CA2122643C - Pll frequency synthesizer capable of changing an output frequency at a high speed - Google Patents

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

21 22~4~
PLL FREQUENCY ~YI~ln~ ZER CAPABLE OF CE~ANGING
AN OUTPUT FREQUENCY AT A HIGH SPEED

Back~ro~ln~l of the Inven~ion:
This is a divisional application of ~n~ n Application No. 2,053,748 filed October 18, lg91.
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 r, ~uen.:y synthesizer aomprises 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 ~requency.
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 ~ designated dividing number D which defines the output frequency, where D represents a positive integer.
5 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 dividing the output signal by a factor l/D. The phase-frequency 10 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 15 difference. In other words, the phase-frequency difference signal indicates one of lag and lead phases which the divided signal has in comparison with the reference signal. Responsive to the phase-frequency difference signal, the control voltage supplying circuit 20 supplies the control voltage signal to the voltage controlled oscillator.
More specifically, the control voltage supplying circuit comprises a current f low control circuit and a loop filter. Responsive to the 25 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 f low control signal indicates the ~2~
.

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 5 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 f ilter capacitor which is selectively charged and l0 discharged when the current flow control signal indicates the f low-out and the f low-in of the current, respectively .
In a conventional PLL frequency synthesi2er, changing of the output f requency is carried out by 15 changing step ~y 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 20 frequency. Accordingly, the conventional PL1 frequency synthesizer is defective in that it is impossible to change the output frequency at a high speed.
Summary of the Invention:
It is therefore an object of the present 25 invention to provide a PLL frequency synthesizer which is capable of changing an output frequency at a high speed.
Other objects of this invention will become clear as the description proceeds.

i 2 ~ 22643 According to an aspect of this lnvention, a frequency synthe~izer compriaes a reference aignal generator for genera-ting a reference signal with a reference frequency, a voltage-controlled oacillator reaponaive to a control voltage ~ignal for generating an output signal having an output frequency, a variable freguency divider aupplied with the output signal and responaive to a de~ignated dividing number ~ef;n;n~ the output frequency for frequency dividing the output aignal on the basis of the designated dividing number to produce a divided signal, and a phase-frequency comparator supplied with the reference signal and the divided signal for detecting a phase- frequency difference between the reference signal and the divided ~ignal to produce a phase-fretluency difference indicative of the phase-frequency difference. The phase-frequency difference signal compriae~ a pulse sequence having pulaea for a predeter-mined cycle that are equal to a controllable number. The fre-quency syntheaizer al~o comprisea a modifying meana connected to the phase-frequency comparator, and responsive to a control signal enable for modifying the phaae-frequency difference aignal into a modified ~ignal. The control enable ~ignal ia present while the controllable o~cillating frequency is changed. The modifying means gradually decreases the control-l;lble number to at last a predet~rm;n~ final nu~ber when the control enable signal is pre~ent. The control voltage aupply-ing circuit i8 aupplied with the modified aignal for aupplying the control voltage signal to the voltage-controlled oscillator in responEIe to the modified aignal.
Brief DescriDtion of the Draw; n~Ta Figure l ia a block diagram of a conv~n~onAl PLL
frequency ayn~h~ai.er;
Figure 2 i~3 a block diagram of a PLL f requency synthe~izer according to a firat ~ t of the instant invention;
~.~
,.i ~ 2 1 22643 4a Figure 3 is a block diagram o~ a pulse-removing circuit ~or use in the PL~ frequency sy~th~si~r illustrated i~ Figure 2;
Figures 4 (a) through (e) collectively show a time 5 chart for use in describi~g operation o~ ~he pulse-removing circuit illustrated in Figure 3;

.i~

~;).! ~ 2 ~ 3 s Fig. 5 is a block diagr~m of a PLL frequency synthesizer according to a second embodiment of the instant invention;
Fig. 6 is a block diagram of a charge pump S circuit for use in the PLL frequency synthesizer illustrated in Fig. 5;
Fig. 7 is a block diagram of a charge-up control circuit o~ a current controlling circuit for use in the PLL frequency synthesizer illustrated in Fig. 5;
Figs. 8(a) through (c) collectively show a time chart for use in describing operation of the charge-up control circuit illustrated in Fig. 7;
Fig. 9 is a block diagram of a PLL frequency synthesizer according to a third embodiment of the lS instant invention;
Fig. lO shows a time chart for use in descri~ing operation of a charge pump circuit of the PLL
frequency synthesi2er illustrated in Fig. 9;
Fig. ll is a block diagram of a PLL frequency synthesizer according to a fourth ' ~(1i nt of the instant invention;
Fig. 12 shows a time chart for use in describing operation of the PLL frequency synthesizer illustrated in Figs. l and ll;
Fig. 13 is a block diagram of a PLL frequency synthesizer according to a fifth ~rnho~i -It of the instant invention; and .

Fig. 14 shows a block diagram of a PLL
frequency synthesizer which modifies the PLL frequency synthesizer illustrated in Fig. 12.
Description of the Pref erred Embodiments:
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 f requency .
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 21~6~3 positive integer. The variable frequency divider 23 frequency divides the output signal on the basis of the designated dividing numoer D to produce a divided signal.
In other words, the variable frequency divider 23 is for 5 frequency dividing the output signal by a f actor 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 10 signal to produce a phase-frequency difference signal indicative of the phase-frequency difference. In other words, the phase-frequency difference signal indicates one of lag and lead phases which the divided signal has in comparison with the reference signal.
The phase-frequency difference signal is supplied with 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 theref rom/to to produce a current f low control 2 0 signal . The current f low 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 25 lead phase. The current flow control signal is supplied with the loop filter 26. The loop filter filters the current flow control signal into a filtered signal as the control voltage signal. Ilore particularly, the loop _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ . .. . ..

~ ~%~6~

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 5 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 apparent from the above description, the PLL
frequency synthesizer produces the output signal having the 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 15 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 20 that it is impossible to change the output frequency at a high speed, as mentioned in the preamble of the instant specif ication .
Referring to Fig. 2, the description will proceed to a PLL frequency synthesizer according to a 25 first 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 S 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 10 with the reference signal from the reference signal generator 21 and first removing data Al indicative of a first pulse number. Responsive to the first removing data Al, the first pulse removing circuit 31 removes pulses from the reference signal that are equal in number 15 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 20 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 25 number for a second predet~rm; n~d 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.

4~

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 indic~tive 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 (~ig. 2~ as a first input pulse signal. The first inverter 311 inverts the f irst input pulse signal to produce a f irst 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 2~2~6~

fre~uency 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 5 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 10 pulse number. The second time-up signal is supplied to the second AND gate 323 which is supplied with the second input pulse signal. 3~esponsive to the second input pulse signal and the second time-up signal, 'che second AND gate 323 produces a second AND ' ed signal as the second pulse 15 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 20 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 to dl.
It will be assumed that the first counter 312 is supplied with the first pulse removing data indicative 25 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 flrst time-up ~12~6~3 signal of ~ 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 S in response to another leading edge of the next succeeding pulse of the f irst inverted pulse signal . The first counter 312 has a counting delay time denoted to d2 . Under the circumstances, the f irst AND gate 313 produces the first AND'ed signal as the first pulse 10 removed signal as shown in Fig. 4(d). That is, the first AND gate 313 produces the first pulse removed signal having pulses which is one-fifth as large as those of the first input pulse signal. In other words, the first pulse removing circuit 31 removes four pulses from the 15 reference signal for the first predetermined cycle which has five pulses. The first AND gate 313 has a gate delay time denoted to d3.
If the first pulse removing data indicates the first pulse number equal to zero, the first counter 312 20 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 to which the first input pulse signal is delayed for the gate delay time t3 by 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 f irst pulse removing circuit 31 and the description thereof is therefore omitted.
Referring to Fig. S, the description will , . . ~

~2~1~43 proceed to a PLL frequency synthesizer according to a second: '-Q~ -nt 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 a current controlling circuit 35.
The current controlling circuit 35 is connected to the charge pump circuit 25 and is supplied with a current command C. Responsive to the current command C, the current controlling circuit 35 controls the current for the charge pump circuit 25. More specifically, the current command C is present while the output ~requency is changed. The current controlling circuit 35 controls the current so as to increase the current when the current command C is present and then to gradually decrease the current af ter the current command C becomes absent. A little more in detail, the current controlling circuit 35 comprises a charge-up control circuit and a discharge control circuit.
Turning to Fig. 6, the charge pump circuit 25 comprises a complementary-symmetry metal-oxide-semiconductor field-effect transistor (CMOSFET) which consists of a p-channel MOSFET 36 and an n-channel MOSFET
37. Each of the MOSFETs 36 and 3~ has a gate ~P~min~l, a drain ~P~;n~l, and a source ~Prm; n~l. The p-channel and the n-channel MOSFETs 36 and 37 are ~oined at their drain t~rm;n.qls which are connected to the loop filter 26 (Fig.
5~. The p-channel MOSFET 36 has the source t~rm;n;~ll _ _ _ _ _ _ _ _ , . . . _ . .. . ...

~~fi~'~

which is connected to the charge-up control circuit of the current controlling circuit 35. The n-channel MOSFET
37 has the source t~rm; nAl which is connected to the discharge control circuit of the current controlling 5 circuit 3 5 . Both of the p-channel and the n-channel MOSFETs 36 and 37 have the gate t~rmi nAl C which are connected to the phase-frequency comparator 24 (Fig. 5).
More particularly, the phase-frequency difference signal comprises lag and lead phase signals which indicate the 10 lag and the lead phases, respectively. The lag phase signal is supplied to the gate terminal of the p-channel MOSFET 36. The lead phase signal is supplied to the gate ,rmin~l of the n-channel MOSFET 37.
Supplied with the lag phase signal, the 15 p-channel MOSFET 36 is put into an ON state, thereby the current flows from the charge-up control circuit to the loop filter 26 through the p-channel MOSFET 36. Supplied with the lead phase signal, the n-channel MOSFET 37 is put into an ON state, thereby the current flows from the 20 loop filter 26 to the discharge control circuit through the n-channel MOSFET 37.
Turning to Fig. 7, the charge-up control circuit of the current controlling circuit 36 comprises a switching element 41, an operational amplifier 42, an 25 n-channel MOSFET 43, first and second p-channel MOSFETs 44 and 45, first through third resistors 46, 47, and g8, and a capacitor 49. The operational amplifier 42 has an inverting input t~rminAl, a noninverting input t~rminAl, and an amplifier output t~rm; nAl .
The switching element 41 has a control input terminal supplied with the current command C. The switching element 41 is supplied with a positive source 5 voltage +VDD. The switchin~ element 41 is connected to the noninverting input ~rm;nAl of the operational amplifier 42. When the current command C is present or has a logic one value, the switching element 41 turns on, thereby the positive source voltage +VDD is supplied to 10 the noninverting input terminal of the operational amplifier 42. The inverting input terminal of the operational amplifier 42 is an end of the first resistor 46 which has another end grounded. The noninverting input terminal of the operational amplifier 42 is 15 connected to an end of the second resistor 47 which has another end grounded. The second resistor 47 is connected to the capacitor 49 in parallel. The noninverting input terminal of the operational amplifier 42 is also connected to an end of the third resistor 47 20 which has another end supplied with the positive source voltage ~VDD. The amplifier output t~rminAl of the operational amplif ier 42 is connected to the gate tPrm;nAl of the n-channel MOSFET 43 which has the source t~rm;nAl connected to the inverting input tf~rm;nAl of the 25 operational amplifier 42.
The n-channel ~qOSFET 43 has the drain tf~rm;nAl which is connected to the drain t~rm;nAl of the first p-channel ~OSFET 44. The drain t~-rm;n~l of the n-channel _ _ _ _ MOSFET 43 is also connected to the g21te tlorm;n~lc of the first and the second p-channel MOSFETs 44 and 45. Both of the first ~nd the second p-channel MOSFETs 44 and 45 have the source terminals which are supplied with the 5 positive source voltage +VDD. The second p-channel MOSFET 45 has the drain terminal which is connected to the source terminal of the p-channel MOSFET 36 (Fig. 6).
Therefore, ~ combination of the first and the second p-channel MOSFETs 44 and 45 composes a current Miller lû circuit. The second p-channel MOSFET 45 has a gate width which is N times as large as that of the first p-channel MOSFET 44, where N represents a predetermined number.
When a first drain current Dl flows through the n-channel MOSFET 43, through the second p-channel MOSFET 45 flows a 15 second drain current D2 which is N times as large as the f irst drain current Dl .
Turning to Figs. 8(a) through (c), description will be made as regards operation of the charge-up control circuit of the current controlling circuit 36 20 illustrated in Fig. 7. It will be assumed that the first through the third resistors 46 to 48 have first through third resistance values, respectively, which are denoted to R46, R47, and R48. The third resistance value R48 is larger than the second resistance value R47. Figs. 8(a) 25 through (c) show the current command C, and the first and the second drain currents Dl and D2 ~ respectively .
When the current command C is present or becomes the logic one value at a time instant to ~ the ~ 2~3 switching element 41 turns on. Therefore, the capacitor 49 is charged up to a voltage of VDD. Through the n-channel MOSFET 43 flows the first drain current Dl which has a current value of VDD/R46. Therefore, through 5 the second p-channel MOSFET 45 flows the second drain current D2 which has a current value of N x VDD/R46.
When the current command C becomes absent or the logic zero value at a time instant t1, the switching element 41 turns off. Therefore, the capacitor 49 is 10 exponentially discharged up to a voltage of R47/(R47 +
R48~ ) x VDD. As a result, the first drain current D
exponentially decreases up to ~ current value of ( 47/( 47 48) ) DD/ 46 cco ngly, the second drain current D2 exponentially decreases up to a current value of N x (R47/(R47 + R4g) ) x VDD/R46 The discharge control circuit of the current controlling circuit 35 is similar in structure and operation to the charge-up control circuit illustrated in Fig. 7 except that the n-channel MOSFET 43 is replaced 20 with a p-channel MOSFET, the p-channel MOSFETs 44 and 45 are replaced with n-channel MOSFETs, and the positive source voltage +VDD and the ground are exchanged each other .
Referring to Fig. 9, the description will 25 proceed to a PLL frequency synthesizer according to a third embodiment of this invention. The PLL frequency gynthesizer i5 similar in ::~LLU~,~Ul~:: and operation to the conventional PLL frequency synthesizer illustrated in _ ., . .. _ _ . , .. . ... , , _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2~2~3 Flg. 1 except that the charge pump circuit is modified from that illustrated in Fig. 1 as will later ~ecome clear. The charge pump circuit is therefore depicted at 25a .
The illustrated charge pump circuit 25a comprises a control circuit 51, a constant current circuit 52, an integrating circuit 53, and a sample and hold circuit 54.
The control circuit 51 is connected to the phase-frequency comparator 24. Responsive to the phase-frequency difference slgnal, the control circuit 51 produces first through third control signals. The first through the third control signals are supplied to the constant current circuit 52, the integrating circuit 53, and the sample and hold circuit 54, respectively.
Responsive to the first control signal, the constant current circuit 52 produces a constant current.
The constant current is supplied to the integrating circuit 53. The integrating circuit 53 has an initial voltage. The integrating circuit 53 integrates the constant current to an integrated voltage. The integrating circuit 53 is reset from the integrated voltage to the initial voltage in response to the second control signal. The integrated voltage is supplied to 2 5 the sample and hold circuit 54 . Responsive to the third control signal, the sample and hold circuit 44 samples the integrated voltage to hold a sampled voltage as a held signal. The sample and hold circuit 44 supplies the ~12264~

held signal to the loop filter 26 as the current flow control signal.
Referring to Fig. 10 in addition to Fig. 9, description will be made as regards operation of the S charge pump circuit 25~. In Fig. 10, the lag phase signal, the constant current, the integrated voltage, and the held signal are depicted along first through fourth lines .
As shown in the first line in Fig. lO, the lag lO phase signal is supplied from the phase-frequency comparator 24 to the control circuit Sl during a first time duration between a first time instant tl and a second time instant t2 Under the circumstances, the control circuit 51 produces the first control signal lS during a second time duration between the first time instant tl and a third time instant t3. Responsive to the first control signal, the constant current circuit 52 produces the constant current during the second time duration as shown in the second line in Fig. lO.
20 Therefore, the integrating circuit 53 integrates the constant current to the integrated voltage during the second time duration as shown in the third line in Fig.
10. After the third time instant t3, the integrating circuit 53 keeps the integrated voltage. At a fourth 2 S time instant t4, the control circuit 51 produces the third control signal. Responsive to the third control signal, the sample and hold circuit 44 samples the integrated voltage to hold a sampled voltage as a held 212~3 ZO, signal at a fifth time instant t5. The held signal is supplied to the loop filter 26 as the current flow control signal . At a sixth time instant t6 ~ the control circuit 51 produces the second control signal.
S T~cr.5ncive to the second control signal, the integrating circuit 53 is reset from the integrated voltage to the initial voltage. Therefore, the charge pump circuit 25a increases a level of the current flow control signal in response to the lag phase signal. In the similar 10 operation, the charge pump circuit 25a decreases the level of the current flow control signal in response to the lead phase signal.
It is understood that it is possible to set a desired value to ~ gain of the charge pump circuit 25a by lS changing the ratio of the first time duration to the second time duration.
Referring to Fig. 11, the description will proceed to a PLL frequency synthesizer according to a fourth f~mhor~; -nt of this invention. The PLL frequency 20 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 a control circuit 60.
The control circuit 60 is connected between the 25 phase-frequency comparator 24 and the charge pump circuit 25. The control circuit 60 is supplied with a control enable signsl CE . R-~cr~-nci ve to the control enable signal CE, the control circuit 60 acts as a modifying ~12~43 arrangement for modifying the phase-frequency difference signal into a modified signal. Instead of the phase-frequency difference signal, the modified signal is supplied to the charge pump circuit 25. Therefore, the 5 charge pump circuit 25 controls, in response to the modified signal, flow-in and flow-out of the current supplied therefrom/to to produce the current flow control signal. A little more in detail, the control circuit 60 is operable in one of a f irst mode and a second mode . At lO first, the first mode will be described. The second mode will be described later in the following.
In the ~irst mode, the control enable signal CE
is present while the output frequency is changed. The phase-frequency difference signal comprises a pulse 15 sequence having pulses for a predetermined cycle that ~re equal in number to a controllable number. When the control enable signal CE is present, the control circuit 60 gradually decreases the controllable number until at last a predet~rmin~fl final number. The predet-rm;ned 20 final number is, for example, one sixty-fourths as large as the controllable number before being decreased.
Fig. 12 shows transient responses of the output frequency in the conventional PLL frequency synthesizer (Fig. l) and the PLL frequency synthesizer according to 25 the fourth embodiment. In Fig. 12, a transient response for the conventional PLL frequency synthesizer is depicted at a solid line. Another transient response for the PLL frequency synthesizer according to the fourth 2~

~mho~; r-~t is depicted at a dotted line . As apparent from Fig. 12, it is possible to raise up the tran3ient response by decreasing the controllable number.
In the second mode, the control enable signal CE is present in a steady state of the PLL frequency synthesizer. The phase-frequency difference signal comprises a pulse sequence having pulses each of which has a controllable pulse width. The control circuit 60 narrows the controllable pulse width when the control enable signal CE is present. It is possible to improve unstableness in the PLL frequency synthesizer.
Referring to Fig. 13, the description will proceed to a PLL ~requency synthesizer according to a fifth embodiment of this invention. The PLL fre~uency synthesizer is similar in structure and operation to the conventional PLL frequency synthesizer except that the PLL frequency synthesizer further comprises a switch 65, a reset circuit 66, a control circuit 70, and a digital-analog converter 71.
The control circuit 70 is supplied with the designated dividing number D, the divided signal, the phase-frequency difference signal. On changing the output frequency, the control circuit 70 produces first and second control signals. The control circuit 70 also produces voltage data on changing the output frequency.
The voltage data represents a control voltage which is defined by the designated dividing number D on the basis of the phase-frequency difference signal. The control _ _ _ _ _, . _ . , .. _ . .. , ... . .. , _ _ _ _ 23 2 ~ 22643 circuit 70 determines the voltage data by carrying out binary search on the phase-frequency difference signal.
The switch 65 is inserted between the loop filter 26 and the voltage controlled oscillator 22. The 5 switch 65 is suppiied with the first control signal. The switch 65 switches off in response to the first control s ignal .
The loop filter 26 comprises first and second resistors 261 and 262 and a filter capacitor 263. The 10 first resistor 261 has one end connected to the charge pump circuit 25 and has another end connected to the switch 65 and to one end of the second resistor. The second resistor 262 has another end connected to one end of the filter capacitor 263 which has another end 15 grounded.
The digital-analog converter 71 is supplied with the voltage data from the control circuit 70. The digital-analog converter 71 is activated only when the output frequency is changed. The digital-analog 20 converter 71 converts the voltage data into the control voltage. ~he control voltage is supplied to the voltage controlled oscillator 22 and the filter capacitor 263 of the loop f ilter 26 .
Responsive to the second control signal, the 25 charge pump circuit 25 is unactivated or put into a high impedance state.
The reset circuit 66 is inserted between the reference signal generator 29 and the phase-frequency ~ 2~

comparator 24. On the basis of the divided signal, the control circuit 70 controls the reset circuit 66 so as to make the reset circuit 66 produce a reset signal which has a phase equal to that of the divided signal. The 5 reset signal is supplied to the phase-frequency comparator 24 instead of the reference signal.
In the PLL frequency synthesizer illustrated in Fig. 13, the digital-analog converter 71 supplies the control voltage to both of the voltage controlled oscillator 22 and the filter capacitor 263 of the loop filter 26. The digital-analog converter 71 may be modified into first and second digital-analog converters 71 and 72 as shown in Fig. 14. Under the circumstances, the first digital-analog converter 71 supplies the 15 control voltage to the voltage controlled oscillator 22.
The second digital-analog converter 72 supplies the control voltage to the filter capacitor 263 of the loop f ilter 2 6 .

Claims (2)

1. A frequency synthesizer comprising:
a reference signal generator for generating a refer-ence signal with a reference frequency;
a voltage-controlled oscillator responsive to a con-trol 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 phase-frequency comparator supplied with said reference signal and the divided signal for detecting 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, the phase-frequency difference signal comprising a pulse sequence having pulses for a predetermined cycle that are equal in number to a controllable number;
modifying means connected to the phase-frequency com-parator and responsive to a control enable signal for modifying the phase-frequency difference signal into a modified signal, the control enable signal being present while the controllable oscillating frequency is changed, the modifying means gradually decreasing the controllable number to at last a predetermined final number when the control enable signal is present; and a control voltage supplying circuit supplied with the modified signal for supplying the control voltage signal to the voltage-controlled oscillator in response to the modified signal.

2. A frequency synthesizer comprising:
a reference signal generator for generating a refer-ence signal with a reference frequency;
a voltage-controlled oscillator responsive to a con-trol 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 phase-frequency comparator supplied with the reference signal and the divided signal for detecting 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, the phase-frequency difference signal comprising a pulse sequence having pulses, each of which has a controllable pulse width;
modifying means connected to the phase-frequency comparator and responsive to a control enable signal for modifying the phase-frequency difference signal into a modified signal, the modifying means narrowing the controllable pulse width in response to the control enable signal; and a control voltage supplying circuit supplied with the modified signal for supplying the control voltage signal to the voltage-controlled oscillator in response to the modified signal.