circuitry has been developed within
                                                              funded projects HGDAT and IBMS2 by the ministry of
                                                              education and research in Germany (BMBF).
                              Vtune
                                                              [1] Wireless LAN Association, "Wireless Networking
                                                                  Standards and Organisations", www.wlana.org, April
                                                                  2002
                                                              [2] Su, M. Zargari, P. Yue, S. Rabii, D. Weber, B. Kaczynski,
                                                                  S. Mehta, K. Singh, S. Mendis, B. Wooley, "A 5 GHz
                                                                  CMOS TRansceiver for IEEE 802.11a Wireless LAN",
                                                                  ISSCC 2002 Digest of Technical Papers, Paper 5.4, Feb.
                                                                  2002
                                                              [3] Bernd-Ulrich Klepser, Manfred Punzenberger, Thomas
                                                                  Rühlicke, Markus Zannoth, “ 5-GHz and 2.4-GHz Dual-
                                                                  Band     RF-Transceiver     for     WLAN        802.11a/b/g
                                                                  Applications”, to be published in the Proceedings of the
                                      on/off
                                                                  RFIC Conference, 2003.
                                                              [4] C.    Grewing,      A.    Hanke,      S.    V.     Waasen,
Fig. 9: VCO with switchable LC-Tank                               "Schaltungsanordnung       zum       Bereitstellen    eines
                                                                  komplexwertigen         Lokaloszillator-Signals        und
Summarizing all tolerances having impact on the                   Empfänger", Patent pending Nr.: P2001,0617WO
frequency accuracy after band selection algorithm             [5] Kral, F. Behbahani, A.A. Abidi, " RF-CMOS oscillators
performed a fine-tuning sensitivity of 200 MHz/V is               with switched tuning", in Custom IC Conf., Orlando, FL,
sufficient for locking the PLL under all conditions.              pp. 569-572, 2000
For a conventional VCO at 5 GHz, a tuning range of            [6] J. Kucera, B.-U. H. Klepser, “3.6GHz VCOs for multi-
1500 MHz would be required to cover all defined bands             band GSM Transceivers”, Proceedings of the ESCCIRC,
over production tolerances. With a given tuning range of          2001
                                                              [7]   B.-U.H. Klepser, M. Scholz, E. Götz, "A 10-GHz SiGe BiCMOS
1 V, the resulting tuning sensitivity would be as large as          Phase-Locked-Loop Frequency Synthesizer", IEEE Journal of
1500 MHz/V. These high tuning sensitivity would have                Solid-State circuits, Vol. 37, No.3 March 2002 pp. 331
significant drawbacks with respect to crosstalk, pushing
and phase noise, which clearly demonstrates the
advantage of the presented VCO architecture.
The phase noise floor normalized to 1 Hz of both PLLs is
measured to be –215 dBc/Hz, which is comparable to a                 XO          Regulator
                                                                                                VCO            PLL
state-of-the-art SiGe-PLL [7]. The rms phase error of the
whole synthesizer including quartz oscillator and voltage              TX-Filter                           VCO        LNA
regulator is measured to be 1° at the 5 GHz modulator
output and 0.6° at the 2.4 GHz modulator output. With                                        RX-Filter
these values the requirements for EVM can be exceeded
for the 54 MBit/s mode of the 802.11a/g standards.                                                          LO-Generation
                                                                       RX-PGC
4.        Conclusion
                                                                                         LNA
                                                                                                      Modulator
A transceiver has been realized in a 0.25 um, 40 GHz                    RSSI
BiCMOS technology for the WLAN standards
802.11a/b/g. The chip integrates a complete dual band
transmitter, dual band receiver, two phase locked loops
with integrated VCOs, level detector and voltage regulator      Fig. 10: Chip photograph of the WLAN transceiver
(see Fig. 10). The chip exceeds all IEEE specifications for
54 MBit/s modes. At a mean output power of –4 dBm, the
measured transmit EVM is –32dB at 5 GHz and -30 dB at
2.4 GHz, respectively. For both 2.4 GHz and 5 GHz

				
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Description: A fully integrated radio transceiver chip for the 2.4 and 5 GHz WLAN standards 802.11a/b/g is presented in a 0.25 �m 40 GHz BiCMOS technology. The chip integrates the LNAs, mixers, channel filters, PGC, synthesizers with VCOs and reference oscillator, transmitters, anti-aliasing filters and voltage regulators. The presented transceiver exceeds all IEEE requirements for the 802.11a/b/g CCK and OFDM standards. At 2.4 GHz, an operational range of –85 to 8 dBm and of -74 dBm to 4 dBm have been demonstrated for the 11 Mb/s CCK and 54Mb/s OFDM modes, respectively. Furthermore a maximum EVM of -32 dB have been measured.