Abstract:
The circuit for controlling the oscillation frequency of an oscillation loop ( 66 ) has among others the following components—a first tunable capacitor unit ( 80 ) for providing a selectable amount of capacitance to the oscillator loop in accordance with a stored setting, and for controlling the oscillation frequency of the oscillator loop, and—a volatile storage unit ( 84 ) adaptated to store the setting of the tunable capacitor unit. The circuit further comprises a supply line ( 52 ) to the volatile storage unit and at least one other supply line ( 44 ) for the other components of said circuit. The supply line to the volatile storage unit is independent of said at least one other supply line, so that the volatile storage unit can be powered independently of other components of said circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a circuit, a method and a radio transceiver for controlling an oscillation loop, and a clock circuit and a mobile phone incorporating this circuit. 
     More precisely, the invention relates to a circuit for controlling the oscillation frequency of an oscillation loop, the circuit having among others the following components:
         a first tunable capacitor unit for supplying a selectable amount of capacitance to the oscillation loop in accordance with a stored setting, and for controlling the oscillation frequency of the oscillation loop, and   a volatile storage unit adaptated to store the setting of the tunable capacitor unit.       

     BACKGROUND OF THE INVENTION 
     Patent application US 2003/0132809 discloses a circuit for a real time clock of a computer. Typically, in the known circuit, all the components and in particular the volatile storage unit, are powered through a common supply line. 
     In situations where power consumption should be kept lowest possible, the power supply line of the circuit is cut off to save energy during the sleep mode. However, in the known circuit, the setting stored in the volatile storage unit is lost when the supply line is cut off. As a result, it is necessary to reload the setting in the volatile storage unit when the power is restored. Reloading the setting in the volatile storage unit each time the power is restored is a cumbersome process. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a circuit for controlling the oscillation frequency of an oscillation loop where reloading the setting in the volatile storage unit after a power cut is facilitated. 
     The invention provides a circuit for controlling the oscillation frequency of an oscillation loop wherein the circuit comprises a supply line to the volatile storage unit and at least another supply line to the other components of said circuit, the supply line to the volatile storage unit being independent of said at least one other supply line, so that the volatile storage unit can be powered independently of other components of said circuit. 
     In the above circuit, it is possible to cut off the supply line to the energy consuming components of the circuit while maintaining the power supply to the volatile storage unit. As a result, it is possible to reduce the power consumption of the circuit while keeping the setting stored in the volatile storage unit. Thereafter, when the power supply to all the components of the circuit is restored, it is not necessary to reload the setting in the volatile storage unit. This makes reloading of the setting after power cuts unnecessary after a sleep mode. 
     Furthermore, since it is not necessary to reload the setting in the volatile storage unit during the restoration of the power supply, the process is faster. 
     A circuit, wherein the supply line to the volatile storage unit supplies only power to the volatile storage unit, has the advantages of maximizing the saving of power consumption. 
     A circuit which comprises a second tunable capacitor unit that has a temperature-sensitive element to automatically reduce changes in the oscillation frequency of the oscillation loop resulting from temperature variations has the advantage that it reduces the sensitivity of the oscillation frequency of the oscillation loop to account for temperature variations. 
     A circuit which comprises:
         an automatic frequency control module ( 92 ) adapted to tune the oscillation frequency of the oscillation loop in accordance with the frequency of a received radio signal, and   a third tunable capacitor unit ( 90 ) adapted to supply a selectable amount of capacitance to the oscillation loop under the control of the automatic frequency control module,
 
and has the advantage of making the claimed circuit suitable for use with radio transceivers.
       

     The invention also relates to a clock circuit for a mobile phone comprising:
         an oscillation loop, and   a power supply unit,   wherein:   the clock circuit further comprises a circuit for controlling the oscillation frequency of the oscillation loop according to the invention, and   the power supply unit comprises at least two power outputs, one of which is connected to the supply line to the volatile storage unit and the other one is connected to the supply line to the other components of the circuit.       

     A circuit, wherein the power supply unit comprises a battery and a voltage regulator to supply a constant voltage on the output connected to the supply line to the volatile storage unit, has the advantage that it makes the clock circuit more reliable than other solutions based on a power storage capacitor. 
     A circuit where the oscillation loop comprises a crystal resonator, has the advantage that it makes the clock circuit more cost-effective to implement. 
     The invention also relates to a radio transceiver chip adapted to be used in a circuit as mentioned above, and a mobile phone comprising such a clock circuit. 
     The invention also relates to a method of controlling the oscillation frequency of an oscillation loop, the method comprising the steps of:
         tuning a tunable capacitor unit to supply a selectable amount of capacitance to the oscillation loop in accordance with a stored setting, to control the oscillation frequency of the oscillation loop, and   loading the setting of the tunable capacitor unit in a volatile storage unit,   wherein the method further comprises steps of cutting off the supply line to the other components needed to control the oscillation frequency of the oscillation loop and simultaneously continuing to supply power to the volatile storage unit simultaneously.       

     These and other aspects of the invention will be apparent from the following description, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a mobile phone comprising a circuit for controlling the oscillation frequency of an oscillation loop according to the invention; and 
         FIG. 2  is a flow chart of a method of controlling the oscillation frequency of an oscillation loop according to the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows part of a radio telecommunication apparatus  4 . By way of illustration, the radio telecommunication apparatus is a GSM radio cellular mobile phone  4 . Phone  4  is able to communicate with a base station  6  of a radio cellular phone network using radio signals  8 . To do so, phone  4  implements a TDMA (Time Division Multiple Access) technique. 
     Base station  6  is equipped with a transmitter and a receiver to transmit and receive radio signals  8  from phone  4 . Radio signals  8  are organized in frames of 1250 bits of information. To transmit or receive radio signals, base station  6  uses a reference frequency. 
       FIG. 1  shows only the details necessary to understand the invention. 
     To receive or transmit such radio signals, phone  4  comprises a tunable radio transceiver  14 , a controllable power supply unit  16 , and a baseband processor  18 . 
     Transceiver  14  is connected to an antenna  20  to receive and transmit radio signals. 
     Transceiver  14  is able to convert a received radio signal into a baseband signal and viceversa. In other words, the main task of transceiver  14  is to remove a carrier from the radio signal or to add such a carrier to a baseband signal. To realize such a conversion, transceiver  14  also uses a reference frequency closest possible to the reference frequency of base station  6 . 
     Baseband signals are exchanged between processor  18  and transceiver  14  through a line  22  connecting transceiver  14  to processor  18 . 
     For setting or tuning transceiver  14 , processor  18  is connected to transceiver  14  via a control bus  24 . For example, the bus  24  is a three-wire bus used to transmit control messages called “telegraphs”. Such telegraphs may be used to change a frequency channel of transceiver  14 . 
     To transmit control messages on bus  24 , processor  18  comprises a conventional transceiver control module  26 . 
     Processor  18  also comprises a power monitoring module  28  to control the power supply unit  16 . More particularly, module  28  is designed to automatically switch phone  4  to an idle mode when phone  4  is turned on but not in use. By idle mode is meant hereinafter a working mode in which power consumption of phone  4  is reduced as much as possible. Typically, during idle mode, phone  4  regularly shifts from a monitoring mode to a sleeping mode and back to the monitoring mode. During the monitoring mode, transceiver  14  is turned on to check if new messages sent by the base station  6  are to be received and processed by phone  4 . During the sleeping mode, transceiver  14  is turned off to save power. For example, phone  4  shifts from the sleeping mode to the monitoring mode every 470 ms with a monitoring duration of 36.9 ms at the most. 
     Phone  4  may also be manually reactivated or awakened from the idle mode by the user of phone  4  by pressing a key. 
     Power supply unit  16  is adapted to supply power to every electronic component of the phone  4  such as transceiver  14  and processor  18 . Typically, for mobile phones, power supply unit  16  comprises a rechargeable battery  40  connected to a voltage regulator  42 . Regulator  42  regulates the output voltage of the battery in order to generate a constant voltage of, for example, 2.8 Volts. Power supply unit  16  has a first power output  43 , which is connected to regulator  42  through a controllable switch  46 . Switch  46  is under the control of module  28 , so that a 2.8 Volts power supply can be switched on or off. The first power output is connected to an external power supply line  44 . 
     Power supply unit  16  also comprises another voltage regulator  50  and a second power output  51  connected to regulator  50  through a controllable switch  54 . Regulator  50  generates a lower constant voltage of, for example, 1.8 Volts. Switch  54  is under the control of module  28  to switch the 1.8 Volts power supply on or off. The second power output  51  is connected to an external supply line  52 . 
       FIG. 1  shows only the supply lines necessary for the understanding of the invention. 
     Power supply unit  16  may be manually turned on or off using an on/off button  60 . 
     To keep the reference frequency of transceiver  14  closest possible to the reference frequency of base station  6 , phone  4  comprises a tunable clock circuit  64 . Circuit  64  comprises an oscillation loop  66  to generate an oscillating signal and a circuit  68  for controlling the oscillation frequency of the oscillation loop. 
     Oscillation loop  66  comprises a 26 MHz crystal resonator  70  and an inverting amplifier  72 . Resonator  70  and amplifier  72  are connected in series to form a loop. 
     More precisely, a terminal  74  of resonator  70  is connected to an input of the inverting amplifier  72 , whereas another terminal  76  of resonator  70  is connected to an output of the inverting amplifier  72 . 
     Resonator  70  is a stand-alone component located outside the transceiver  14  packaging. 
     Circuit  68  comprises a digitally tunable capacitor unit  80  and a volatile storage unit  82  to store the setting of unit  80  to tune the oscillation frequency of loop  66 . 
     Unit  80  is, for example, a digitally tunable capacitor bank such as the one described in US 2003/0132809. One terminal of unit  80  is connected to ground and another terminal of unit  80  is directly connected to the output of the inverting amplifier  72  to provide a selectable amount of capacitance to oscillation loop  66 . 
     Unit  80  is powered through supply line  44 . 
     Volatile storage unit  82  stores the setting of capacitor unit  80 . More precisely, storage unit  82  latches a set of control signals on a bus  84 , which is connected to the digital control inputs of unit  80 . 
     For example, storage unit  82  is a set of latches like the ones described in US 2003/0132809; 
     Storage unit  82  is powered through supply line  52 , which is dedicated to this function. 
     Units  80  and  82  are on-chip units manufactured in the same chip as the one of transceiver  14 . The term “on-chip” means that these components are manufactured on a semiconductor chip. 
     Processor  18  comprises a setting module  86  to initialize or restore the setting in storage unit  82 . Setting module  86  is able to load a new setting in storage unit  82  each time the power supply on line  52  is restored. 
     Circuit  68  comprises a conventional tunable capacitor unit  90  associated with an automatic frequency control module  92  to keep the difference between the reference frequency of phone  4  and the reference frequency of base station  6  smallest possible. 
     Capacitor unit  90  is connected between terminal  74  and ground to provide a voltage controlled capacitance for the oscillator loop  66 . Unit  90  is, for example, a stand-alone component independent of the transceiver packaging. 
     Module  92  is designed to tune capacitor  90  according to information on the received radio signal  8  and on the present oscillation frequency of loop  66 . To receive information on the oscillation frequency of loop  66 , one input of module  92  is connected to the output of amplifier  72  through a buffer amplifier  94 . Amplifier  94  is manufactured on the chip of transceiver  14 . 
     The algorithm implemented in module  92  is conventional. For example, such an algorithm is described in: Francis D. Natali, “AFC Tracking Algorithms”, IEEE Transactions on Communications, Vol.Com-32, No 8, Aug. 1984, pages 935-947. 
     Module  92  is implemented in processor  18 . 
     Finally, to obviate oscillation frequency fluctuations due to temperature variations, circuit  68  comprises a temperature-variable capacitor unit  100  built with a temperature-sensitive element  102 . Capacitor unit  100  is connected in series in loop  66  between the output of amplifier  72  and the terminal  76 . 
     Temperature-sensitive element  102  is used to automatically compensate for oscillation frequency fluctuations due to temperature variations. For example, temperature-sensitive element  102  is a group of thermo-resistors. 
     Unit  100  is a stand-alone component. 
     The operation of phone  4  will now be described with reference to  FIG. 2 . 
     Initially, phone  4  is turned off and none of the components of phone  4  is supplied with power. Thus, the setting stored in storage unit  82  has been lost. 
     Thereafter, the user manually switches phone  4  on, using button  60 . Processor  18  starts with an initialization stage  110  during which module  86  loads a new setting in storage unit  82 . 
     Subsequently, processor  18  shifts to an active mode  112  during which every component is powered and ready for use. More precisely, during the active mode  112 , switches  46  and  54  are closed, so that loop  66  and circuit  68  are powered. 
     In step  114 , when circuit  68  is powered, the capacitance of unit  80  is selected according to the setting stored in storage unit  82  during the initialization stage. Therefore, oscillation loop  66  generates an oscillating signal according to the selected capacitance. 
     In step  116 , still during the active mode  112 , module  92  tunes capacitor unit  90  to keep the difference between the reference frequencies of phone  4  and base station  6  smallest possible. 
     In parallel, in a step  118  the oscillation frequency fluctuations of loop  66  are automatically compensated for by capacitor unit  100  and its associated temperature-sensitive element  102 . 
     In the active mode  112 , the user uses phone  4 , for example, to make a call, to play or to manually update the phone configuration. During the active mode, the power consumption of clock circuit  64  is at a maximum since all of its components are powered. 
     To reduce power consumption, processor  18  automatically switches into an idle mode  120  if the user does not use phone  4  for a predetermined period of time. During the idle mode  120 , processor  18  regularly alternates between a monitoring mode  122 , and a sleeping mode  124 . 
     During the monitoring mode  122 , in a step  126 , module  28  switches off or maintains the power supply to every component of phone  4  except the ones necessary to check if new radio signals are to be received. More particularly, during step  126 , module  28  closes switch  46  and keeps switch  54  closed, so that the clock circuit  64  is fully powered and is able to generate the reference frequency necessary to receive radio signals. 
     In a step  128 , if new radio signals are to be received, like in a telephone call, processor  18  automatically returns to the active mode  112 . Otherwise, module  28  proceeds to the sleeping mode  124 . 
     In the sleeping mode, in a step  130 , module  28  opens switch  46  to reduce the power consumption of clock circuit  64 . 
     However, in parallel, in a step  132 , module  28  keeps switch  54  closed so that storage unit  82  is the only component of clock circuit  64 , that remains powered. As a result, the setting stored in storage unit  82  is preserved while the power consumption of clock circuit  64  is reduced considerably. 
     Since the setting in storage unit  82  is not lost even during the sleeping mode, when returning to the monitoring mode or to the active mode, it is not necessary to reload the setting in storage unit  82 . Therefore, the transition from the sleeping mode to the monitoring mode or the transition from the sleeping mode to the active mode is faster. 
     The use of a lower voltage to power storage unit  82  reduces the power consumption. 
     Furthermore, the use of a constant voltage to power storage unit  82  makes the circuit  68  more reliable. In another embodiment, the or a storage unit is powered by a capacitor, which is charged during the monitoring mode and discharged during the sleep mode. In this embodiment, regulator  50  is no longer necessary. However, a supplementary supply line is required to charge the capacitor. 
     Circuit  68  has been described in the particular case of a clock circuit for a mobile phone. However, circuit  68  can be used with other devices and pieces of equipment where saving power consumption is important. As an example, circuit  68  could be used in a computer. For such an application, capacitor unit  90  and module  92  are not needed. 
     Finally, if temperature compensation is not necessary, capacitor unit  100  may be omitted.