Abstract:
A phase locked loop comprising: an oscillator for generating an output signal of a frequency that is dependent on an input to the oscillator; sampling means for generating a sequence of digital values representing the output of the oscillator at moments synchronized with a reference frequency; a difference unit for generating a feedback signal representing the difference between successive values in the sequence; and an integrator for integrating the difference between the feedback signal and a signal of a desired output frequency; the signal input to the oscillator being dependent on the output of the integrator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/EP2008/060847 filed Aug. 19, 2008, claiming priority based on English Patent Application No. 0717883.3, filed Sep. 13, 2007, the contents of all of which are incorporated herein by reference in their entirety. 
     This invention relates to a phase locked loop. Embodiments of the phase locked loop can be implemented efficiently using digital components to provide at least some functions in the loop. 
     BACKGROUND OF THE INVENTION 
     Many transmitters, such as early Bluetooth on-chip transmitter designs, were based on a “Cartesian Loop” transmitter/receiver design. However, this design has a number of drawbacks due to the relatively high content of analogue circuitry that they contain. As silicon processes shrink and are further optimised for digital circuitry, these drawbacks increase. 
     A more recent transmitter design is the polar transmitter, as shown in  FIG. 1 . The input signals are encoded to produce two output signals, one (θ) conveying frequency information and the other (r) conveying amplitude information. The frequency information signal is fed into a phase locked loop (PLL) which controls the generation of a signal of a corresponding frequency by an oscillator and the amplitude information signal is fed into an amplitude modulator (AM). The output of the oscillator is then amplitude modulated by the signal r. 
     The PLL typically consists of the following blocks
         An oscillator whose frequency is controlled by a signal of some sort. The oscillator is typically a voltage controlled oscillator, whose output frequency is controlled by an input voltage.   A phase comparator, to compare the phase of the VCO output—perhaps after division—with an input reference frequency. If the phase of an output related signal is locked to a reference, its differential (its frequency) is ipso facto locked.   A loop filter. For the overall loop to be stable and usable, a loop filter is implemented.       

     In analogue PLLs the phase comparator is difficult to design so as to meet the desired tolerances whilst maintaining acceptable power consumption. 
     Particularly because of the way processes are optimised, it is attractive that both the PLL and the AM are implemented in digital form, and are controllable using digital input signals. However, it has proved difficult to implement digital forms of these components in practice. 
     A Texas Instruments digital PLL design (“All-digital Tx frequency synthesizer and discrete-time receiver for Bluetooth radio in 130-nm CMOS”, Staszewski, Muhammad, Leipold, Hung Ho, Wallberg, Fernando, Maggio et al.) provides a VCO that is controlled by a digital word (a very wide digital word), and a phase detection system that achieves very fine phase resolution. However, this circuit is relatively complex. 
     There is therefore a need for a PLL that is more readily implemented and that has relatively low power consumption. Preferably such a PLL could be readily integrated with an amplitude modulator, so that the amplitude can be controlled using a digital input. 
     According to one aspect of the present invention there is provided a phase locked loop and an amplitude modulator as set out in the accompanying claims. 
     SUMMARY OF THE INVENTION 
     The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a transmitter using polar modulation; 
         FIG. 2  shows a phase locked loop; and 
         FIG. 3  shows an amplitude modulator. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The loop shown in  FIG. 2  comprises a series of integrators  1 ,  2  and a filter  3  which collectively perform in the digital domain functions equivalent to the phase detector, charge pump and loop filter of a conventional PLL. A signal at a wanted frequency is received at  4  and a loop feedback signal is provided at  5 . These are weighted by selected values a and d in weighting units  6  and  7 , and difference between the resulting signals is determined by a difference unit  8 . The output of the difference unit is integrated by an integration unit  1 , which acts as a second-order loop filter. The integration may be performed by having the integration unit output the value:
 
z −1 /(1−z −1 )
 
where z −1  represents the data word applied to the integration unit on the previous clock cycle. A summation unit  11  adds the output of the integrator  1  to two further inputs formed by weighting the signals at  4  and  5  by respective selected values b and e in weighting units  9  and  10 . The output of the summation unit  11  is passed to a second integrator  2 . The integrator  2  could apply the same function as the integrator  1 . An effect of the second integrator is to provide loop gain. A second summation unit  14  adds the output of the integrator  2  to two further inputs formed by weighting the signals at  4  and  5  by respective selected values c and f in weighting units  12  and  13 . The output of the summation unit  14  is filtered by a low-pass filter  3 . The low-pass filtering may be performed by having the low-pass filter output the value:
 
α/(1−(1−α)z −1 )
 
where α is a filter constant and z −1  represents the data word applied to the filter unit on the previous clock cycle. An effect of the low-pass filter is to attenuate out-of-band information. The roll-off of the filter can be at around 4 times the desired bandwidth.
 
     Since the loop filter is implemented in the digital domain it can cover a wide range of frequencies and can be programmable, for example in at least the range from mHz to MHz. Also, the filter can be implemented to perfectly reflect a desired response characteristic, without any variations due to manufacturing tolerances. These are significant advantages over PLLs in which the loop filter is implemented in the analogue domain. 
     The output of the filter  3  is multiplied in multiplication unit  16  with a signal of a selected value k representing a trimming signal for trimming the VCO  21 . 
     The values a, b, c, d, e f, k and α are selected to provide the PLL with satisfactorily stable and effective characteristics, and can be determined using conventional theory. Some examples of suitable values for a, b, c, d and α with corresponding approximate loop bandwidths are as follows: 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 approx loop 
               
               
                 alpha 
                 a 
                 b 
                 c 
                 d 
                 e 
                 BW (MHz) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.25 
                 0.0062500 
                 0.0491411 
                 0.7311611 
                 −0.0062500 
                 −0.0833333 
                 2 
               
               
                 0.125 
                 0.0015625 
                 0.0245706 
                 0.7311611 
                 −0.0015625 
                 −0.0416667 
                 1 
               
               
                 0.0625 
                 0.0003906 
                 0.0122853 
                 0.7311611 
                 −0.0003906 
                 −0.0208333 
                 0.5 
               
               
                 0.03125 
                 0.0000977 
                 0.0061426 
                 0.7311611 
                 −0.0000977 
                 −0.0104167 
                 0.25 
               
               
                 0.015625 
                 0.0000244 
                 0.0030713 
                 0.7311611 
                 −0.0000244 
                 −0.0052083 
                 0.125 
               
               
                   
               
             
          
         
       
     
     The signals passing between units  1 ,  11 ,  2 ,  14 ,  3  and  16  are of a sufficient width to preserve enough information to maintain satisfactory operation of the loop. Due to the nature of the operations performed in these steps those signals will typically contain more bits than the input signals  4 ,  5 . Around 20 bits has been found to be suitable. To simplify subsequent operations in the loop, the output of the multiplication unit  16  is truncated by a truncation unit  17  to reduce its bit length whilst retaining the most significant information in the signal. The truncation unit could simply discard the less significant bits of the signal, or could apply a more complex function. The output of the truncation unit could, for instance, be around 2 to 8 bits wide. 
     The output of the truncation unit is passed to a digital-to-analogue converter  18 , which generates an output signal that is representative of its input. The output of the DAC is passed to a low-pass filter  20 , which attenuates any extraneous high-frequency components. The roll-off of the filter  20  could be at around 8 times the loop bandwidth. The output of the filter  20  is passed to the control input of the VCO  21 , so that the voltage level of the filter&#39;s output controls the operating frequency of the VCO. The VCO may include one or more dividers to achieve the desired output frequency from a core oscillator. 
     The components  20  and  21  operate in the analogue domain as indicated at  19 . The remainder of the components of the PLL operate in the digital domain. It has been found that implementing all the stages of the PLL in the digital domain apart from the final control of the VCO, in this way, provides a particularly efficient implementation of a PLL. 
     The output of the VCO could be used for any appropriate purpose. For example, it could be mixed with data signals that are to be transmitted in order to up-convert those signals, or with received data signals in order to down-convert them. In the present example the output of the VCO is used in a polar transmitter of the type shown generally in  FIG. 1 . 
     The output of the VCO is split and passes to two delay blocks  23  and  24  which apply a respective delay to the signal. The delay applied by each delay block is dependent on control signals  29 ,  30 . In the polar transmitter design, the delay applied by block  23  relative to that applied by block  24  is used to set the output amplitude, as is described below. The relative delays are selected to cause the outputs to collectively represent the signal r illustrated in  FIG. 1  when they are modulated. Outputs  31 ,  32  from the delay blocks pass to an amplitude modulation stage, which is described below with reference to  FIG. 3 . To achieve this, the delay blocks implement respective delays that are offset by equal but opposite amounts from two pre-set values in dependence on the desired amplitude, the amplitude modulation being achieved as the difference between the signals. For example, delay block  23  could implement a delay in the range from 110 to 290 ps, being 110 ps plus the currently set delay offset, and delay block  24  could implement a delay in the range from 290 to 110 ps, being 290 ps minus the currently set delay offset. The delay blocks could be implemented in the analogue domain. 
     The outputs of the delay blocks also pass to a frequency detection section implemented in the digital domain by blocks  25 - 28 ,  33  and  34 . This performs a similar task to the phase detection system of a conventional PLL. 
     Each of the outputs from the delay blocks passes to a respective Gray coded counter  33 ,  34 . These count up independently, clocked by a reference frequency received at  35 . Thus, if the reference frequency is f 1  and the frequency of the VCO is f 0 , the value held by of the counter will increase in each clock cycle by (f 0 /f 1 ), moderated by the delay applied in blocks  23  and  24 . Thus, the value in each counter represents the phase offset between the reference frequency and the effective frequency of the VCO as input to the respective counter. In practice, the counter has a finite length so roll-over of the counter must also be accounted for. This is described in more detail below. 
     The values held in the counters are each sampled by respective latches  25 ,  26 , the outputs of which are passed to a summation unit. The summation unit generates two outputs. A first output at  36  is formed as the sum of the inputs from latches  25  and  26 . A second output at  37  is formed as the difference between the inputs from latches  25  and  26 . The value of output  36  is used in the PLL. The value of output  37  is used as feedback for an amplitude control loop of the transmitter. 
     Output  36  passes to a difference unit  28 . The difference unit  28  outputs a value representing the difference between the data words applied to it in successive clock cycles, i.e. the value:
 
z 0 −z −1  
 
where z 0  represents the input to the unit in the present clock cycle and z −1  represents the input to the unit in the previous clock cycle.
 
     The width of the data words passing between the units  23 ,  33 ,  25 ,  27 ,  24 ,  34 ,  26  and  28 , and at the output of unit  28  can take any suitable size, but are preferably as small as possible whilst still achieving the required precision. They could, for example, be 2, 3 or 4 bits wide. 
     As discussed above, since the counters  33 ,  34  are clocked by the reference frequency, the values held by the counters will represent the offset between the phase of the signals output from the delay units and the reference frequency. When those are summed in summation unit  27  the effect of the delay units is in effect cancelled. When successive samples are subtracted from each other by difference unit  28  the output is dependent on the relative phase of the reference frequency and the VCO output frequency, but the output of the difference unit represents a frequency, rather than a phase as would be the case at the analogous point in a conventional PLL. This frequency forms the feedback frequency signal at  5 . By integrating that signal as described above, the phase data is recovered. 
     As indicated above, the counters  23  and  24  are of finite length and will therefore roll over when they reach their maximum values. This needs to be accommodated by the system. A convenient way to do so is by means of analysis units  38 ,  39 . These monitor the inputs from the respective latches and detect from those inputs when the counters have rolled over. Provided the counter is of a length greater than the maximum count increment in a particular clock cycle due to a phase disparity roll-over can be detected by the latch outputs moving from the upper end of their range to the lower end of their range between two clock cycles. When that is detected the analysis unit(s) signal(s) the difference unit  28  to treat the sample on which rollover has occurred differently when comparing it with the preceding sample than when comparing with the succeeding sample. This may be implemented by means of a look-up table in the difference unit  28 . It should be noted that the difference unit runs at the reference frequency, not the frequency of the VCO, allowing it to use less power in a typical implementation if the reference frequency is lower than that of the VCO. 
     The circuit can be further simplified by eliminating the counters  33 ,  34  (and hence the analysis units  38 ,  39 ) and having the latches simply sample the outputs from the delay units  23 ,  24 . Provided the range over which the frequency of the VCO can vary is within the resolution obtainable by simply comparing successive samples from the latches, this method achieves equivalent results with a relatively simple circuit, saving power and circuit area. 
     Due to the way in which the reference signal  35  and the output of the VCO are compared, the reference signal need not have the same frequency as the VCO: indeed, it could be of an unrelated frequency. For example, the reference signal could be around 200 MHz when the VCO is operating at around 2.5 GHz. This makes the implementation of the circuit especially convenient. Since there is no need to provide a reference frequency of a particular value, a suitable signal that may exist elsewhere in the circuit could be used to provide the reference frequency. Since the reference frequency can be selected to be relatively low, the power consumption of the frequency detection circuitry can be kept relatively low. The reference frequency could even be modulated provided the modulation is known, since it can then be accounted for digitally in the frequency detection section. 
       FIG. 3  shows a modulation stage that can be used for amplitude modulating the outputs  31 ,  32 . One pair of transistor amplifiers receives signal  31  and its inverse,  31 ′. Another pair of transistor amplifiers receives signal  32  and its inverse  32 ′. These are combined in a balun and output, for example to an antenna for transmission. 
     The oscillator could operate in the digital domain, in which case there would be no need to convert its output to the digital domain for use in the frequency detection section. However, if the oscillator is an analogue oscillator then its output is converted to the digital domain, conveniently after the delay units  23 ,  24 . 
     The oscillator could be controlled other than by voltage, with appropriate variation of its input signal. 
     The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.