PLL frequency synthesizer capable of changing an output frequency at a high speed

Abstract: In a frequency synthesizer, a first pulse removing circuit (31) is connected between a reference signal generator (21) and a phase-frequency comparator (24). A second pulse removing circuit (32) is connected between a variable frequency divider (23) and the phase-frequency comparator. Responsive to first removing data indicative of a first pulse number, the first pulse removing circuit removes pulses from the reference signal that are equal in number to the first pulse number for a first predetermined cycle to produce a first pulse removed signal. Responsive to second removing data indicative of a second pulse number, the second pulse removing circuit removes pulses from the divided signal that are equal in number to the second pulse number for a second predetermined cycle to produce a second pulse removed signal. Responsive to a current command, a current controlling circuit may control current supplied from/to a charge pump circuit (25). A control circuit may be connected between the phase-frequency comparator and the charge pump circuit. A switch may be inserted between the loop filter and the voltage controlled oscillator. When the switch switches off a PLL, a D/A converter supplies a control voltage to the voltage controlled oscillator and a filter capacitor of the loop filter. The charge pump circuit may comprise a control circuit, a constant current circuit, an integrating circuit, and a sample and hold circuit.

Description: Background of the Invention: This invention relates to a frequency synthesizer with a phase-locked loop (PLL). Such a frequency synthesizer is called a PLL frequency synthesizer. As well known in the art, the PLL frequency synthesizer comprises a reference signal generator, a voltage controlled oscillator, a variable frequency divider, a phase-frequency comparator, and a control voltage supplying circuit. The reference signal generator generates a reference signal with a reference frequency. Responsive to a control voltage signal, the voltage controlled oscillator generates a voltage controlled signal having a controllable oscillating frequency. The PLL frequency synthesizer produces the voltage controlled signal as an output signal. Therefore, the output signal has an output frequency equal to the controllable oscillating frequency. The output signal is supplied to the variable frequency divider. The variable frequency divider is also supplied with a designated dividing number D which defines the output frequency, where D represents a positive integer. The variable frequency divider frequency divides the output signal on the basis of the designated dividing number D to produce a divided signal. In other words, the variable frequency divider is for frequency multiplying the output signal by a factor l/D. The phase-frequency comparator is supplied with the reference signal and the divided signal. The phase-frequency comparator detects a phase-frequency difference between the reference signal and the divided signal to produce a phase-frequency difference signal indicative of the phase-frequency difference. In other words, the phase-frequency difference signal indicates either a lag or lead phase which the divided signal has in comparison with the reference signal. Responsive to the phase-frequency difference signal, the control voltage supplying circuit supplies the control voltage signal to the voltage controlled oscillator. More specifically, the control voltage supplying circuit comprises a current flow control circuit and a loop filter. Responsive to the phase-frequency difference signal, the current flow control circuit controls flow-in and flow-out of current supplied therefrom/to to produce a current flow control signal. The current flow control signal indicates the flow-out of the current when the phase-frequency difference signal indicates the lag phase. The current flow control signal indicates the flow-in of the current when the phase-frequency difference signal indicates the lead phase. Supplied with the current flow control signal, the loop filter filters the current flow control signal into a filtered signal as the control voltage signal. More particularly, the loop filter comprises a filter capacitor which is selectively charged and discharged when the current flow control signal indicates the flow-out and the flow-in of the current, respectively. In a conventional PLL frequency synthesizer, changing of the output frequency is carried out by changing step by step the designated dividing number D. Therefore, the PLL frequency synthesizer has a variable delay amount on changing the output frequency. As a result, a frequency error of the output frequency occurs in the PLL frequency synthesizer on changing the output frequency. Accordingly, the conventional PLL frequency synthesizer is defective in that it is impossible to change the output frequency at a high speed. Published European patent application No. 0376847 describes a PLL synthesiser in which the control signal for the voltage controlled oscillator (which outputs the synthesiser output) is generated by a PLL controller and low-pass filtered before being input to the voltage controlled oscillator. A switchable connection links the PLL controller output directly to the input of the voltage controlled oscillator, bypassing the low-pass filter. When the PLL controller is instructed to change the output frequency, the low-pass filter would tend to smooth the resulting change in the control signal for the voltage controlled oscillator, and thus slow the response of the circuit. The PLL controller output is therefore switched directly to the input of the voltage controlled oscillator when the output frequency is changed, so as to accelerate the response of the circuit. US Patent No. 4,745,372 describes a PLL synthesiser in which the operating mode can be switched between two modes in which the feedback loop gain is changed. A phase comparator compares a reference signal and a divided signal from the synthesiser output and produces a phase lock signal. In response to the phase lock signal, a mode switching signal is generated for switching the circuit between a high-gain mode and a low-gain mode. The high-gain mode is employed during output frequency changes in order to reduce response time. Summary of the Invention: The invention provides a frequency synthesiser as defined in claim 1, to which reference should now be made. Preferred features of the invention are defined in appendant dependent claims. The invention thus advantageously provides a frequency synthesizer comprising a reference signal generator for generating a reference signal with a reference frequency, a voltage controlled oscillator responsive to a control voltage signal for generating an output signal having an output frequency, a variable frequency divider supplied with the output signal and responsive to a designated dividing number defining the output frequency for frequency dividing the output signal on the basis of the designated dividing number to produce a divided signal, a first pulse removing circuit supplied with the reference signal and responsive to first removing data indicative of a first pulse number for removing pulses from the reference signal that are equal in number to the first pulse number for a first predetermined cycle to produce a first pulse removed signal, a second pulse removing circuit supplied with the divided signal and responsive to second removing data indicative of a second pulse number for removing pulses from the divided signal that are equal in number to the second pulse number for a second predetermined cycle to produce a second pulse removed signal, a phase-frequency comparator supplied with the first and the second pulse removed signals for detecting a phase-frequency difference between the first and the second pulse removed signals to produce a phase-frequency difference signal indicative of the phase-frequency difference, and a control voltage supplying circuit supplied with the phase-frequency difference signal for supplying the control voltage signal to the voltage controlled oscillator in response to the phase-frequency difference signal. The invention may thus provide a PLL frequency synthesizer which is capable of changing its output frequency at a high speed. Other advantages of this invention will become clear as the description proceeds. Specific embodiments of the invention will now be described in more detail by way of example, with reference to the drawings, in which; Fig. 1 is a block diagram of a conventional PLL frequency synthesizer; Fig. 2 is a block diagram of a PLL frequency synthesizer according to an embodiment of the instant invention; Fig. 3 is a block diagram of a pulse removing circuit for use in the PLL frequency synthesizer illustrated in Fig. 2; Figs. 4(a) through (e) collectively show a time chart for use in describing operation of the pulse removing circuit illustrated in Fig. 3; Referring to Fig. 1, a conventional PLL frequency synthesizer will be described at first in order to facilitate an understanding of the present invention. The PLL frequency synthesizer comprises a reference signal generator 21, a voltage controlled oscillator 22, a variable frequency divider 23, a phase-frequency comparator 24, a charge pump circuit 25, and a loop filter 26. The reference signal generator 21 generates a reference signal with a reference frequency. The voltage controlled oscillator 22 is supplied with a control voltage signal in the manner which will become clear as the description proceeds. Responsive to the control voltage signal, the voltage controlled oscillator 22 generates a voltage controlled signal having a controllable oscillating frequency. The PLL frequency synthesizer produces the voltage controlled signal as an output signal. Therefore, the output signal has an output frequency equal to the controllable oscillating frequency. The output signal is supplied to the variable frequency divider 23. The variable frequency divider 23 is also supplied with a designated dividing number D which defines the output frequency, where D represents a positive integer. The variable frequency divider 23 frequency divides the output signal on the basis of the designated dividing number D to produce a divided signal. In other words, the variable frequency divider 23 is for frequency multiplying the output signal by a factor l/D. The phase-frequency comparator 24 is supplied with the reference signal and the divided signal. The phase-frequency comparator 24 detects a phase-frequency difference between the reference signal and the divided signal to produce a phase-frequency difference signal indicative of the phase-frequency difference. In other words, the phase-frequency difference signal indicates whether the divided signal has either a lag or a lead phase in comparison with the reference signal. The phase-frequency difference signal is supplied to the charge pump circuit 25. The charge pump circuit 25 acts as a current flow control circuit which is for controlling flow-in and flow-out of current supplied therefrom/to to produce a current flow control signal. The current flow control signal indicates the flow-out of the current when the phase-frequency difference signal indicates the lag phase. The current flow control signal indicates the flow-in of the current when the phase-frequency difference signal indicates the lead phase. The current flow control signal is supplied to the loop filter 26. The loop filter filters the current flow control signal into a filtered signal which forms the control voltage signal. More particularly, the loop filter 26 comprises a filter capacitor (not shown) which is selectively charged and discharged when the current flow control signal indicates the flow-out and the flow-in of the current, respectively. At any rate, a combination of the charge pump circuit 25 and the loop filter 26 serves as a control voltage supplying circuit for supplying the control voltage signal to the voltage controlled oscillator in response to the phase-frequency difference signal. As is apparent from the above description, the PLL frequency synthesizer produces an output signal having an output frequency which is equal to D times as large as the reference frequency of the reference signal. It is therefore possible to change the output frequency by changing the designated dividing number D. In the conventional PLL frequency synthesizer, changing of the output frequency is carried out by changing step by step the designated dividing number D. As a result, the conventional PLL frequency synthesizer is defective in that it is impossible to change the output frequency at a high speed, as mentioned in the preamble of the instant specification. Referring to Fig. 2, the description will proceed to a PLL frequency synthesizer according to an embodiment of this invention. The PLL frequency synthesizer is similar in structure and operation to the conventional PLL frequency synthesizer illustrated in Fig. 1 except that the PLL frequency synthesizer further comprises first and second pulse removing circuits 31 and 32. The first pulse removing circuit 31 is connected between the reference signal generator 21 and the phase-frequency comparator 24. The second pulse removing circuit 32 is connected between the variable frequency divider 23 and the phase-frequency comparator 24. The first pulse removing circuit 31 is supplied with the reference signal from the reference signal generator 21 and first removing data A1 indicative of a first pulse number. Responsive to the first removing data A1, the first pulse removing circuit 31 removes pulses from the reference signal that are equal in number to the first pulse number for a first predetermined cycle to produce a first pulse removed signal. Instead of the reference signal, the first pulse removed signal is supplied to the phase-frequency comparator 24. The second pulse removing circuit 32 is supplied with the divided signal from the variable frequency divider 23 and second removing data A2 indicative of a second pulse number. The second pulse removing circuit 32 removes pulses from the divided signal that are equal in number to the second pulse number for a second predetermined cycle to produce a second pulse removed signal. Instead of the divided signal, the second pulse removed signal is supplied to the phase-frequency comparator 24. Therefore, the phase-frequency comparator 24 detects a phase-frequency difference between the first and the second pulse removed signals to produce a phase-frequency difference signal indicative of the phase-frequency difference. Turning to Fig. 3, the first pulse removing circuit 31 comprises a first inverter 311, a first counter 312, and a first AND gate 313. The first inverter 311 is supplied with the reference signal from the reference signal generator 21 (Fig. 2) as a first input pulse signal. The first inverter 311 inverts the first input pulse signal to produce a first inverted pulse signal. The first inverter 311 is connected to the first counter 312 which is supplied with the first pulse removing data. The first counter 312 counts up a first count in synchronism with the first inverted pulse signal. The first counter 312 produces a first time-up signal when the first count increases up to the first pulse number. The first time-up signal is supplied to the first AND gate 313 which is supplied with the first input pulse signal. Responsive to the first input pulse signal and the first time-up signal, the first AND gate 313 produces a first AND'ed signal as the first pulse removed signal. Similarly, the second pulse removing circuit 32 comprises a second inverter 321, a second counter 322, and a second AND gate 323. The second inverter 321 is supplied with the divided signal from the variable frequency divider 23 (Fig. 2) as a second input pulse signal. The second inverter 321 inverts the second input pulse signal to produce a second inverted pulse signal. The second inverter 321 is connected to the second counter 322 which is supplied with the second pulse removing data. The second counter 322 counts up a second count in synchronism with the second inverted pulse signal. The second counter 322 produces a second time-up signal when the second count increases up to the second pulse number. The second time-up signal is supplied to the second AND gate 323 which is supplied with the second input pulse signal. Responsive to the second input pulse signal and the second time-up signal, the second AND gate 323 produces a second AND'ed signal as the second pulse removed signal. Turning to Figs. 4(a) through (e), description will be made as regards operation of the first pulse removing circuit 31. The first input pulse signal is depicted along Fig. 4(a). The first input pulse signal is inverted by the first inverter 311 into the first inverted pulse signal as shown in Fig. 4(b). The first inverter 311 has an inverting delay time denoted as d1. It will be assumed that the first counter 312 is supplied with the first pulse removing data indicative of the first pulse number equal to four. In this event, the first counter 312 counts up the first count at a leading edge of each pulse in the first inverted pulse signal. The first counter 312 produces the first time-up signal of a logical "1" level when the first count increases up to the first pulse number, namely, four, as shown in Fig. 4(c). The first counter 312 is reset to produce the first time-up signal of a logical "0" level in response to another leading edge of the next succeeding pulse of the first inverted pulse signal. The first counter 312 has a counting delay time denoted as d2. Under the circumstances, the first AND gate 313 produces the first AND'ed signal as the first pulse removed signal as shown in Fig. 4(d). That is, the first AND gate 313 produces the first pulse removed signal having pulses which are one-fifth as frequent as those of the first input pulse signal. In other words, the first pulse removing circuit 31 removes four pulses from the reference signal for the first predetermined cycle which has five pulses. The first AND gate 313 has a gate delay time denoted as d3. If the first pulse removing data indicates the first pulse number equal to zero, the first counter 312 continuously produces the first time-up signal with the logical "1" level. In this event, the first AND gate 313 produces the first pulse removed signal in which the first input pulse signal is delayed for the gate delay time d3 the first AND gate 313 as shown in Fig. 4(e). Operation of the second pulse removing circuit 32 is similar to that of the first pulse removing circuit 31 and the description thereof is therefore omitted.

Claims: A frequency synthesizer comprising: a reference signal generator (21) for generating a reference signal with a reference frequency; a voltage controlled oscillator (22) responsive to a control voltage signal for generating an output signal having an output frequency; a variable frequency divider (23) supplied with said output signal and responsive to a designated dividing number (D) defining said output frequency for frequency dividing said output signal on the basis of said designated dividing number to produce a divided signal; a phase-frequency comparator (24) for producing a phase-frequency difference signal; and a control voltage supplying circuit (25,26) supplied with said phase-frequency difference signal for supplying said control vcltage signal to said voltage controlled oscillator in response to said phase-frequency difference signal; characterised by; a first pulse removing circuit (31) supplied with said reference signal and responsive to first removing data (A 1 ) indicative of a first pulse number for removing pulses from said reference signal that are equal in number to said first pulse number for a first predetermined cycle to produce a first pulse removed signal; and a second pulse removing circuit (32) supplied with said divided signal and responsive to second removing data (A 2 ) indicative of a second pulse number for removing pulses from said divided signal that are equal in number to said second pulse number for a second predetermined cycle to produce a second pulse removed signal; the phase-frequency ccmparator (24) being supplied with said first and said second pulse removed signals and being for detecting a phase-frequency difference between said first and said second pulse removed signals to produce the phase-frequency difference signal, which is indicative of said phase-frequency difference. A frequency synthesizer as claimed in Claim 1, wherein said first pulse removing circuit (31) comprises: a first inverter (311) supplied with said reference signal as a first input pulse signal for inverting said first input pulse signal to produce a first inverted pulse signal; a first counter (312) connected to said first inverter and supplied with said first pulse removing data for counting up a first count in synchronism with said first inverted pulse signal, said first counter producing a first time-up signal when said first count increases up to said first pulse number; and a first AND gate (313) responsive to said first input pulse signal and said first time-up signal for producing a first AND'ed signal as said first pulse removed signal. A frequency synthesizer as claimed in Claim 1, wherein said second pulse removing circuit (32) comprises: a second inverter (321) supplied with said divided signal as a second input pulse signal for inverting said second input pulse signal to produce a second inverted pulse signal; a second counter (322) connected to said second inverter and supplied with said second pulse removing data for counting up a second count in synchronism with said second inverted pulse signal, said second counter producing a second time-up signal when said second count increases up to said second pulse number; and a second AND gate (323) responsive to said second input pulse signal and said second time-up signal for producing a second AND'ed signal as said second pulse removed signal.

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