Receiver and frequency synthesizer-tuner for the same

Abstract: An apparatus for acquiring tuning for a tuning frequency at a high speed is disclosed. In a receiver including a receiving unit (4) incorporating a VCO (4a), control section (2) and a tuning control unit (5), the tuning control unit holds the inputted tuning frequency information at the designated address in accordance with the storage designation signal supplied from said control unit and of controlling the oscillating frequency of the VCO based on the tuning frequency information held at the designated address in accordance with the read designation signal supplied from said control unit. This receiver can shorten the tuning time in exchanging the tuning frequency. Further, in a frequency synthesizer tuner comprising a frequency comparing means for acquiring a signal indicative of a frequency difference between a target tuning frequency and an oscillation frequency of VCO in a frequency comparison loop so that the tuning frequency is adjusted, a count time setting means (107,Fig.8) is provided for setting the count time for measuring the oscillating frequency of the VCO used by the oscillating comparison means, and the count time is increased whenever measurement of the oscillating frequency of the VCO is repeated by the frequency comparison means. This frequency synthesizer tuner can greatly shorten the time taken for the oscillating frequency of the VCO to coincide with the target tuning frequency.

Description: SPECIFICATION The present invention relates to a frequency synthesizer tuner having the features of the preamble of claim 1. The features are disclosed in GB-A-2122822. The aforementioned document discloses a frequency control device including an analogue phase locked loop, which operates on initial switch-on, and a digital frequency locked loop which takes over from the analogue loop when it is locked to control the same voltage controlled oscillator so as to endure faster locking. Control of the oscillator successively by the two loops uses a switching circuit with two inputs, one for the analogue loop, one for the digital loop. This switching unit includes a sampler-coder, an up-down counter, a register and a D/A connector. The sampler-coder input is the one for the phase loop, and the inputs for the counter are those for the frequency loop. The reference signal for both control loops is an external signal of very accurate mean frequency but having a high jitter. In the phase loop configuration the sampler-coder, up-down counter and register are driven by a high-frequency signal of about 1 MHz. In the frequency loop configuration the sampler-coder is ineffective and a lower frequency signal of about 1 Hz drives the register. This device is useful in digital telephone exchanges. The switching circuit is controlled first by the phase loop and then by the digital frequency loop. Once locking-on has been detected, it is this circuit which decides between "phase loop" and "frequency loop". When phase locking occurs, the mean frequency of the external signal and the loop output frequency are already very close, and it is necessary to wait for a large number of periods before the difference in the number of passages at 0 is significant. DE37 22 328 A1 discloses a frequency synthesizer tuner comprising a circuit arrangement for obtaining a clock signal from a data signal, wherein a phase comparator circuit and a controllable oscillator form a phase control loop, the phase comparator circuit is provided with the data signal and the controllable oscillator is provided with a clock signal. The data signal and the clock signal are compared with one another in a digital frequency comparator circuit, the output of which is fed to a D/A converter. The output signal of said converter and the output signal of the phase comparator circuit are provided to the controllable oscillator. The field of application of the invention will now be explained. A receiver for FM and AM generally includes a signal receiving section, a tuning control section and a control section. Fig. 11 shows a conventional receiver in block form. As seen from the figure, a receiver 51 includes a receiving section 52, a tuning control section 53 and a control section 54. The control section 52 includes a voltage controlled oscillator (hereinafter referred to as "VCO") and can vary a tuning frequency in response to a control signal from the tuning control section. The receiving section 52 is tuned to the frequency of a selected channel and demodulates a demodulated signal from the received signal. The tuning control section 53 controls to set the tuning frequency of the receiving section 52 for a frequency to be selected and conventionally uses a PLL (Phased Locked Loop). The control section 54 which can be constructed by a microcomputer serves to perform setting the tuning control section 53 for a selected frequency, processing of input information inputted from a user and processing for the operation of whole appliances such as display processing. In such a receiver, when the received frequency is to be varied, first, the frequency-division data Mi defining a new oscillation frequency of VCO is inputted to the tuning control section 53 from the control section 54. The tuning control section 53 detects a phase difference between a reference signal internally created and a comparison signal resulting from division of the oscillation frequency of VCO from the receiving section 52 by the frequency-division data Mi and supplies the control signal output according to the phase difference to the VCO of the receiving section 52, thereby controlling the oscillating frequency of the VCO. Specifically, the PLL loop constituted by the receiving section 52 and tuning control section 53 controls the oscillating frequency of VCO to converged into a value to be set and maintained. When the oscillating frequency of VCO reaches the frequency to be set by control by the tuning control section 53, the tuning control section 53 produces a lock signal representative of this fact to the control section 54. Thus, the control section recognizes completion of channel. Thus, the time required to select a designated channel is defined by the time from when the division data Mi to be set is produced from the control section 54 to when the lock signal is inputted to teh control section 54. It is well known that the receiver using a PLL loop can be tuned to plural channels by suitably setting the frequency Mi of the program divider constituting the loop. With development of multi-function of the receiver in recent years, it has become necessary to vary the receiving frequency continuously and tune the receiver to plural frequencies continuously in a short time. For example, in the RDS system which has been put into practice in Europe, a receiver, which has a function of network-follow based on a list of substitutive channels in the same network obtained from received data, is tuned to frequencies of plural alternative channels to check its receiving state in order to determine a channel providing a good receiving state previously. Further, another receiver, which has a function of deciding coincidence of a present received channel with an alternative channel on the basis of respective demodulated audio outputs, performs coincidence decision processing of changing the receiving frequency continuously to receive the present receiving channel and the substitutive channel alternately and comparing demodulated outputs from both channels. In these conventional receivers, such continuous channel-tuning processing considers, as a single process, the process from when the frequency dividing data Mi to be set is produced from the control section 54 to when the lock signal or a signal representative of completion of acquisition of a demodulated output of the substitutive channel is inputted to the control section 54, and performs such processes in parallel to change the receiving frequency continuously. As described above, when the tuning frequency is to be changed, the conventional receivers transmit the frequency division data Mi to the tuning control section 53. The frequency division data Mi of the receiver having a wide receiving band, which is generally represented by ten bits or more, takes a longer time for its transmission. This make it impossible to shorten the tuning time for channel tuning. Particularly, where the tuning frequency for channel tuning is to be changed, it is necessary to perform the entire processing in a short time so that the time required to select each channel must be shortened. A frequency synthesizer tuner generally adopts a PLL circuit in a receiving section for demodulating a received signal in tune with a target channel tuning frequency. Fig. 12 shows the theory of a PLL circuit used in the conventional frequency synthesizer tuner. The PLL circuit used in the conventional frequency synthesizer, as shown in Fig. 12, includes a phase comparator 101, a low-pass filter (LPF) 102, a voltage controlled oscillator (VCO) 103 and a programmable divider 104 constituting a frequency divider. In operation, a reference signal f ref and frequency-dividing output f v = f 0 /M i (Mi represents the frequency-division value providing a target tuning frequency) are supplied to the phase comparator 101. The phase comparator 101 detects the phase difference Δ between these two input signals f ref and f v . The LPF 102 converts the phase difference signal Δ into a direct current which is in turn supplied to the VCO 103. The oscillating frequency of the VCO 103 varies with the voltage controlled signal sent from the LPF 102. The oscillating output of the VCO 103 is sent to the phase comparator 101 through the programmable divider 104 again to repeat the above operation of phase comparison. As a result of repetition of such phase comparison, the oscillating frequency of the VCO 103 is finally converged into f 0 = M i x f ref . Therefore, if various frequency division values M i are prepared so that the oscillating frequency f 0 coincides with target tuning frequencies, the frequency division value M i of the programmable divider 104 has only to vary to change a target channel freely. As understood from the equation f 0 = M i x f ref , the tuning accuracy of the frequency synthesizer tuner depends on the reference frequency f ref . The frequency synthesizer tuner using PLL must satisfy contradictory conditions that in signal reception, the PLL system is stable, the time ("lock-up time") taken to reach a target frequency is as small as possible, and noise in the system in locking is small. Therefore, if the loop constant of the PLL circuit is set taking only the lock-up time seriously into consideration, the system is not stable and so suited to practical use. For this reason, in the ordinary frequency synthesizer tuner, the loop constant of the PLL circuit was set so as to satisfy the above respective conditions in a practical use. Thus, it was difficult to shorten the lock-up time so that in changing the tuning frequency, muting processing is carried out in order to prevent noise from being generated, thereby attenuating a demodulated output as necessity requires. In order to solve the above problem, the inventors of this application proposed a frequency synthesizer tuner which can greatly shorten the lock-up time and does not require the muting processing in changing the tuning frequency. Fig. 13 shows the principle of the PLL circuit adopted in the previous proposal. The PLL circuit shown in Fig. 13 adds, to the circuit shown in Fig. 12, a frequency comparing means 5 for acquiring a frequency difference between the oscillation frequency f 0 of the VCO 103 and the target tuning frequency f i . Namely, a frequency comparison loop constituted by the frequency comparison means 105 is newly added to the ordinary phase comparison loop. In Fig. 13, like reference numerals refer to like parts in Fig. 12. The PLL circuit of Fig. 13 operates as follows. At the start of exchanging the tuning frequency , a switch 106 is located in an off state so that the phase comparison loop by the phase comparator 101 is previously opened. In this state, first, the frequency difference Δf between the oscillating frequency f 0 of VCO 103 and a target tuning frequency f i is acquired by the frequency comparison means 105. The frequency difference signal Δf is fed back to the LPF 102. Thus, the oscillating frequency of the VCO 3 varies towards the target tuning frequency f i . A lock detector (not shown) detects that the oscillation frequency f 0 has substantially reached the target tuning frequency f i . Then, the switch 106 is turned on to operate the phase comparison loop by the phase comparator 101 so that the ordinary PLL operation starts. In such a configuration, control is carried out through the frequency comparison loop until the oscillation frequency f 0 of the VCO 3 becomes substantially equal to the target tuning frequency f i . This makes the phase comparison operation unnecessary so that the oscillation frequency f 0 of the VCO 3 can be shifted to the tuning frequency f i at a high speed. Thus, the lock-up time can be greatly shortened. The above frequency synthesizer tuner was sufficiently satisfactory in performance. However, from the standpoint of high-speed change of a receiving channel, the lock-up time is desired to be as small as possible. For example, when the channel is changed while a person is listening to the radio during motor driving at high speed, an RDS (radio data system) using multiplex broadcasting put into practice in Europe has a function of searching another channel which is broadcasting the same program as that has been heard until then and automatically tuning the receiving channel. In order to realize such a function, it is necessary to check the receiving status of substitutive candidates which is broadcasting the same program. As the above lock-up time is smaller, the checking can be made at a higher speed, thus making unnecessary the muting in changing the receiving channel. In the PLL circuit of the above frequency synthesizer tuner, as shown in Fig. 14, the frequency comparison means 105 uses the one period T ref (hereinafter referred to as "reference period", T ref = 1/f ref ) of a reference frequency f ref as a count time T c . By counting the number of oscillating pulses during the count time T c = T ref , the oscillating frequency f 0 of the VCO 103 is acquired so as to calculate its frequency difference Δf from the target tuning frequency f i . In Fig. 14, symbol W t represents the wait time from when the oscillating frequency of the VCO 3 is changed to be stabilized to when detection of the next frequency difference can be performed. The reason why the reference period T ref is used as the count time for measuring the oscillation frequency f 0 of the VCO 103 is as follows. Assuming that the target tuning frequency is f i , the frequency division value M i set for the programmable divider 104 in the phase comparison loop can be expressed by M i = f i /f ref = f i · T ref As seen form the equation, the frequency division value M i represents the number of pulses at the tuning frequency f i during the reference period T ref . In terms of the difference between the number of oscillating pulses counted in the VCO 103 during the reference period T ref and the frequency division value, the frequency difference between the target tuning frequency f i and the oscillating frequency f 0 can be easily calculated. The processing of detecting the frequency difference must be performed at least several times until the oscillating frequency of the VCO 103 becomes substantially equal to the target tuning frequency f i . Therefore, in order that the oscillating frequency of the VCO 103 is varied to reach the target tuning frequency f i , the processing time corresponding to at least several periods of the reference period T ref is required. If the total time required to reach the target tuning frequency can be shortened, the lock-up time can be further shortened correspondingly. On the basis of a further research after the frequency synthesizer tuner was proposed previously, the inventors of the present invention has developed a new frequency synthesizer tuner which can further shorten the time taken for the oscillating frequency of the VCO 3 to reach the target tuning frequency f i to decrease the lock-up time. Thus, the object of the present invention is to provide a frequency synthesizer tuner which can further shorten the lock-up time. In order to attain the object of the present invention, there is provided a frequency synthesizer tuner as set forth in claim 1. Preferred embodiments thereof are subject-matter of the dependent claims. In the above frequency synthesizer tuner, at least the frequency comparing means is preferably constituted as a digital circuit, and a D/A converter is preferably provided at the output of the frequency comparing means so that the input/output conversion characteristic of the D/A converter is the same as the voltage - frequency characteristic of the VCO. In accordance with the present invention, the time taken for the oscillating frequency of the VCO is shortened to reach a target tuning frequency to reduce the lock-up time. In the preferred embodiment of the present invention, a conversion error due to a difference in the conversion characteristic between the VCO and D/A converter is removed so that the lock-up time can be further decreased. The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the theory of a PLL used in the frequency synthesizer tuner according to the present invention; Fig. 2 is timing chart of the counting operation in the frequency synthesizer tuner in the circuit in Fig. 1; Fig. 3 is a graph showing the conversion characteristics in a VCO and D/A converter; Fig. 4 is a block diagram showing one example of the PLL circuit in the frequency synthesizer tuner in Fig. 1; Figs. 5A to 5J are timing charts showing the operation of the PLL circuit in Fig. 4; Fig. 6 is a concrete example of the Mi dividing unit 114 in Fig. 4; Fig. 7 is a concrete example of the frequency difference detector 115 in Fig. 4; Fig. 8 is a concrete example of the LPF 102 in Fig. 4; Fig. 9 is a concrete example of the D/A converter 108 in Fig. 4; Fig. 10 is a graph showing the I/O conversion characteristic in the D/A converter in Fig. 9; Fig. 11 is a schematic block diagram showing a conventional receiver; Fig. 12 is a block diagram showing the principle of the PLL circuit used in a conventional frequency synthesizer tuner; Fig. 13 is a block diagram showing the principle of the PLL circuit used in another conventional frequency synthesizer tuner; and Fig. 14 is a timing chart for explaining the counting operation the PLL circuit in Fig. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawings, an explanation will be given of embodiments according to the present invention described above. Fig. 1 is a block diagram of the theoretical configuration of the PLL circuit used in the frequency synthesizer tuner according to the present invention. Referring to Fig. 1, an explanation will be given of the means adopted to solve the problem relative to the second aspect described above. In Fig. 1, like reference numerals refer to like parts in Fig. 13. The present invention relates to a frequency synthesizer tuner comprising a frequency comparing means 105 for acquiring a signal indicative of a frequency difference Δf between a target tuning frequency f i and an oscillating frequency f 0 of VCO 103; a frequency comparison loop for feeding back the acquired frequency difference signal Δf to the VCO 103 to control the oscillating frequency f 0 of the VCO 103 so that the tuning frequency f i is adjusted; and a count time setting means 107 for setting the count time T c for measuring the oscillating frequency of the VCO 103 used by the oscillating comparison means 105, wherein the count time T c is increased whenever measurement of the oscillating frequency of the VCO 103 is repeated by the frequency comparison means 105. As described above, it is difficult to cause the oscillating frequency f 0 of the VCO 3 to approach the target tuning frequency f i with high accuracy by the one round detection of frequency difference using the frequency comparing loop. In view of this fact, it is not desired to detect the frequency with high accuracy from the start with the count time lengthened. Alternatively, the total time taken for the oscillating frequency f 0 of VCO 103 to reach the target tuning frequency f i can be shortened by increasing the count time T c gradually in such a manner that at the first time, the frequency difference is detected for a short time with the count time T c shortened, and at the second detection of frequency difference, the count time is made longer than the first detection to enhance the accuracy. For this purpose, in accordance with the present invention, the count time setting means 107 which can vary the count time T c is provided to increase the count time T c whenever the count operation is repeated by the said frequency comparing means 105. In such a configuration, as shown in Fig. 2, it is assumed that by the operation of three-times detection of frequency difference, the oscillating frequency of VCO 103 has reached the target tuning frequency f i . The count time is set at the first time T c = 1/4·T ref , at the second time T c = 1/2·T ref and at the third time T c = T ref . The total time taken for the three times of count operation is (1/4·T ref + 1/2·T ref + T ref ) = 1. 75 T c except the wait times W t . On the other hand, in the prior art, the total time taken for the three-times of count operation is 3T c . The present invention permits the oscillating frequency of the VCO 103 to be coincident with the target tuning frequency f i . In the above frequency synthesizer tuner, at least the frequency comparison means 105 is preferably constituted as a digital circuit, and a D/A converter 108 (encircled by dotted line in Fig. 1) is preferably provided at the output of the frequency comparing means 105 so that the input/output conversion characteristic of the D/A converter 108 is the same as the voltage - frequency characteristic of the VCO 103. As described above, in the frequency control by the frequency comparing loop, in order to acquire the frequency difference from the target tuning frequency f i , the oscillating pulses of the VCO 103 must be counted. This counting processing can be more easily carried out by a digital circuit than an analog circuit. For this reason, in many cases, the frequency comparing means 105 is constituted by the digital circuit. In this case, for coincident of signal format with the LPF 102 which is an analog circuit receiving the output from the frequency comparison means, a D/A converter 108 indicated by a block of dotted line in Fig. 1 is provided on the output side of the frequency comparison means 105. Meanwhile, the D/A converter used in an acoustic device such as a radio receiver is caused to have a linear input/output conversion characteristic indicated by dotted line in Fig. 3. But, the VCO 103 is commonly a non-linear element whose V (voltage) - f(frequency) characteristic is non-linear indicated by solid line in Fig. 3. Therefore, provision of the D/A converter 108 having the linear conversion characteristic gives rise to an error due to the difference in conversion characteristic. A redundant time will be spent until the oscillating frequency f 0 will be converged into the target tuning frequency f i . For this reason, in the present invention, the input/output conversion characteristic of the D/A converter 108 is made the same as the V -f characteristic of the VCO 103. Such a configuration can remove the error due to the difference in conversion characteristic, thus enhancing the conversion accuracy to this degree. Thus, the time taken for the oscillating frequency f 0 of the VCO 103 to converge into the target frequency f i can be further shortened. Now referring to the drawings, an explanation will be given of more concrete embodiments of the second aspect of the present invention. Fig. 4 shows an example of the PLL in the frequency synthesizer tuner according to the present invention. In Fig. 4, like reference numerals refer to like elements in Fig. 1. In Fig. 4, the PLL circuit includes a phase comparator 101, an LPF 102, a VCO 103, a program divider 104 constituting a frequency divider 104, a frequency comparing means 105, a D/A converter 108, a synchronizing control unit 109, a master clock oscillator 110 and a data holding unit 112. The synchronizing control unit 109 also serves as the count time setting means 107. A switch for conversion from the frequency comparison loop into the phase comparison loop is incorporated in the LPF 102 the concrete example of which is shown in Fig. 8. The frequency comparison means 105 includes a VCO counter 113 which counts the oscillating frequency of VCO 103 on the basis of the count pulse time P c sent from the synchronizing control unit 109, an M i division section 14 for 1/n dividing the frequency division value M i for setting the tuning frequency f i sent from the data holding unit 112, a frequency difference detector 115 for taking subtraction between the frequency division value M i /n sent from the M i division section 114 and the pulse count value N i of the VCO 103 sent from the VCO counter 114, an adder/subtracter 117 and a latch circuit 118 constituting the digital charge pump 116, and a lock detector 119 for detecting substantial coincidence between the oscillating frequency f 0 of the VCO 103 and the tuning frequency f i . Concrete examples of the M i division section 114 and the frequency difference detector 115 will be described later (see Figs. 6 and 7). The synchronizing control unit 109, under the control of CPU (not shown), creates a pulse P c with a pulse width of T c which is 1/n (n ≤ 1) of reference period T ref using a master clock from the master clock oscillator 111 and supplies it to the VCO counter 113 as a count time pulse Pc, and also produce various control signals inclusive of a reset signal RS, the frequency division value n of the count time pulse P c and a synchronizing signal. The reference divider 111 divides the master clock produced by the master clock oscillator 110 into a reference pulse having a reference frequency f ref to be produced. The data holding section 112 is a circuit such as a shift register for storing and holding the frequency division value M i for setting the target tuning frequency f i . The frequency division value M i is transferred from the CPU (not shown). Referring to the timing chart of Fig. 5, an explanation will be given of the operation of the above circuit of Fig. 4 in connection with an example in which the oscillating frequency of the present oscillation frequency f 0 = f i ' into a new tuning frequency f i instructed by the CPU. First, in order to convert the oscillating frequency of the VCO 103 into f 0 = f i , the frequency division value M i providing the new tuning frequency f i is supplied to the data holding unit 112 from the CPU (not shown) and stored there. Further, the CPU not shown supplies to the synchronizing control unit 109 an end discrimination signal indicative of end of the data transmission as shown in Fig. 5A. In response to the end discrimination signal End, the synchronizing control section 109 starts the operation of varying the oscillation frequency of the VCO 103 controlling the respective constituent circuit units as follows. In synchronism with the end discrimination signal End, the synchronization control unit 109 produces the reset signal RS as shown in Fig. 5H. This reset signal RS is a signal for converting the control of the PLL circuit to the side of frequency comparison loop. While the reset signal RS is at "H" level, the operation of the reference divider 111, programmable divider 104 and the phase comparator 101 is stopped. In response to the end discrimination signal End, the synchronizing control unit 109 sends to the VCO counter 113 the first count time pulse P c having a pulse width of 1/n·T ref resulting from n division of the reference period T ref and supplies this division value to the M i division unit 114. For example, assuming that n = 2 for the first count operation, the synchronizing control unit 109 creates the count time pulse P c having a pulse width of T c = 1/n·T ref = 1/2·T ref to be supplied to the VCO counter 113 and sends n = 2 to the M i division unit 114. The VCO counter 113 counts the number of oscillating pulses supplied from the VCO 103 and supplies the pulse count value N i /n to the frequency difference detector 115. Specifically, assuming that the pulse count value of the oscillating output from the VCO 103 is N i during the reference period T ref which corresponds to the pulse width T c of the count time pulse P c , the pulse count value when it is counted by the count time pulse P c having a pulse width of the above T c = 1/n·T ref is N i /n. Thus, the VCO counter 113 supplies the pulse count value N i /n to the frequency difference detector 115. On the other hand, in response to the frequency division value n of the count time pulse P c from the synchronizing control unit 109, the M i division section 114 divides the frequency division value M i , providing the target tuning frequency f i sent from the data holding section 112, by this value of n thereby converting the frequency division value M i into the count pulse value M i /n having the tuning frequency f i corresponding to the present count time T c . The count pulse value is sent to the frequency difference detector 115. The frequency difference detector 115 takes subtraction between the pulse count value M i /n having the target frequency f i sent from the M i division unit 114 and the pulse count value N i /n having the present oscillating frequency sent from the VCO counter 113, thus acquiring the difference value (M i /n) - (N i /n). The frequency difference Δf obtained by multiplying the difference value (M i /n) - (N i /n) by n will be supplied to the charge pump 116. The reason why the difference value (M i /n) - (N i /n) is multiplied by n is as follows. Since the difference value (M i /n) - (N i /n) is a frequency difference during 1/n of the reference period T ref , it is 1/n of the inherent frequency difference Δf when counted during the reference period T ref . For this reason, the difference value (M i /n) - (N i /n) is multiplied by n to be restored to the inherent frequency difference Δf = Mi - Ni to be outputted. Thus, the frequency difference Δf Mi - N i can be obtained in a short count time T c which is 1/n when counted during the reference period T ref , but with deteriorated accuracy. The digital charge pump 116 includes the adder/subtracter 117 and the latch circuit 118. The adder/subtracter 117 carries out the addition/subtraction between the present oscillating frequency f 0 of the VCO 103 held in the latch circuit 118 and the frequency difference Δf supplied from the frequency difference detector 115. The frequency data obtained as a result of the addition/subtraction is held in the latch circuit 118. The frequency difference Δf supplied from the frequency difference detector 115 is a positive or negative value. The positive value is added to the oscillating frequency f 0 whereas the negative value is subtracted from the oscillating frequency f 0 . The result of addition or subtraction is shown in Fig. 5E (It should be noted that Fig. 5E shows the case of addition. The frequency data after addition/subtraction newly held in the latch circuit 118, which is an analog signal, is converted into a digital signal in the D/A converter 108. The digital signal thus obtained is applied to the VCO 103. As a result, the oscillating frequency of the VCO 103 abruptly varies from the present oscillating frequency f i ' toward the target frequency f i as shown in Fig 5F. Meanwhile, the VCO 103 constitutes a resonating circuit including a varicap, a capacitor, a coil, etc. The oscillating frequency of the VCO 103, therefore, cannot be varied instantaneously, but gradually varied toward the target frequency at a time constant defined by the circuit. If the next count operation is started before the varying oscillating frequency of the VCO 103 is stabilized, correct frequency control cannot be carried out. In order to obviate this inconvenience, the synchronization control unit 109 creates a wait time W t so as to stop the next count operation until the oscillating frequency of the VCO 103 is stabilized, and as shown in Fig. 5B, operates to produce a next count time pulse P c after passage of the wait time W t . On the other hand, the frequency difference Δf acquired by the frequency difference detector 115 is also supplied to the lock detector 119. The lock detector 119 which can be constituted by a counter decides whether or not the supplied frequency difference Δf is within a predetermined lock range, i.e., the oscillating frequency of the VCO 103 has substantially agreed with the target tuning frequency f i . The lock detector 115 produces an "H" level signal indicative of the unlock state if the oscillating frequency is not within the lock range, whereas it produces an "L" level signal indicative of the lock state if the oscillating frequency is within the lock range. After passage of the wait time W t , the synchronizing control unit 109 produces a count time pulse P c for the second count operation as shown in Fig. 5B. Then, because of the frequency comparison operation at the first time, the oscillating frequency of the VCO 103 is already very close to the target tuning frequency f i as shown in Fig. 5F. For this reason, in the second count operation, in order to enhance the accuracy of detecting the frequency for correct frequency control, the pulse width of the count time P c is made longer than in the first count operation. In the example shown, in second time the count operation, n is set at 1 (n = 1). Namely, in the second time count operation, the synchronizing control unit 109 produces a count time pulse P c having a pulse width (T c = T ref ) twice as long as that in the first count operation. The count pulse P c is sent to the VCO counter 113. The M i division unit 114 divides M i by the frequency division value of the count time pulse P c and supplies the value M i /n to the frequency difference detector 115. In the manner similar to the first count operation, the oscillating frequency of the VCO counter 103 is counted, and the frequency difference Δf is computed by the frequency difference detector 115. Thus, the frequency of the VCO 103 is controlled. The second count operation, in which the pulse width of the count time pulse P c is twice as long as that in the first count operation, requires a time twice as long as that in the first count operation, but can be performed with the accuracy of detection twice that in the first count operation. Thus, as a result of the frequency control based on the second count operation, the oscillating frequency f 0 of the VCO 103 becomes approximately equal to the target tuning frequency f i as shown in Fig. 5F. However, in the example at issue, as shown in Fig. 5D, the frequency difference Δf is not yet within the lock range. For this reason, the synchronizing control unit 109 produces a count time pulse P c having a pulse width T c = T ref so that the oscillating frequency of the VCO 103 is controlled. As a result of the third count operation, as shown in Fig. 5D, the frequency difference Δf is within the lock range so that the oscillating frequency f 0 of the VCO 103 becomes approximately equal to the target tuning frequency f i . The lock detector 119 detects this fact and exchanges its output level from the "H" level into the "L" level indicative of the lock state. The lock detector 119 sends this signal to the synchronizing control unit 109 and the required constituent circuits. In response to the lock detection signal from the lock detector 119, the synchronizing control unit 109 exchanges the reset signal RS from the "H" level to the "L" level as shown in Fig. 5H. This starts the operation of the phase comparator 101 and turns on the switch 106 incorporated in the LPF 102 (Fig. 12). The LPF 102 is set in a status permitting reception of a phase difference signal Δ from the phase comparator 101 also. Further, the reference divider 111 and program divider 104 which have stopped hitherto start the operation for frequency division so that the control using the phase comparison loop is started in place of the frequency comparison loop. When the control using the phase comparison loop is started, the oscillating frequency f 0 of the VCO 103 is adjusted finely on the basis of the phase difference signal Δ. Accordingly, the oscillating frequency f 0 of the VCO 103 is converged into the status completely synchronized with the target frequency f 0 . As described hitherto, in accordance with the second aspect of the present invention, since the count time T c is gradually lengthened with an increment in the number of the counting operation, the oscillating frequency f 0 can reach the tuning frequency f i in a very short time. The lock up time of the PLL circuit can be shortened correspondingly. In the present invention, simultaneously when the frequency comparison loop is exchanged into the phase comparison loop, as shown in Figs. 5I and 5J, the frequency operation of the reference divider 111 and program divider 104 are started. In this way, the phase error in the above exchange can be reduced, thus leading the PLL circuit into a synchronizing state. The frequency comparison loop intends to control the oscillating output of the VCO 103 on the basis of the frequency difference Δf, whereas the phase difference comparison loop intends to control the oscillating output of the VCO 103 on the basis of the phase difference Δ. Both are operated on the basis of entirely different control theories. Therefore, even if the frequencies of the comparison signals f ref and f v inputted to the phase comparator 101 are equal, inadvertent exchange from the control by the frequency comparison loop to that by the phase comparison loop will provide a phase difference between both signals as a phase error. In order to obviate such inconvenience, when control is exchanged from the frequency comparison loop to the phase comparison loop, as shown in Figs. 12I and 12J, the frequency division operation of the reference divider 111 and the program divider 104 are also started. Thus, the phase difference in the above exchange can be removed. This shortens the time to the phase synchronization of the both signals, and can also the lockup time. Fig. 6 shows a typical example of the M i division section 114. The M i division unit 114 serves to divide the frequency division value M i providing the target tuning frequency f i by the frequency division value n of the count time pulse P c and includes a RAM 120 and a divider 121. The divider 121 divides the frequency value M i stored in the RAM 120 by the frequency division value n of the count time pulse P c sent from the synchronizing control unit 109 and supplies the division value M i /n thus obtained as a pulse count value of the tuning frequency f i in the present count time T c to the frequency difference detector 115. Fig. 7 is a typical example of the frequency difference detector 115. The frequency difference detector 115 includes a subtracter 122 and a multiplier 123. The subtracter 122 takes subtraction between the pulse count value M i /n sent from the M i division unit 114 and the pulse count value N i /n sent from the VCO counter 113. The difference value (M i /n) - (N i /n) is multiplied by n by the multiplier 123 so that it is converted into frequency difference Δf = M i - n i during the reference period Tref. Fig. 8 shows a typical example of the LPF 102. As seen from Fig. 8, the LPF 102 includes a first amplifier 124 serving as a low pass filter and a second amplifier 125 serving as an adder/subtracter. The switch 106 for turning on/off the phase comparison loop (Fig. 1) is connected between an input and output of the first amplifier 124. The switch 106 is turned on/off by the reset signal RS (Fig. 5H) outputted from the synchronizing control unit 109. To the input terminal of the first amplifier 124, a phase difference signal Δ is inputted, whereas to the non-inverting terminal of the second amplifier 125, an analog signal from the D/A converter 108 is inputted. When the reset signal RS supplied from the synchronizing control unit 109 is at the "H" level, i.e., in the control state by the frequency comparison loop, the switch 106 is turned on or closed to shortcircuit the input and output of the first amplifier 124. The phase comparator 101 connected to the input terminal of the first amplifier 124 has a circuit configuration providing high output impedance. Thus, when the switch 106 remains closed, the second amplifier 125 operates as a buffer amplifier so that only the output from the D/A converter 108 inputted to the input terminal of the second amplifier 125. On the other hand, when the reset signal RS supplied from the synchronizing control unit 109 is at the "L" level as shown in Fig. 12H, i.e., in the control state by the phase comparison loop, the switch 106 is turned off (or opened) so that the first amplifier 124 serves as a low pass filter. When the reset signal RS becomes "L" level, the phase comparator 101 also starts the phase comparison operation (Figs. 5I and 5J). Therefore, when the switch 106 is opened, the phase difference signal Δf sent from the phase comparator 101 is converted in a DC signal by the first amplifier 124 serving as a low pass filter, and the DC signal is supplied to the inverting input terminal of the second amplifier 125. Then, in place of the control hitherto made by the frequency comparison loop, the control by the phase comparison loop is started. When the control by the phase comparison loop is started, the analog signal inputted to the non-inverting input terminal of the second amplifier 125 from the D/A converter 108 is fixed at the frequency data held in the latch circuit 118 when the frequency comparison loop is shifted into the phase comparison loop, i.e., the value substantially equal to the target tuning frequency f i . Thus, the above phase difference signal Δf is added to or subtracted from this analog signal so that the oscillating frequency f 0 of the VCO 103 is finely adjusted to be accurately coincident with the target frequency f i . As described above, in accordance with the second aspect of the present invention, the PLL circuit can be locked up in a very short time as described above. When the input/output conversion characteristic of the D/A converter 108 is different from that of the VCO 103, it takes a redundant time for the oscillating frequency of the VCO 103 to reach the target tuning frequency f i . In order to remove the conversion error, the D/A converter preferably has such a circuit configuration as shown in Fig. 9. The D/A converter 108 in Fig. 9 converts a digital signal into an analog signal in such a manner that the data inputted from the digital charge pump 116 is decoded by the decoder 126 and the corresponding switches 127 l to 127 n are turned on or off to shortcircuit or open resisters R i to R n . In this case, as shown in Fig. 10, the resistors R l to R n are previously weighted so the input/output conversion characteristic of the D/A converter is substantially equal to the voltage-frequency conversion characteristic of the VCO 103. In place of weighting the resistors R l to R n , as indicated by dotted line in Fig. 9, an index conversion circuit 128 or 129 using the B - E characteristic of a transistor may be connected at the input side or output side.

Claims: A frequency synthesizer tuner comprising: a frequency comparison loop and a phase lock loop inclusive of a phase comparison loop, the phase comparison loop including phase comparison means (101) for comparing the phases of a divided frequency (fv) of an oscillating frequency (fo), and a reference signal (fref) for feeding back a phase difference signal (Δ) to a VCO (103), the frequency comparison loop including a frequency comparing means (105) for acquiring a signal (Δf) indicative of a difference between a target frequency (fi) and an oscillating frequency (fo) of the VCO so that the tuning frequency (fi) is adjusted by feeding back the acquired signal to the VCO characterized in that the arrangement is adapted so that the phase comparison is executed after the frequency comparison, and that said tuner further comprises a count time setting means (107) for setting the count time (Tc) that is a time of counting the number of pulses at the oscillating frequency of the VCO (103) used by the frequency comparing means (105) in the frequency comparison loop, wherein the count time (Tc) is increased whenever measurement of the oscillating frequency (fo) of the VCO (103) is repeated by the frequency comparing means (105) so that the total time taken for said oscillating frequency (fo) to reach said target tuning frequency (fi) can be shortened. A frequency synthesizer tuner according to claim 1, wherein at least the frequency comparing means (105) is constituted as a digital circuit, and a D/A converter (108) is provided at the output of the frequency comparing means (105) so that the input/output conversion characteristic of the D/A converter (108) is the same as the voltage - frequency characteristic of the VCO (103). A frequency synthesizer tuner according to claim 2, wherein said D/A converter (108) includes a plurality of resistors weighted so that its input/output conversion characteristic of the D/A converter is the same as the voltage - frequency characteristic of the VCO. A frequency synthesizer tuner according to claim 2, wherein said D/A converter (108) includes an index conversion circuit (128, 129) connected to its input side or output side so that its input/output conversion characteristic of the D/A converter (108) is the same as the voltage - frequency characteristic of the VCO (103).

Inventors: ICHIKAWA TOSHIHITO, YAMAMAOTO YUJI, ICHIKAWA, TOSHIHITO, YAMAMAOTO, YUJI, ICHIKAWA, TOSHIHITO, C/O PIONEER ELECTRONIC CORP., Yamamaoto, Yuji, c/o Pioneer Electronic Corp.
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1996
Main classification: H03J5
IPC codes: H03L7, H03J1