WO1992012578A1

WO1992012578A1 - Frequency control system

Info

Publication number: WO1992012578A1
Application number: PCT/US1991/009674
Authority: WO
Inventors: Theodore T. Sanecki; Ralph D. Smallwood
Original assignee: Motorola, Inc.
Priority date: 1990-12-31
Filing date: 1991-12-26
Publication date: 1992-07-23
Also published as: CA2073341C; MX9102892A; KR920704447A; GB2258961B; KR970007361B1; GB9218094D0; JPH05505088A; GB2258961A

Abstract

A frequency control system for maintaining a receiver oscillation frequency of a variable frequency oscillator (580) of a receiver unit in a desired frequency relationship with oscillation frequency of a signal transmitted thereto. The frequency control system is operative only during those times in which the receiver unit receives an information-containing signal of desired characteristics such as a data signal or a voice signal.

Description

FREQUENCY CONTROL SYSTEM

Background of the Invention

The present invention relates generally to automatic frequency control systems, and, more particularly, to an automatic frequency control system operative responsive to detection of an information-containing signal of desired characteristics.

A communication system which transmits information between two locations includes, at minimum, a transmitter and a receiver wherein the transmitter and the receiver are interconnected by a transmission channel upon which an information signal may be transmitted.

In one type of communication system, a radio communication system, the transmission channel is comprised of a radio-frequency channel which interconnects the transmitter and the receiver. To transmit an information signal, referred to as a baseband signal, upon the radio-frequency channel, the baseband signal must be converted into a form which may be transmitted upon the radio-frequency channel. Such a conversion process is referred to as modulation wherein the baseband signal is impressed upon a radio-frequency electromagnetic wave. The frequency of the radio-frequency electromagnetic wave is of a value within a range of values of frequencies defining the radio-frequency channel. The radio- frequency electromagnetic wave is commonly referred to as a carrier signal, and the carrier signal, once modulated by the baseband signal, is referred to as a modulated, information signal wherein the information content of the modulated, information signal occupies a range of frequencies, referred to as a modulation spectrum centered at, or close to, the frequency of the carrier signal. Once the baseband signal is modulated upon the carrier signal, the resultant, modulated, information signal may be transmitted through free space upon the radio-frequency channel to transmit thereby information between the transmitter and the receiver of the communication system.

Various techniques have been developed to modulate the baseband signal upon the carrier signal. Such modulation techniques include amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and complex modulation (CM).

The receiver of the radio communication system receives the modulated, information signal transmitted over the radio- frequency channel. The receiver contains circuitry to detect, or to recreate otherwise, the information signal from the modulated, information signal transmitted over the radio-frequency channel. This process of detection or recreation of the baseband signal is referred to as demodulation. Typically, the receiver includes circuitry for performing the demodulation process (demodulation circuitry), and, additionally, down conversion circuitry. The down conversion circuitry converts downward in frequency the modulated, information signal to permit proper operation of the demodulation circuitry.

A plurality of transmitters may each modulate and transmit a signal over different radio-frequency channels. To permit demodulation of only a single, desired signal by the demodulation circuitry of the receiver, the receiver additionally contains timing circuitry to permit demodulation of only those signals within a desired range of frequencies. Such tuning circuitry typically includes filter circuitry which form frequency passbands for passing only those signals having frequency components within the frequencies defined by the passbands of the filter circuitry. The receiver further contains additional filter circuitry to prevent passage of signals generated during down conversion and demodulation of the received signal.

The broad range of frequencies over which the modulated, information signals may be transmitted is referred to as the electromagnetic frequency spectrum. The electromagnetic frequency spectrum is divided into frequency bands each of which defines a range of frequencies of the electromagnetic frequency spectrum. The frequency bands are further divided into channels, referred to as transmission channels. To minimize interference between simultaneously-transmitted, modulated, information signals, transmission of radio-frequency signals in certain ones of the frequency bands of the electromagnetic frequency spectrum is regulated. Portions of a 200 MHz frequency band of the electromagnetic frequency spectrum (extending between 800 MHz and 1000 MHz) are allocated for radiotelephone communication. Radiotelephone communication, may, for example, be effectuated by radiotelephones utilized in a cellular, communication system. Such radiotelephones contain circuitry which permits simultaneous generation and reception of modulated, information signals, thereby permitting two-way communication between the radiotelephone and a remotely- located receiver.

Detailed descriptions of cellular, communication systems are found in, e.g., texts entitled: Mobile Cellular

Telecommunications Systems by William C.Y. Lee, copyright 1989, The Bell System Technical Journal , Volume 58, No. 1, January 1979, entitled "Advanced Mobile Phone Service", and Cellular System Mobile Station Compatibility Specification, Volume IS-3-D, published by the Electronic Industries Association, copyright, 1987. hi general, a cellular communication system is created by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated, information signals transmitted by radiotelephones, and to transmit modulated, information signals to the radiotelephones. Transmission of modulated, information signals between a radiotelephone and a base station permits two-way communication therebetween.

Careful selection of the position at which each of the base stations of the cellular, communication system is located is required such that at least one base station is within the transmission range of a radiotelephone positioned at any location throughout the geographical area. Because the base stations are spaced-apart throughout the geographical area, portions of the geographical area, by proximity, are associated with individual ones of the base stations. Portions of the geographical area positioned proximate to each of the spaced-apart base stations define "cells" wherein the plurality of cells, each associated with a base station, together form the geographical area encompassed by the cellular, communication system. A radiotelephone positioned within the boimdaries of any of the cells of the cellular, communication system may transmit, and receive, modulated, information signals to, and from, at least one base station.

Typically, communication between the radiotelephone and the base station first occurs on control channels in which the base station transmits data, such as, for example, data referred to as wideband data (e.g., eight kilobit per second data) to the radiotelephone. Such data includes instructions to cause the radiotelephone to receive and to transmit signals upon particular radio-frequency channels, referred to as voice channels. When operating on a voice channel, the base station sends voice information to the radiotelephone, along with a SAT (for Supervisory Audio Tone) signal; in some cellular, communication systems of increased capacity, the base station sends voice information along with a DSAT (for Digital Supervisory Audio Tone) signal. The SAT signal is described in more detail in the above-listed references. The DSAT signal is a 100 bit per second data under voice signal which is functionally similar to the SAT signal. For example, the DSAT signal can be comprised of a 24 bit, digitally-encoded word that is repeated continuously at a rate of 100 bits per second. The use of a DSAT signal is advantageous for the reason that the signed can be passed by a channel of reduced bandwidth, whereas a SAT signal cannot be passed by a channel of reduced bandwidth. Of significance, each of the signals, when received and properly detected by a radiotelephone, are indicative of valid communication (i.e., transfer of information) between the base station and the radiotelephone.

Although numerous radiotelephones may transmit modulated, information signals simultaneously at different transmission frequencies (i.e., each of the plurality of radiotelephones may simultaneously modulate baseband signals as long as the signals are transmitted upon different radio- frequency channels), each modulated, information signal, during transmission thereof, occupies a portion of the frequency band (i.e., a radio-frequency channel). Overlapping of simultaneously- transmitted, modulated, information signals, whether by transmission of the signals at the same frequency or by frequency drift of one or more signals, is impermissible as overlapping of simultaneously transmitted signals may prevent detection of any of the simultaneously-transmitted, modulated, information signals. To prevent overlapping of simultaneously-transmitted, modulated, information signals, the transmission channels defined in the frequency band allocated for radiotelephone communications are divided, in the United States, into 30 KHz channels, and, in Japan into 25 KHz channels. Schemes have been developed to divide further the transmission channels into 10 KHz channels in the United States, and 12.5 KHz channels in Japan. By reducing the bandwidth of the channels, the number of channels over which signals may be transmitted is increased. Such increase increases the phone-call capacity of a cellular, communication system.

To prevent overlapping of simultaneously transmitted, modulated, information signals, whether the signals are transmitted over transmission channels of 30, 25, 12.5, 10 KHz, or other frequency bandwidths, requires minimization of the frequency drift of any transmitted signal. Frequency drift of transmitted signals is typically caused by variations in the oscillation frequency of oscillators utilized to generate (or to down convert) the electromagnetic waves (i.e., the carrier signals) upon which the information signals are modulated. Such variation in oscillator frequencies are caused, for example, by changes in ambient conditions such as temperature changes and supply voltage variations.

By decreasing the channel bandwidth to increase the capacity of the cellular, communication system, a need to minimize frequency drift of a transmitted signal becomes even more pressing as less frequency drift may be tolerated on channels of reduced bandwidths.

Systems and methods of frequency control for minimizing frequency drift to minimize thereby frequency drift problems are known and are frequently utilized in many existing communication systems. Generally, one oscillator, of a transmitter/receiver pair is of a construction, or is contained in an environment in which the ambient conditions are controlled, such that changes in ambient conditions do not cause significant alteration of the oscillator frequency. The characteristic frequency of the selected oscillator is then utilized as a reference frequency to which other oscillators are "locked", i.e., maintained in a relative frequency relationship therewith.

In the particular instance of cellular, radiotelephone communications as above-described, an oscillator forming a portion of the circuitry of each of the base stations positioned throughout the geographical area is of a construction (typically, the reference oscillator of a base station is an "ovenized" oscillator) and may be maintained within an environment in which the ambient conditions are closely controlled to control precisely thereby the oscillating frequency of the oscillator. A radio-frequency signal generated therefrom is of a characteristic frequency which exhibits a minimal drift.

Radiotelephones positioned within the transmission range of a signal transmitted by a base station may utilize the carrier signal frequency of the radio-frequency signal transmitted thereto by the base station as the frequency to which the oscillators of the radiotelephone are locked, to control thereby the oscillating frequencies of the oscillators of the radiotelephone. The reference frequency determined by the carrier signal frequency of the radio- frequency signal transmitted by the base station may be utilized by the radiotelephone, for example, as a reference from which the transmit frequency of the radiotelephone may be offset to allow the signal transmitted by the radiotelephone to be as precise of a frequency as the base station frequency, and as a reference to which the oscillating signals supplied to the mixing circuitry of the down conversion circuitry may be locked. Such a system is commonly referred to as an automatic frequency control (AFC) system.

Existing automatic frequency control systems have a provision to enable or disable AFC operation, depending on the level of the strength of the received signal. When the signal level received is below a certain threshold level, the frequency control system of the receiver is disabled. When the signal level received is above the threshold level, the frequency control system of the radiotelephone receiver is enabled. When the frequency control system is enabled, the reference oscillator frequency of the radiotelephone is adjusted in accordance with the incoming receive signal from the base station, with the objective of correcting the radiotelephone frequency to the base station frequency. When the frequency control system is disabled, the reference oscillator frequency is not adjusted by the frequency control circuitry.

In the case of a cellular radiotelephone, the receive signal strength indicator (RSSI) signal is used to enable or disable operation of the automatic frequency control system. Typically, a cellular radiotelephone generates a receive signal strength indicator (RSSI) signal, which is a DC voltage proportional to the power level of the signal received by the receiver. If the signal strength, as indicated by the RSSI signal is above the threshold, the AFC is enabled; conversely, if below, the AFC is disabled. This determination of RSSI level is done after a control channel has been selected, or when the radiotelephone is operating on a voice channel. Details of the control channel scan and selection processes, voice channel transmission, RSSI signal, and other call processing functions are more fully described in the above- listed texts.

While such a frequency control system is normally suitable to determine accurately the carrier signal frequency of the transmitted signal, such a frequency control system may operate inadequately in several situations.

First, because the frequency control system is operative only when the RSSI signal is above a pie-determined level, a signal received by the radiotelephone receiver which is less than a certain, minimum power level is not utilized as a reference to which the reference oscillator of the radiotelephone is locked.

This occurs in spite of the fact that the AFC circuitry is usually capable of achieving frequency lock with incoming receive signals having a level considerably below the AFC threshold level. The threshold level cannot be simply reduced because the weakest signal level at which lock occurs is usually below the level at which the RSSI can accurately determine signal strength. At weak signal levels, the RSSI signal becomes non-linear, the RSSI signal does not have the necessary dynamic range, and the noise received by the receiver causes a fluctuating RSSI signal, all of which degrades the accuracy of the RSSI signal.

Additionally, the presence of a spurious signal (such as a signal generated by another service, a signal caused by intermodulation, or a spurious response of a radiotelephone) may cause a frequency control system operative responsive to determination of a signal strength indication to "lock" onto an incorrect frequency.

Further, when no signal is received by the radiotelephone, and noise is in excess of a pre-determined level, such a frequency control system may again "lock" onto an erroneous frequency. Further, when a simultaneously-transmitted signal on an adjacent channel drifts in frequency onto the desired transmission channel, the frequency control system operative responsive to an indication of signal strength may "lock" onto the signal which has drifted in frequency onto the desired transmission channel. Still further, when a simultaneously-transmitted signal on a channel adjacent to a signal transmitted upon a desired transmission channel is much greater in magnitude than the signal transmitted upon the desired transmission channel, a frequency control system operative responsive to a signal strength indication may again "lock" onto the signal of much greater magnitude.

What is needed, therefore, is a frequency control scheme operative to determine the frequency of oscillation of oscillators of a radiotelephone only when the signal received by the radiotelephone is a desired, information signal.

Summary of the Invention

The present invention, accordingly, advantageously provides a frequency control system for correcting frequency differences between a transmitter and a receiver operative responsive to detection of a transmitted, information-containing signal of desired characteristics. The present invention further advantageously provides a frequency control system operable to determine a frequency, to be utilized as a reference, of a signal supplied to a receiver only when an information-containing signal of desired characteristics is detected. The present invention still further advantageously provides a method for maintaining a receiver oscillator frequency of a receiver unit in a desired frequency relationship with an oscillation frequency of a signal transmitted thereto.

In accordance with the present invention, a frequency control system operable to maintain a receiver oscillation frequency of a receiver unit in a desired frequency relationship with a oscillator frequency of a signal transmitted thereto is disclosed. A frequency control system includes at least one variable frequency oscillator oscillating at the receiver oscillation frequency and receives the signal transmitted to the receiver unit. A signal detector detects when the signal transmitted to, and received by, the receiver unit is comprised of an information- containing signal of the desired characteristics. Responsive to detection by the signal detector of the information-containing signal of desired characteristics, the receiver oscillation frequency of the variable frequency oscillator is altered to maintain thereby the receiver oscillation frequency in the desired relationship with the oscillation frequency of the signal transmitted thereto.

Brief Description of the Drawings

The present invention will be better understood when read in light of the accompanying drawings, in which:

FIG. 1 is a graphical representation of a series of adjacently-positioned transmission channels upon which a plurality of modulated, information signals are simultaneously transmitted;

FIG. 2 is a graphical representation of a single signal transmitted upon a single transmission channel representative of one such signal which may be received by a radiotelephone having the frequency control system of the present invention;

FIG. 3 is a graphical representation, similar to that of FIG. 2, but illustrating a signal shifted in frequency to be partially beyond the passband of filter circuitry of a radiotelephone;

FIGS. 4A and 4B are graphical representations illustrating small-strength, modulated, information signals transmitted upon a transmission channel upon which significant amounts noise is present; FIG. 5 is a graphical representation of a single transmission channel upon which is transmitted a modulated, information signal, and, additionally, a spurious signal;

FIGS. 6A and 6B are graphical representations of two adjacently-positioned transmission channels upon which a small-strength signal is transmitted on a first of the transmission channels, and a strong-strength signal is transmitted upon the transmission channel adjacent to thereto wherein FIG. 6A is representative of the signals as received by a receiver, and FIG. 6B is representative of the signals after down conversion and filtering thereof by receiver down conversion and filtering circuitry;

FIG. 7 is a simplified block diagram of the frequency control system of the present invention; FIG. 8 is a block diagram of a transceiver, such as a cellular, radiotelephone, which includes the frequency control system of the present invention; and

FIG. 9 is a logical flow diagram of the method of the present invention.

Detailed Description of the Preferred Embodiment

Referring first to the graphical representation of FIG. 1, a portion of the frequency band allocated for cellular, radiotelephone communication is illustrated wherein ordinate axis 10 is scaled in terms of dBm, and abscissa axis 14 is scaled in terms of kilohertz. (Alternately, axis 10 may be scaled in terms of volts, watts, or some other such amplitude-indicative scale.) The illustrated portion of the frequency band is divided into adjacently- positioned channels 18, 22, 26, 30, 34, 38 and 42. Vertically extending lines in the figure indicate the boundaries between adjacent ones of the channels 18-42. Channels 18, 22, 26, 30 and 42 are of conventional bandwidths such as, as indicated hereinabove, the 30 KHz bandwidth transmission channels of the frequency band allocated for radiotelephone communication in the United States, or the 25 KHz bandwidth transmission channels of the frequency band allocated for radiotelephone communications in Japan. Channels 34 and 38 are of reduced bandwidths compared with the size of the bandwidths of channels 18-30 and 42, and are representative of the transmission channels to be utilized in radiotelephone communication systems of increased capacity such as 10 KHz bandwidth channels proposed for use in the United States, and 12.5 KHz bandwidth channels proposed for use in Japan.

Signals are transmitted upon each of the channels 18-42. A signal comprised of modulation spectrum 46 centered about carrier frequency 48 is transmitted upon channel 18. A signal comprised of modulation spectrum 50 is centered about carrier frequency 52 upon channel 22. A signal comprised of modulation spectrum 54 is centered about carrier frequency 56 upon channel 26. A signal comprised of modulation spectrum 58 is centered about carrier frequency 60 upon channel 30. A signal comprised of modulation spectrum 62 is centered about carrier frequency 64 upon channel 34. A signal comprised of modulation spectrum 66 is centered about carrier frequency 68 upon channel 38. A signal comprised of modulation spectrum 70 is centered about carrier frequency 72 upon channel 42.

The signals transmitted upon channels 18, 22, 26, and 42 are representative of modulated, information signals comprised of voice signals (and also SAT signals) of conventional bandwidths; signals transmitted upon channels 34 and 38 are representative of modulated, information signals comprised of voice signals (and also DSAT signals) of reduced bandwidths. Because voice signals are transmitted upon channels 18-26 and 34-42, channels 18-26 and 34-42 are referred to as voice channels. Signals 46-70 are of various magnitudes to indicated that the signals received by a receiver may be of varying signal strengths. The signal transmitted upon channel 30 is representative of a modulated, information signal comprised of a control signal of a conventional bandwidth. (In the United States, such control signal is typically comprised of ten kilobit per second data, and, in Japan, such control signal is typically comprised of eight kilobit per second data.) Because a control signal is transmitted upon channel 30, channel 30 is referred to as a control channel. It is noted that, although the control signal is illustrated in the figure as being of a magnitude less than the magnitudes of signals transmitted upon channels 18-26 and 38-42, such distinction is shown for purposes of illustration only. It is further noted that the bandwidth of a control channel, such as control channel 30, is conventionally of a bandwidth similar to the bandwidth of a voice channel of conventional bandwidth, such as voice channels 18-26 and 42. FIG 2 is a graphical representation of a single transmission channel wherein, similar to the graphical representation of FIG. 1, the ordinate axis, here axis 80, is scaled in terms of dBm, and the abscissa axis, here axis 84, is scaled in terms of kilohertz. (Again, the ordinate axis may be alternately scaled in terms of volts or watts.) Modulation spectrum 88 and carrier frequency 90 are representative of a single signal received by a receiver, such as the receiver portion of a radiotelephone, which has been down converted in frequency by down conversion circuitry as described hereinabove. Vertically-extending lines 92 and 96, shown in hatch in the figure, represent the cut-off frequencies of the receiver filter circuitry such as the filter circuitry of the receiver tuning circuitry. Although the signal is shifted downward in frequency by the receiver down conversion circuitry, as long as the bandwidth of the signal is within the bandwidth of the filter circuitry of the receiver, the shape of the modulation spectrum is the same as the shape of the modulation spectrum as transmitted over a transmission channel, such as any of the channels 18-42 of FIG. 1. As the receiver down conversion circuitry typically includes mixer circuitry, the center frequency, here represented by center frequency 90, of the down converted signal is dependent upon the frequency of the mixing signal supplied to the mixer circuitry. Control of the frequency of the mixing signal formed of oscillating signals generated by receiver oscillators is required to ensure that the center frequency of the resultant, down converted signal is within the passband of the filter circuitry of the receiver. FIG. 3 is a graphical representation wherein, again similar to the graphical representation of FIG. 2, the ordinate axis, here axis 100, is scaled in terms of dBm, and the abscissa axis, here axis 104, is scaled in terms of kilohertz. (Again, the ordinate axis could be alternately scaled in terms of volte or watts.) A signal, comprised of modulation spectrum 108 and carrier frequency 110 is similar to the corresponding signal of FIG. 2 and is representative of a signal which has been down- converted in frequency by down-conversion circuitry. Vertically extending lines 112 and 116, shown in hatch, are, similar to lines 92 and 96 of FIG. 2, representative of the cut-ofif frequencies of filter circuitry of a receiver. As illustrated, a portion of the signal is beyond the cut-off frequency of the filter. This is caused by frequency error of the oscillating frequency of the oscillators of the receiver circuitry. The frequency error causes the resultant, down-converted signal to be offset in frequency from the center frequency of the filter passband. Portions of signal 108 not within the passband of the filter are not passed by the filter, and the information-content of signal 108 is distorted as a portion of the signal is truncated. The passband amplitude and phase response of the filter also distorts the information content of the signal. The amount of distortion depends on the amount of frequency error in the local oscillator— the greater the frequency error, the greater is the distortion produced. To keep the distortion below a certain amount, the frequency error must be kept below a certain percentage of the size of the passband. When the voice channel bandwidth is reduced, less frequency error is tolerable to keep the distortion below acceptable limits because the passband must also be reduced. Because such frequency variation of the oscillators can result in signals, such as signal 108, being partially, or wholly, outside the passbands of filter circuitry of the receiver, to distort, or completely lose, the information-content of the signal, such variance in the oscillation frequency of the oscillators of the receivers must be minimized.

Operation of a frequency control system minimizes differences in frequency between the oscillating frequencies of oscillators of the receiver and the oscillating frequencies of signals generated by a remote transmitter (here a base station of a cellular, communication system). Because the oscillation frequency of the oscillators of a base station may more conveniently be controlled to prevent variances in the oscillating frequency thereof, such a frequency control system is oftentimes utilized to form a portion of many conventional radiotelephones to automatically control the frequencies of the oscillators of the radiotelephones.

Existing frequency control systems compare the IF signal frequency with a reference, generate a correction signal, and use the correction signal to correct the local oscillators in the receiver. The reference referred to above may be a circuit consisting of resonators having high frequency stability (such as quartz or ceramic resonators), or may consist of a frequency reference signal and a comparator.

In such systems, the IF signal from the limiter is also applied to a comparator. (Frequency dividers may be used in the IF signal path.) When a base station signal is present, the IF signal frequency is representative of the base station frequency, and the local oscillator frequency. Because the base station has very high frequency stability, most of the variation in IF signal frequency is due to variation (drift) in the local oscillator frequency, so that the correction signal varies in response to the local oscillator frequency, for the most part. When a base station signal is absent, the IF signal frequency is a random frequency determined by the upper and lower cutoff frequencies of the the IF passband filter. A randomly fluctuating correction signal is then present at the output of the comparator. If this signal were applied to the local oscillator to correct its frequency, the local oscillator frequency would randomly fluctuate. To prevent this, conventional frequency control systems have a provision to disable frequency control when there is no signal present. This is done by enabling frequency control only when the RSSI signal level is above a certain threshold, and disabling frequency control when the RSSI signal is below the threshold.

FIG. 4A is a graphical representation of a single transmission channel, here referred to by reference numeral 124. Similar to the graphical representations of FIGs. 1-3, the ordinate axis, here axis 128, is scaled in terms of dBm (or, alternately, volts or watts), and the abscissa axis, here axis 132, is scaled in terms of kilohertz. Vertically extending lines 136 and 140, shown in hatch, indicate the boundaries of transmission channel 124. Transmission channel 124 may form either a voice channel or a control channel, as the signal transmitted upon channel 124 may comprise either a voice signal or a control signal. Signal 124 is of a small magnitude, and is indicative of a small-strength signal.

Further illustrated in the figure is gaussian, or white, noise, indicated by line 148. The magnitude of the noise is greater than the magnitude of modulation spectrum 144 of the signal. Both the signal and the noise contained on transmission channel 124 are, however, down converted in frequency and passed by the filter circuitry of the receiver.

If the RSSI level of the received signal (the information signal summed together with the noise) is higher than the threshold level, the frequency control system of existing designs would be enabled. However, because the noise is masking the information signal, and the frequency detected by the frequency control system to be the carrier frequency of the received signal would actually be a randomly fluctuating frequency. Use of such a frequency to lock the oscillators of the radiotelephone thereto would similarly cause the radiotelephone oscillators to fluctuate randomly in frequency.

The graphical representation of FIG. 4B is similar to that of FIG. 4A and includes a similarly-numbered transmission channel 124 having boundaries indicated by lines 136 and 140. A signal comprised of modulation spectrum 144 and carrier frequency 146 is transmitted upon channel 124. In FIG. 4B, noise present upon the channel is not flat in frequency, but, as indicated by curve 154, is of a maximum at a frequency indicated by reference numeral 156. Again, if the RSSI level of the received signal is higher than a threshold level, the frequency control system of existing designs would be enabled. However, the frequency detected by the frequency control system to be the carrier frequency would not be at the true carrier frequency 146 of the information signal, but, rather, would be at the frequency indicated by reference numeral 156. Use of such a frequency to lock the oscillators of the radiotelephone thereto would similarly cause the radiotelephone oscillators to lock onto the incorrect frequency.

A similar situation would occur in the event that no information signal is received by the receiver, and only the noise, indicated in the figures by lines 148 and 156 is received.

FIG. 5 is a graphical representation of a single transmission channel 164 having boundaries thereof indicated by vertically extending lines 168 and 172 shown in hatch. Again, similar to the graphical representations of the preceding figures, the ordinate axis, here axis 176, is scaled in terms of dBm (again, the ordinate axis may alternately be scaled in terms of volts or watts), and the abscissa axis, here axis 180, is scaled in terms of kilohertz. Transmission channel 164 may be either a voice or a control channel, as a signal comprised of modulation spectrum 184 and carrier frequency 186 may comprise either a voice signal or control signal.

FIG. 5 additionally illustrates a second signal comprised of modulation spectrum 192 and center frequency 194 which is representative of a spurious signal. The maximum signal strength of the spurious signal is greater than the maximum signal strength of the desired information signal, and the maximum signal strength of the spurious signal is located at the spurious signal carrier frequency 194. When the spurious signal is of a power level great enough to cause the RSSI signal to be above the threshold level, the frequency detected by the frequency control system to be the carrier frequency would not be at the true carrier frequency 188 of the information signal, but, rather, would be at the spurious signal carrier frequency 194. FIG. 6A is a graphical representation wherein, again, similar to the graphical representations of the previous figures, the ordinate axis, here axis 200, is scaled in terms of dBm (again, the ordinate axis may be alternately scaled in terms of volts or watts), and the abscissa axis, here axis 202, is scaled in terms of kilohertz. Two adjacent transmission channels 204 and 208 are illustrated wherein channel 204 is of a frequency bandwidth having boundaries indicated by vertically extending lines 212 and 216, shown in hatch, and channel 208 is of a frequency bandwidth having boundaries indicated by vertically extending lines 216 and 220, shown in hatch. Transmission channels 204 and 208 may comprise either voice or control channels, as a signal comprised of modulation spectrum 224 and carrier frequency 226, transmitted upon channel 204, and a signal comprised of modulation spectrum 228 and carrier frequency 230, transmitted upon channel 208, may each comprise either voice or control signals. As illustrated, the signal transmitted upon channel 208 is much larger in magnitude than the signal transmitted upon channel 204. Because the magnitude of the signal transmitted upon channel 208 is so great relative to the magnitude to the signal transmitted upon channel 204, component portions of the larger signal, after down conversion to an intermediate frequency, are still present, although in distorted form, as indicated in the graphical representation of FIG. 6B. FIG. 6B is similar to that of FIG. 6A, but illustrates, in graphical form, signals down converted in frequency,, and filtered by filter circuitry of a radiotelephone receiver. The signal transmitted upon channel 204 is down converted, and passed in undistorted form, as indicated by modulation spectrum 224B and carrier frequency 226B. The much larger signal transmitted upon transmission channel 208 is down converted and partially passed, as indicated by modulation spectrum 228B and carrier frequency 230B. The signal is partially passed by the radiotelephone receiver filter circuitry as the filter circuitry has only finite rejection at the ad acent channel. When the passed signal is of a power level great enough to cause the RSSI signal to be above the threshold level, the frequency detected by the frequency control system to be the carrier frequency would not be at the true carrier frequency 226B of the information signal, but, rather, would be at the adjacent channel signal carrier frequency 230B.

The frequency control system of the present invention, by generating a frequency control signal only when a desired, information signal is detected, prevents an erroneous indication of the reference frequency of a received signal as described with reference to the diagrams of FIGs. 4-6. The presence of excessive noise (FIGS. 4A-4B), spurious signals (FIG. 5), and adjacent- channel interference (FIGS. 6A-6B) all prevent proper reception of a transmitted information signal. By disabling the frequency control system when an actual, desired, information signal is not detected by the receiver, the control system does not "lock" onto an incorrect frequency.

FIG. 7 is a simplified block diagram of the frequency control system of the present invention. A signal received by a receiver and down converted in frequency to an intermediate frequency level is supplied on line 304 to frequency comparator 306 and demodulator 308. Frequency comparator 306 compares the frequency of the signal supplied thereto on line 304 with the frequency of the oscillating signal supplied to frequency comparator 306 on line 310. Frequency comparator 306 generates a signal on line 312 responsive to such comparison.

Demodulator 308 demodulates the signal supplied thereto on line 304, and generates a demodulated signal on line 316. Line 316 is coupled to information signal detector 320 which is operative to determine times when the demodulated signal supplied thereto is a desired information signal. When information signal detector 320 detects the presence of such a desired signal, a signal is generated on line 324. Lines 324 and 312 are coupled to frequency control 332.

Frequency control 332 generates an output signal on line 338 which is supplied to oscillator 344 to vary the oscillation frequency of oscillator 344 responsive to the value of the signal transmitted thereto on line 338. Frequency control 332 generates a signal on line 338 to vary the oscillation frequency of oscillator 344 only during those times in which a signal on line 324 indicates the presence of an information signal of desired characteristics. Because frequency control 332 generates a signal on line 338 to alter the oscillating frequency of oscillator 344 only when an information signal of desired characteristics is supplied on line 304, distortion of, or the lack of presence of an information signal, prevents alterations of the oscillation .frequency of oscillator 344.

FIG. 8 is a block diagram of a transceiver, referred to generally by reference numeral 400, which incorporates the frequency control system of the present invention. More particularly, the block diagram of FIG. 8 illustrates a radiotelephone operable in a cellular, communication system of either conventional capacity or of increased capacity. It is to be noted, however, that the frequency control system of the present invention is operable in any of many other transceiver constructions, as well as receiver constructions (such as, for example, pager constructions).

Transceiver 400 includes antenna 404 for receiving radio- frequency signals transmitted thereto. Antenna 404 generates a signal on line 408 indicative of the signals received by antenna 404. line 408 is coupled to filter 412 which forms a passband to pass signals of desired frequencies, h a preferred embodiment of the present invention, the passband of filter 412 is between 843 and 870 MHz. Filter 412 generates a filtered signal on line 416 which is supplied to radio frequency amplifier/mixer 420. Amplifier/mixer 420, while preferably comprised of a single circuit, is diagramatically shown in the figure by amplifier 422 and mixer 424. Amplifier 422 amplifies the signal supplied thereto on line 416 and the amplified signal is supplied to mixer 424. An oscillating signal is further provided to mixer 424 on line 428 which is generated by oscillator 430. Mixer 424 mixes downwardly in frequency the signal amplified by amplifier 422 and generates a mixed signal on line 432. The frequency of the signal generated on line 432 is referred to as the "first IF" (for first intermediate frequency signal). The signal generated on line 432 is similar to the signal supplied on line 416, but is amplified and shifted downwardly in frequency. (It is noted that, alternately, circuit 420 may be comprised of only a mixer circuit, or an amplifier/filter/mixer-combination circuit.)

The signal generated on line 432 is supplied to filter 436 which generates a filtered signal on line 440. In a preferred embodiment of the present invention, filter 436 includes a passband centered about the frequency of 55 MHz. The filtered signal generated on line 440 is supplied to intermediate frequency amplifier 444 which generates an amplified signal on line 448. The amplified signal generated on line 448 is supplied to second mixer 452. Mixer 452 further receives an oscillating signal on line 456 generated by oscillator 460 which is, in turn, coupled to 2d LO oscillator synthesizer 462. In a preferred embodiment of the present invention, oscillator 460 generates an oscillating signal of an oscillation frequency of 54.540 MHz.

Mixer 452 generates a signal on line 464, which in the preferred embodiment is of a frequency of 460 kHz, and is referred to as the "2d IF" frequency. Line 464 is alternately coupled to filter 468 or filter 472. Filter 468 includes a passband of a conventional bandwidth (in the United States, 30 KHz and in Japan, 25 KHz), and filter 472 includes a passband of a reduced bandwidth (proposed in the United States to be 10 KHz, and proposed in Japan to be 12.5 KHz.). Filter 468 generates a filtered signal on line 476 when filter 468 is supplied with the signal generated on line 464. Similarly, filter 472 generates a filtered signal on line 480 when the signal generated on line 464 is supplied to filter 472. The filtered signal generated on line 476 or the filtered signal generated on line 480 is supplied to intermediate frequency amplifier 484. Amplifier 484 (which is preferably comprised of several cascaded stages) generates an amplified signal on line 488. Additionally, amplifier 484 includes circuitry to generate a signal on line 492 indicative of the magnitude of the signal supplied thereto. The signal generated on line 492 is referred to as a receive signal strength indicator (RSSI) signal.

The amplified signal generated on line 488 is supplied to filter 494, and a filtered signal generated thereat is generated on line 496. The filtered signal generated on line 496 is supplied to limiter 500 (which is preferably comprised of several cascaded stages) which generates a voltage limited signal on line 504. Additionally, and similar to amplifier 484, limiter 500 includes circuitry to generate a signal on line 506 indicative of the magnitude of the signal supplied thereto. The signal generated on line 506 is, similar to the signal generated on line 492, referred to as a receive signal strength indicator (RSSI) signal.

The signal generated on line 504 is supplied to FM demodulator 508, and, on line 510, to automatic frequency control (AFC) unit 512. AFC unit 512 is conventional in nature and is operative as described previously. FM demodulator 508 demodulates or recreates otherwise the information content of a signal supplied thereto on line 504. FM demodulator 508 generates a signal on line 516 which is supplied to receive (Rx) voice processor circuit 518 and to receive (Rx) data interface circuit 520.

The RSSI signals generated on lines 492 and 506 are supplied to RSSI circuit 522. RSSI circuit 522 generates a signal on line 526 responsive to values of the signals supplied thereto on lines 492 and 506. line 526 is coupled to A D converter 530 which converts the analog signal supplied thereto into a digital signal on line 534. Voice processor circuit 518 may, for example be comprised of a custom-designed integrated circuit, which performs functions such as deemphasis, expansion, volume control (and muting), etc. A signal output by voice processor circuit 518 on line 538 is supplied to earpiece 542. Data interface 520, which also may be comprised of a custom-designed integrated circuit, interfaces data signals supplied thereto on line 516 for input to processor 546 on line 550. Interface 520 is additionally coupled to bus 554 (as is, additionally, voice processing circuit 518). Data interface 520 performs functions such as SAT detection, DSAT conditioning, and eight kilobit per second data decoding.

Processor 546 receives input signals, not only on line 550, but also on line 534 (indicative of the RSSI signal level) and a signal generated by AFC unit 512 on line 558. Processor 546 is also coupled to bus 554.

Bus 554 interconnects processor 546 not only to processing circuit 518 and data interface circuit 520, but additionally to transmit (Tx) data interface circuit 562 and Transmit (Tx) voice processing circuit 566. Circuit 562 is analogous to circuit 520, but performs functions such as SAT signal generation, DSAT signal conditioning and eight kilobit per second data encoding. Tx data interface 562 is additionally coupled by line 568 to receive signals foπned by Rx data interface 520, and by line 569 from processor 546. Circuit 566 is analogous to circuit 518, but performs functions such as pre-emphasis, compression, maximum deviation limiting, and splatter filtering. Bus 554 is also coupled to display 570 and keypad 574 to permit manual input to processor 546 and to display information generated therefrom.

Processor 546 is additionally coupled to bus 576 to interconnect the processor with reference oscillator 580 and programmable synthesizer 584. When data interface circuit 520 detects the presence of valid information, here a SAT signal or 8 kilobit per second data, such detection is supplied to processor 546. A software algorithm embodied in processor 546 detects the presence of a DSAT signal. Responsive to detection of any of such signals, processor 546 generates signals on bus 576 to alter the oscillating frequency of reference oscillator 580 in a manner indicated by the signal generated by AFC unit 512 on line 558.

When valid information (here, namely, the SAT signal, 8 kilobit per second data, or the DSAT signal) is not detected by data interface circuit 520 or the software algorithm embodied in processor 546, processor 546 does not generate a signal on bus 576 to alter the oscillating frequency of oscillator 580. Because the oscillating frequency of reference oscillator 580 is not altered unless a voice or data signal is received by the receive portion of the radiotelephone, erroneous frequency alterations responsive to reception of undesired signals (such as noise, or any of the conditions described in connection with FIGs. 4-6) do not result.

Oscillator 580 generates an oscillating signal on line 588 which is supplied to 2d LO oscillator synthesizer 462, programmable synthesizer 584 and AFC unit 512. Changes in oscillating frequency of oscillator 580 cause, thereby, changes in oscillating frequencies of oscillator 460 and oscillator 430 (which is coupled to programmable synthesizer 584 by line 592). The oscillating signal generated by oscillator 580 and supplied to AFC unit 512 is compared with the signal supplied to AFC unit 512 on line 526 (division and comparison circuitry internal to unit 512 is not shown), and the signal generated on line 558 is indicative of such comparison as is conventional with AFC units.

FIG. 8 further illustrates a transmit portion of the radiotelephone. As the transmit portion is conventional in nature, a detailed description of operation thereof is omitted, but review of the figure shows the transmit portion to receive signals generated by Tx data interface circuit 562 and Tx voice processing circuit 566 on line 596 (such as from speaker 598). The transmit portion includes offset synthesizer 600, offset oscillator 604, offset mixer 608 (which is supplied an oscillating signal generated by oscillator 430 and amplified by amplifier 612), filter 616, exciter 620, amplifier 624, directional coupler 628, and filter 632 (filters 632 and 412 may together form a duplexer).

Output power of a signal generated by transceiver 400 is controlled by power control unit 636 which is provided input signals generated by RF detector 640 and by processor 546 on line 644. Offset synthesizer 600 is additionally coupled to line 588 to be supplied the oscillating signal generated by oscillator 580.

Turning now to the logical flow diagram of FIG. 9, the method steps of the method for maintaining a receiver oscillation frequency of a receiver unit in a desired frequency relationship with an oscillation frequency of a signal transmitted thereto are listed. First, and as indicated by block 700, the signal transmitted to the receiver unit is received. Next, and as indicated by block 704, times when the signal transmitted to the receiver unit is comprised of an information-containing signal of desired characteristics is detected. Next, and as indicated by block 708, the receiver oscillation frequency of the variable frequency oscillator is altered to maintain thereby the receiver oscillator frequency in the desired relationship with the oscillation frequency of the signal transmitted thereto.

While the present invention has been described in connection with the preferred embodiments shown in the various figures, it is to be understood that other similar embodiments may be used and modifications and additions may be made to the described embodiment for performing the same functions of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims. What is claimed is:

Claims

1. A frequency control system operable to maintain a receiver oscillation frequency of a receiver unit in a desired frequency relationship with an oscillation frequency of a signal transmitted thereto, said frequency control system comprising:

means, including at least one variable frequency oscillator oscillating at said receiver oscillation frequency, for receiving the signal transmitted to the receiver unit;

means forming a signal detector for detecting when the signal transmitted to, and received by, the receiver unit is comprised of an information-containing signal of desired characteristics;

means, operative responsive to detection by the signal detector of the information-∞ntaining signal of desired characteristics, for altering the receiver oscillation frequency of the variable frequency oscillator to maintain thereby the receiver oscillation frequency in said desired relationship with the oscillation frequency of the signal transmitted thereto.

2. The frequency control system of claim 1 wherein said information-containing signal of desired characteristics comprises a signal transmitted upon a control channel.

3. The frequency control system of claim 1 wherein said information-containing signal of desired characteristics comprises a signal transmitted upon a voice channel.

4. The frequency control system of claim 1 wherein said information-containing signal of desired characteristics comprises a frequency modulated signal.

5. The frequency control system of claim 1 wherein said means for detecting comprises an audio processor circuit.

6. The frequency control system of claim 1 wherein said means for altering comprises means for calculating a difference signal indicative of the frequency difference between the receiver oscillating frequency and the oscillation frequency of the signal transmitted thereto.

7. The frequency control system of claim 6 wherein said means for altering generates a frequency-correction, output signal.

8. The frequency control system of claim 7 wherein said frequency-correction, output signal is applied to the at least one variable frequency oscillator responsive to times when the signal detector detects the information-containing signal of desired characteristics.

9. The frequency control system of claim 1 wherein said means for receiving comprises means forming demodulation circuitry for demodulating the signal received thereat.

10. The frequency control system of claim 9 wherein said means for receiving comprises means for passing signals within either a first bandwidth or a second bandwidth of frequencies, respectively.