United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,802,463
`
`Zuckerman
`
`[45] Date of Patent:
`
`Sep. 1, 1998
`
`USOO5802463A
`
`[54] APPARATUS AND METHOD FOR
`RECEIVING A MODULATED RADIO
`FREQUENCY SIGNAL BY CONVERTING
`THE RADIO FREQUENCY SIGNAL TO A
`INTERMEDIATE FREQUENCY
`
`[75]
`
`Inventor: Lawrence H_ zuckerman‘ pleasanton.
`Calif.
`
`[73] Assignee: Advanced Micro Devices, Inc..
`Sunnyvale. Calif.
`
`[21] Appl. No.: 699,991
`_
`[22] Filed:
`Aug. 20, 1996
`[51]
`Int. cm ....................................................... H041; 1/16
`[52]
`455/208: 455/318
`
`[58] Field of Search ..................................... 455/203. 205.
`455/208. 318‘ 324‘ 313 314 315
`'
`‘
`
`[56]
`
`Refemnces Cited
`U.S. PATENT DOCUMENTS
`
`Primary Examiner—Amel.ia Au
`Attorney, Agent, or Firm—Foley & Lardner
`
`[57]
`ABSTRACT
`transceiver is
`A Very low intermediate frequency (IF)
`described for use in a wireless LAN. cellular telephone.
`
`cordless telephone. and other radio transceiver applications.
`The transceiver preferably directly down-converts the RF
`signal to lower frequency such as a very low IF signal. which
`can be handled by nansceiver components advantageously
`integrated with the communication control system such as
`an MAC or serial communications controller. Preferably. the
`very low IF signal is above peak modulation deviation and
`b 1
`th
`h
`1 interval for the communication s
`t
`"['T1z:.):leryel<)CwaIlli“:igna.l may be up converted so that tT1:cII{[LF
`signal can be more reliably demodulatecl Alternatively. the
`RF signal can be additionally convened to a second IF
`frequency before. the Very ‘°‘”. IF frequency to mdu.°e the
`effects of 110156 in the transceiver. Alternanvely. an image
`rejection mixer circuit can be employed to provide some
`rejection selectivity for the adjacent channels on one side of
`the local oscillator. Alternatively. a phasing circuit can be
`added in the receiver front end to assist in the isolation of the
`
`4,653,117
`4,852,123
`5,109,531
`
`3/1937 Heck ....................................... 455/209
`.. 375/223
`7/1989 Bickley et a1.
`
`4/1992 Heck ....................................... 455/203
`
`l°°“1 °5°m‘“°’ signal f’°‘“ ‘he ‘""°“”a'
`
`19 Claims, 6 Drawing Sheets
`
`5
`
`E
`E
`:
`:
`
`;
`;
`.:
`
`
` —-.--...‘
`IL‘
`:
`:
`;
`
`RECEIVE
`
`RECEIVER
`DATA OUT
`
`IFIN
`TRANSMIT
`
`§{j'lT“5'1‘“
`mum
`(HANNEUN
`
`3
`:
`:
`
`I -
`
`
`
`CHANNEL OUT
`
`T6
`
`
`
`TRANSMIT
`DATA OUT
`
`
` CONTROLLER
`
` RECEIVE
`
`DATA TN
`
`
`APPLE 1004
`
` 1
`
`1
`
`APPLE 1004
`
`

`
`U.S. Patent
`
`Sep. 1, 1998
`
`Sheet 1 of 5
`
`36A,20co,5
`
`-__.._......_..-....-..-_-.._-.‘
`
`E3:
`
`So<.—<aE5:
`
`=___
`
`=_as
`
`zzmzsz._=_m__=E
`
`so=.=.<_c
`
`
`
`5$2.25:5
`
`=55:
`
`:5<2:
`
`#5:
`
`z_5.3
`
`.E§=.=8
`
`_6:
`
`2
`
`
`
`
`
`
`
`

`
`U.S. Patent
`
`Sep. 1, 1993
`
`Sheet 2 of 6
`
`364:203,5
`
`---—__-_--____..__-—__1
`
`E8
`
`33..
`
`:3E.6.
`
`2
`
`<253
`
`2.E%2
`
`5:85
`
`3
`
`m:<2:
`
`._.”:_8.u=_
`
`.83
`
`HE:
`
`8>
`
`2
`
`525:
`
`:2E32
`
`a
`
`E:25:
`
`:3:
`
`3E
`
`55>“
`
`/
`
`2
`
`E
`
`3
`
`
`
`

`
`QMU
`
`..lH8taD1
`
`Sep. 1, 1998
`
`Sheet 3 of 6
`
`5,802,463
`
`rm--Q-----------—---Inn
`
`
`
`fiu_I.IIII|IIII..IoIInl.l.nI|..l'I.|.|.l.I.InI.lI'.I'uI.
`
`
`
`‘-I-IIIIIIIIlI'IIlIII"ll:II'.'-""I""-
`
`E:2=z8
`
`
`
`----.--.---------------.-.-_2.....IIIl!l...I.|I||.Il....|...|IlS----vu----..
`
`4
`
`
`
`

`
`__
`
`364.,2.MI
`
`
`
`
` 009_.wlucl:..ulcn.u.uI.s.u-:u....u......A....M..50_EaU_as
`
`6_cm_4_w_«H._
`
`r#ll:
`
`Dam_.|I
`
`nW2.
`
`tLme
`
`U
`
`QM:
`
`9:5:1_—FM_...255%9._____2.asS_E._§§.a._j
`59.2555ESE3-.5%2__"---E._E3:
`
`5
`
`
`

`
`U.S. Patent
`
`Sep. 1, 1993
`
`Sheet 5 of 6
`
`.2::2=§55.5.5:=...:_§.
`---42--2..--1----I}
`
`
`6
`
`

`
`U.S. Patent
`
`Sep. 1, 1998
`
`Sheet 6 of 6
`
`5,802,463
`
`E
`
`~Q
`Q 5-—c-c‘-—-—¢
`'_
`§
`--on--can
`-a-a-.——-—cJ———— ———-—-——.-—uu—————
`
`§§Es<2:
`
`.285
`
`55.3
`
`.228
`
`2.32
`
`5::
`
`2_
`
`:52E:5:
`
`...9:
`
`3
`
`IllIL-.
`
`''I l
`
`I
`
`asE<<E
`
`E::28.QEél
`EaEu-8.,I
`
`~_
`
`N:
`
`5625:
`
`.=.2332m:
`
`>
`
`1%!
`__=35::
`
`m<.===_
`
`<8.
`
`25.2
`
`S.
`
`_
`
`:2
`
`5:3
`
`<25:
`
`7
`
`
`
`
`

`
`5.802.463
`
`1
`APPARATUS AND METHOD FOR
`RECEIVING A MODULATED RADIO
`FREQUENCY SIGNAL BY CONVERTING
`THE RADIO FREQUENCY SIGNAL TO A
`VERY LOW INTERMEDIATE FREQUENCY
`SIGNAL
`
`5
`
`TECHNICAL FIELD
`
`The present invention relates generally to a receiver for
`electromagnetic waves. More particularly. the present inven-
`tion relates to a receiver for use in a wireless local area
`
`10
`
`network. or cordless telephone.
`
`BACKGROUND OF THE INVENTION
`
`Computer systems such as workstations, personal
`computers. notebook computers. personal digital assistants
`(PDAS) or other electronic processing units may be inter-
`connected via a wireless local area network (LAN). Each
`computer system includes a communications control system
`(e.g.. a media access controller (MAC). a serial or non-serial
`communications controller or other communications mod-
`
`erating device) and a wireless transceiver. The communica-
`tions control system governs the exchange of data between
`the computer system and the wireless transceiver. For
`example.
`the communications control system selects the
`channel at which the transceiver operates. organizes data
`packets for transmission and reception across the LAN.
`performs error correction on received data packets. controls
`retransrnissions of data packets in the event of transmission
`or reception errors. and temporarily stores incoming and
`outgoing data for the computer system and transceiver.
`Typically. transceivers for wireless LANs are superhet-
`erodyne radio frequency (RF) receivers. Superheterodyne
`receivers down-convert the received signal (e.g.. the RF
`signfl) to one or more intermediate frequency (IF) signals.
`The IF signals have fixed. or at least restricted. frequencies
`which allow the IF signals to be more easily filtered.
`amplified. and otherwise processed. In conventional super-
`heterodyne receivers. an antenna provides RF signals which
`are fed into a band pass RF filter which selectively passes
`only the RF signals (both desired and otherwise) and noise
`within a bandwidth of interest while attenuating other RF
`signals and noise outside this bandwidth. thereby reducing
`the necessary dynamic range of the succeeding stages. The
`band-limited RF signals and noise are then amplified by a
`low noise amplifier (LNA). To assist the RF filter in attenu-
`axing electrical noise and signals that are amplified by the
`LNA and fall within the image frequency band——which are
`especially critical because they can pass unfiltered through
`the intermediate frequency (IF) section—the amplified RF
`signal from the LNA is filtered by an image filter. A mixer
`mixes the amplified RF signal with a local oscillator (L0)
`frequency signal to convert the band-limited RF signals to
`an IF band. along with undesired mixing products.
`The IF signals from the mixer are generally coupled to an
`IF filter. which passes mainly the sub-band containing the
`desired signals. This (and any succeeding) IF filter passes
`without further attenuation the remnants of any undesired
`signals and noise present in the image sub-band of the image
`band which were insufficiently filtered by the RF filter and
`image filter. In a second mixer. the filtered IF signal mixes
`with a diiferent. typically lower LO frequency.
`thereby
`converting the signal into a lower IF frequency signal. which
`is amplified and filtered by another IF amplifier and IF filter.
`In the process of propagation through the IF filters and
`amplifiers. the desired IF signal present in the sub-band
`
`20
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`2
`
`passed by the filters is selected in favor of signals and noise
`present at other sub-bands in the IF. It should be understood
`however. that unwanted signals present at the image chan-
`nels of either the first and second conversions and not
`adequately filtered by the RF filters and first IF filters will
`appear directly in the 2nd IF sub-band of the desired signal
`and therefore cannot be filtered. The selected 1F signal is
`typically demodulated and translated into a baseband infor-
`mation signal for use by the communications control system
`Although many variations of the superheterodyne design
`exist. prior art superheterodyne designs generally share the
`characteristics. restrictions and advantages of the described
`superheterodyne receiver.
`One disadvantage of the conventional superheterodyne
`design is that it cannot easily be fully integrated onto a
`Silicon integrated circuit (IC. or microchip). Most superhet-
`erodyne receivers require significant pre-conversion filters
`and high quality narrow band IF filters which operate at high
`frequencies. For instance. most superheterodyne receivers
`require sizeable pre-conversion filters and voluminous. high
`quality. narrow band IF filters which operate at high fre-
`quencies. In light of the diificulty in fabricating high-quality
`filters at RF frequencies on an IC. these filters are typically
`built using tuned circuits which require discrete filtering
`components such as capacitors and inductors. Alternatively.
`external surface acoustic wave (SAW) filters which are
`somewhat large. expensive. and lossy are often utilized.
`Additionally. superheterodyne receivers require high fre-
`quency oscillators and mixers which are diflicult to integrate
`on a single IC substrate (especially CMOS) due to noise
`isolation. power considerations. and resonator Q. Typically.
`high frequency oscillators and mixers must be provided as
`discrete components. These discrete components increase
`the size. materials cost. assembly cost. and power consump-
`tion of receivers built utilizing superheterodyne design tech-
`niques.
`Certain alternative prior art RF transceivers eliminate the
`need for IF stages for some types of modulation by directly
`converting the RF signal to a baseband signal. In direct
`conversion receivers. the RF signal is heterodyned with a
`Local Oscillator (L.O.) signal having the same frequency
`and phase as the desired RF signal. Such receivers require
`that the RF signal be mixed in quadrature for adequate RF
`signal selection and demodulation. In a direct conversion
`receiver. locally generated orthogonal carriers mu st be tuned
`to the frequency and phase of the incoming signal’s carrier
`(or virtual carrier. as the case may be). and be heterodyned
`with the incoming signal in two separate mixers in order to
`obtain zero IF signals and baseband signals. The baseband
`signals are detected as beats in quadrature between the
`transmitted carrier frequency signal and local carrier fre-
`quency signals in quadrature. The matching of these two
`mixer characteristics and the phase and amplitude accuracy
`of the two orthogonal local oscillators signals need narrow
`tolerance components in order to produce baseband signals
`which are suitable for demodulation under weak signal
`conditions; components with such tolerances are more dif-
`ficult to integrate than those producing only single phase
`mixing.
`Thus. there is a need for a receiver which does not require
`extensive pre-conversion filtering. quadrature mixing. or
`expensive high frequency IF filters. Further. there is a need
`for a receiver architecture which includes components which
`can be readily integrated with a digital communications
`control system such as a MAC. There is also a need for a
`receiver which reliably down-converts the RF signal to a
`lower frequency signal which can be readily processed by
`
`8
`
`

`
`5.802.463
`
`3
`
`receiver components integrated with the communications
`control system.
`
`SUMMARY OF THE INVENTION
`
`The present invention relates to a receiver for use in a
`wireless network upon which information is communicated
`on a modulated carrier wave at one of a plurality of channel
`frequencies. The receiver includes an RF band input. a
`frequency synthesizer. a mixer. a band pass filter. and a
`decoder. The mixer has an RF input coupled to the RF hand
`signals coming from the antenna. a local oscillator input
`coupled to the frequency synthesizer to receive a synthesizer
`signal. and a mixer output. The synthesizer signal has a
`frequency approximate one of the channel frequencies. and
`the mixer provides a very low intermediate fiequency signal
`at the mixer output. The very low intermediate frequency
`signal has a frequency above the peak modulation deviation.
`The band pass filter has a filter input and a filter output. The
`filter input is operatively coupled to the mixer output. The
`decoder is operatively coupled to the filter output and
`receives a signal corresponding to the very low intermediate
`frequency signal. The decoder provides a decoded signal
`indicative of the information on the modulated carrier wave.
`
`The present invention further relates to a receiver for
`receiving a baseband frequency signal
`transmitted on a
`modulated carrier wave having one of a plurality of channel
`frequencies. The receiver includes an antenna means for
`receiving the modulated carrier wave. a frequency source
`means such as a phase locked loop synthesizer for providing
`a local oscillator signal. an intermediate frequency means
`(such as a mixer) for receiving the local oscillator signal and
`the modulated carrier and generating a very low intermedi-
`ate frequency signal. and a demodulator means for receiving
`the very low intermediate frequency signal and providing a
`demodulated signal indicative of the baseband frequency
`signal transmitted on the modulated carrier wave. The local
`oscillator signal has a frequency approximate the one of the
`channel frequencies. and the very low intermediate fre-
`quency signal has a frequency above the peak modulation
`deviation.
`
`The present invention further relates to a method of
`receiving information on a modulated carrier wave having
`one of a plurality of channel frequencies for use in a wireless
`network. The method includes the steps of receiving the
`modulated carrier wave signal. amplifying the modulated
`carrier wave signal to generate an amplified modulated
`carrier signal. generating a mixing signal (local oscillator)
`having a frequency approximate the one of the channel
`frequencies. mixing the mixing signal and the amplified
`modulated carrier signal to form a very low intermediate
`frequency signal having a frequency above the peak modu-
`lation deviation and important sidebands. and demodulating
`a result signal related to the very low intermediate frequency
`signal to obtain the information on the modulated carrier
`wave.
`
`the receiver
`In one aspect of the present invention.
`directly converts the radio frequency (RF) signal to a very
`low intermediate frequency (IF) signal (e.g.. to a frequency
`between the peak modulation deviation and considerably
`less than the channel
`interval). The receiver preferably
`operates to down-convert a high frequency electromagnetic
`signal such as 2.4-2.5 GHz RF signal to a 340-660 kHz
`signal (depending on the symbol or bit pattern being trans-
`mitted via frequency shift keyed (FSK) modulation). The
`receiver may be used in a variety of applications such as
`cordless telephones. wireless networks. or other radio
`
`4
`devices. and with a variety of radio frequencies and modu-
`lation techniques.
`In another aspect of the present invention. the transceiver
`does not require pre-conversion filtering. The receiver mixes
`the carrier wave or RF signal with a synthesizer signal or
`mixing signal having a frequency one-half of a channel
`interval above the RF signal. Alternatively. the synthesizer
`signal may have a frequency of some other fraction of the
`channel interval of the RF signal. The mixer advantageously
`provides a very low IF signal having a frequency higher than
`the sum of the highest frequency contained in the baseband
`information and the peak deviation. yet well below the
`channel spacing interval. Preferably. the very low interme-
`diate frequency is low enough to eliminate the need for
`pre-conversion filtering or image reject phasing and yet high
`enough to reliably include the baseband signal.
`In still another aspect of the present invention. the very
`low IF transceiver architecture allows a significant number
`of transceiver components to be integrated with the digital
`communication control system An integrated audio band
`pass filter advantageously filters the very low IF signal to
`eliminate components from other channels at the receiver
`front end. The very low IF signal can be reliably demodu-
`lated without requiring quadrature mixing. Further.
`the
`advantageous architecture allows high gain amplification to
`be performed in a relatively quiet region of the frequency
`spectrum by an integrated amplifier.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will hereafter be described with reference
`
`to the accompanying drawings. wherein like numerals
`denote like elements. and:
`
`FIG. 1 is a general block diagram of a communications
`system node including a transceiver and a communications
`control system in accordance with a first exemplary embodi-
`ment of the present invention;
`FIG. 2 is a more detailed block diag;ram of the commu-
`nications system node illustrated in FIG. 1 in accordance
`with a second exemplary embodiment of the present inven-
`tion;
`
`FIG. 3 is a block diagram of the communications system
`node in accordance with a third exemplary embodiment of
`the present invention;
`FIG. 4 is a detailed block diagram of a communications
`system node in accordance with a fourth exemplary embodi-
`ment of the present invention;
`FIG. 5 is an image rejection circuit for use in the com-
`munication system nodes illustrated in FIGS. 1. 2. and 3
`according to fifth and sixth exemplary embodiments of the
`present invention;
`FIG. 6 is a local oscillator leakage rejection circuit for use
`in the communication system nodes illustrated in FIGS. 1. 2.
`and 3 according to seventh and eighth exemplary embodi-
`ments of the present invention.
`
`DETAILED DESCRIPTION OF PREFERRED
`EXEMPLARY EMBODIMENTS OF THE
`PRESENT INVENTION
`
`FIG. 1 is a general block diagram of a communications
`system node 14 such as a wireless network (not shown).
`cordless telephone or other radio frequency (RF) device.
`System node 14 is preferably part of a wide area network
`(WAN). cellular network or wireless LAN such as an IEEE
`802.11 standard radio LAN or other radio frequency appli-
`cation such as a cordless or wireless telephone system.
`
`10
`
`20
`
`25
`
`30
`
`35
`
`45
`
`S0
`
`55
`
`65
`
`9
`
`

`
`5.802.463
`
`5
`Alternatively. communications system node 14 may be
`utilized in other RF communication apparatus.
`System node 14 preferably operates at a radio frequency
`between 2.4 and 2.5 GHz at channel intervals of 1 MHz.
`Information is preferably modulated with two or four level
`Gaussian frequency shift keyed (GFSK) modulation having
`a peak deviation of +/-160 KHz of the RF signal or carrier
`wave. Symbols representing the information are preferably
`encoded as periods of +160 KHZ to -160 KHz of frequency
`deviation (e.g.. a peak-to-peak deviation of 320 KHz) at a
`data rate of 1 Megabit per second (2-level) or 2 Megabits per
`second (4-level). Alternatively.
`the information may be
`amplitude modulated. phase modulated. or otherwise
`encoded on the RF signal.
`Communications system node 14 includes a radio trans-
`ceiver 12. a communications control system or controller 16.
`and an antenna 18. The antenna may be incorporated directly
`into transceiver 12. Communications controller 16 is pref-
`erably a communications controller or media access con-
`troller (MAC) such as an Am79C93O PC Net Wirelessm
`MAC device or other generation MAC device manufactured
`by Advanced Micro Devices. Sunnyvale. Calif. Communi-
`cations controller 16 is preferably a single substrate. inte-
`grated digital controller with a plurality of inputs and
`outputs. such as a transmit data output 19A. a receive data
`input 1913. and a channel selection output 19C. Controller 16
`is preferably fabricated in CMOS and is operable in a variety
`of communications applications.
`In the simplest exemplary embodiment. radio transceiver
`12 includes a mixer 28. a transmit/receive (‘T ") switch 20
`and the remaining receiver and transmitter circuitry in a
`module or circuit 15. Circuit 15 includes a transmit data
`input 17A. a receive data output 17B. a channel selection
`input 17C. a receiver IF input terminal 17D. and a local
`oscillator (“L0") synthesizer output 17E. An RF input of
`mixer 28 is coupled to antenna 18 through the T/R switch 20.
`The L0 input of mixer 28 is coupled to the LO synthesizer
`output 17E. An IF output of mixer 28 is coupled to the
`receiver IF input 17D. The L0 synthesizer output 17E is also
`connected to the transmit terminal of the T/R switch. often
`through a power amplifier (not shown) whose gain may or
`may not be amplitude modulated. Transmit data input 17A
`is coupled to transmit data output 19A. and receive data
`output 17B is coupled to receive data input 19B. Also not
`shown could be a bandpass filter connected between the
`antenna 18 and the TIR switch 20. whose purpose is to
`confine the signals passing in either direction to those in the
`frequency band of interest. Channel selection output 19C is
`coupled to channel selection input 17C. Also not shown
`could be a receiver preamplifier connected between the T/R
`switch 20 receive output and the mixer 28 RF input. whose
`purpose is to help provide adequate receiver sensitivity.
`Preferably. a number of components in transceiver 12 are
`integrated on the semiconductor substrate of controller 16.
`In operation. transceiver 12 performs RF. IF. modulation
`and demodulation signal prooes sing for communication sys-
`tem node 14. In the receive mode. antenna 18 receives a
`modulated carrier wave or RF signal and provides same to
`mixer 28 through T/R switch 20. The L0 synthesizer con-
`tained in circuit 15 provides a mixing signal to 17E corre-
`sponding to a selected channel in the appropriate frequency
`range (such as between 2.4 and 2.5 GHz at 1 MHz channel
`intervals). The synthesizer signal corresponds to a channel
`selection signal provided by the controller 16 at output 19C
`to input 17C of 15. In the receive mode. the LO synthesizer
`signal provided at output 17B is preferably one-half of the
`channel interval above the carrier. or center. frequency of the
`
`6
`selected channel. For example. if the desired RF signal is
`centered on 2433 MHz. the LO synthesizer is set to 2433.5
`MHz in accordance with the channel selection signal at
`output 19C. Alternatively. the LO synthesizer’s frequency
`may be offset below the center frequency of the desired RF
`signal by one-half the channel interval. or related to the
`channel interval by a diflerent ratio such as one quarter. This
`amount of the LO synthesizer’s otfset must be greater than
`the highest significant frequency component (measured as
`the diflerence in frequency between the sideband energy and
`the carrier) contained in the sidebands of the incoming
`modulated carrier wave. For example. in this exemplary
`embodiment. significant sideband energy extends to almost
`plus and minus 500 KHz from the carrier frequency. Thus.
`in the receive mode an L0 synthesizer offset of 500 KHz
`(half the channel width) was chosen.
`Mixer 28 provides essentially the multiplicative product
`of the signals from its two inputs to receiver IF input 17D.
`which works out to be replicas of the RF signal at frequen-
`cies which are the sum and difference of it and the LO.
`
`Depending upon its exact design. the mixer also provides the
`original signals. preferably at reduced level. Only the dif-
`ference frequency will be discussed here. as the other three
`frequency components are removed by the first component
`after the IF input 17D. This diiference frequency is the very
`low IF signal. which extends from nearly DC to nearly 1
`MHZ. corresponding to sidebands from nearly 500 KHz plus
`and minus the carrier frequency. respectively.
`There are at least two effects of utilizing such a low
`intermediate frequency. The positive effect is that the signal
`can be processed (i.e.
`filtered. amplified.
`further
`heterodyned) by relatively inexpensive and compact inte-
`grated circuitry and other circuitry. The negative effect is
`that appearing at receiver IF input 17D with equivalent
`frequency and amplitude response will also be any signals in
`the RF input band appearing at antenna 18 which are above
`the LO frequency emanating from 17E; as these signals will
`differ in frequency by the same amounts as those below the
`LO. Any low pass filter following input 17D whose band-
`pass cutoff is just below 1 MHz will tend to eliminate all
`signals which were more than 1 MHz below the LO fre-
`quency and all signals which were more than 1 MHZ above
`the LO frequency; signals on the desired channel will be
`passed unattenuated. but so. also. will be signals on the next
`higher channel. The superheterodyne receiver mixing pro-
`cess always provides equal response to the desired signal
`band and this undesired image band. The usual design
`strategy is to place the LO frequency significantly far from
`the desired RF input frequency so that the image band can
`be removed by filtering prior to the mixer 28. In the case of
`this particular design. the LO frequency is so close (in fact.
`just far enough away to avoid all modulation sidebands) that
`the image band is immediately adjacent to the desired signal
`at all times and thus cannot be filtered out prior to the mixer
`by any known practical method. However. responses to this
`and any image band can be reduced through the use of image
`reject mixing.
`This situation is normally considered to be intolerable. but
`in some circumstances (including the one for which this
`receiver was designed). the required (or specified) receiver
`system selectivity permits full passband response to the
`adjacent channels (usually to lower the cost of selectivity
`elements). For this design. even though the image rejection
`ratio is 0 dB. this is not a problem; as the image band is
`always at one of the adjacent channels. The particular
`specification is the upcoming IEEE P802.l1 Wireless LAN
`standard. but this idea applies to any selectivity specification
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`10
`
`10
`
`

`
`5 . 802.463
`
`7
`
`or band plan. official or otherwise. which allows zero
`selectivity for enough bandwidth to include the carrier and
`both sidebands.
`
`This very low intermediate frequency should be above
`DC (zero Hz) and yet low enough so as not to require
`expensive IF filters and other components. Further. the very
`low intermediate frequency should be large enough to
`include the full range of the baseband frequency. Thus.
`circuit 15 is designed to cause mixer 28 to convert the RF
`signal to a very low IF signal which can be reliably pro-
`cessed to obtain the information encoded at the baseband
`
`frequency or modulated on the radio frequency.
`Circuit 15 filters. amplifies. otherwise processes. and
`decodes the very low IF signal to produce a receive data
`signal at output 17B indicative of the information modulated
`at the baseband frequency on the RF signal received by
`antenna 18. The received data is received by communica-
`tions controller module 16 at receive data input 19B. The
`received data can be a serial or parallel digital bit stream.
`pulse signal. or analog signal representing the symbols or
`information modulated on the RF signal.
`In the transmit mode. communications controller module
`16 provides transmit data at transmit data output 19A to
`transmit data input 17A of circuit 15. The transmit data can
`be a bit stream. pulse signal or analog signal representing
`signals or information to be modulated on the RF signal. The
`transmit digital data entering circuit 15 at 17C modulates the
`voltage controller oscillator (VCO) within the synthesizer;
`so that the latter’s output at output 17E is frequency-shift-
`key (FSK) modulated.
`instead of unrnodulated as was
`required for the receive mode. This modulated signal is sent
`17E to the TR Switch 20. sometimes through a power
`amplifier (not shown). which in this transmit mode is set to
`connect the modulated signal to the antenna.
`FIG. 2 is a schematic block diagram of transceiver 12 for
`use in communications system node 14 in accordance with
`a second exemplary embodiment of the present invention.
`Communications system node 14 includes antenna 18. t:rans-
`ceiver 12 and media access controller (MAC) or communi-
`cations controller 16. Transceiver 12 includes a transmit]
`receive switch 20. a transmit power amplifier 22. a receive
`low noise amplifier 24. a mixer 28. a voltage controlled
`oscillator 30. a loop filter 32. a prescaler 34. /N/A 37. a phase
`comparator 36. a frequency reference generator 38. a band
`pass filtu 40. a high gain amplifier 44. a data decoder 46.
`and a data encoder 48. Preferably. filter 32. /N/A 37. phase
`comparator 36. reference frequency generator 38. band pass
`filter 40. high gain amplifier 44. data decoder 46. and data
`encoder 48 are integrated in CMOS with cont:roller 16 on a
`single integrated circuit substrate 13.
`Transceiver 12 and communications system node 14 are
`configured to utilize GFSK modulation to transmit inforrna-
`tion over the wireless network (not shown). Transceiver 12
`is a frequency hopping device which receives a channel
`selection signal in accordance with the IEEE 802.11 stan-
`dard. VCO 30. loop filter 32. prescaler 34. /N/A 37. and
`phase comparator 36 cooperate to form a phase locked loop
`frequency synthesizer capable of producing high frequency
`(RF) signals for antenna'18 and mixer 28. For example.
`VCO 30. prescaler 34. /N/A 37. phase comparator 36. and
`loop filter 32 are preferably arranged to provide any channel
`frequency to mixer 28 from 2.3995 to 2.4995 GHZ in 1.0
`MHz steps in Receive mode and any channel frequency to
`Transmit Power Amplifier 30 from 2.4 GHZ to 2.5 GHz in
`1 MHz steps in Transmit mode. VCO 30. transmit power
`amplifier 22. and T/R switch 20 preferably provide the RF
`signal directly to antenna 18.
`
`8
`The operation of transceiver 12 is discussed below with
`reference to FIG. 2 as follows. In the receive mode. antenna
`18 receives a modulated carrier wave signal or RF signal at
`any 1 MHz channel between 2.4 and 2.5 GHZ (e.g.. one of
`100 channels). Transmit and receive switch 20 is set to
`receive by controller 16 so that the signals on antenna 18 are
`provided to the receive low noise amplifier 24. Switch 20
`also prevents signals from being reflected through transmit
`power amplifier 22 from mixer 28 to antenna 18. Switch 20
`is preferably a Silicon (Si) or Gallium Arsenide (GAs)
`device having less than 1 microsecond operation time.
`Switch 20 is preferably controlled by controller 16 in
`accordance with a IEEE 802.11 standard protocol.
`Antenna 18. or its matching circuit normally provides
`some RF filtering for system node 14 and is tuned to the 2.4
`to 2.5 GHZ range. Antenna 18 can be a printed circuit
`antenna mounted on a PCMCIA card or other printed circuit
`board. Antenna 18 is impedance matched. in some instances
`by microstrip techniques. Antenna 18 and transceiver 12
`generally have a 200-300 foot indoor range for receiving
`and transmitting the RF signal. Between antenna 18 and T/R
`switch 20 may be a low cost ceramic or other type filter (not
`shown) for increased RF filtering capabilities.
`Switch 20 provides the modulated carrier wave (RF
`signal) to low noise receive amplifier 24. which amplifies the
`modulated carrier wave and establishes the receiver system
`noise figure. Amplifier 24 provides the amplified RF signal
`to mixer 28. Preferably. amplifier 24 is a low noise amplifier
`having a maximum noise figure of less than 5 dB. Preferably.
`amplifier 24 has a large reverse isolation for reducing the
`leakage of the VCO 30 signal and prescaler 34 noise back
`through the mixer 28 and T/R switch 20 and into antenna 18
`during receive mode. Amplifier 24 preferably has a gain of
`10 to 15 dB.
`
`VCO 30. loop filter 32. prescaler 34. /N/A 37 and phase
`comparator 36 cooperate to provide a synthesizer or local
`oscillator (L.O.) mixing signal to mixer 28. The mixing
`signal has a frequency of 0.5 MHz (e.g.. 1/: of the channel
`interval) above the desired channel frequency which is being
`tuned. Alternatively. the mixing signal may be below the
`desired frequency. The mixing signal is preferably close
`enough to the desired channel frequency so that mixer 28
`produces a very low intermediate frequency signal which is
`above DC and below approximately 1 MHz. In addition to
`receiving the desired signal centered at 0.5 MHZ below the
`mixing signal frequency (or 0.5 MHz above the mixing
`signal frequency if the latter has been placed 0.5 MHz below
`the desired carrier frequency) appearing at
`the IF with
`identical response will be any signals at the channel centered
`at 0.5 MHz above the mixing signal frequency. as this is the
`image band. In other words. the image band delivers virtu-
`ally identical signals to the IF. and the image rejection is 0
`dB because the RF filtering is much too wide to provide
`rejection. While this state of affairs is normally intolerable.
`it is perfectly acceptable in this case;
`inasmuch as the
`applicable IEEE P802.ll specification (as well as other
`possible specifications) provides for 0 dB selectivity at the
`channels immediately adjac