CA2066420C - Dual mode automatic frequency control - Google Patents

Abstract

A frequency control system (126) for locking the frequency of a receiver, susch as a receiver portion of a transceiver, to the frequency of a transmitter, such as a base station. The frequency control system (126) is operable to determine a center, or other reference, frequency of a signal, either a conventional analog signal, or discrete, encoded signal. The frequency control system may be advantageously embodied in a dual-mode radiotelephone operable to receive both conventional, analog signals and dis-crete, encoded signals.

Description

WO 92/02991 PCT/l,rS91~04874 206~2i~

DUAi MODE AUrONiATiC FREQUEi~iCY CONTROL
Backaround of the Invention The present invention relates ~ienerally to automatic frequ~ncy control apparaius, and, more particularly, to an automatic frequency con:rol system for correcting frequency dirr~tences between a receiver and a l,~ns,l,ilL~r which 15 transmits either an analos i~rvrlllation signal or a discrete, encoded information signal.
An i~rvr~ldLion sign.ll is illl~ ssed upon an elecl,ui,,a~,letic wave by a process referred to as modulation.
In a modulation process, tha information signal is combined 20 with an electromagnetic wave (referred to as the carrier wave), and the resultant, combined signal is an eiectromagnetic wave whi~h varies in some manner according to the values of the il,rurl~a~ion signal. Various modulation techniques have been de~eloped to moduiata the i~lru~llaLion 25 signal upon the electromagnetic wave; amplitude modulation, frequency modulation, and phase modulation are three of such modulatiûn techniques.
In general, an ampliltude modulated signal is formed by modulatin3 the informatiorl signal upon the electromagnetic 30 wave such that the inforn1ation si3nal modifies the amplitude of thQ ~lectromagnetic wa~/e co"t,apondi"g to the value of the information si~nal. The flequency of the elevLlul,,ay,,t,lic wave does not vary, and the information content of the modulated signal is contained in the shape, or ampiitude, of the signal.

-2- 20~642~ --The shape of the signal is referred to as the envelope of the signal, arid the changes in the amplitude of the modulated signal change the envelope formed thereby. A frequency modulated signal is formed by altering the frequency of the 5 ~e ullld~llet;~; wave corresponding to the value of the ~ urllldliûn signal. The a" ' ~de of the ~ .l,ui"a~n~lic wave does not vary and the i"~or",alion content of the modulated signal is co" ,ed in the variation of the frequency of the si~nal. A phase modulated si~nal is formed by altering the 10 phase of the r~'u~ L~ui"ay"~tic wave c~ spor,~iny to the value of the i"lur",alion signal.~ The amplitude of the ele~;t,ul,,a~neli~ wave does not vary, and the i"~ur",~lion content of the modulated signal is contained in the variation of the phase of the signal. Because the amplitudes of a frequency 15 modulated and a phase modulated signal do not vary these modulated signals are referred to as cons~ant envelope signals.
A receiver which receives the modulated information signal includes circuitry to detect or to otherwise recreate the i,,~r,~,aLiûn signal modulated upon the Gl~c~,u",agnetic 20 wave. This process is referred to as demodulation and various receiver circuits permits demodulation of information signals modulated upon an rJl~ctru,,,agnetic wave acc~"Ji,)g to the various modulation techniques.
Many different modulated infûrmation signals may be 25 simultaneously transmitted by a plurality of transi"ill~r~ at a plurality of different frequencies.
Portions of a 100 megahertz band of the G~cl,u",aynetic frequency spectrum (extending between 800 ",egahe,l~ and 900 megahertz) are allocated for radiotelephone communication 30 such as for example by radiotelephones utilized in a cellular, communication system. A dcl;otelephone contains circuitry both to qenerate and to receive modulated i,~or",ation signals.
A cellular communication system is created by positioning numerous base stations at spaced-apart locations WO 92/02991 . PCr~US91/rJ4874 3 2B~2~
throughout a geographic~ll area. Each of the base stations is constructed to receive ~nd transmit modulated information si~nals simultaneously t~ and from radiotslephones to permit two-way communication therebetween. The base stations are positioned at locations such that a ,d~iv;~laFhone at any location in the geographical area is within the reception range of at least one of the b;~se station receivers.
The geo~,ap~, ' area is divided into portions and one base station is posili~ne~l in each portion. Each portion of the geographical area define~ thereby is referrsd to as a ~cell~.
As ,,,e,,Liùned hereinabove a portion of the 100 megahertz frequency band is allocated for cellular communications. Although numerous modulated information signals may be simultalleously transmitted at different I,di-sl";.sion frequenciesl eâch occupies a finite portion of the - fr~quency band. Overlapp:.,g of simultaneously transmitted modulated information signals is not permitted as inl~,r~,~nce between ove!rlapping signals on the same fr~quency could prevent detection of either of the modulated inlur,r~dliùn signals by a receiver.
The frequency bancl is divided into channels each of which is of a thirty kilohertz banu~.;dLl,. Presently one signal is permitted to be transmitted in each thirty kilohertz-wide channel of the frequency band. Additionally a first portion ~ 8l " ~g between 824 megahertz and 849 megahertz of the frequency band is alloca~ed for the transmission of modulated i"~ur,nd~ion signals from ~ phone to a base station. A
second portion, e~L~I,.li,,g between 869 "~e~ahe,k and 894 ",~gah~ of the frequency band is allocated for the l,dl~s",is:,ion of modulated i~ur~ldlion signals from a base s~ation to a r 'i~ rhonle. 832 trans",is:,;on chann~ls are formed in the first frequency band portion and 832 s",ission channels are formed in the second frequency band WO 92/02991 PCI`/US91/04874 - =2~6~I~s? 4 portion, thereby permittin~ a maximum of 832 simultaneous, two-way communications within a geographical area.
A modulated si~nal l,dn:""iL~d upon any one of the l,d,~s",;ssion channels must be of a bal '~.;dl~, less than the S bar,u~.;dLl, of the l,dnsl";ssi~n channel (i.e., less than thirty kilohertz). Oscillators which oscillate at frequencies to ~enerate the ~'e ~.u",ag"etic carriers thereby are susc~pliL~lo to frequency vd,idLi~ns. Such variations, referred to as frequency drift, can cause the transmitted signal to extend 10 beyond the boundaries of the L,d,~s",;ss;on channel.
I"~,t,ased usage of the cellular, communication systems has resulted, in many in:,~dnces, in full utilization of every l,dna",;ssion channel allocated for cellular"; ~ .'ephone communication. Other frequency bands of the electromagnetic 15 spectrum are similarly oftentimes fully utilized.
Various attempts have been made to increase the information-transmission capacity of the cellular, r~ ot~lephone communication systems as well as other communication systems utilizins other frequency bands of the 20 electromagnetic spectrum. However, existing cellular radiotelephone communication systems are comprised of ~Jiolt,lephones and base stations having circuitry which transmits and receives frequency mocl~ t~ analog si~nals.
Only one modulated i,~""aLion signal may be lldils,,,ill~d upon 25 a l,~,~s",;;,sion channel at a time. Significant i,~cf~ases in the information-t,ransmission capacity of cellular, radiotelephone communication systems has accordingly, b~en limited.
Discrete modulation techniques have been do~ ped, however, to permit l,dns",;ssion of more than one signal at the 30 same frequency. A cellular, Id~iot~lEphone communication system capable of transmitting modulated information signals formed by discrete modulation L~cl,~ es would allow l~di~s",;~sion of more than one signal on a transmission WO 92/02991 PCI~/US91/04874 .
5 2a6~2 channel. The capacity of such a communication syst~3m can be si~ni~i~antly increased.
In general, a discr~3te modulation tecl~nique encodes a continuous, i"rur",dLi~n signal into discrete signals and then 5 modulates the discrete si~nals upon an electromagnetic wave to form thereby a modulated i"~ur",a~ion signal. Discrete si~nals of more than on~ tulllla~ion signal may be modulated upon electromagnetic waves of identical carrier frequencies and Llelilsll,ilk,d sequentially to two or more ( 'iut~l~phones.
Frequency drift ma~ be a sreater problem when certain disctete, encoded signal~; are transmitted. Oscillators which generate electromagnetic waves upon which i"rur",ation si~nals are modulated al-e susce~, ' !e to ~requency drift responsive to changes in ambient condilions, such as, for example, temperature ch~nges and supply voltage variations. A
frequency drift of a magtitude which Ille~ e~ a conventional, analog signal within the Iboundaries of a l,dns",;ssion channel, may u~l~"li",es be of a rrlagnitude which causes a corresponding discrete, ~ncoded signal to exteltd beyond the boundaries of a l,ai,s",;ssion channel. Fr~3quency drift of modulated information signals generated by discrete modulation techniques mé~y be more s~sce~:' !e to interference problems than are modulated information signals generated by conventional analog mod~llation ItlCIIIl 7~es.
Additionally, circuitry for demodulating transmitted information signals of certain discrete, encoded-type signals requires less frequency error than the frequency error pel,-,illdd of conventional, analog signals. Quantitatively, the US cellular standard freql~ency error per",iLled of conventional, 3û analog signals is 2.5 parts per mi~lion, whereas frequency error pe""illdd of discret~3, encoded signal~ i5 Rppr~Xi)llately 0.2 parts per million.
Sy~tems and metho~s of fr~quency control for minimizing frequency drift to minimize thereby frequency drift problems WO 92/02991 ~6Ç~ PCI/US91/04874 are known and are frequently utilized in many existing communication systems. Generally, one oscillator, referred to as th~ ref~rence oscillator, within a transmitter is controlled such that frequency drift of a signal generated thereby is within an ~"1~ ''2 range. Other 05 ~ of the radio may then be locked to the frequency of the reference oscillator.
In the particular instance . of celiular" ' : 'ap'~one communications as above-des~, iL,ed, the oscillators of the base stations positioned throughout the geographical area to modulate an information signal thereupon may be precisely controlled to minimize drift of the frequency of the ~!e '~ui,,a~,,etic wave generated thereby. The receivers of the ~ddiu~ phones may utilize the frequency of the modulated information si~nal from the base station as a ~ rt,nce frequency. The reference frequency is utilizad by the - ~dui~,t~lAphone, for example, as a reference from which the transmit frequency of the radiotelephone may be offset allowin~ the signal transmitted by the c..J;~,t~lGphone to be as precise of a frequency as the base station frequency.
2û In order to increase the capacity of cellular, communication systems, existing base stations having circuitry to transmit and receive only com/~r,liunal analog signals are to be converted to base stations which additionally permit l,dns",;Jsion and reception of discrete, encoded, modulated i,,for,,,aliùn sisnals. Radiot~'~phones are being developed to permit l-dns-";ssion and reception of both conventional analo~ signals, and discrete, encoded signals. As the cellular~system base stations are gradually converted, and radiot~'eFhones are similarly dov .loped, some channels of a cell will be c~",prised of receivers having circuitry permitting reception of discrete, encoded modulated in~or",dliol1 signals, and other channels will be col"~,,ised of receivers having circuitry permitting only reception of conventional analog modulated information signals. Similarly, some ~0 92/02991 2 ~ Q PCI~US9}/04874 radiotelephones operatl~d in the cellular, communication systems will contain ~ircuitry psrmitting transmission and reception of both discrete, encoded modulated i"~r",dLion signals and conventional, analog modulated illf~rllldlion signals. Other radiotelephones will contain circuitry permittin~ transmission and reception of only conventional, analog, modulated information signals.
A dual-mode Idd;Gt~ phone permitting t,dr,s",;s~ion of both conventional, anal~g modulated information signals, and discrete, encoded modulated information signals may be constructed havin~ both first circuitry for l,dns",i~sion and reception of the convelltional, analos modulated information signals, and second circuitry for transmission and reception of discrete, encoded modulated i"~""alion signals. When a ~a ii~t~!~laphone receives a discrete encoded signal, a digital signal processor may bl~ conveniently utilized to decode the signal. At the same time, the digital signal processor may be utilized to derive an error signal to cortect the reference frequency in the radiot~lephone.
While a disital si~nal p~oc~ssor may be utilized to generate the error sign;ll to correct the reference frequency when an analog signal is received, the digital signal processor ~ c;~ es larger amounts of power than conventional analog circuitry used to determine the reference frequency of a conventional, anaiog mc~dulated informatiorl signal. The conventional, analog circuitry is, ,however, ~ns~ rc~ ry for det~r", ,il~g a reference frequency for discrete encoded, information signals.
A r~'ic: 'eFhone ~perable to receive both conventional analog and discrete, enl~oded, modulated in~.r",dLion signals ~aving circuitry to determine a re~erence frequency of either type of transmitted signlal, and, additionally, having minimal powe~ co~sunr,~tion r6~uirenlents would bs advantageous.

What is needed, tlleref~re, is a frequency controls scheme which requires minimal power (.,~ , but also may be alternately operated to determine the referenee frequency of either uullvcll~iulldl, analog modulated information signals or discrete, encoded modulated informatinn signals transmitted to the S l,,.li"trl~
Sumlma~ of the Invention It is, ac~ulil~ly, an object of the present invention to provide a frequency control system for correcting frequency differences between a transmitter and a receiver operable to transmit and receive both analog and lû discrete, encoded, modulated il~ullllatiull signals.
It is a further object of the present invention to provide a frequency control system operable to determine a reference frequency of a sigmal transmitted either by conventional, analog modulation techniques, or by discretemn~ ati~n techniques whicll requires a minimal power l.,Ull:~UllllJIiUll for operation thereof.
It is a yet further objec~ of the present invention to provide a dual-mode radiotelephone operable to receive both a cuuvcllliu~ldl, analog information signal amd a discrete, encoded signal having frequency control circuitry of minimal power lcului.cl~
In accordance ~-vith the present invention, there is provided a frequency control system operable to maintain a receiver oscillation frequency of a variable oscillator forming a portion of a receiver in a desired frequency relationship with an oscillatiorl frequency of a modulated signal transmitted tothe receiver and received thereat. The frequency control system comprises converting means, analog detection circuitry, discrete signal detection circuitry, and altering means. The frequency control system is operable to determine a reference frequency signal trans,mitted either by c ull~ iullal, analog modulation techniques, or by discrete m~ atir)n techniques. Analog signal detection circuitry is operative when the modulated signal comprises a frequency modulated signal, and has a phase detector for detecting frequency a~ lh,~ of the electrical sigmal indicative of the modulated signal amd for .
~9~ 206~420 generating a first reference signal. Discrete signal detection circuitry is operative, only when the modulated signal comprises a discretely-encoded, modulated signaJ, to generale a second reference signal. The meams for altering the oscillation frequency of the variable oscillator is responsive to the levels of the first frequency reference signal when the modulated signal comprises a frequency modulated signal and responsive to the levels of the second frequency reference signal when the :modulated signal comprises a discretely-encoded, modulated signal, to maint;lin the variable oscillator of the receiver in the desired frequency relationship with the oscillation frequency of the modulated signal.
Brief Description of the Drawings The present invention will be better understood when read in light of the a~w~pallyi~lg drawings in v~hich:
Fig. 1 is a graphical Ic~lC~CIIlaLivl. of an amplitude modulated i.,r."",AIi.", signal l~ ta~ivc of one such signal which may be utilized by the frequency control system of the present invention;
Figs. 2A and 2B are graphical 1~1~,3~,.1Lli~ions of const~mt envelope signals wherein Fig. 2A is a frequency modulated signal IGylG~Gl.t.ltivc of one such signal that may be utilized by the frequency control system of the present invention, and Fig. 2B is a phase modulated signal IGyl~,~cllLdLive of another such signal that may be utilized by the frequency control system of the present invention;
Fig. 3 is a graphical IG~ La~i~lll of the f~ t~ ti~n points of a discrete encoding scheme which may be utilized to encode an inff~rnl~ti~m signalto form thereby a discrete, encoded signal;
Fig. 4 is a graphical IG~llC~CLLLaLiOn of the frequency modulated signal of Fig. 2A graphed as a function of frequency;

~, -10- 206642~3 Fig. 5 is a graphical representation of a DQPSK signal, which is a combination of an amplitude modulated siQnal and a phase modulated signal, ~raphed as a function of frequency;
Fig. 6 is a graphical ~preser,ldLion of two adjacent 5 I-,lnsn,;~ion channels of a frequency band, wherein a conventional, modulated i"rur."alion signal is l.~ina",illed upon a first of the l-d-lsll-;~sioll channels, and a discrete, encoded modulated infur",alion si~nal is l-~ d upon a second of the transmission channels;
Fig. 7 is a block di~gram of the frequency control system of th~ present invention;
FiQ. 8 is a partial block, partial schematic illustration of a preferred ei.,L- ` "ent c~f the present invention; and FiQ. 9 is a partial block, partial scl~ei"a~ic illustration of an alternate preferred ei~ odii"er,i of the present invention.
Des~riJ.lion of the Preferred Embocli"~e"~:, Turning first now to the graphical repr~seriLd~ions of Fi~s. 1 and 2A-2B, waveforms ,~p,~senLdli~e of three types of 2û modulated information signals are shown. Waveforms similar to the waveforms of Figs. 1 and 2A-2B (or, more particularly, a waveform similar to the waveform of Fig. 2A, and a waveform similar to the combination of Fig. 1 and Fig. 2B) may be utilized by the system of the pre~;ent invention for C,or,~Lir~Q frequency dirrd-~nces between a recl~iver operable to receive such waveforms, and a transmitter operable to transmit such waveforms. The waveforms are actually plots of voltage, scaled in terms of millivolts, on ordinate axes 10, as a functiûn of time, plotted along abscissa axes 12.
3û Waveform 14 of Fig. 1 is an amplitude modulated signal formed by modulatinQ an il~r~r",aLiorl siQnal upon an electromagnetic wave whelein the amplitude (i.e., voltaQe) of the waveform 14 varies responsive to values of the i,~rul ",ation siQnal modulat,~d thcr~.pon. The information--.

wo 92/02991 PCr~US91/04874 - ~1 2~42~
containing portion of w~iveform 14 is, thereby, contained in the amplitude of the waveform such ~hat variations in the amplitude of the wavef~rm 14 correspond to variations in the amplitude of the information signal. The amplitude of 5 waveform 14, referred to as the envelope of the waveform, is ,~rt,ser,lt,d in Fig. 1 by curve 16. Curve 16 is similar in shape to the i"r~r",dLion signal which, when modulated upon an ol~_L,u,,,a~,~etic wave forms waveform 14. Waveform 14 does not vary in frequency, and the frequency of waveform 14 10 co--t,s~,oncls to the freql~ency of the unmodulated wave upon which the information signal is modulated. Such frequency is referred to as the carri~r frequency of waveform 14, and the electromagnetic wave is referred to as the carrier wave.
Waveform 18 of Fig. 2A is a frequency modulated signal 15 formed by modulating an i-,~ur,,,dLion signal upon an electromagnetic wave. The a",~ d~ of waveform 18 does not vary; however, the freql~ency of waveform 18 varies responsive to values of the inform~tion signal modulated thereupon.
Variations in frequency of waveform 18, thus, form the 20 information-containing portion of the waveform. The variation in frequency of waveforrn 18 caused by modulation of the information signal upon the electromagnetic wave is, however, slight, compared to the frequency of the o!~cl,ui"ay"atic wave.
Hence, waveform 18 maly, similar to the waveform 14 of Fig. 1, 25 be characterized by the frequency of the electromagnetic wave upon which the information signal is modulated; such frequency is referred to as the carrier frequency of the waveform 18, and the electromagnetic wave is referred to as the carrier wav~.
Waveform 19 of Fig. 2B is a phase modulated signal 30 formed by modulating a~ ur~aLi~n signal upon an electromagnet,c wave. rhe amplitude of waveform ~9 does not vary; however, the phase of the waveform 19 varies responsive to values of the in~ormation sig~la~ modulated thereupon.
Variations in phase of the waveform, thus, form the WO 92/02991 PCI/I~S91/04~74 2 ~ fiJ ~ - 1 2 -i,lr~r",alion-containing portion of waveform 19. It is to be noted that the abrupt phase change of waveform 19 of Fig. 2B
is for purposes of illustration only, and that an actual phase modulated signal would exhibit a gradual phas~ change. The phase variation of waveform 19 does not significantly alter the carrier frequency of the signal. Therefore, wave 1g, once rrlod~ t~d, may (similar to waveform 14 of Fi0. 1 and waveform 18 of Fig. 2A) be said to be cllard~;t~riLed by the carrier frequency.
Turning now to the graphical ,~pr~senLaLion of Fig. 3, the col~si " '-n points of a discrete encoding scheme for encoding an inforn~ation signal is illustrated. As ",e,~lioned he,t,;,~above, by encoding an information signal into a series of discrete, encoded signals, more than one sign~l may be 15 transmitted at a particular frequency to increase significantly thereby the information-transmission capacity of a particular frequency band.
Fig. 3 illustrates an eight-level phase shift keying (PSK) system in which an i"rur",dLion signal may take the form of 20 any of eight different levels (i.e., phases). Other discrete, encoding schemes are, of course, similarly possible. In this system, the i,,~u,,,,aLiùn signal is encoded into two parallel bit streams, referred to as l(t) and Q(t). At the sampling instants tj, I(tj) and Q(t;) form a vector whose possible values (i.e., 2~ vector tips) are graphically represented in Fig. 3. Ordinate axis 2û and abscissa axis 22 are scaled in terms of the magnitude of Q(t) and l(t). Such a vector may be modulated upon an ui"agnetic wave to form thereby a modulated information signal wherein the information content of the signal is 30 co",j~rised of a series of discrete signal levels (or phases).
The encoding scheme of Fig. 3 illustrates the standard selected for digital, cellular radiotelephone communication systems to be implemented in the United States. With particular respect to the United States standard, only four -92/02991 PCI`/US91/04874 - 13- 2~B~20 differential chan~es bet~Neen any two sequential vectors are permitted. Such an encl~ding scheme is ref~rred to as a dirr~,~r,lial quaternary phase shift keying (DQPSK) system.
Fig. 4 is a ~raphi~ epres~r,ldlion of waveform 18 of Fig.
5 2A plotted as a function of frequency. Ordinate axis 50 of Fig.

4 ,~pr~s~"l:, the power of a signal, scaled in terms of ll-"-..dll~, plotted as a function of frequency, scaled in terms of hertz, on abscissa axis 52. The waveform is centered about a center frequency, fc, indicated by reference numeral 54.
Sidebands 56 and 58 fDrm the information-containing portion of the waveform. The ~andwidth of the FM signal is indicated by segment 59.
Turning now to th~ graphical ,~prese"ldli~n of Fig. 5, a wav~form upon which a DQPSK signal is modulated is plotted as a function of frequency. A DQPSK modulated signal is a composite modulated si,c~nal having both amplitude modulation components (similar to Fig. 1) and phase modulation coi"~on~r,l~ (similar to Fig. 2B). The power of the waveform, scaled in terms of milli~vaKs on ordinate axis 60, is plotted as a function of frequency, scaled in terms of hertz, on abscissa axis 62. The signal is centered about a center frequency, fc, indicated by reference numeral 64. Center frequency 64 defines sicleba,~ds 66 anc~ 68. The bar~.;dll, of the DQPSK
signal is indicated by segment 69.
Fig. 6 is a graphi~:al r~presenldli~n of two adjacent l,~ns",;ssion channels wherein each l,ans",ission channel is of a banu~.;dll, of thirty kilc)hertz. Hatched lines 74, 76, and 78 of Fig. 6 indicate the respective boundaries of the adjacent channels 70 and 72, wh~rein line 76 indicates the boundary between channels 70 an~ 72. Similar to the graphs of Figs. 4-5, the waveforms ploti~d in Fig. 6 are plots of power as a function of frequency. I-or purposes of illustration, the wavefurm piotted in transmission channel 70 is a frequency modulated signal having center frequency 8û and sidebands 82 WO 92/0~991 - PCT`/US91/04874 ~ ~ - 14 -and 84. As described hereinabove sicleba~cls 82 and 84 represent the information-containing portion of the modulated information signal.
A certain amount of frequency drift of a transmitted 5 signal is permiKed while still maintaining sidebands of the si~nal within the thirty kilohertz bar,~.;.lLI, of channel 70. For an analog, frequency modulated signal plotted in ~,dns",;ssion channel 70 per",:ss; ~ ~requency drift of th~ signal is indicated by arrow 86 pictured above center frequency 80.
10 Arrow 86 illustFates the allowable positioning of the center frequency 80 of the signal while still ",d;"ldirii"g the signal within the boundaries of channel 70. Drift of the carrier wave of the modulated i"~u""dli~n signal which does not exceed in frequency the drift indicated by arrow 86 ",ai"i ,s the sidebands 82 and 84 within ~he thirty kilohertz bal1ci.. kll1, of the channel 70. Quantitatively the frequency drift permitted of the signal is approximately 2100 hertz on either side of the center frequency. The signal centered about center frequency fc indicated by reference numeral 8û indicates a frequency 20 modulated signal which has drifted upwards in frequency but still within the permissible frequency drift. Sidebands 82 and 84 are ",ainl~; ,ed within the boundaries 74 and 76 of I,di1s",;Osion channel 70. It is to be noted however that further increase in the drift of the waveform would cause 25 sideband 84 to extend beyond the boundary 76 s~pa,c~ g transmission channels 70 and 72. Such a drift can result in overlapping of signals of the adjacent channels and thereby cause interference therebetween.
Transmission channel 72 shown in the right-hand portion 30 of Fig. 6 defines a thirty kilohertz transmission channel eAI~n ,9 between hatched lines 76 and 78. For purposes of illustration a DQPSK modulated i,~ur."ation signal is positioned within the transmission channel 72. The modulated information signal similar to the signal of Fi~. 5 is centered - 15 0~642~
about a center frequen~:y, fc, indicated by reference numeral 90.
Center frequency 90 defines sidebancis 92 and 94.
The frequency drift permitted of a discrete signal such as the signal illustrateci in l,d,~s".;~sion channel 72 is less 5 than the frequency drift permitted of a conventional analog signal (such as the siçlnal illustrated in tran "";ssion channel 70). Arrow 96 picture~ above center frequency 90 iliustrates the pe""iLLed frequenc~ drift of the discrete, encoded signal of l,dns",;ssion channel 7:~. Arrow 96 cor,tl~l,on~i:, to arrow 86 10 pictured above impuise spike 80 of l,d~ls",;~sion channel 70, and defines the permitted frequency drift of the discrete, encoded signal. Quantitatively, the frequency drift permitted of a discrete, encoded signal is ap~,u~i,,,~tuly 200 hertz on either side of the cr~nt~3r frequency. This per",issil,le drift is 15 app,uAi",ai~ly one order of masnitude less drift than that per",iLLed of a conventional analog signal.
The much smaller frequency drift permitted of a DQPSK
signal is due, not only because of spilling of the signal into adjacent channels, but, also, because receiver circuitry for 2û receiving and demodulaiing a DQPSK signal cannot receive and demodulate a DQPSK signal as accurately when it is shifted in frequency.
While frequency c:ontrol (i.e. Iocking) is advantageous (and sûmetimes necessary to prevent overlapping of signals) to 25 minimize frequency drift of a conventional analog signal, frequency control is virtually always nec~ssary when certain discrete, encoded signals are transmitted. Digital signal processors may be constructed ~o provide an i" iicaLion of the center frequency ~or ot~ler reference frequency) of any 30 transmitted signal whetller the signal is a conventional analog signal or a discrete, enco~ed signal. The power consumption of a digital signal p,oc~ssor is, however, significant. When ~iscr~t~, encod~d signals are tr~nsmitt~d, a digital signal WO 92/02991 PCI`/US91/04~74 O

processor is required to be operative only intermittently when the discrete, encoded signals are L,clns,,,iLL~d.
Turning now to the block diagram of Fig. 7, the elements of the frequency control system of the present invention are shown in functional block form. The frequency control system embodying the present invention is operative to determine the center, or other reference, frequency of either a discrete, . encoded modulated i"~ur",alion signal, or a co"~n,nli~nal, analog modulated i"~u""ation signal. A digital signal processor is operative only when the transmitted information signal is a discrete, encoded signal, and, then, only when the discrete, encoded signal is received, thereby ~, ,i",i~ g the power consumption of the prul~essor.
Alternately, when receiving an analog i"~r",d~ion signal, the digital signal processor may be utilized to del~r",;"e the reference frequency of the transmitted signal.
The l,ailsl"ilL~d signal, either a conventional, analog signal, or a discrete, encoded signal, is Ll~ilsl,,iLL~d to an antenna tor other ele.;L,u",a~ Lic wave receiving device) 110.
The signal received by antenna 110 is filtered and amplified, if necessary, and supplied to first down conversion circuit 112.
Down conversion circuit 112 converts the transmission frequency signal (which may, for example, be of 890 megahertz) into a signal of a lower frequency, such as, for example, 45 megahertz. Down conversion circuit 112 generates the lower frequency signal on line 114 which is coupled to second down conversion circuit 116. Second down conversion circuit 116 converts the signal supplied thereto on line 114 to a baseband signal. Down conversion circuit 116 generates an in-phase signal on line 11~ and a signal in-phase quadrature therewith on line 120. The in-phase signal generated on line 118 is supplied to baseband filter 122, and the quadrature signal ~enerated on line 120 is supplied to baseband filter 124. Down conversion circuit 116 and filters 92/02991 -- ~
2~6~20 122 and 124 may togetlher comprise a single integrated circuit chip, raferred to as a zero intermadiate frequency (ZIF) circuit illustrated by block 126, shown in hatch.
Fiitered signals gen~?,al~?d by filters 122 and 124 are çienerated on lines 128 i~nd 130, respectii~ly. Filters 122 and 124 contain pass~al1ds to pass signals of desired frequencies.
When antenna 110 receives a discrete, encoded signal, the filtered signal ~ener~lled by filters 122 and 124 are supplied to analog-to-digital convl?rter t32 and 133. The digital si~nals gene,dL,?d by A/D converters 132 and 133 are supplied to digital signal processor 134 on lines 136 and 137.
Di~ital signal processor processes the di~ital signal supplied thereto, and generates an audio signal on line 138 i"~icati~a of the information signal transmitted in discrete, encoded form to antenna 110. Digital signal processor 134 also generates an output signal on line 14t~ which is indicative of the center, or other reference, frequen~,y of the transmitted signal. The signal generated on line 140 may be utilized to lock the frequency of the receivel~ to the center, or other reference, frequency of the transmitted signal.
When the signal I,~1ns",i~l-?d to antenna 110 is a conventional, analog sigl1al, the filtered signals generated by filters 122 and 124 are supplied to up-conversion circuit 142.
Up conversion circuit 142 converts the filtered, in-phase 25 and filtered, quadrature F~hase signals generated on lines 128 and 130, respectively, to a higher-frequency signal, and such signal is gene,dl~d on line 143. The signal generated on line 143 is supplied to the d~?modulation circuit 144. The demodulation circuit 144 demodulates the signal supplied thereto by conventional irequency demodulation techniques.
Conversion of the basebanci s~nais generated on lines 128 and 130 into a higher frequency signal is required for demodulation by con~entional dr~modulation circuitry.

WO 92/02991 ~ PCI/US91/04874 Demodulation circuit 144 generates an audio signal on line 145 which is representative of the information si~nal portion of a conventional, analog modulated signal received by antenna 110.
The signal ~enerated by up-conversion circuit 142 on line 143 is also supplied to phase detector 146. Phase detector 146 COlllpdlt~S the frequency of the up conversion circuit 143 output to a frequency of the signal generated by the offset loop circuit 147 to generate an output signal on line 149 whicll is indicative of the Genter, or other ~felence, frequency of the transmitted signal.
Because a ttansmitter which transmits a modulated i"~ur",dlion signal, and, which in particular instance of a cellular communication system comprises a base station, generally is of a size which permits means for preventing frequency drift caused by ambient conditions and voltage irregularities to form a portion of a l,dns",ilLer, the center, or other reference, frequency of the transmitted signal may be used as a reference frequency by the receiver.
Turning now to the partial block, partial scl~ei"alic illustration of Fig. 8, a preferred ei,.L-~ "ent of the frequency control system of the present invention is shown. A
modulated, information signal, either a conventional, analog signal or a discrete, encoded signal is ~IdllsllliLL~d by L,dns",iLL~r 150 and is received by antenna 152.
Antenna 152 supplies the received signals on line 154 to filter 156. ~Filter 156 forms a passband to pass signals of frequencies within a desired frequency range on line 158. The . signals passed by filter 156 are supplied to mixer circuit 160 to down convert the modulated signal received by antenna 152.
Mixer 160 receives an oscilldLi"g signal on line 162 ~ne,dLt,d by voltage control oscillator 164. Voltage control oscillator 164 forms a portion of a conventional phase locked loop ~PLL) circuit including phase detector 166, filter 168, and dividing Pt~r/ussl/o4s74 wo 92/02991 1~ ,9 206~20 c;rcuits 170 and 17~. The si~nal mixed by mixer 160 is suppli~d on iine 163 to filter 165. Filter 1~5 contains a p~sband to pass si~nals of desired frequencies on line 166.
Line 166 is coupled to an i"~t,r",edidLe frequency input of zero inL~r,l,e~ialt3 frequency (Z1F) section circuit 180. Mixer 160, oscillator 164 and the r~sso~ d PLL circuit, and filter 165 ars enclosed by block 112, shown in hatch, to correspond with down conversion circuit 112 shown in the block diagram of Fig.
7.
Circuitry forming .I second PLL circuit, ct;i"~ ri~ed of voltage controlled oscillator 182, lowpass filter 184, phase detector 186 and dividinig circuits 188 ant~ 190, provides an c~ ,9 signal to the second LO input of circuitry 180. The second PLL circuit and Icircuitry 180 are enclosed by block 126, 1~ shown in hatch, to ct""3spond with the zero intermediate frequency circuit 126 of Fig. 7.
Reference oscillattlr 192 generates an oscil.aling signal on line 194 which is di~ided by dividing circuitry 196 to supply a reference oscillating signal to the intermediate frequency reference input of circuitry 180. Line 194 also is coupled to the oscillator circuitry conne~ 3d to the second LO input of circuitry 180, and to the PLL circuit which provides an oscillating signal on line 162 to mixer 160 to provide thereby an oscillating signal to each of the PLL circuits.
Intermediate frequt~ncy section circuitry 180 generates I
and Q output signals on lines 198 and 200, respectively, when the modulated si~nal rec0ived by arltenna 152 is coi"~rised of a discrete, encoded signal. When the modulated signal received by antenna 152 is comprised of a conventional, analog signal, intermediate frequency section circuitry 180 generates an audio out~ut on line 202. Line 202 is coupled to audio processi"g circuitry (not shown). When antenna 152 receives the conventional, analo~ modulated signal, sir~uitry 18~
further generates a phas~ detection output which is supplied to WO 92/02991 . PCI /US91/04874 filter 204. Filter 204 generates a filtered output signal on line 206 indicative of the phase, or frequency, of the signal received by antenna 152.
When circuitry 180 gen~ldlc,s I and Q output signals on lines 198 and 200, the ~enerated output signals are supplied to analo~-to-di~ital converters 208 and 210, r3spectively. A/D
CO~ r::, 208 and 210 provide digital signals to di~ital signal processor 212. While Fig. 8 illustrates parallel conne~ ns between COh~ r:~ 208 and 210, and p~ucesso( 212, it is to be noted that serial connections are simiiarly possible.
Plucessor 212 further receives phase angle, or frequency, information indicated by the filtered signal generated by filter 204 which is supplied to the processor 212 through analog-to-digital converter 214. Digital signal processor 212 processes the I and a signals and the frequency i"~u""alion supplied thereto, and ~enerates output signals which are converted to an analog signal by digital-to-analo~ converter 216.
The analog signal generdl~d by D/A converter 216 is supplied on line 218 to frequency control switch 220. Line 206 is additionally coupled to frequency control switch 220.
Switch 220 is actuated by an external signal supplied thereto on line 222 which alternately connects line 206 or line 218 to the ,~rt,nce oscillator 192. The signals supplied on lines 206 and 218 are il~di~.dli~ of the frequency of the signals received by antenna 152. As des~,iiJed hereinabove, the signal on line 206 is indicative of the frequency of the received signal when the si~nal is a conventional, analog signal, and the signal supplied on line 218 is indicative of a discrete, encoded signal received by antenna 152. The signal supplied on lin~s 206 and 218, respectively, are utilized to alter the oscillating frequency of oscillator 192. More particularly, the changes in frequency of oscillator 192 correspond to the changes in the frequency of the signal l,dns",il~d to antenna 152.

2i~ 2~

Switch 220 may, f~r example, be comprised of any electronic controliable, c~r other, switch. For example, switch 220 may be cc""pl~sed c)f CMOS L,dns",;~sion gates arranged in the form of a 2:1 m~ti~'qYer.
Turning now to the partial block, block schematic illustration of Fig. 9, an alternate preferred ~IlIL " "en~ of the present invention is illu~strated. Similar to the el"t ' "ent of Fi~. 8, a signal, either al conventional, analo~ modulated information signal, or a discrete, encoded modulated information signal is transmitted by transmitter 250 and is received by antenna 25~~. Antenna 252 supplies the received signals on line 254 to t~and~,ass filter 256. Bandpass filter 256 forms a pass~and to pa<;s signals of a desired frequency range on line 258 to mixer 260 to ~down-convert~ the modulated signals received by antenna 252. Mixer 260 receives an signal on lin~ 262 generated by voltage control oscillator 264. Voltage control oscillator 264 forms a portion of a conventional PLL circuit including phase detector 266, filter 268, and divider circuits 270 and 272. The signal mixed by mix~r 260 is supplie~ on line 274 to bandpass filter 276.
Bandpass filter 276 con:ains a pas:,L~and to pass signals of desired frequencies on line 278 to the i"Lt:r",e~idLe frequency input of intermediate flequency circuitry 280.
Circuitry forming c~ PLL comprised of voltage control oscillator 282, lowpass filter 284, phase detector 286, and divider circuits 288 and 290 provide an osc;lldLi"g signal to the second LO input of circuitry 280.
Reference oscillator 292 generates an oscillating signal on line 294 which is divided by dividing circuitry 296 and supplied to an intermec~iate frequ~ncy ,e~rence input of circuitry 280. Line 294 is also coupled to the oscillator conne~li"g the second L.O input of circuitry 280, and to the PLL
circuit~y which provides an osciilating si~nai on line 262 to -WO g2/02991 PCI/US91/04874 ~ ~, -22- 206~420 mixer 260 to provide thereby an os' " "~9 sisnal to each of the PLL circuits.
Intermediate frequency section circuitry 280 ~enerates I
and Q outputs on lines 298 and 300, respec~ively, when the modulated signal received by antenna 252 is cG-",urised of a discrete, encoded signal. When a rn~du'^'~ rulllld~;on signal received by antenna 252 is co"",,ised of a conventional, analog signal, i"l~""e~idl~ frequency section circuitry 280 generdt~s an audio output signal on line 302. Line 302 is coupled to audio r;,ucesai,~g circuitry (not shown). When antenna 252 receives a conventional, analog modulated information siynal, circuitry 28û further ~enerates a phase detection output which is supplied tû filter 304. Filter 304 generates a filtered output signal on line 306 indicative of the phase, or frequency, of the signal received by antenna 252.
I"lt"",edial~ frequency s~ction circuitry 280 generates I
and Q output signals on lines 298 and 300, respectively, when the modulated signal received by antenna 252 is c~"",rised of a discrete, encod~d signal. The I and Q output signals formed on lines 298 and 300, respectively, are supplied to analog-to-digital converters 308 and 310. AID converters 308 and 310 provide digital signals to digital signal processor 312. While Fig. 9 illustrates parallel conneoLions between converters 308 and 310, and processor 312, it is to be noted that serial connecli~ns are similarly possible. Digital signal processor 312 pr~cesses the I and Q signals supplied thereto, and gen~,dles outputs which are converted to an analo~ signal by digital-to-analog converter 316. The analog signal generated by digital-to-analog'converter 316 is supplied on line 318 to 3û reference oscillator 292.
The ~ G~ nl of Fig. 9 differs from ~hat of Fig. 8 in that instead of supplying the signals yenerdled on lines 306 and 318 to a switch, the signal generated on line 318 is supplied directly to reference oscillator 292, and the signal WO 92/02991 PCI-/r~S91/04874 ~ 2Q~2~-supplied on line 306 is supplied to offset voltage control oscillatar 320. Offset osciliator 320 ~nerates an oscillating signal on line 322 whic~l is supplied to an image reject mixer 324. Reference oscillatl~r 292 is fixed to a preset value, and 5 the LO loop is locked tt) the frequency of r~rt,nce oscillator 292. When antenna 25~! receives a ctj~J~ ional, analog modulated i"rtjr",dLion signal, the digital signal processor 312 is disabled, and the I arld Q signals are not utilized; rather, the phase detection output signal generated by circuitry 280, is 10 supplied to filter 304. The filtered signal generated by filter 304 is supplied on line 306 to offset oscillator 320 which, when connected as illustrated, alters the oscilld~i"g frequency of the second LO input. When antenna 252, conversely, receives a discrete, encoded mtdulated information signal, oscillator 15 320 is disabled, causin~ unbalancing of the image reject mixer 324 such that the second LO feedback signal passes through circuit 324, and the second LO divided by N circuit is IJ~u~rd~ d. This locl<s the second LO to the frequency of r3ference oscillator 292, and digital signal p(tjcessor 312 20 generates a control signal on line 318 to alter the frequency of oscillator 292 corresponloling to the frequency of the signal received by antenna 252.
Reference oscillatar 192 of Fig. 8, and reference oscillator 292 of Fig. 9, may, for example, be ct~i"prised of a 25 current controlled oscillator when the signal supplied on lines 206 and 218, and line ~18 are current signals. Alternately, o- " ' .:, 192 and 292 may be comprised of data driven os~;llaltjra when the signals supplied on lines 206 and 218, and on line 318 are compris~d of data si~nals. A data driven 30 reference oscillator is advantageous for the reason that noise ç,t~ne,~l~d on a control line dD9S not ca~se frequency variations.
While the present invention has been dest,,i~ed in co~nection with the prt~lerred embo~imer~ts of ~he vario~ls figures, it is to be undt~rstood that other similar e".t ~ "enl~

WO 92/02991 PCr~US91/04874 -24- 2~!6642~
may be used and Illod;~i-,dlio~is and additions may be made to the described embodiments for performin~ the same function of the present invention without deviatin~ therefrom.
Therefore, the present invention should not be limited to any 5 sin~le e",~odii"e"l, but rather construed in breadth and scope in accordance with the recitation of the appended claims.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A frequency control system operable to maintain a receiver oscillation frequency of at least one variable oscillator forming a portion of areceiver in a desired frequency relationship with an oscillation frequency of a modulated signal transmitted to the receiver and received thereat, said frequency control system comprising:
means for converting the modulated signal received by the receiver into an electrical signal of signal characteristics indicative of the modulated signal;
analog signal detection circuitry coupled to receive the electrical signal generated by the means for converting and operative when the modulated signal comprises a frequency modulated signal, said analog signal detection circuitry having a phase detector for detecting frequency characteristics of the electrical signal indicative of the modulated signal and for generating a first reference signal of levels indicative of the frequency characteristics detected thereat;
discrete signal detection circuitry coupled to receive the electrical signal generated by the means for converting and operative only when the modulated signal comprises a discretely-encoded, modulated signal, said discrete signal detection circuitry for determining frequency characteristics of the electrical signal indicative of the modulated signal and for generating a second reference signal of levels indicative of the frequency characteristics determined thereat;and means for altering the oscillation frequency of the variable oscillator responsive to the levels of the first frequency reference signal when the modulated signal comprises a frequency modulated signal and responsive to the levels of the second frequency reference signal when the modulated signal comprises a discretely-encoded, modulated signal, to maintain thereby the variable oscillator of the receiver in the desired frequency relationship with the oscillation frequency of the modulated signal.

2. The frequency control system of claim 1 wherein said means for converting comprises two-stage down-conversion circuitry having a first stage and a second stage.

3. The frequency control system of claim 2 wherein a second stage of the two-stage down-conversion circuitry forms a baseband information signal.

4. The frequency control system of claim 1 further comprising means forming a switch coupled at one side thereof to the variable oscillator and at another side thereof to receive alternately the first reference signal when the information signal is comprised of the frequency modulated signal or the second reference signal when the information signal is comprised of a discrete, encoded signal.

5. The frequency control system of claim 4 wherein said switch is actuated by a signal transmitted by the transmitter.

6. The frequency control system of claim 4 wherein said switch comprises a multiplexer.

7. The frequency control system of claim 1 wherein said means forming discrete signal detection circuitry comprises a digital signal processor.

8. The frequency control system of claim 7 further comprising at least one analog-to-digital converter for converting signals supplied to the digital signal processor into digital form.

9. The frequency control system of claim 7 further comprising at least one digital-to-analog converter for converting the second frequency reference signal generated by the digital signal processor into analog form.

10. The frequency control system of claim 9 wherein said digital-to-analog converter generates a signal of a preset value when the digital signal processor fails to generate the second frequency reference signal.

11. The frequency control system of claim 9 wherein said digital-to-analog converter generates a signal responsive to the first frequency reference signal generated by the phase detector of the analog signal detection circuitry when the digital signal processor fails to generate the second frequency reference signal.

12. In a dual mode transceiver having receiver circuitry including a variable oscillator oscillating at a receiver oscillation frequency forming a portion thereof, the receiver circuitry operative to receive either a frequency modulated signal or a discretely-encoded, modulated signal, the combination with the receiver circuitry of a frequency control system operable to maintain the receiver oscillation frequency of the variable oscillator in a desired frequency relationship with an oscillation frequency of the frequency modulated signal when the frequency modulated signal is received by the receiver circuitryor with an oscillation frequency of the discretely-encoded modulated signal when the discretely-encoded, modulated signal is received by the receiver circuitry, said frequency control system comprising:
analog signal detection circuitry coupled to receive the electrical signal representative of the frequency modulated signal when the frequency modulated signal is received by the receiver circuitry, said analog signal detection circuitry having a phase detector for detecting frequency characteristics of the electrical signal representative of the frequency modulated signal and for generating a first reference signal of levels indicative of the frequency characteristics detected thereat;
discrete signal detection circuitry comprised of a digital signal processor coupled between an analog-to-digital converter and a digital-to-analog converter, the analog-to-digital converter being operative to convert and electrical signal representative of the discretely-encoded, modulated signal when the discretely-encoded, modulated signal is received by the receiver circuitry and to apply digital signals indicative of such to the digital signal processor, the digital signal processor being operative only when the modulated signal comprises the discretely-encoded, modulated signal for determining frequency characteristics of the electrical signal indicative of the discretely-encoded, modulated signal and for generating a second reference signal of levels indicative of the frequency characteristics determined thereat, and the digital-to-analog converter for converting the second reference signal into analog form; and a multiplexer forming a switch coupled at one side to the variable oscillator and at a second side to receive, alternately, the first reference signal or the second reference signal, for supplying the first reference signal to the variable oscillator when the frequency modulated signal is received by the receiver circuitry, or alternately, for supplying the second reference signal to the variable oscillator when the discretely-encoded, modulated signal is received bythe receiver circuitry.

13. A method for maintaining a receiver oscillating frequency of a variable oscillator forming a portion of a receiver in a desired frequency relationship with an oscillation frequency of a modulated signal transmitted to the receiver and received thereat, said method comprising the steps of:
converting the modulated signal received by the receiver into an electrical signal of signal characteristics indicative of the modulated signal;
operating analog signal detection circuitry when the modulated signal comprises a frequency modulated signal, thereby to detect frequency characteristics of the electrical signal indicative of the modulated signal, andgenerating a first reference signal of levels indicative of the frequency characteristics detected thereat;
operating discrete signal detection circuitry only when the modulated signal comprises a discretely-encoded, modulated signal, thereby to determine frequency characteristics of the electrical signal indicative of the modulated signal and generating a second reference signal of levels indicative of the frequency characteristics determined thereat; and altering the oscillation frequency of the variable oscillator responsive to the levels of the first frequency reference signal when the modulated signal comprises a frequency modulated signal or, alternately, responsive to the levelsof the second frequency reference signal when the modulated signal comprises a discretely-encoded, modulated signal, to maintain thereby the receiver oscillation frequency of the variable oscillator of the receiver in the desired frequency relationship with the oscillation frequency of the modulated signal.