This application is the National Stage of International Application No. PCT/EP02/05722, International Filing Date, May 24, 2002, which designated the United States of America, and which international application was published under PCT Article 21(2) as WO Publication No. WO 03/100978 A1. 
   FIELD OF THE INVENTION 
   The present invention relates to a phase locked loop circuit. 
   BACKGROUND TO THE INVENTION 
   The present trend in portable communications devices such as mobile telephones is to increasingly lightweight devices with increased talk-time between battery recharge cycles. Such developments require ever more efficient radio-frequency (RF) amplifiers to minimise power consumption. In cellular systems such as GSM, the modulation scheme is a constant amplitude scheme, also referred to as constant envelope modulation, which permits use of efficient non-linear amplifiers. However, recent types of communication system such as EDGE and UMTS use non-constant envelope modulation schemes. The drawback is that the amplification of non-constant envelope RF signals requires the use of linear power amplifiers, which are inherently less efficient. The lower power efficiency of linear amplifiers translates into higher power consumption and higher heat dissipation. 
   A variety of linearisation architectures and schemes exist, including fixed and adaptive pre-distortion, adaptive bias, envelope elimination and restoration, polar loop and Cartesian loop transmitters. Details of such devices are shown in “Increasing Talk-Time with Efficient Linear PA&#39;s”, IEE Seminar on TETRA Market and Technology Developments, Mann S, Beach M, Warr P and McGeehan J, Institution of Electrical Engineers, 2000, which is incorporated herein by reference. However, many of these devices and techniques are unsuitable for battery operated portable devices such as mobile telephones, or are incapable of meeting current RF design standards, such as the TETRA linearity standard, ETSI publication ETS 300 396-2; “Trans-European Trunked Radio (TETRA);—Voice plus Data (V+D)—Part 2: Air Interface (AI)”; March 1996. 
   Envelope elimination and restoration (EER) transmitters separate envelope and phase information from an input modulated signal. The phase information is then passed through a power amplifier as a constant envelope signal, permitting the use of efficient, non-linear amplifiers, while the envelope signal is added to the power amplifier output. In order to correct AM-PM distortion, phase feedback is employed and the power amplifier is effectively placed within phase-locked loop. 
   A problem arises in the spurious emissions are generated by the power amplifier in the period before the loop is locked at the beginning of a transmission. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided a phase-locked loop circuit comprising an oscillator controlled in dependence on the output of a phase detector, an output amplifier for amplifying the output of the oscillator and a feedback path to the phase detector from the output of the output amplifier, characterised by a second feedback path from the output of the oscillator to the phase detector, by-passing the output amplifier, and control means for disabling the output amplifier when the loop circuit is not locked and interrupting the second feedback path when the loop circuit has become locked. 
   Thus, unwanted components in the output of the output amplifier can be avoided by ensuring that the amplifier does not become active until the loop has achieved lock. 
   Preferably, the second feedback path includes a variable gain amplifier, the control means being configured to interrupt the second feedback path by reducing the gain of the variable gain amplifier. More preferably, the control means is configured to interrupt the second feedback path by ramping down the gain of the variable gain amplifier. Still more preferably, the control means is configured to ramp up the gain of output amplifier on enabling thereof, the ramping down of the gain of the variable gain amplifier overlapping the ramping up of the gain of output amplifier. Phase control means may be included for matching the phase of the output of the variable gain amplifier to that of the output of the output amplifier when both are operating. Such phase control means may comprise a variable delay in the second feedback path, a phase detector receiving a signal from the second feedback path downstream of the variable delay and a signal from the first feedback path and a low-pass filter for filtering the output of the phase detector to provide a delay control input signal for the variable delay. 
   Preferably, the first and second feedback paths share a common portion. More preferably, the first and second feedback paths are united by a summer and/or the common portion includes a frequency down converter. 
   A circuit according to the present invention may be advantageously employed in an envelope elimination and restoration transmitter such that the first feedback path provides a feedback signal for a closed loop envelope restoration circuit and the control means includes an envelope controller for controlling the gain of the output amplifier. 
   A circuit according to the present invention may be advantageously employed in a mobile phone such that the output amplifier is an RF power amplifier. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a mobile telephone handset; 
       FIG. 2  is a schematic diagram of mobile telephone circuitry for use in the telephone handset of  FIG. 1 ; 
       FIG. 3  is a block diagram of a first embodiment of the present invention; 
       FIG. 4  illustrate the initiation of a transmission by the embodiment shown in  FIG. 3 ; and 
       FIG. 5  is a block diagram of a second embodiment of the present invention. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the invention will now be described by way of example with reference to the accompanying drawings. 
   Referring to  FIG. 1 , a mobile station in the form of a mobile telephone handset  1  includes a microphone  2 , keypad  3 , with soft keys  4  which can be programmed to perform different functions, an LCD display  5 , a speaker  6  and an antenna  7  which is contained within the housing. 
   The mobile station  1  is operable to communicate through cellular radio links with individual public land mobile networks (PLMNs) operating according to communication schemes such as UMTS and EDGE. 
     FIG. 2  illustrates the major circuit components of the telephone handset  1 . Signal processing is carried out under the control of a digital micro-controller  9  which has an associated flash memory  10 . Electrical analogue audio signals are produced by microphone  2  and amplified by pre-amplifier  11 . Similarly, analogue audio signals are fed to the speaker  6  through an amplifier  12 . The micro-controller  9  receives instruction signals from the keypad and soft keys  3 ,  4  and controls operation of the LCD display  5 . 
   Information concerning the identity of the user is held on a smart card  13  in the form of a GSM SIM card which contains the usual GSM international mobile subscriber identity (IMSI) and an encryption key K i  that is used for encoding the radio transmission in a manner well known per se. The SIM card is removably received in a SIM card reader  14 . 
   The mobile telephone circuitry includes a codec  15  and an rf stage  16  including a power amplifier stage  17  feeding the antenna  7 . The codec  15  receives analogue signals from the microphone amplifier  11 , digitises them into an appropriate signal format and feeds them to the power amplifier stage  17  in the rf stage  16  for transmission through the antenna  7  to the PLMN shown in  FIG. 1 . Similarly, signals received from the PLMN are fed through the antenna  7  to be demodulated in the rf stage  16  and fed to codec  15 , so as to produce analogue signals fed to the amplifier  12  and speaker  6 . 
   Referring to  FIG. 3 , the power amplifier stage  17  comprises an envelope elimination and restoration (EER) transmitter  18  which separates the envelope and phase components of an input modulated IF signal into two separate forward paths  19 ,  20 . A common feedback path  21  is used for control of both the envelope and phase components of the RF output by the amplifier stage  17 . 
   The envelope forward path  19  comprises first and second envelope detectors  22 ,  23  which detect the envelopes of the input modulated IF signal and the feedback signal from the feedback path  21  respectively. The outputs of the envelope detectors  22 ,  23  are fed to respective inputs of a comparator  24 . The output of the comparator  24  is filtered by a low-pass filter  25  and applied to an envelope controller  26 . 
   The envelope controller  26  comprises a fast power supply modulator which directly modulates the supply voltage of the power amplifier  27  itself. 
   The phase forward path  20  comprises first and second limiters  28 ,  29  which limit the input modulated IF signal and the feedback signal respectively to produce respective constant-amplitude signals. The constant amplitude signals are applied to a phase detector  30  and the output of the phase detector  30  is filtered by a low-pass filter  31  and applied to a voltage-controlled oscillator  32  as is conventional in a phase-lock loop. The RF signal produced by the voltage-controlled oscillator  32  is input into the power amplifier  27  which amplifies it in dependence on the signal input to the envelope controller  26 . 
   The output of the voltage-controlled oscillator  32  is also fed to a variable gain amplifier  33  which forms a branch of the common feedback path  21 . A summer  34  receives the output of the variable gain amplifier  33  and the power amplifier  27  on respective inputs. The output of the summer  34  is connected to one input of a mixer  35 . The other input of the mixer  35  receives a local oscillator signal. The output of the mixer is low-pass filtered by a feedback path filter  36  to select a low frequency mixing product. Thus, the mixer  35  and filter  36  act to down convert the RF output of the amplifier  27  to the IF signal frequency. 
   The output of the feedback path filter  36  is fed to the inputs of the second envelope detector  23  and the second limited  29  to complete the feedback paths of the envelope and phase control loops. 
   A lock detector  40  is provided to detect when the phase locked loop is locked. The lock detector  40  provides a control signal to the envelope controller  26  which disables the power amplifier  27  when the loop is not locked and enables the power amplifier  27  when the loop is locked. 
   The output of the lock detector  40  is also input into an amplifier control circuit  41  which produces a gain control signal for the variable gain amplifier  33 . When the loop is not locked, the amplifier control circuit  41  outputs a circuit that keeps the variable gain amplifier&#39;s gain at a maximum. However, when lock is achieved and the output of the lock detector  40  changes state, the amplifier control circuit  41  outputs a signal that causes the gain of the variable gain amplifier  33  to decay to zero. 
   Referring to  FIG. 4 , when a transmission is not being made, the power amplifier  27  is disabled and the voltage-controlled oscillator  32  runs freely, although it could also be disabled in the absence of an input IF signal. 
   When an IF signal is initially input, the voltage-controlled oscillator  32  is locked to the input IF signal by the action of the loop comprising the voltage-controlled oscillator  32 , the variable gain amplifier  33 , the mixer  35 , the feedback path filter  36 , the second limiter  29 , the phase detector  30  and the low-pass filter  31 . Since the power amplifier  27  is not producing an output, the summer  34  can be disregarded. During this period the frequency and phase of the voltage-controlled oscillator  32  vary until lock is achieved. Once lock has been achieved and the frequency and phase of the voltage-controlled oscillator  32  have stabilised, the output of the lock detector  40  changes state and the power amplifier  27  is enabled. The gain of the power amplifier  27  is ramped up to avoid sharp transitions in the output RF signal. While the gain of the power amplifier  27  is ramping up, the gain of the variable gain amplifier is ramped down by the amplifier control circuit  41  so that the feedback signal becomes dominated by the output of the power amplifier  27  and then completely dependent on the output of the power amplifier  27 . 
   The gain of the variable gain amplifier  33  is ramped down while the gain of the power amplifier  27  is ramping up to ensure that the amplitude of feedback signal is always sufficient to be limited by the second limiter  29 . 
   Referring to  FIG. 5 , a second embodiment is substantially the same as the first embodiment, described above, except that a delay locked loop is added to the control the phase of the signal input to the variable gain amplifier  33 . The delay locked loop is used so that during the transition from control on the basis of the voltage-controlled oscillator output to control on the basis of the power amplifier output, the output from the variable gain amplifier  33  does not tend to cancel the feedback from the power amplifier  27  due to a significant phase difference between the signals. 
   The delay locked loop comprises a voltage-controlled delay  37  for controllably delaying the output of the voltage-controlled oscillator  32  input into the variable gain amplifier  33 , a phase detector  38  connected to receive the input to the variable gain amplifier  33  and the output of the power amplifier  27  as its inputs and a low-pass filter  39  for filtering the output of the phase detector  38  to provide a control signal for the voltage-controlled delay  37 . 
   In another embodiment, the phase detector of the delay locked loop receives the outputs of the variable gain amplifier and the power amplifier as its inputs.
A phase-locked loop circuit has an output amplifier ( 27) and a main feedback path from the output of the output amplifier ( 27). A subsidiary feedback path is provided directly from the output of the circuit's VCO ( 32). At the start of operation, the output amplifier ( 27) is disabled and the subsidiary feedback path is used until lock is achieved. Then the output amplifier ( 27) is enabled and the main feedback loop is used. This avoids spurious outputs from the outer amplifier while to loop locks.