PLL frequency synthesizer

Abstract: A frequency synthesizer capable of switching the output frequency thereof at high speed has a second phase/frequency comparator (32) and a second variable frequency divider (34) in addition to a first phase/frequency comparator (20) which is included in a PLL. The synthesizer switches over the time constant of a loop filter (24) by the output of the second phase/frequency comparator, i.e., independently of the PLL which is responsive to the output of the first phase/frequency comparator (20). As a result, the time constant of the loop filter is switched over at an advanced timing to thereby reduce the PLL tuning time.

Description: BACKGROUND OF THE INVENTION The present invention relates to a frequency synthesizer using a phase locked loop (PLL) and, more particularly, to a PLL synthesizer capable of switching the output frequency thereof at high speed. A conventional PLL synthesizer has a reference frequency oscillator, a fixed frequency divider for dividing the output frequency of the oscillator, a voltage controlled oscillator (VCO), a variable frequency divider for dividing the output frequency of the VCO, a phase/frequency comparator for comparing the outputs of the fixed and variable frequency dividers to output a difference therebetween, a charge pump to which the difference or deviation is applied, and a loop filter receiving and smoothing the output of the charge pump and feeding back the output thereof to the VCO as a control voltage. It is a common practice with this type of PLL synthesizer to cause the loop filter to select either one of a greater and a smaller time constant in order to enhance high-speed frequency switching and to stabilize the frequency. More specifically, the time constant of the loop filter is reduced at the time of frequency switching so as to promote rapid tuning. After the frequency switching, a lock signal is fed from the Phase/frequency comparator to the loop filter to increase the time constant of the filter, thereby stabilizing the frequency. In practice, however, at the time when the switching operation is completed, the control voltage applied to the VCO has been deviated from an expected value due to the delay particular to detection and switching. Therefore, the initial frequency just after the switching is different from predetermined one with the result that the switchover of the time constant of the loop filter is delayed. This in turn delays the tuning time of the PLL and prevents the frequency to be switched over at high speed. Another phase-locked loop circuit having a switchable feedback loop is described in: Proceedings of the IEEE 1987, New York, pages 461-464; Ramon S. Co et al.:"A CMOS IC for Multichannel Voice/Data Communication over a Single Twisted-Pair Wire". This document describes (Figure 5a) a phase-locked loop circuit for locking onto a predetermined, known carrier frequency (42 kHz) of a data transmission line. The data is frequency multiplexed on 42kHz and 6kHz carriers. The phase-locked loop comprises a fixed divider chain for dividing the output of a VCO to produce a "42kHz" signal and a "6kHz" signal (these frequencies are only accurate when the phase-locked loop is locked to the carrier frequencies). The divided "42kHz" signal is supplied to the feedback loop of the phase-locked loop, which is switchable between a wide-bandwidth, high-gain feedback filter (first filter) and a narrow-bandwidth, low-gain feedback filter (second filter). The divided "6kHz" signal is supplied to a bandwidth control logic unit which compares it with the 6kHz carrier signal and, when the two are within 100 Hz of each other, switches the feedback filter from the first to the second filter. The purpose of this switchable prior art filter is to allow, using the first filter, the loop to first acquire, or find, the 42kHz carrier frequency, and then when it is near the carrier frequency, to switch to the second filter in order to lock onto the carrier frequency more accurately, with less jitter. This resolves to some extent the compromise involved in using only one feedback filter. If only a wide bandwidth filter were used, the phase-locked loop would be able to acquire the carrier frequency but would be unable to lock onto it accurately. If only a narrow bandwidth filter were used, the loop might never be able to acquire the carrier frequency. European patent application No. 0211594 describes a phase-locked loop circuit in which two alternative feedback loops are provided, each comprising an identical filter/hold element. The circuit is designed to generate carrier frequencies for a "burst-mode" radio system, repeatedly switching back-and-forth between two different frequencies, F₁ and F₂. The circuit switches to generate each output frequency by changing a divisor provided by a control unit to a variable divider which divides the output of a VCO. The divided output is provided to the feedback loop. When the divisor is changed, the circuit simultaneously switches the feedback loop from one filter/hold element to the other. While one frequency, F₁, is being generated, the hold portion of the temporarily inoperative filter/hold element stores the control voltage value it previously supplied to the VCO when the other frequency, F₂, was last being generated. Thus, when the circuit switches back to the other frequency, F₂, the filter/hold element can immediately provide an approximately correct control voltage to the VCO for generating frequency F₂. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a PLL frequency synthesizer capable of switching the output frequency thereof at high speed. It is another object of the present invention to provide a generally improved PLL frequency synthesizer. According to the invention, there are provided a PLL synthesiser and a method according to the appended independent claims. Preferred features of the invention are defined in the dependent subclaims BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: Fig. 1 is a block diagram schematically showing a conventional PLL frequency synthesizer; Fig. 2 is a schematic block diagram showing a PLL frequency synthesizer embodying the present invention; and Fig. 3 is a circuit diagram modeling a loop filter included in the embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT To better understand the present invention, a brief reference will be made to a prior art PLL frequency synthesizer, shown in Fig. 1. As shown, the prior art frequency synthesizer, generally 10, has a reference frequency oscillator 12, a fixed frequency divider 14, a VCO 16, a variable frequency divider 18, a phase/frequency comparator 20, a charge pump 22, and a loop filter 24. The fixed frequency divider 14 divides the output of the reference frequency oscillator while the variable frequency divider 18 divides the output of the VCO 16. The phase/frequency comparator 20 compares the outputs of the frequency dividers 14 and 18 to produce an output representative of a phase or frequency difference therebetween. The loop filter 24 smoothes the output of the charge pump 22 and then feeds it back to the VCO 16 as a control voltage C. Divisor data is applied to the variable frequency divider 18 via an input terminal 26. A lock signal L is fed from the comparator 20 to the loop filter 24 to switch over the time constant of the latter. The loop filter 24 has two different time constants one of which is selected at the time of switching and the other is selected after the switching. This is to promote rapid switching and to stabilize the frequency. Specifically, at the time when the frequency is switched over, the loop filter 24 selects a smaller time constant to effect rapid tuning. After the switchover, the loop filter 24 selects a greater time constant in response to the lock signal L fed from the phase/frequency comparator 20, thereby stabilizing the frequency. In practice, however, at the time when the switching operation is completed, the control voltage C applied to the VCO 16 has been deviated from an expected value due to the delay particular to detection and switching, as stated earlier. Therefore, the initial frequency just after the switching is different from predetermined one with the result that the switchover of the time constant of the loop filter 24 is delayed. This in turn delays the tuning time of the PLL and prevents the frequency to be switched over at high speed. Referring to Fig. 2, a PLL frequency synthesizer embodying the present invention is shown and generally designated by the reference numeral 30. In Fig. 2, blocks functionally equivalent to the blocks of Fig. 1 are designated by the same reference numerals, and redundant description will be avoided for simplicity. Let the phase/frequency comparator 20 and the variable frequency divider 18 shown in Fig. 1 be referred to as a first phase/frequency comparator and a first variable frequency divider, respectively. As shown in Fig. 2, the frequency synthesizer 30 has a second phase/frequency comparator 3 2 and a second variable frequency divider 34 in addition to the first phase/frequency comparator 20 and first variable frequency divider 18. The second variable frequency divider 34 divides the output of a VCO 16 while a fixed frequency divider 14 divides the output of a reference frequency oscillator 12. The second phase/ frequency comparator 32 compares the outputs of the two frequency dividers 14 and 34. A loop filter 24 switches over the time constant thereof on the basis of the result of comparison outputted by the comparator 32. Particular divisor data is applied to each of input terminals 26 and 36. The output of the frequency synthesizer 30 appears on an output terminal 38. In operation, the first variable frequency divider 18 divides the output of the VCO 16 by a particular divisor applied thereto via the input terminal 26. The fixed frequency divider 14 divides the output of the reference frequency oscillator 12. The first phase/frequency comparator 20 compares the outputs of the frequency dividers 18 and 14 and delivers the output thereof representative of their difference to a charge pump 22. In response, the charge pump 22 controls the loop filter 20 such that a capacitor included in the loop filter 20 is charged or discharged. The resulting output voltage of the loop filter 20 is fed back to the VCO 16 to effect necessary frequency control. The output of the VCO 16 is also applied to the second variable frequency divider 34. The frequency divider 34, therefore, divides the output of the VCO 16 by a divisor applied to the input terminal 36. The second phase/frequency comparator 32 compares the output of the variable frequency divider 34 with the output of the fixed frequency divider 14. When the difference between the two frequencies becomes smaller than a predetermined value, the comparator 32 feeds a switching signal SW to the loop filter 24 to switch over the time constant of the latter. Specifically, in the initial stage of frequency switching operation, the loop filter 24 selects a smaller time constant since the difference or deviation determined by the comparator 32 is greater than the predetermined value. As the switching operation proceeds, the deviation is sequentially reduced. When the deviation becomes smaller than the predetermined value, the comparator 32 causes the loop filter 24 to select a greater time constant by the switching signal SW. At the same time, the switching signal SW is supplied to the first variable frequency divider 18 and fixed frequency divider 14 to reset them and also to the first phase/frequency comparator 20 to initialize it. Assume that the divisors for frequency division applied to the input terminals 26 and 36 are respectively D₁ and D₂, and that the divisor initially set in the first variable frequency divider 18 is D₀. It is to be noted that the divisors D₁ and D₂ are stored in a ROM, not shown, and each is fed to one of the input terminals 26 and 36 via a buffer. Also, assume that the predetermined deviation to be detected by the second phase/frequency comparator 32 is t₁, and that the time necessary for the loop filter 24 to be switched over is t₂. Further, assume that the control voltages V₁ and V₂ are supplied to the VCO 16 when the loop including the second phase/frequency comparator 32 is stabilized with the divisors D₁ and D₂ applied to the second variable frequency divider 34, respectively, and that the maximum voltage which the charge pump 22 can output is V max . As shown in Fig. 3, let the loop filter 24 be modeled as a filter made up of resistors R₁ and R₂, a capacitor C, an input terminal 40, and an output terminal 42. When the control voltage D₀ is lower than the control voltage D₁, there holds an equation: t₁ + t₂ = - (R₁ + R₂) C l n V₁ - V max V₂ - V max Assuming that the voltage appearing on the output terminal 42 is initially V₀, then V(t) = V max + (R₁/(R₁ + R₂))·(V o - V max ) · e -t/(R1+R2)c When t₁, t₂, R₁, R₂, C and V₁ are given, V₂ is obtained and, therefore, optimum D₂ is produced. Preferably, a frequency intermediate between the frequencies D₀ and D₁ set in the first variable frequency divider 18 before and at the time of switching, respectively, should be set in the second variable frequency divider 34. Then, the second phase/frequency comparator 32 will output the switching signal SW to switch over the loop filter 24 before the PLL is established in response to the output of the first phase/frequency comparator 20. This is successful in reducing the tuning time of the PLL despite the delay particular to the detection and switching. In summary, in accordance with the present invention, a PLL frequency synthesizer has a second phase/ frequency comparator and a second variable frequency divider in addition to a first phase/frequency comparator which is included in a PLL. The synthesizer switches over the time constant of a loop filter by the output of the second phase/frequency comparator, i. e., independently of the PLL which is responsive to the output of the first phase/frequency comparator. As a result, the time constant of the loop filter is switched over at an advanced timing to thereby reduce the PLL tuning time.

Claims: A phase-locked loop (PLL) synthesizer having a switchable output frequency comprising: a reference oscillator (12) for generating a reference oscillation signal; a voltage-controlled oscillator (VCO) (16) for generating a VCO oscillation signal; a variable frequency divider (18) for frequency dividing said VCO signal in accordance with a divisor; a comparator (20) for phase/frequency comparing said reference oscillation signal with the output of said divider means; and a filter (24) for filtering the output of said comparator to produce a filtered output and supplying it to said VCO as said control signal, the time constant of said filter being changeable; characterized by a second variable frequency divider (34) for frequency dividing said VCO signal in accordance with a second divisor (D₂); and a second comparator (32) for phase/frequency comparing said reference oscillation signal with the output of said second divider, the time constant of said filter (24) being changeable in response to the output of said second comparator; and in that said second divisor (D₂) of said second variable frequency divider (34) is intermediate between divisors (D₀,D₁) of said first variable frequency divider (18) before and at the time of switching, respectively. A PLL synthesizer as claimed in claim 1, wherein said second divisor (D₂) is so selected that the frequency difference between said reference oscillation signal and the output of said second divider (34) is smaller than the frequency difference between said reference oscillation signal and the output of said first divider (18). A PLL synthesizer as claimed in claim 1 or 2, in which said reference oscillator comprises a third divider (14) for frequency dividing said reference oscillation signal in accordance with a predetermined divisor. A method of switching the frequency of an oscillation signal comprising the steps of: (a) generating a reference oscillation signal; (b) generating a second oscillation signal whose frequency changes in response to a control signal; (c) responsive to a divisor signal, frequency dividing said second oscillation signal according to a first divisor to produce a first divided signal, the frequency of the first divided signal being switchable in response to a change of the value of the first divisor between first and second values (D₀,D₁); (d) frequency and/or phase comparing said reference oscillation signal with said divided signal to produce a comparison signal; and (e) filtering said comparison signal to produce a filtered signal and supplying said filtered signal to the generating step (b) as said control signal, the time constant of the filtering step (e) being changeable; characterized by (f) responsive to a second divisor signal, frequency -dividing said second oscillation signal according to a second divisor (D2) to produce a second divided signal, the value of the second divisor lying between the first and second values of the first divisor (D o ,D₁); (g) frequency and/or phase comparing said reference oscillation signal with said second divided signal to produce a second comparison signal; and (h) changing the time constant of the filtering step (e) in response to said second comparison signal.

Inventors: NORIMATSU HIDENHIKO C O NEC CO, NORIMATSU, HIDEHIKO , C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H03L7
IPC codes: H03L7