Abstract:
A controllable current source circuit and a phase locked loop contained such a circuit are disclosed. The current source circuit such a single switched driver stage for a switched actuation of the loop filter. A continuously switched-on driver stage is introduced into one of several paths of the current source circuit, so that a continuously weaker current is drawn by the loop filter. The currents are preferably stabilized via a current mirror circuit.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a current source circuit and phase locked loop equipped with such a circuit, by means of which the phase angle of an output signal is controlled with respect to the phase angle of a reference signal, if required with a desired phase offset. 
     The general design of such phase locked loops is known from “IEEE Transactions on Communications”, Vol. COM-28, No. 11, November 1980: “Charge-Pump Phase-Lock Loops” by F. M. Gardner, pages 1849 to 1858, where a current source circuit (charge pump) is connected between the outputs of a phase detector and a loop filter which controls an oscillator and contains a capacitance, and this current source circuit feeds the loop filter with current or draws current from the loop filter depending on the respective phase comparison result. 
     The general design of such phase locked loops is also known from “IEEE Journal of Solid-State Circuits”: “A 2.7-V GSM RF Transceiver IC”, Yamawaki et al, Vol. 32, No. Dec. 12, 1997, pages 2089-2096, in which the output signal of the oscillator whose phase is to be controlled is applied, after mixing with the local signal, to the phase comparator, to whose other input a reference signal in the form of an intermediate frequency signal is applied. A current source circuit is connected between the output of the phase comparator and the loop filter which actuates the oscillator, and this current source circuit contains transistor switches in the same way as in the document already discussed, which switch the current supply of the loop filter as a function of the respective phase angle, so that the loop filter capacitance is either charged or discharged. However, the switching of the driver transistors (charge pumps) is in each case associated with switching interference signals so that the overall signal-to-noise ratio deteriorates owing to the switching processes carried out by the driver transistors. In addition, the required circuit complexity is rather high owing to the switching transistors and their circuitry. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of providing a current source circuit, in particular for a phase locked loop, in which the number of switching processes is reduced, and the signal-to-noise ratio is thus improved. 
     This object is achieved by the features cited in patent claim 1. Furthermore, a phase comparator is provided, according to claim 7. Advantageous refinements of the invention are specified in the dependent claims. 
     Only a single switch is now provided in the current source circuit according to the invention, which is switched as a function of a control signal, for example the phase comparison result, and either charges or discharges the component connected on the output side, for example a loop filter. The other function, that is to say the discharging or the charging of the component, is carried out by a continuously switched-on driver transistor. In consequence, this driver transistor does not carry out any switching processes and thus produces no switching noise. The total switching noise produced is thus reduced, and the signal-to-noise ratio is improved. The circuit also provides the advantage that the component which produces the control signal, for example the phase comparison circuit, now need be equipped with only a single output, to which the driver switch is connected, being switched as a function of the control signal, for example the phase comparison result. The required circuit and wiring complexity is thus also reduced at the same time. 
     The continuously switched-on driver stage is preferably designed in such a way that it carries a current which differs from the current carried by the switched driver stage when it is switched on. This results in a net current difference which acts as a bias current and is used for frequency coverage during a search and lock-on process. In consequence, the circuit design is even further simplified. Yet another simplification can be achieved by the continuously switched-on driver stage being part of a current mirror circuit into which a desired stabilized current flows. This current mirror circuit is preferably also connected to the phase comparison circuit and causes a stabilized current to flow into it. 
     The total number of circuits and control components required is thus reduced, so that temperature drift problems, matching problems and the like are also reduced. Good circuit characteristics are thus ensured. 
    
    
     The invention will be explained in more detail in the following text using an exemplary embodiment and with reference to the drawings, in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic illustration of a phase locked loop having a current source circuit designed according to the invention, and 
     FIG. 2 shows a block diagram of an exemplary embodiment of the phase comparator designed according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As is shown in FIG. 1 the phase locked loop contains a phase comparator  1  to one input  2  of which a reference signal  3  is applied. An input signal whose phase angle is to be controlled, in this case the possibly frequency-converted output signal  5  from a voltage-controlled oscillator  10 , is applied to the other input  4  of the phase comparator  1 . The phase comparator  1  comprises a phase comparison circuit  6  and a current source circuit  7  (driver or charge pump stage) and, via its output  8 , emits the output signal to a loop filter  9 , whose capacitance is charged or discharged depending on the phase comparator output signal. The loop filter  9  controls the oscillator  10  which produces the output signal  5 , whose phase is locked to the reference signal  3 . 
     FIG. 2 shows a block diagram of the phase detector  1 , which comprises three functional blocks: the phase comparison stage  6 , the current source circuit  7  and a current mirror circuit  11 . The phase comparator  1  is connected between a supply voltage connection  12  and a ground potential connection  13 . The phase comparison stage  6  contains a switching element which switches between two output states, for example a JK flipflop or, as shown in FIG. 2, an exclusive-OR gate with the symmetrical design shown, to whose input connections  2 ,  4  the two signals  3 ,  5  to be compared with one another are applied via the lines shown, and which is connected to the supply voltage connection  12  via the illustrated switching transistors and emitter negative feedback and depletion resistors. The phase comparison stage  6  has only a single output  15  which is switched to either a high potential or a low potential depending on the phase angle. One switching state corresponds, for example, to the reference signal leading the signal to be measured, while the other state is assumed when the phase is lagging. 
     The phase comparison stage output  15  is connected to the current source circuit  7 , which is designed as a switched charge pump and comprises a switching transistor  17  whose base is connected to the connection  15  and which, when switched on, feeds current to the phase comparator output  8 , so that the capacitance of the loop filter  9  is charged. 
     In order to discharge the loop filter capacitance, the current source circuit  7  contains a continuously operated drive stage, which is connected between the output connection  8  and ground potential  13 , in the form of a transistor  18  through which a constant discharge current flows. 
     The transistor  18  can be provided with its own control circuit for stabilization of the current flowing through it, but is preferably a component of a current mirror circuit  19  having a plurality of transistors  18 ,  20  whose bases and emitters are each connected to one another in a manner known per se. The base connection of the current mirror circuit  19  is connected to an input connection  14  to which, for example, the primary side of the current mirror circuit  19  is connected. In this case, the collector and the base of the transistor on the primary side are connected to the connection  14  and to a current source which causes the desired stabilized current to flow. The emitter of this transistor is connected to the connection  13  via an emitter resistor, analogous to the circuitry of the transistors  18 ,  20 . The external current feed means that it is simple to change and adjust the respective current level that is stabilized in the current mirror circuit  19 . The primary side of the current mirror circuit may also, of course, be designed as part of the current source circuit  7 , on the same chip. 
     As can be seen from FIG. 2, the collectors of a plurality of transistors  20  are jointly connected to one connection  21  of the phase comparison stage  6 , so that the current mirror circuit  19  is also used for current control of the current flowing in the phase comparator. The illustrated quadruple arrangement of the transistors  20  results in a stabilized current being caused to flow in the phase comparison circuit  6  which is four times as great as the discharge current which flows continuously through the transistor  18 . This current is also caused to flow into the output stage, which is connected to the output  15 , of the phase comparison circuit  6 , provided this path is switched on on the basis of the phase angle at that time. This output stage forms a current mirror circuit with the transistor  17 , so that when it is switched on, the transistor  17  carries this current increased by a factor of four and feeds it to the connection  8 . The transistor  18  draws one quarter of this current increased by a factor of four so that, effectively, a charging current is fed to the output  8  which is three times as great as the discharge current carried away via the transistor  18  when the transistor  17  is switched off. 
     The circuit arrangement can also be reversed in such a way that the transistor  17  is continuously switched on and feeds charging current continuously to the loop filter  9 . In this case, the transistor  18  represents the switched current source, whose current connection is connected to the connection  15 . In this case, the connection  15  is preferably connected to the base of the transistor  22  shown in FIG. 2 while, in contrast, the transistor  17  forms a part of the current mirror circuit  19 . In this case as well, the phase comparison circuit  6  need have only a single output connection, and only one current source (transistor) is switched, so that the circuit and wiring complexity is low and the switching noise caused by the switching processes is reduced. Furthermore, no inverter is required to invert the potential at the output connection  15  in order to control a second transistor. 
     In general, in the case of phase detectors whose output state can be represented, as in the present case, by an output signal, it can be said that the current supply to the loop filter can be expressed as follows: 
     
       
           I=Q·I   sc −(1 −Q )· I   dc   +I   b . 
       
     
     In this case, Q denotes the output state of the phase detector which may be either 1 or 0; I sc  denotes the source charge pump current, that is to say the charging current; I dc  denotes the drain charge pump current, that is to say the discharge current; and I b  denotes a bias current, which is fixed at a constant value. This equation can be rewritten as follows: 
     
       
           I=Q ·( I   sc   +I   dc )+( I   b   −I   dc ). 
       
     
     This equation means that, with regard to the invention, it is possible to operate with a single switched charge pump, which carries the current (I sc +I dc ) when switched on, in conjunction with an unswitched constant current source which carries a fixed constant bias current (I b −I dc ). In this case, the same result is achieved in terms of charge as with two switched charge pumps with an additional bias current. For the exemplary embodiment shown in FIG. 2, in which the emitter current carried continuously and constantly by the transistor  18  is one quarter of the charge pump current provided by the charge pump  16 , this gives the following result for the current flow to and from the loop filter  9 : 
       I=Q·I   c   −I   c /4 =Q·I   c /2−(1 −Q )I c /2 +I   c /4. 
     With regard to the currents carried, the illustrated circuit is thus equivalent to the conventional circuit, in which two switched charge pumps are used with a current of I c /2 and an additional fixed, constant bias current of I c /4. As is known, the function of this bias current is to produce the frequency shift during the search and lock-on process.