Method for reducing the settling time in PLL circuits

Abstract: A method for reducing the settling time in PLL circuits, particularly for use in an RF transceiver, which comprise a phase comparator (2), a filter (4), a digital-analog converter (8) and an adder (5) which are suitable to produce in output a voltage (V c ) for controlling a voltage-controlled oscillator (6) provided by means of a varactor, characterized in that it comprises the steps of: determining the dependency of the control voltage (V c ) of the voltage-controlled oscillator (6) on the frequency of a selected channel of a transmitter; generating a law describing the variation of the output current (I DAC ) of said digital-analog converter (8) such that the voltage (V DAC ) obtained from the output current of the digital-analog converter, added to an output voltage (V f ) of said filter (4), is such as to keep said filter voltage (V f ) constant, in order to reduce the settling time of the PLL circuit as a selected channel varies.

Description: The present invention relates to a method for reducing the settling time in PLL circuits. More particularly, the invention relates to a method for reducing the settling time in PLL circuits used for example in digital cordless telephones (for example of the DECT type). It is known that in recent years the world of wireless communications has changed very rapidly thanks to advances in silicon-based technologies, which have given great impulse to the use of electronic solutions of the monolithic type. In particular, this technological impulse and market demand have led to the use of architectures of the RF transceiver which include the integration of the local oscillator, of the RF and IF transmitter and receiver section, and of part of the baseband circuits. One of the critical blocks of an integrated transmitter is the PLL circuit. In applications requiring a particularly short settling time, the frequency synthesizer is generally provided according to the block diagram shown in Figure 1, in which the reference numeral 1 designates a clock signal which is input to a phase comparator 2 which is followed in a cascade configuration by a charge pump circuit 3 and by a filtering circuit 4. The voltage signal in output from the filtering circuit 4 is input to an adder 5 which receives, on its inverting input, a signal which arrives from a DAC to which a digital word WORD is input. The output of the adder 5 is sent to a voltage-controlled oscillator 6 which feeds back its output, by means of a divider 7, to the phase comparator 2. The above-described block diagram differs from a conventional layout in that there is a digital-analog converter and an adder. The function of these elements is to perform the analog algebraic sum of the voltage V F that arrives from the filter 4 and of a reference voltage V DAC produced by the current I DAC on the feedback resistor R F of the adder circuit 5. In formal terms, this is stated as follows: V C = V F - R F I DAC (w) where w is the digital word WORD input to the DAC 8. The purpose of this structure is to provide, during channel transitions, by means of a suitable control of the DAC 8, a voltage R F I DAC which follows the variations of the control voltage, so as to leave the voltage across the filter 4 ideally constant. In this manner, the settling time of the PLL circuit shown in Figure 1 is limited only by the settling time of the voltage-controlled oscillator 6 and of the DAC 8 and is therefore very short. In the real case, the voltage supplied by the DAC 8 is affected by a quantization error and accordingly, assuming that the channel transitions are instantaneous with respect to the reaction times of the PLL, this quantization error leads directly to an initial offset of the synthesized frequency (with respect to the steady-state value), according to a relation in which the frequency deviation is equal to the quantization error multiplied by the gain factor of the voltage-controlled oscillator 6. If the locking conditions of the PLL are still met, this deviation is recovered by the synthesizer loop with a settling time which is directly proportional to said frequency deviation. However, the efficiency of the system increases as the frequency deviation at the beginning of the transient decreases. However, the need to cover the required synthesizing band regardless of process tolerances forces high values of the gain factor of the voltage-controlled oscillator 6. This entails that in order to obtain a low settling time it is necessary to minimize the quantization error. This is achieved in conventional applications by using a DAC with a large number of bits. This solution, however, entails several evident drawbacks. First of all, a large number of bits increases the complexity of the control logic of the system in which the PLL circuit is placed. Moreover, this solution also entails a higher consumption of current for an equal noise contribution and in most cases entails the use of complicated circuit topologies, with a consequent increase in silicon area occupation. EP-A-0 360 442 discloses a method for reducing the settling time in PLL circuits as defined in the preamble of claim 1. The aim of the present invention is therefore to provide a method for reducing the settling time in PLL circuits in which it is possible to minimize frequency deviation in said circuits. Within the scope of this aim, an object of the present invention is to provide a method for reducing the settling time in PLL circuits which allows to use a DAC control logic which is proportionally simpler than in known solutions. Another object of the present invention is to provide a method for reducing the settling time in PLL circuits which allows to achieve reduced current consumption. Another object of the present invention is to provide a method for reducing the settling time in PLL circuits which allows to obtain less complicated DAC circuit topologies than known solutions and therefore with smaller silicon area occupations. Another object of the present invention is to provide a method for reducing the settling time in PLL circuits which is highly reliable, relatively easy to provide and at competitive costs. This aim, these objects and others which will become apparent hereinafter are achieved by a method for reducing the settling time in PLL circuits, particularly for use in an RF transceiver, as defined in claim 1. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of the method according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a block diagram of a PLL circuit to which the method according to the present invention has been applied; Figure 2 is a chart which plots, by way of example, the control voltage of a voltage-controlled oscillator with respect to the frequency for a voltage-controlled oscillator of the varactor type; Figure 3 is a block diagram of a DAC topology applied in the method according to the present invention; Figure 4 is an exemplifying chart which plots the output characteristic of a seven-bit DAC used to optimize the settling times of a PLL having a VCO provided by means of a varactor, in the method according to the invention; and Figure 5 is an exemplifying diagram of the frequency offset of the PLL circuit produced by a channel variation. With reference to the above figures, the method according to the present invention is as follows. As mentioned, a voltage V C for controlling the voltage-controlled oscillator 6 is correlated to the voltage V F at the output of the filter 4 and to the voltage R F I DAC produced by the DAC 8. This relation shows that if the law of variation of the DAC 8 is such as to keep the quantity V C +RI DAC constant, then the voltage of the filter 4 remains constant and the settling time of the loop becomes ideally zero. The method according to the invention entails, once the law of variation V C (f) of the voltage for controlling the voltage-controlled oscillator 6 as a function of the channel frequency is known, the generation of a current I DAC which satisfies the previously described relation that links the control voltage of the voltage-controlled oscillator to the output voltage of the filter 4 and to the current produced by the DAC 8 for each one of the selected channels. It is assumed that there is a one-to-one relation between the word WORD of the DAC 8 and the frequency of the selected channel. If the voltage-controlled oscillator 6 is implemented by means of a varactor, the curve V C (f) that describes the dependency of the control voltage of the voltage-controlled oscillator 6 on the selected channel frequency is of the type shown in Figure 2. Since this curve has a quadratic development, in order to minimize the variation of the voltage across the filter 4 the characteristic curve of the DAC 8 must also follow the same law. The quadratic law of the DAC 8 has been obtained by using known circuits according to the conceptual diagram shown in Figure 3. In this diagram, it can be seen that the polynomial rule of the DAC 8 is generated by raising to the power of two a linear combination of a constant current reference 10 and of a current signal that arrives from a linear DAC 9. In order to make the system efficient, the design of the individual blocks and of the weights of the linear combination obtained by performing the sum of the node 11 has been carefully performed so as to make the control voltage V C related to a given channel correspond to each word WORD of the DAC 9. The operation for raising to the power of two has been obtained with analog methods by using a conventional translinear circuit 12. In this manner, the quantization law induced by the linear DAC is particularly efficient, since it tends to produce the same initial frequency deviation for each selected channel. This characteristic is particularly advantageous for determining the dimensions of the DAC 9 and of the associated control circuits, since the number of bits of the DAC no longer needs to include redundant configurations in order to deal with the different weight of the quantization effect of the selected channel, as occurs instead in conventional PLL implementations. Figure 4 plots, by way of example, the output characteristic of a 7-bit DAC used to optimize the settling times of a PLL with a VCO provided by means of a varactor. Figure 5 instead plots the frequency offset of the PLL due to a channel variation. This offset is clearly a function of the channel and is to be understood as an initial frequency deviation with respect to the new selected channel. The frequency offset plotted in Figure 5 must be compared to the frequency offset that would be produced if the DAC were inactive, the minimum value of which is clearly equal to the channel separation interval. For example, in the case of application to a cordless telephone of the DECT type, the channels are separated by an interval of 1.728 MHz and therefore the use of the DAC having the characteristic plotted in the figure induces a significant improvement in the PLL settling time. It should in fact be noted that the use of a linear DAC 9 having the same number of bits would not have been efficient, since the initial frequency offset would have been greater than the channel separation interval over approximately half of the frequency interval being considered. Therefore, it is evident that with an equal number of bits of the digital-analog converter DAC it is possible to improve the performance of a PLL with a varactor VCO in terms of settling and precision of the synthesized frequency. In practice it has been observed that the method according to the invention allows to ensure a very short PLL settling time without modifying the loop filter. The method thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Thus, for example, since the voltage-controlled oscillator must have the same law as the varactor, for example a quadratic law as described earlier, it is necessary to provide, as mentioned, a suitable translinear circuit which follows the law of the varactor. Therefore, if the varactor has a cubic law, the translinear circuit must be able to provide a law such that the output current of the DAC has a cubic behavior. All the details may furthermore be replaced with other technically equivalent elements. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A method for reducing the settling time in PLL circuits, particularly for use in an RF transceiver, which comprise a constant current reference (10), a phase comparator (2), a filter (4), a digital-analog converter (8) and an adder (5) which are suitable to produce in output a voltage for controlling a voltage-controlled oscillator (6) provided by means of a varactor, comprising the steps of: determining the dependency of the control voltage (V C ) of said voltage-controlled oscillator (6) on the frequency of a selected channel of a transmitter; generating a law (V C (f)) describing the variation of the output current (I DAC ) of said digital-analog converter such that the voltage obtained from the output current of said digital-analog converter (8), added to an output voltage of said filter (V F ), is such as to keep said filter voltage constant, in order to reduce the settling time of said PLL circuit when changing a selected channel, characterized in that said output current (I DAC ) of said digital-analog converter is generated using a translinear circuit (12) arranged upstream of said adder (5), downstream of said constant current reference (10) and being part of said digital-analog converter (8), said law of variation of the output current of said digital-analog converter (8) being nonlinear. The method according to claim 1, characterized in that said law of variation of the output current (I DAC ) of the digital-analog converter (8) is generated by raising to the power of two a linear combination of a constant current reference (10) and of a current signal which originates from a linear digital-analog converter (9). The method according to claim 1, characterized in that each digital word (WORD) in input to the digital-analog converter (8) has a one-to-one correspondence with the control voltage (V C ) of said voltage-controlled oscillator (6) in relation to a channel of said transmitter. A PLL circuit with reduced settling time, particularly for use in RF transceivers, comprising: a constant current reference (10), a phase comparator (2), which is cascade-connected to a charge pump circuit (3) and to a filter (4); a digital-analog converter (8); an adder (5); and a voltage-controlled oscillator (6) provided by means of a varactor; the output current signal (I DAC ) of said converter being input to said adder (5) together with a voltage signal (V F ) output by said filter (4), said output current signal of said digital-analog converter being variable according to a law (V C (f)) such that the voltage (V DAC ) obtained from the output current (I DAC ) of said digital-analog converter (8), added to an output voltage (V F ) of said filter (4), is such as to keep said filter voltage constant, in order to reduce the settling time of said PLL circuit when a selected channel changes, characterized in that said digital-analog converter (8) is nonlinear and comprises a translinear circuit (12) arranged upstream of said adder (5) and downstream of said constant current reference (10) and adapted to output said DAC output current (I DAC ), the law of variation of the output current of said digital-analog converter (8) being nonlinear. The circuit according to claim 4, characterized in that said nonlinear digital-analog converter (8) is constituted by a linear digital-analog converter (9) and by said translinear circuit (12) suitable to provide said linear combination followed by said raising to the power of two. The circuit according to claim 5, characterized in that the output current signal (I DAC ) of said nonlinear digital-analog converter (8) is obtained by raising to the power of two a linear combination of. an output current signal of the linear digital-analog converter (9) and of a constant current reference signal (10). The circuit according to claim 4, characterized in that each digital word (WORD) in input to the nonlinear digital-analog converter (8) has a one-to-one correspondence with the control voltage (V C ) of said voltage-controlled oscillator (6) in relation to a channel selected in a transceiver. The circuit according to claim 5, characterized in that contiguous channels of said transceiver correspond to contiguous digital words (WORD) of said nonlinear digital-analog converter (8).

Inventors: COSENTINO GAETANO, FILORAMO PIETRO, COSENTINO, GAETANO, FILORAMO, PIETRO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H03L7
IPC codes: H03L7