Phase-locked loop

Abstract: A phase-locked loop circuit (10) includes a variable-frequency oscillator (12) that generates a first oscillator signal, a reference signal source (14) that generates a second oscillator signal, a control block (20) that generates a select signal, and a frequency divider (16) that receives as an input signal one of the first and second oscillator signals. The frequency divider (16) also receives the select signal from the control block (20). The frequency divider (16) generates a plurality of frequency-divided signals in response to the input signal, and passes through a selected one of the plurality of frequency-divided signals as an output signal in response to the select signal. The frequency divider (16) also synchronises its output signal to its input signal. The phase-locked loop (10) also includes a frequency comparator (18) that receives the output signal of the frequency divider (16) and a signal derived from one of the first and second oscillator signals. The frequency comparator (18) compares the output signal of the frequency divider (16) to the signal derived from one of the first and second oscillator signals, and provides a feedback signal to the variable-frequency oscillator (12) reflecting this comparison. In this phase-locked loop circuit (10), the number of signal regenerators introduced by the programmable frequency divider (16) is effectively limited to one, thereby reducing the jitter introduced by the frequency divider (16).

Description: The present invention relates to a phase-locked loop. US-A-4,590,602 discloses a wide range, variable rate clock recovery circuit for NRZ data having a PLL and a frequency synthesizer which share control of a common VCO in single loop realization. Narrow PLL bandwidths and short acquisition time may be achieved employing the frequency synthesizer to initially control the VCO to produce an estimate of the data frequency which is accurate to within the bandwidth of the PLL. Once this VCO frequency is attained, the PLL disables the frequency synthesizer control of the VCO and provides fine tuning control of the VCO output frequency itself. Single loop realization is achieved with a wide range VCO which includes a narrow range VCO, a frequency divider, and an auto-ranging circuit. EP-A-0,841,754 discloses a digitally-controlled oscillator (DCO), such as may be used in clock generator or clock recovery circuitry in an integrated circuit. The DCO is a single-stage oscillator including a variable load implemented as a binary-weighted array of switched capacitors. Each capacitor has a plate connected to a common node (X), and a plate that receives a signal corresponding to one bit of a digital control word (DCOCW). The common capacitor node (X) is also connected to the input of a Schmitt trigger that produces the output clock signal (OUT-CLK) and a feedback signal that is applied to logic that inverts the common node of the capacitors. The switching time at the input of Schmitt trigger depends upon the variable load presented by the array of switched capacitors, which is controlled by the digital control word (DCOCW). As a result, the clock signal (OUTCLK) is digitally synthesized by a single stage of the DCO. A digital phase-locked loop (PLL) clock generator circuit including a phase detector, digital loop filter in combination with the DCO, and a programmable frequency divider providing a feedback path from the output of the DCO to the phase detector is also disclosed. JP-A-0,254,021 discloses a system in which, with a reference clock given, each frequency stage divides the clock and a multiplexer selects one of them and gives an output. Then an end point detection means detects an end point being the end of the period of an output. An end point signal is given at the end of period to activate an initalizing means thereby initializing each frequency division stage. Before the end point signal is given, if a strobe for frequency division switching comes from an external software, it is stored in a latch to switch the multiplexer when the end point signal arrives. US-A-4,087,677 discloses a system in which in the transmitter portion a multiplexer multiplexes first digital data and first teletype signals to provide first digital data signals. The first digital signals are converted into second digital data signals having a different form than the first digital data signals by a Viterbi coder. A PSK modulator responsive to the second digital data signals provides a first PSK signal for transmission. In the receiver portion a PSK demodulator receives a second PSK signal containing third digital data signals having multiplexed second digital data and second teletype signals with the third digital data signals being extracted from the second PSK signal. A Viterbi decoder converts the third digital data signals into fourth digital data signals having a different digital form than the third digital data signals. A PSK demultiplexer is responsive to the fourth digital data signals to separate the second digital data and the second teletype signals for utilization. The arrangement to extract the third digital data signals from the second PSK signals includes the PSK demodulator and a Costas' type phase locked loop including a number controlled oscillator, a digital phase shifter, an accumulator arrangement including three accumulators accumulating different data and errors, a clock recovery circuit and a loop filter controlling the number controlled oscillator. Phase-locked loop (PLL) circuits are used for a variety of purposes, including signal demodulation, frequency synthesis, pulse synchronisation of signals from mass storage devices, and regeneration of "clean" signals. A PLL circuit typically compares a variable-frequency signal to a reference signal to determine a frequency difference, which is then fed back to the variable-frequency signal generator to synchronise the two signals. Often, logic circuit elements such as frequency dividers are placed between at least one of the two signal sources and the frequency comparator. Each such intervening logic circuit element, known as a regenerator, has an uncertainty in its output signal timing and therefore contributes to an overall signal timing uncertainty in the PLL circuit, thereby contributing to jitter in the phase-locked loop. To increase the versatility of a phase-locked loop circuit, it is desirable to introduce flexibility in the frequency divide ratios used to generate the signals used by the frequency comparator. This requires the introduction of multiplexers after the frequency dividers to select the signal with the desired frequency division ratio. These multiplexers are also regenerators, further contributing to the jitter of the phase-locked loop circuit. The present invention seeks to provide an improved phase-locked loop circuit. According to an aspect of the present invention, there is provided a phase locked loop as specified in claim 1. According to another aspect of the present invention, there is provided a method of synchronising a variable frequency signal as specified in claim 7. According to another aspect of the present invention, there is provided a frequency divider as specified in claim 12. The preferred embodiment can provide a phase-locked loop circuit with selectable frequency divide ratios and reduced jitter. In one embodiment, the phase-locked loop circuit includes a variable-frequency oscillator that generates a first oscillator signal, a reference signal source that generates a second oscillator signal, a control block that generates a select signal, and a frequency divider that receives as an input signal one of the first and second oscillator signals. The frequency divider also receives the select signal from the control block. The frequency divider generates a plurality of frequency-divided signals in response to the input and selects a selected one of the plurality of frequency-divided signals as an output signal in response to the select signal. The frequency divider also synchronises the output signal to its input signal. The phase-locked loop also includes a frequency comparator that receives the output signal of the frequency divider and a signal derived from one of the first and second oscillator signals. The frequency comparator compares the output signal of the frequency divider to the signal derived from one of the first and second oscillator signals, and provides a feedback signal to the variable-frequency oscillator reflecting this comparison. An advantage of the preferred embodiment is that, while a programmable frequency divider is used to maximise the flexibility of the phase-locked loop circuit, the number of regenerators introduced by the frequency divider is effectively limited to one, thereby reducing the jitter caused by the frequency divider. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a block diagram of an embodiment of a phase-locked loop circuit; FIGURE 2 is a block diagram of one embodiment of a reference clock source for use in the phase-locked loop circuit; and FIGURE 3 is a block diagram of a frequency divider for use in the phase-locked loop circuit. The preferred embodiments of the present invention and their advantages are best understood by referring to FIGURES 1, 2 and 3 of the drawings. Referring to FIGURE 1, a block diagram of a phase-locked loop (PLL) circuit 10 is shown. PLL circuit 10 includes a voltage-controlled oscillator (VCO) 12, a reference clock source 14, a programmable frequency divider 16, a frequency comparator 18 and a control block 20. Reference clock source 14 provides a reference clock signal REF to frequency comparator 18. In one embodiment, illustrated in FIGURE 2, reference clock source 14 includes an oscillator 22 and a frequency divider 24. Frequency divider 24 reduces the frequency of the output signal from oscillator 22 by a predetermined ratio. Frequency divider 24 may be a programmable frequency divider constructed in a manner similar to frequency divider 16, described below, in which case frequency divider 24 receives a ratio select signal RSEL2 from control block 20. Voltage-controlled oscillator 12 provides a variable-frequency oscillating signal (S1) to frequency divider 16. VCO 12 may be any standard voltage-controlled oscillator, the design of which is well known in the art, or any other type of controlled, variable-frequency oscillator. Frequency divider 16 reduces the frequency of the signal from VCO 12 by a selected ratio, which is determined by a ratio select signal RSEL1 from control block 20. This ratio is preferably selected so that the frequency of the output signal from frequency divider 16 approximately matches the frequency of the output signal from reference clock source 14, or one of its harmonics. The output of frequency divider 16 (S2) is provided to frequency comparator 18. The design and operation of frequency divider 16 will be described more fully below. Frequency comparator 18 compares the frequency of reference clock signal REF and variable-frequency signal S2. This may be accomplished by continually comparing the phases of the two signals in a manner well known to those skilled in the art of phase-locked loops. Alternatively, any other well-known frequency comparison method may be used. The resulting frequency error signal FERR is provided as feedback to VCO 12 to adjust the frequency of its output signal. In this manner, VCO 12 is adjusted to produce an output signal with a frequency that is as near as possible to a multiple of the frequency of reference clock signal REF. Referring to FIGURE 3, a block diagram of frequency divider 16 is shown. Frequency divider 16 includes an optional predivide block 25 and a series of frequency divide blocks 26a through 26f. Output signal S1 from VCO 12 is optionally provided to predivide block 25, which divides the frequency of signal S1 by a predetermined ratio. In this example, a predivide block 25 with a division ratio of four is used. It will be understood, however, that predivide block 25 is not essential to the operation of the preferred embodiment described herein. The output signal S3 from predivide block 25 is provided to the clock input of frequency divide block 26a. Alternatively, if no frequency predivision is necessary, signal S1 is provided directly to the data input of frequency divide block 26a. Each frequency divide block 26a through 26f reduces the frequency of its input signal by a predetermined ratio, such as two, to produce an output signal. In the example shown in FIGURE 3, each frequency divide block 26a through 26f is a D flip-flop, with its inverted output signal Q-bar tied to its data input signal D. In this arrangement, each frequency divide block 26a through 26f generates an output signal Q which has a frequency equal to half that of its clock input signal. Thus, each frequency divide block 26b through 26f receives as its clock input signal the output signal Q from the previous frequency divide block 26a through 26e in the series. In this example, the output signal Q from frequency divide block 26f will have a frequency that is equal to the frequency of signal S3 divided by 64. While D flip-flops have been used to illustrate one embodiment, it will be appreciated that each frequency divide block 26a through 26f may comprise any other logic circuitry, including but not limited to a JK flip-flop, configured to reduce the frequency of an input signal by a predetermined ratio. In one alternative embodiment, a synchronous divider chain may be implemented. In this embodiment, frequency divide blocks 26a through 26f are all clocked by a common clock signal, such as signal S3. Each frequency divide block 26b through 26f may then receive as input signals the output signals from all of the preceding frequency divide blocks in the chain. Each frequency divide block 26b through 26f may be configured, using appropriate logic circuitry, to toggle its output when the output signals from all preceding frequency divide blocks are HIGH. It will be appreciated that this type of synchronous divider chain, which is well known to those skilled in the art, provides a reduction in frequency by a factor of two at each frequency divide block in the chain. However, the advantages of the preferred embodiment may be realised with or without the use of a synchronous divider chain. It will also be appreciated that the divide ratio of each frequency divide block need not be the same as that of all the other frequency divide blocks. For example, a divide-by-3 block may be used in one or more frequency divide blocks 26a through 26f. In addition, the number of frequency divide blocks may be altered to suit any particular implementation, for example to achieve a greater total frequency divide ratio or to reduce the number of integrated circuit components that constitute frequency divider 16. Signal S3 and the output signals from frequency divide blocks 26a, 26b and 26c are provided to the data inputs of a 4-to-1 multiplexer 28. Select signals RSEL1(A) and RSEL1(B) from control block 20 determine which of the input signals is passed through as the output of multiplexer 28. Likewise, the outputs of frequency divide blocks 26d, 26e and 26f are provided to three of the data inputs of a 4-to-1 multiplexer 30. Select signals RSEL1(C) and RSEL1(D) from control block 20 determine which of the input signals is passed through as the output of multiplexer 30. It will be understood that, for purposes of economy in signal routing, signal RSEL1(C) may be identical to signal RSEL1(A), and signal RSEL1(D) may be identical to signal RSEL1(B). The output signals from multiplexers 28 and 30 are provided to the inputs of a 2-to-1 multiplexer 32. Select signal RSEL1(E) from control block 20 determines which of the input signals is passed through as the output signal S2 of frequency divider 16. The output of multiplexer 32 is received by a retiming device 33. Retiming device 33 may be, for example, a flip-flop or some other signal latching mechanism that produces an output signal 52 that corresponds to the input signal received from multiplexer 32. Retiming device 33 also receives a clock signal that synchronises the output of retiming device 33 to signal S1 from VCO 12. In this example, the clock signal received by retiming device 33 is the output signal 53 from predivide block 25. It will be understood, however, that signal S1 from VCO 12 may be used as a clock signal for retiming device 33, particularly in an embodiment in which predivide block 25 is not implemented. Multiplexers 28, 30 and 32 allow control block 20 to select any one of seven different frequency divide ratios for the output of frequency divider 16. Two stages of multiplexing are shown here because some integrated circuit technologies allow the stacking of a limited number of logic levels, thereby limiting the maximum size of a single multiplexer to 4-to-1, or in some cases 2-to-1. Thus, it will be understood that, in some technologies, it may be possible alternatively to implement multiplexers 28, 30 and 32 as a single 8-to-1 (or 7-to-1) multiplexer. Alternatively, in some technologies each multiplexer 28 or 30 may be implemented as three 2-to-1 multiplexers. Regardless of the implementation, it will be understood that the advantages provided by the described embodiments and described below may be realised by providing a synchronising clock signal to a retiming device 33 following the final output multiplexer 32. The use of a clock signal to synchronise or retime the output of frequency divider 16 effectively reduces the number of regenerators between signal S3 and frequency comparator 18 to one, namely retiming device 33. In the embodiment in which predivide block 25 is not implemented, the number of regenerators between VCO 12 and frequency comparator 18 is effectively reduced to one. This significantly reduces the uncertainty in the timing of the output signal S2 from frequency divider 16, thereby reducing the jitter in PLL circuit 10. Thus, PLL circuit 10 provides the advantages of flexibility in the frequency divide ratio used and a very low level of jitter. It will be understood that, in some logic design technologies such as source coupled FET logic (SCFL), each of the signals described herein may be a complementary, differential pair of logic signals. Furthermore, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A phase-locked loop comprising: a frequency divider (16) coupled to receive a variable-frequency oscillating signal (S1, S3) from a voltage controlled oscillator (12); and a frequency comparator (18) coupled to receive an output signal (S2) from the frequency divider (16); wherein the frequency divider comprises a first frequency divide block (26a) for receiving the variable-frequency oscillating signal (S1, S3) and for outputting a first frequency divided signal to a second frequency divide block (26b); at least one multiplexer (28, 32) for receiving the oscillating signal (S1, S3), the first frequency divided signal, and a second frequency divided signal from the second frequency divide block (26b); and a single retiming device (33) coupled between the voltage controlled oscillator (12) and the frequency comparator (18), the retiming device being operable to receive an output of the at least one multiplexer (28, 33), to receive the oscillating signal (S1, S3) as a retiming signal, and to output the output signal (S2) to the frequency comparator (18). A phase-locked loop according to claim 1, including a control unit (20) operable to control the operation of the or each multiplexer (28, 33) so as to select the frequency divided signal outputted from the frequency divider (16). A phase-locked loop according to claim 1 or 2, including a predivide unit (25) at the output of the voltage controlled oscillator (12) for predividing the signal from the voltage controlled oscillator prior to input to the first frequency divide block (26a). A phase-locked loop according to claim 1, 2 or 3, including a series of n frequency divider blocks (26a-26f) coupled to one another in series wherein the output of one of said frequency divider blocks is coupled to the or one of said multiplexers (28, 32) and to the input of a succeeding frequency divider block in the series. A phase-locked loop according to any preceding claim, including a reference clock unit (14) provided with a clock frequency divider unit (24) for dividing the clock signal; the clock unit being coupled to the frequency comparator (18). A phase-locked loop as in claim 5, wherein the or a control unit (20) is operable to control the clock frequency divider unit (24) and thereby the frequency of the clock signal fed to the frequency comparator. A method of reducing jitter in a phase-locked loop synchronised output signal, comprising: providing a frequency divider (16) comprising a first frequency divide block (26a), a second frequency divide block (26b), one or more multiplexers (28, 32), and a retiming device (33); providing a frequency comparator (18); receiving at the first frequency divide block an oscillating signal (S1, S3) from a voltage controlled oscillator (12); receiving at the second frequency divide block a first frequency divided signal from the first frequency divide block; receiving at the one or more multiplexers the first frequency divided signal, a second frequency divided signal from the second frequency divide block, and the oscillating signal; receiving at the retiming device the oscillating signal (S1,S3) and an output from the one or more multiplexers; using the received oscillating signal (S1,S3) to retime the received output from the one or more multiplexers; and receiving at the frequency comparator an output signal (S2) from the retiming device. A method according to claim 7, including the step of controlling the operation of the or each multiplexer (28, 32) so as to select a frequency of the output signal from the retiming device. A method according to claim 7 or 8, including the step of predividing the signal from the voltage controlled oscillator prior to input to the first frequency divide block (26a). A method as in claim 7, 8 or 9, including the steps of: receiving a frequency divide ratio select signal at the or one of said multiplexers (28, 32); and selecting the first or the second frequency divided signals as an output signal by the or one of said multiplexers (28, 32) in response to the frequency divide ratio select signal. A method according to any one of claims 7 to 10, including the step of predividing the oscillating signal received by the retiming device. A frequency divider comprising: an input port operable to receive an oscillating input signal; a plurality of frequency divide blocks (26a-26f) connected in series, each of the frequency divide blocks being operable to generate a block output signal having a frequency less than a frequency of the oscillating input signal by a selected frequency divide ratio, a first one of the frequency divide blocks (26a) being coupled to the input port and operable to receive the input signal, each subsequent frequency divide block (26b-26f) being operable to receive a block input signal from a preceding frequency divide block; a multiplexer system (28, 32) operable to receive a plurality of the block output signals, and operable to receive a select signal, and operable to provide a selected one of the block output signals as an output signal of the multiplexer system in response to the select signal; and a single retiming device (33) coupled to the multiplexer system and to the input port, the retiming device (33) being operable to generate a retimed output signal of the frequency divider the retimed output signal of the frequency divider being synchronised by the retiming device (33) with the oscillating input signal. A frequency divider as in claim 12 comprising a frequency predivide block coupled between the input port and the first frequency divide block, and having a frequency reduction ratio greater than three. A frequency divider as in claim 12 or 13, wherein each one of the frequency divide blocks (26a-26f) and/or the retiming device (33) comprises a flip-flop.

Inventors: MCNEILL DAVID E, TURUDIC ANDY, MCNEILL, DAVID E., TURUDIC, ANDY
Applicants: TRIQUINT SEMICONDUCTOR INC, TRIQUINT SEMICONDUCTOR, INC.
Application year: 2000
Main classification: H03L7
IPC codes: H03K23, H03L7