PLL frequency synthesizer

Abstract: In case of switching a frequency of an output signal of a voltage control oscillator, an arithmetic unit outputs to a variable frequency divider such a frequency divisor that the voltage control oscillator outputs a signal having a predetermined frequency ranging between frequencies before and after switching and thereafter the frequency divisor to be output to the variable frequency divider is changed to such a frequency divisor that the voltage control oscillator outputs a signal having the frequency after switching when the frequency of the output signal from the voltage control oscillator detected by a counter and a register reaches a frequency previously stored in a memory circuit.

Description: The present invention relates to a PLL frequency synthesizer, and more particularly to a PLL frequency synthesizer for switching frequencies at high speed. A phase looked-loop (PLL) frequency synthesizer is used in a transmitter and/or a receiver in a radio apparatus and the like for the purpose of generating a desired frequency. The PLL frequency synthesizer is constituted by a reference oscillator, a voltage control oscillator (VCO), a phase comparator and a low-pass filter (LPF). The phase comparator detects a phase difference between an output frequency of the reference oscillator and an output frequency of the VCO. A signal corresponding with this phase difference is supplied as a control voltage to the voltage control oscillator through the LPF. When this type of PLL frequency synthesizer is used in, e.g., in mobile communication, high speed frequency switching is desired. Then, a PLL frequency synthesizer for switching frequencies at high speed is disclosed in, for example, Japanese Laid-Open Patent Application No. 214925/1991 (JP-A-03-214925). In the PLL frequency synthesizer, a switch is provided in parallel with the LPF as a loop filter. The switch is turned on when the frequency locking is started in the frequency switching operation and the input/output of the LPF is short-circuited, thereby supplying the control voltage corresponding with the phase difference to the VCO without the LPF. In the PLL frequency synthesizer, however, the response characteristics of the loop become unstable or switching noise is generated when the switch is turned off. Further, since the voltage which has already been charged in the loop filter is different from the control voltage for generating a desired frequency, the control voltage supplied to the VCO fluctuates. For the VCO, one having a high modulation sensitivity with which the oscillation frequency greatly varies due to a small change in the control voltage is usually used. When the noise is generated or the control voltage fluctuates as described above, the oscillation frequency therefore fluctuates even if it becomes closer to a desired frequency. The time for locking the frequency is thus prolonged. In addition, the conventional PLL frequency synthesizer stabilizes the oscillation frequency at a desired frequency while repeating overshooting of the oscillation frequency to lock the frequency. Accordingly, there occurs such a problem that this overshooting prolongs the frequency-locking time. It is therefore an aim of the present invention to provide a PLL frequency synthesizer with which the noise or the fluctuation of the control voltage due to change-over of the switch is not generated even though the PLL frequency synthesizer switches frequency at high speed. It is another aim of the present invention to provide a PLL frequency synthesizer which enables a stable frequency-locking operation even though the PLL frequency synthesizer switches frequency at high speed. It is still another aim of the present invention to provide a PLL frequency synthesizer which can suppress the overshooting of the oscillation frequency when the PLL frequency synthesizer switches frequency at high speed. It is a further aim of the present invention to provide a PLL frequency synthesizer which can switch frequencies at high speed. A development of the frequency synthesizer discussed above is described in patent document EP-A-0461358. That synthesizer comprises: a voltage controlled oscillator for outputting an oscillation signal having an oscillation frequency corresponding to a voltage of a control signal; a variable frequency divider for dividing the frequency of said oscillation signal by a variable frequency divisor to provide a first frequency-division signal; a reference frequency oscillator for supplying a reference signal having a reference frequency; a reference frequency divider for dividing the frequency of the reference signal by a reference frequency divisor to provide a second frequency-division signal; a phase comparator for comparing the first and second frequency-division signals to detect a phase difference therebetween and providing a corresponding phase-difference voltage signal; a loop filter for filtering the phase-difference signal to provide the control signal; and a control circuit operable, in the case of changing the variable frequency divisor from a first value to a second value, to change the variable frequency divisor firstly from the first value to a value intermediate the first and second values, and then from the intermediate value to the second value. In some of the synthesizers described in EP-A-0461358, the time at which the variable frequency divisor is changed from the intermediate value to the second value is calculated or determined based on various constants employed in the loop filter and is set in advance in the control circuit. In another of the synthesisers described in that document, the phases of the outputs of the variable frequency divider and the reference frequency divider are detected, and switching takes place when the phases become synchronised. The previously indicated aims of the invention are achieved by a PLL frequency synthesizer according to claim 1. The PLL frequency synthesiser of the present invention is characterised in that: the reference frequency divisor has a fixed value; the control circuit includes a frequency detector for detecting the frequency of said oscillation signal and control means responsive to the detected frequency of the oscillation signal for changing the variable frequency divisor; and the control means is operable to determine to change the variable frequency divisor from the intermediate value to the second value when said detected frequency reaches a predetermined frequency. The control means may be operable to determine to change the variable frequency divisor from the intermediate value to the second value when the frequency of the oscillation signal first reaches the predetermined frequency, or when the frequency of the oscillation signal has reached the predetermined frequency a predetermined plurality of times. The frequency detector preferably includes: a counter for counting the oscillation signal; and a register for accumulating a counted value of the counter. The frequency detector may further include a second variable frequency divider for dividing the frequency of the oscillation signal, prior to supply to the counter, by a frequency divisor smaller than the frequency divisor supplied to the first-mentioned variable frequency divider. Also, the control means preferably includes a memory for storing a switching frequency; and a comparator for comparing the stored switching frequency with the detected frequency. The predetermined frequency may be determined from the target oscillation frequency after switching, or from the target oscillation frequencies before and after switching. These and other aims, features and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings, and provided purely by way of example. In the drawings: Fig. 1 is a structural block diagram showing a conventional PLL frequency synthesizer; Fig. 2 is a structural block diagram showing a preferred embodiment according to the present invention; Fig. 3 is a view showing output frequencies of a PLL frequency synthesizer according to the present invention; Fig. 4 is an operational flowchart of an arithmetic unit illustrated in Fig. 2; Fig. 5 is a structural block diagram showing another embodiment according to the present invention; Fig. 6 is a structural block diagram showing still another embodiment according to the present invention; and Fig. 7 is a structural block diagram showing a further embodiment according to the present invention. In the drawings, the same reference numerals denote the same structural elements. First, to facilitate an understanding of the present invention, a conventional PLL frequency synthesizer will be described with reference to Fig. 1. In Fig. 1, there is shown a PLL frequency synthesizer represented by simplifying the PLL frequency synthesizer disclosed in the above-described Japanese Laid-Open Patent Application No. 214925/1991 for facilitating its explanation. In Fig. 1, the PLL frequency synthesizer includes a phase locked loop (PLL) circuit 27, an analog switch 28 and a phase difference detecting circuit 29. The PLL circuit 27 is constituted by: a reference frequency oscillator 21; a phase comparator 22; a voltage control oscillator (VCO) 23; a loop filter 24; a fixed frequency divider 25; and a variable frequency divider 26. The VCO 23 controls a frequency of an output signal 102 in accordance with a control voltage 101. The variable frequency divider 26 divides a frequency of the output signal 102 of the VCO 23 and supplies a first frequency division signal 107 to the phase comparator 22. The reference frequency oscillator 21 outputs a reference signal 103 having a reference frequency. The fixed frequency divider 25 divides a frequency of the reference signal 103 and supplies a second frequency division signal 108 to the phase comparator 22. The phase comparator 22 compares a phase of the first frequency division signal 107 with that of the second frequency division signal 108 and supplies a control voltage 104 corresponding with a phase difference. The loop filter 24 provided with a capacitor smoothes the control voltage 104 and supplies a stable control voltage 101. The phase difference detecting circuit 29 supplies a switch control signal 106 in response to a phase difference signal 105 representing the phase difference from the phase comparator 22. The phase difference detecting circuit 29 so controls as to close the analog switch 28 when the phase difference exceeds a predetermined value. Thereafter, the phase difference detecting circuit 29 so controls as to open the analog switch 28 when the phase difference becomes below the predetermined value. The analog switch 28 is provided in parallel with the loop filter 24 and turns on/off the switch in response to the switch control signal 106. In the PLL frequency synthesizer, the analog switch 28 is turned off when the phase difference indicated by the phase difference signal 105 is below a predetermined value. At this time, the response characteristics of the loop become unstable. switching noise is also generated. Further, since the voltage charged in the loop filter 24 is different from the control voltage 104, the control voltage 101 which is to be supplied to the VCO 23 fluctuates. Accordingly, the frequency-locking time is prolonged because of the fluctuation of the control voltage 101, although the oscillation frequency approximates a desired frequency. Further, the repetition of overshooting of the oscillation frequency prolongs the frequency-locking time. The present invention will now be described in connection with Figs. 2 to 6. Fig. 2 is a block diagram showing a preferred embodiment according to the present invention. The PLL frequency synthesizer according to the present invention is constituted by a PLL circuit 7 and a control circuit 16. The PLL circuit 7 includes: a reference frequency oscillator 1; a fixed frequency divider 2; a phase comparator 3; a loop filter 4; a voltage control oscillator 5; and a variable frequency divider 6. The control circuit 16 is composed of: a counter 9; a register 10; a data comparator 11; a memory 12; and a controller 13. The VCO 5 supplies an oscillation signal 202 having an oscillation frequency corresponding with a control voltage 201 to the variable frequency divider 6. The variable frequency divider 6 divides a frequency of the oscillation signal 202 in response to a first control signal 207 and supplies a first frequency division signal 203 to the phase comparator 3. The reference frequency oscillator 1 supplies a reference signal 204 having a reference frequency to the fixed frequency divider 2. The fixed frequency divider 2 divides a frequency of the reference signal 204 with a predetermined frequency division ratio and supplies a second frequency division signal 205 to the phase comparator 3. The phase comparator 3 compares a phase of the first frequency division signal 203 with that of the second frequency division signal 205 and supplies a control voltage 206 corresponding with a phase difference to the loop filter 4. The loop filter 4, which is preferably provided with a capacitor, smoothes the control voltage 206 and outputs a stable control voltage 201. When the oscillation frequency of the oscillation signal 202 is changed, to the controller 13 is supplied from a non-illustrated circuit a data signal 208 indicating a desired frequency which is intended to be finally output from the synthesizer. In response to this supply, the controller 13 outputs the first control signal 207 for controlling a frequency divisor of the variable frequency divider 6. At this time, the controller 13 supplies to the variable frequency divider 6 a control signal which indicates a frequency divisor smaller than a frequency divisor which instructs to output the oscillation frequency. Preferably, a frequency lower than the desired frequency is read out of a memory (not shown) in the controller 13 in response to the data signal 208. Then, the controller 13 calculates the frequency divisor corresponding to the frequency lower than the desired frequency. The PLL circuit 7 so operates as to lock at a frequency lower than a desired frequency in response to the control signal 207. The controller 13 outputs to the counter 9 a reset signal 209 for resetting the counter 9 every predetermined time. Simultaneously, the controller 13 supplies a second control signal 210 to the register 10. The counter 9 counts the oscillation signal 202 and supplies a counted value 211 to the register 10. The counter 9 resets the counted value in accordance with the reset signal 209 from the controller 13. The register 10 accumulates the counted value 211 and supplies data 212 corresponding with the counted value 211 to the data comparator 11 in accordance with the second control signal 210. Since the second control signal 210 and the reset signal 209 are simultaneously output from the controller 13, the data 212 corresponds with the oscillation frequency of the oscillation signal 202. To the memory 12 are stored frequency data corresponding with a plurality of frequencies. The memory circuit 12 supplies frequency data 213 indicating lower frequencies than a desired frequency to the data comparator 11 in response to the data signal 208. The data comparator 11 compares the frequency data 213 with the data 212 from the register 10. When both the data are found to be coincident with each other as a result of comparison, a coincidence signal 214 representing the coincidence is supplied to the controller 13. In response, the controller 13 supplies to the variable frequency divider 6 the first control signal 207 indicating such a frequency divisor that the synthesizer outputs a desired oscillation frequency. In this way, the frequency synthesizer is first controlled so as to oscillate a frequency lower than a desired frequency. The frequency synthesizer is then controlled so as to oscillate the desired frequency when the oscillation frequency approximates the the desired frequency. Preferably, the frequency divisor used for frequency-dividing the oscillation signal of the VCO may be set to such a frequency divisor that the synthesizer oscillates a frequency lower than the desired frequency. When the oscillation frequency becomes close to the desired frequency, the frequency divisor is set to such a frequency divisor that the synthesizer oscillates the desired frequency. The timing at which this frequency divisor is changed is previously obtained by experiments and the like so as to reduce overshooting and rapidly lock to the desired frequency. In the above embodiment, the frequency data 213 which are the timing at which the frequency divisor is changed are obtained by calculating based on a response characteristic of the loop or by experiments in advance and stored in the memory 12. Fig. 3 illustrates the relationship between the oscillation frequency and the time when the frequency divisor is changed in the PLL frequency synthesizer according to the present invention. Referring to Fig. 3, the curve 30 indicated by a solid line represents the oscillation frequency of the voltage control oscillator 5 shown in Fig. 2. The curve 31 indicated by a broken line represents the oscillation frequency for the case where a frequency divisor n1 such that the PLL frequency synthesizer outputs a desired frequency f2 is directed to the variable frequency divider. The curve 32 indicated by an alternate long and short dash line denotes the oscillation frequency for the case where a frequency divisor n2 such that the PLL frequency synthesizer outputs a frequency f3 lower than the desired frequency is directed to the variable frequency divider. The curves 31 and 32 show that the oscillation frequency is converged to a frequency corresponding with a directed frequency divisor while repeating the overshooting, respectively. A description will now be given as to the case where the oscillation frequency is changed from f1 to f2. In the above-mentioned embodiment, the frequency divisor is first set to the frequency divisor n2 as shown by the curve 32, and the oscillation frequency traces the same locus as the curve 32. The frequency divisor is then changed from n2 to n1 when the oscillation frequency becomes f4 at the time t1. As a result, the frequency is rapidly stabilized at the desired frequency f2 with the overshooting being reduced, as compared to the curve 31. The frequency f4 is stored in the memory 12 as a data frequency in accordance with the oscillation frequency f2. That is, the frequency f4 which is the timing for changing the frequency divisor with which the frequency f1 is rapidly changed to f2 is previously obtained by experiments and the like, and the frequency f4 is stored in the memory 12 in accordance with the data signal representing the oscillation frequency f2. Similarly, the frequency which is the timing for changing the frequency divisor corresponding with each oscillation frequency is previously stored in the memory 12. When the data signal 208 indicating the oscillation frequency f2 is supplied to the controller 13, the arithmetic unit 13 sends the first control signal 207 representing the frequency divisor n2 to the variable frequency divider 6. The counter 9 counts the oscillation frequency and supplies the counted value 211 to the register 10. The register 10 sends to the data comparator the data corresponding with the counted value, i.e., the data 212 representing the oscillation frequency. Upon receiving the data signal 208 indicating the oscillation frequency f2, the memory circuit 12 sends the frequency data 213 representing the frequency f4 to the data comparator 11. The data comparator 11 compares the frequency data 213 with the data 212. In other words, the data comparator 11 detects whether the oscillation frequency becomes f4. When the oscillation frequency becomes f4, the data comparator 11 feeds the coincidence signal 214 to the controller 13. In response to this signal, the controller 13 sends the first control signal 207 indicating the frequency divisor n1. Fig. 4 illustrates the operational flowchart of the controller 13. Upon receiving the data signal 208 used for changing the frequency to f2 from f1 (step S401), the controller 13 calculates the first frequency divisor n2 corresponding with the data signal 208 and supplies the first control signal 207 indicating this first frequency divisor to the variable frequency divider 6 (step S402). As described above, the first frequency divisor is such a frequency divisor that a frequency lower than a desired frequency is output. When a predetermined time t lapses (step S403), the reset signal 209 and the second control signal 210 are sent to the counter 9 and the register 10, respectively (step S404). Subsequently, when the coincidence signal 214 is fed from the data comparator 11 (step S405), the second frequency divisor n1 used for outputting a desired frequency is calculated, and the first control signal 207 representing this second frequency divisor is supplied to the variable frequency divider 6 (step S406). Fig. 5 is a block diagram showing another embodiment of this invention. As shown in Fig. 5, the arrangement in which the variable frequency divider 8 is provided within the control circuit 16 is different from the embodiment illustrated in Fig. 3. Since other configurations are substantially similar to those of the embodiment in Fig. 3, explanation of the operation will be omitted in order to avoid repetition. The variable frequency divider 8 submits the frequency of the oscillation signal 202 to the frequency division in accordance with the third control signal 215 from the controller 13 and outputs the thus-divided signal as a third frequency division signal 216 to the counter 9. Preferably, the frequency of the third frequency division signal 216 from the variable frequency divider 8 may be higher than the frequency of the first frequency division signal 203 from the variable frequency divider 6. With this operation, the counting numbers in the counter 9 are decreased. The counter 9 is therefore constituted by a counter whose maximum counting number is small. That is, the burden to the counter 9 is reduced. Fig. 6 is a block diagram showing still another embodiment of the present invention. In Fig. 6, the arrangement in which a memory 14 is provided and the memory 12 is controlled by the controller 13 is different from that of the embodiment shown in Fig. 2. The overshooting in the locking operation of the PLL frequency synthesizer varies depending on the difference between the frequencies before and after the frequency output from the PLL frequency synthesizer is switched. The frequency data 213 should be changed on the basis of the difference. The frequency before switching is stored in the memory 14 by the controller 13 in such a manner that the frequency divisor of the variable frequency divider 6 can be switched at an optimum timing. The controller 13 obtains the difference between the frequencies before and after the switching and finds the timing for switching the frequency divisor of the variable frequency divider 6 on the basis of this difference in frequency. The controller 13 calculates the difference and the frequency supplied to the data comparator 11 from the memory 12. The memory 12 outputs the frequency data addressed by the controller 13, then the controller 13 calculates the frequency divisor of the variable frequency divider 6. Such an operation can reduce the overshooting in the locking operation. Fig. 7 is a block diagram showing a further embodiment of the present invention. In Fig. 7, the difference from the embodiment illustrated in Fig. 5 is that the memory circuit 15 to which data can be written is provided instead of the memory circuit 12. Referring to Fig. 7, the frequency data 213 from the memory 15 is controlled in response to an optimum third frequency division signal 216 of the variable frequency divider 8. Namely, the frequency data 213 which is a reference for comparison in the data comparator 11 can be changed. With this operation, the controller 13 can rewrite the frequency data 213 which is a reference for comparison in the data comparator 11 in accordance with the data signal 213, thus carrying out the appropriate comparison. In the above-described embodiments, a frequency divisor such that the synthesizer can output a frequency lower than a desired frequency is directed to the variable frequency divider, and a frequency divisor corresponding with the desired frequency is indicated to the variable frequency divider 6 when the oscillation frequency becomes a predetermined frequency. In the present invention, however, after a frequency divisor for indicating a frequency lower than a desired frequency is set, the oscillation frequency may be constantly monitored, and the frequency divisor may be changed when the oscillation frequency reaches a frequency slightly lower than a desired frequency. Although the oscillation frequency is changed from a lower frequency to a higher frequency in the above-described embodiments, the oscillation frequency may be changed from a higher frequency to a lower frequency. In this case, first the frequency higher than the desired frequency is selected. In addition, the timing for changing the frequency is not restricted to the first time when the oscillation frequency reaches the frequency data from the memory circuit, and it may be set to the second time or others. As described above, according to preferred embodiments of the present invention, a stable frequency-locking operation is enabled. Further, since the overshooting of the oscillation frequency is suppressed, the frequency can be changed at high speed. In summary, in the preferred embodiments, in case of switching a frequency of an output signal of a voltage control oscillator, an arithmetic unit outputs to a variable frequency divider such a frequency divisor that the voltage control oscillator outputs a signal having a predetermined frequency ranging between frequencies before and after switching and thereafter the frequency divisor to be output to the variable frequency divider is changed to such a frequency divisor that the voltage control oscillator outputs a signal having the frequency after switching when the frequency of the output signal from the voltage control oscillator detected by a counter and a register reaches a frequency previously stored in a memory circuit.

Claims: A PLL frequency synthesizer comprising: a voltage controlled oscillator (5) for outputting an oscillation signal (202) having an oscillation frequency corresponding to a voltage of a control signal (201); a variable frequency divider (6) for dividing the frequency of said oscillation signal by a variable frequency divisor (207) to provide a first frequency-division signal (203); a reference frequency oscillator (1) for supplying a reference signal (204) having a reference frequency; a reference frequency divider (2) for dividing the frequency of the reference signal by a reference frequency divisor to provide a second frequency-division signal (205); a phase comparator (3) for comparing the first and second frequency-division signals to detect a phase difference therebetween and providing a corresponding phase-difference voltage signal (206); a loop filter (4) for filtering the phase-difference signal to provide the control signal; and a control circuit (16) operable, in the case of changing the variable frequency divisor from a first value to a second value, to change the variable frequency divisor firstly from the first value to a value intermediate the first and second values, and then from the intermediate value to the second value; characterised in that: the reference frequency divisor has a fixed value; the control circuit includes a frequency detector (9, 10) for detecting the frequency of said oscillation signal and control means (11, 12, 13) responsive to the detected frequency of the oscillation signal for changing the variable frequency divisor; and the control means is operable to determine to change the variable frequency divisor from the intermediate value to the second value when said detected frequency reaches a predetermined frequency (f4). A synthesizer as set forth in Claim 1, wherein the control means is operable to determine to change the variable frequency divisor from the intermediate value to the second value when the detected frequency first reaches the predetermined frequency. A synthesizer as set forth in Claim 1, wherein the control means is operable to determine to change the variable frequency divisor from the intermediate value to the second value when the detected frequency has reached the predetermined frequency a predetermined plurality of times. A synthesizer as set forth in any preceding claim, wherein the frequency detector includes: a counter (9) for counting the oscillation signal; and a register (10) for accumulating a counted value of the counter. A synthesizer as set forth in claim 4, wherein the frequency detector further includes a second variable frequency divider (8) for dividing the frequency of the oscillation signal, prior to supply to the counter, by a frequency divisor smaller than the frequency divisor supplied to the first-mentioned variable frequency divider. A synthesizer as set forth in any preceding claim, wherein the control means includes: a memory (12) for storing a switching frequency; and a comparator (11) for comparing the stored switching frequency with the detected frequency. A synthesizer as set forth in Claim 6, wherein the predetermined frequency is determined from the target oscillation frequency (f2) after switching. A synthesizer as set forth in Claim 6, wherein the predetermined frequency is determined from the target oscillation frequencies (fl, f2) before and after switching.

Inventors: OSAKA MASAHIKO, OSAKA, MASAHIKO, OSAKA, MASAHIKO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H03L7
IPC codes: H03L7