Automatic frequency control circuit

Abstract: This AFC circuit stabilizes the frequency of an intermediate frequency signal to be applied to a demodulation circuit (2). A frequency conversion circuit (1) responds to a local oscillation signal generated from a voltage-controlled oscillation circuit (309) to convert the frequency of a digital modulation signal and apply the converted frequency to the demodulation circuit. A carrier reproduction circuit (203) in the demodulation circuit reproduces the carrier of the intermediate frequency signal to output a synchronizing detection signal (306). The frequency of the reproduced carrier is frequency-demultiplied by a frequency demultiplying circuit (310). A frequency demultiplication output thereof is counted by a counter (311) for a definite period. Data is output from a microprocessor (307) in response to a count output of the counter and the synchronizing detection signal and then converted into an AFC voltage by a D/A converter (308). The converted voltage is supplied as a control voltage to the voltage-controlled oscillation circuit. A stable intermediate frequency signal can be obtained by frequency-demultiplying the frequency of the carrier and counting the frequency demultiplication output thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to automatic frequency control circuits (hereinafter referred to as AFC circuits) and, more particularly, to an AFC circuit in which the frequency of a frequency-converted digital modulated signal to be supplied as an input to a demodulation circuit is stabilized. Description of the Background Art For demodulation of a digital modulated signal which is modulated by a digital signal, in general, an AFC circuit is employed in order to enhance demodulation characteristics of a demodulation circuit, particularly bit error rate characteristics. The AFC circuit serves to suppress various frequency fluctuations and stabilize a center frequency of a digital modulated signal to be supplied as an input to the demodulation circuit. Fig. 1 is a block diagram showing one example of a conventional AFC circuit. With reference to Fig. 1, a frequency conversion circuit 1 includes an input band-pass filter (BPF) 101, a mixer 102, an IF band-pass filter 103 and an IF amplifier 104. Input band-pass filter 101 is supplied with a digital modulated signal received through an antenna. Input band-pass filter 101 serves to band-limit the applied digital modulated signal, to apply the band-limited signal to mixer 102. Mixer 102 mixes the digital modulated signal with a local oscillation signal applied from a voltage-controlled oscillator (VCO) 309, which will be described later, included in an AFC circuit 3, to supply an intermediate frequency signal as an output to IF band-pass filter 103. IF band-pass filter 103 band-limits the applied intermediate frequency signal, to supply the band-limited signal to IF amplifier 104. IF amplifier 104 amplifies the intermediate frequency signal, to apply the amplified signal to a demodulation circuit 2. Demodulation circuit 2 includes a band-pass filter 201 for band-limitation, a phase detector 202 and a carrier reproduction circuit 203. Band-pass filter 201 limits the band of the intermediate frequency signal, which is a digital modulated signal, to improve a carrier-to-noise ratio (C/N) and enhance demodulation characteristics. The band-limited intermediate frequency signal is applied to phase detector 202 and then multiplied by carrier f L supplied as an input from carrier reproduction circuit 203, so that the digital modulated signal is demodulated. In the example shown in Fig. 1, the digital modulated signal is an example of a quadrature four phase modulated signal, in which two signals I and Q orthogonal to each other are demodulated as demodulation signals. Carrier reproduction circuit 203 synchronizes the carrier with the center frequency of the digital modulated signal in response to the demodulated two signals I and Q, to apply a synchronizing detection signal 306 serving as a determining signal for determining synchronization/non-synchronization to AFC circuit 3. AFC circuit 3 includes a frequency demultiplier 301, a reference oscillator 302, a phase comparator 303, a microprocessor 307, a D/A converter 308 and VCO 309. Frequency demultiplier 301 serves to 1/A frequency-demultiply the digital modulated signal that has passed band-pass filter 201 in demodulation circuit 2. A frequency demultiplication output of frequency demultiplier 301 is applied to phase comparator 303. Phase comparator 303 is supplied with a reference frequency signal by reference oscillator 302. Phase comparator 303 compares in phase the digital modulated signal, which is frequency-demultiplied by frequency demultiplier 301, and the reference frequency signal, to supply an error signal as an output. More specifically, phase comparator 303 supplies as an output an error signal 304 of plus (+) when the phase of the digital modulated signal is in advance of that of the reference frequency signal, and conversely the comparator supplies an error signal 305 of minus (-) when the phase of the digital modulated signal is delayed from that of the reference frequency signal. Error signals 304 and 305 supplied from phase comparator 303 and synchronization detection signal 306 supplied from carrier reproduction circuit 203 of demodulation circuit 2 are applied to microprocessor 307. Microprocessor 307 applies data corresponding to error signals 304 and 305 and synchronization detection signal 306 to D/A converter 308. D/A converter 308 converts the applied data into an analogue signal, to supply an AFC voltage to VCO 309. A description will now be given of an operation of the conventional AFC circuit shown in Fig. 1. First, if synchronization detection signal 306 supplied as an output from carrier reproduction circuit 203 is at the level representing non-synchronization, microprocessor 307 makes the data to be applied to D/A converter 308 variable. D/A converter 308 converts the variable data into an analogue voltage, to supply the converted voltage to VCO 309. VCO 309 sweeps a frequency in response to the variable voltage, to supply the swept frequency to mixer 102. Mixer 102 sweeps a frequency in response to the signal from VCO 309, so that an output signal of frequency demultiplier 301 varies. Phase comparator 303 compares in phase a digital modulated signal and a reference frequency signal. If carrier f L is synchronized with the digital modulated signal in the course that VCO 309 performs sweeping, carrier reproduction circuit 203 causes synchronization detection signal 306 to attain the level representing synchronization. Accordingly, microprocessor 307 stops VCO 309 from sweeping and then controls VCO 309 so as to lower the oscillation frequency of VCO 309 in response to the (+) error signal of those two error signals 304 and 305 applied from phase comparator 303. Conversely, microprocessor 307 controls VCO 309 so as to make the oscillation frequency thereof higher in response to the (-) error signal. As described above, microprocessor 307 controls VCO 309 via D/A converter 308 so that the frequency of the digital modulated signal to be applied to demodulation circuit 2 may be stable. In addition, in order to prevent the oscillation frequency of VCO 309 from being frequently altered when carrier f L is in synchronization with the digital modulated signal, an allowable error range is set in phase comparator 303, thereby suppressing the times that the oscillation frequency of VCO 309 is controlled, to suppress the frequency fluctuation of the reproduced carrier. In the conventional AFC circuit shown in Fig. 1, since the digital modulated signal, which is applied as an input to phase comparator 303 and frequency-demultiplied, has passed band-pass filter 201 for band limitation, the digital modulated signal is influenced by the characteristics of band-pass filter 201. Particularly, because of variations in phase characteristics and amplitude characteristics due to temperature variations, the digital modulated signal undergoes phase modulation or amplitude modulation, so that the frequency of the frequency-demultiplied digital modulated signal becomes higher or lower than the frequency which is inherently demultiplied. This might cause an error in an error signal of phase comparator 303. Further, a frequency demultiplication error occurs in an output of frequency demultiplier 301 in the case of a low C/N ratio, thereby causing a frequency error. Since phase comparator 303 is employed as means for stabilizing a digital modulated signal, reference oscillator 302 becomes necessary, and also the structure of phase comparator 303 becomes complicated by the setting of the allowable error range. Moreover, in order to compress the expansion of a spectrum of the digital modulated signal, the digital modulated signal should finally be frequency-demultiplied up to the frequency of approximately 10kHz. If the frequency of the digital modulated signal is 100MHz or more, the frequency must be demultiplied by 10000 or more, resulting in the disadvantage that the number of stages of frequency demultiplier 301 increases. In addition, there is another disadvantage that jitter is produced in the frequency-demultiplied signal, thereby causing an adverse effect when a phase comparison is made by phase comparator 303. EP-A-0 339 647 discloses a conventional broadcast satellite tuner in which an automatic frequency control is realized. The intermediate frequency signal output by a second IF stage is filtered and amplified prior to demodulation in a PLL FM demodulator. The input signal to the demodulator is frequency-divided and then directly counted. A microprocessor compares the count data with predetermined reference data and controls a VCO in the second IF stage to provide frequency control. SUMMARY OF THE INVENTION A principal object of the present invention is to provide an AFC circuit having a simple structure and implementable at lower cost. Another object of the present invention is to provide an AFC circuit in which a digital modulated signal to be applied to a demodulation circuit can be made stable on a frequency basis, and a reproduced carrier can also be made stable without influence caused by temperature characteristics of circuit elements. The present invention provides an automatic frequency control circuit for stabilizing a frequency of an intermediate frequency signal to be supplied as an input to a demodulation circuit, comprising frequency conversion means for frequency-converting a digital modulated signal to output an intermediate frequency signal, voltage-controlled oscillation means for applying to said frequency conversion means a local oscillation signal having an oscillation frequency varying in response to an applied control voltage, carrier reproduction means for generating a reproduced carrier required for demodulating said digital modulated signal, said reproduced carrier being mixed with said intermediate frequency signal in said demodulation circuit to demodulate said digital modulated signal, said carrier reproduction means outputting a synchronization detection signal indicating synchronization, between the frequency of the digital modulated signal and the reproduced carrier, which provides valid demodulation of the digital modulated signal, frequency demultiplying means arranged for frequency demultiplying the frequency of the reproduced carrier generated by said carrier reproduction means, counting means for counting the frequency-demultiplied signal output by said frequency demultiplying means for a definite period, and control means for applying a said control voltage to said voltage-controlled oscillation means in response to the count output of said counting means and said synchronization detection signal. Therefore, in accordance with the present invention, when the reproduced carrier is in synchronization with the digital modulated signal, the voltage-controlled oscillation means is subject to fine tuning. Thus, the AFC circuit can perform a stable operation without influence caused by temperature characteristics of circuit elements in use since no jitter is produced in the frequency-demultiplied signal. Thus, the AFC circuit in accordance with the present invention can require an extremely simple structure and can be manufactured at lower cost. According to a preferred embodiment of the present invention, the control voltage is output as a digital signal from a microprocessor. This digital signal is converted into an analogue signal by a D/A converter and then applied to a voltage-controlled oscillation circuit. Further, a count output of a counter is stored in a shift register and then serially applied to the microprocessor. The serial application of the count output enables a decrease in the number of pins of the microprocessor. Further, according to a more preferred embodiment of the present invention, a carrier-to-noise ratio of a digital modulated signal is detected, so that sweep speed of a control voltage to be applied to a voltage-controlled oscillation circuit is varied in accordance with the level of the detected carrier-to-noise ratio. Accordingly, when the carrier-to-noise ratio decreases, the sweep speed is reduced. When the carrier-to-noise ratio is higher, the carrier can be synchronized with the digital modulated signal sooner. According to a further preferred embodiment of the present invention, as long as the digital modulated signal is asynchronous with the carrier, a switch is changed over to the digital modulated signal side, so that the digital modulated signal is applied to a frequency demultiplying circuit. If the digital modulated signal is close to a synchronization frequency of the carrier, the switch is changed over to the reproduced carrier side, so that the reproduced carrier is applied to the frequency demultiplying circuit. Thus, in the case of non-synchronization, the oscillation frequency of the voltage-controlled oscillation circuit can be swept at high speed by frequency-demultiplying of the digital modulated signal to count this frequency demultiplication output. When the carrier is close to a synchronization frequency, the switch is changed over. With the reproduced carrier frequency-demultiplied, the voltage-controlled oscillator sweeps the oscillation frequency at low speed until the synchronization of the reproduced carrier is made. When the synchronization of the reproduced carrier is made, a signal obtained by frequency demultiplying the reproduced carrier is counted for a definite period, so that the voltage-controlled oscillator is controlled so as to eliminate errors by the comparison between a count value thereof and a reference value. According to a still further embodiment of the present invention, a control circuit is constituted by a PLL loop including a variable frequency-demultiplying circuit, a reference oscillation circuit, a phase comparison circuit and a microprocessor. The employment of the PLL loop achieves a more stable AFC circuit. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of a conventional AFC Circuit; Fig. 2 is a schematic block diagram of one embodiment of the present invention; Fig. 3 is a diagram showing an example of an AFC voltage for explaining an operation of an AFC circuit shown in Fig. 2; Fig. 4 is a diagram showing another example of a portion in which a count value of a counter shown in Fig. 2 is applied to a microprocessor; Fig. 5 is a block diagram showing another embodiment of the present invention; Fig. 6 is a schematic block diagram showing still another embodiment of the present invention; Fig. 7 is a schematic block diagram showing an embodiment in which an input of a frequency demultiplier can be switched; Fig. 8 is a schematic block diagram showing an embodiment in which an input of a counter can be switched; Fig. 9 is a schematic block diagram showing an embodiment in which a C/N detection circuit is provided; Fig. 10 is a block diagram showing another configuration of a demodulation circuit; Fig. 11 is a schematic block diagram showing an embodiment in which a PLL circuit is employed; Fig. 12 is a flow chart for explaining an operation of the embodiment shown in Fig. 11; Fig. 13 is a diagram showing one example of voltage-to-frequency characteristics of the reference oscillator of Fig. 11; and Fig. 14 is a diagram showing one example of reference oscillator 324 shown in Fig. 11. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 is a schematic block diagram of one embodiment of the present invention. The embodiment shown in Fig. 2 is different from Fig. 1 with respect only to an AFC circuit 30. A frequency conversion circuit 1 and a demodulation circuit 2 are structured similarly to those in Fig. 1. AFC circuit 30 includes a microprocessor 307, a D/A converter 308 and a VCO 309 as in Fig. 1, and also includes a frequency demultiplier 310 and a counter 311. While frequency demultiplier 301 shown in Fig. 1 serves to frequency-demultiply a digital modulation signal of an output of band-pass filter 201 for band limitation, the frequency demultiplier of this embodiment of the present invention serves to m frequency-demultiply carrier f L supplied as an output by a carrier reproduction circuit 203 (where m ≧ 2). A frequency demultiplication output of frequency demultiplier 310 is applied to counter 311 of N bits and counted for a definite period therein. A count output 312 of counter 311 is applied to microprocessor 307. Microprocessor 307 controls data to be supplied as an output to D/A converter 308 in response to a count output applied to a signal line 312 and a synchronization detection signal 306. Fig. 3 is a waveform diagram of an AFC voltage generated in the AFC circuit 30 shown in Fig. 2. A description will now be given of an operation of the AFC circuit shown in Fig. 2 with reference to Fig. 3. First, if synchronization detection signal 306 represents non-synchronization at a logic low level as shown in Fig. 3 (b), microprocessor 307 varies an AFC voltage, to apply data for generating the sawtooth wave shown in Fig. 3 (a) to D/A converter 308. D/A converter 308 converts the applied data into the sawtooth wave, to apply the converted data to VCO 309. VCO 309 sweeps the frequency of the sawtooth wave as the AFC voltage. As a result, a digital modulated signal supplied as an input to frequency conversion circuit 1 is frequency-converted while being swept similarly to the variation of the frequency by VCO 309. The frequency-converted digital modulated signal is supplied as an input to demodulation circuit 2. The swept digital modulated signal passes band-pass filter 201 for band limitation and is then multiplied by the carrier f L supplied as an output from carrier reproduction circuit 203 in a phase detector 202, so that two signals I and Q are demodulated. These two signals are fed back to carrier reproduction circuit 203. When carrier f L is synchronized with the digital modulated signal, synchronization detection signal 306 attains a logic high level to represent synchronization. When synchronization detection signal 306 attains a logic high level, microprocessor 307 stops VCO 309 from sweeping the frequency. Then, microprocessor 307 applies a control signal to counter 311 so as to operate counter 311 for a definite period to count the m frequency-demultiplied carrier. After counting the m frequency-demultiplied carrier for a definite period, counter 311 applies a count value thereof to microprocessor 307 via signal line 312. Microprocessor 307 compares the count value and the reference value set on a program and, if an error occurs, applies data to D/A converter 308 so as to eliminate the error. Thus, the oscillation frequency of VCO 309 is varied. Accordingly, the frequency of the digital modulated signal supplied as an input to demodulation circuit 2 is subjected to fine tuning and then stabilized, resulting in stabilization of the reproduced carrier. As described above, this embodiment has such a feature that since the reproduced carrier f L is directly frequency-demultiplied by frequency demultiplier 310, a frequency demultiplication output can be accurately counted by counter 311 without the influence of the temperature characteristics caused by band-pass filter 201 and without jitter produced in the frequency-demultiplied signal. Since the structure of the embodiment shown in Fig. 2 is extremely simple, the AFC circuit can be manufactured at lower cost. When carrier f L output from carrier reproduction circuit 203 is in synchronization with the digital modulated signal, the times that the oscillation frequency of VCO 309 is altered is decreased, and the frequency fluctuation of the reproduced carrier f L is suppressed. Thus, when the comparison is made between the value obtained by counting of the frequency demultiplied carrier and the reference value, only the error exceeding an allowable error range should be corrected. If the carrier f L is not counted continuously and repetitively but counted for a certain time, for example, at intervals of one second, the times that the oscillation frequency of VCO 309 is altered are further suppressed, so that the frequency of the digital modulated signal to be supplied as an input to demodulation circuit 2 can further be stabilized. Since the program of microprocessor 307 can cope with such an allowable error range, the allowable error range causes no influence on the circuit. A description will now be made of a setting method employing actual numeric values. Assuming that an intermediate frequency signal of the digital modulated signal to be applied to demodulation circuit 2 is 140MHz, the reproduced carrier f L is also 140MHz. In order to prevent degradation of the C/N ratio, a synchronization range of this carrier f L is normally set to be smaller, for example, +/-1MHz. Assuming that a frequency demultiplying ratio of frequency demultiplier 310 is 1/40, and operation time of counter 311 is 2msec, a count value of counter 311 is 7000, and resolution power by which carrier f L can be counted is 20kHz. If the count value 7000 is represented by 1101101011000 in binary, the number has 13 bits. Since the synchronization range of carrier f L is +/- 1MHz, the range of a count value +/-50 (= +/-110010 bits) should be acquired in counter 311, and hence counter 311 of 8 bits should be employed. Therefore, in the case of selecting counter 311, there is a degree of freedom for selecting a counter of 8 - 13 bits in the foregoing example. Such a method is also available that counter 311 of 12 bits is employed, and lower order 8 bits are input to microprocessor 307. If the operation time of counter 311 is appropriately selected, a counter of 7 bits may be employed. As mentioned above, if the frequency demultiplying ratio of frequency demultiplier 310 and the operation time of counter 311 are appropriately selected in consideration of the synchronization range of carrier f L , the extremely simple structure of the AFC circuit can be provided. While the repetitive waveform of the AFC voltage for sweeping VCO 309 is a sawtooth waveform as shown in Fig. 3 (a), the repetitive waveform is not limited to this waveform, and any waveforms such as a triangular waveform or sine waveform, which enable a smooth frequency sweeping of VCO 309, may be applied. Even if the level representing synchronization/nonsynchronization of synchronization detection signal 306 is opposite to the level of the example shown in Fig. 3 (b), this opposite level can be handled on a program, thereby causing no problem. Fig. 4 is a block diagram showing an embodiment for serially applying a count value of the counter to the microprocessor. In the foregoing example, the count value of counter 311 requires a signal line 312 of 7 bits at minimum, and hence microprocessor 307 requires 7 input terminals. In the embodiment shown in Fig. 4, however, a control signal LE9 is applied from microprocessor 307 to a shift register 313. A count output of counter 311 is stored via signal line 312 into shift register 313. A clock signal CK is output from microprocessor 307 to shift register 313. A count value is applied as serial data 314 to microprocessor 307. Accordingly, the number of the input terminals of microprocessor 307 can be reduced. Fig. 5 is a block diagram showing another embodiment of a demodulation circuit. A demodulation circuit 20 shown in Fig. 5 includes low-pass filters 204 and 205 for band limitation of a demodulated base band signal in place of band-pass filter 201 for band limitation shown in Fig. 2. In this case, according to the conventional example, since no band limitation of a digital modulated signal is made, a spectrum becomes wider, and jitter produced in a frequency demultiplied signal becomes larger. Further, it is possible that the AFC circuit malfunctions by application of noise. In this embodiment, however, since the reproduced carrier is utilized, no adverse influence is provided. Fig. 6 is a block diagram showing another embodiment of the present invention. In the embodiment shown in the foregoing example of Fig. 2, when the C/N ratio of the digital modulated signal is higher, the carrier f L output from carrier reproduction circuit 203 is synchronized with the digital modulated signal even if sweep speed of the sawtooth wave shown in Fig. 3 (a) is higher. Conversely, when the C/N ratio thereof is lower, the carrier f L is not synchronized with the digital modulated signal until the sweep speed of the sawtooth wave is decreased. Thus, assuming in general that the C/N ratio of the digital modulated signal is decreased, the sweep speed is reduced. In the embodiment shown in Fig. 6, an AFC circuit 31 includes a C/N detection circuit 315. A digital modulated signal is applied to C/N detection circuit 315 by an input of a frequency conversion circuit 1, so that the C/N ratio of the applied digital modulated signal is detected. A detection output of C/N detection circuit 315 is a digital signal of several bits to be applied to microprocessor 307. Microprocessor 307 determines the level of the C/N ratio in response to the output of C/N detection circuit 315, to set sweep speed of a sawtooth wave corresponding to the determined level and thus control VCO 309. In case where the C/N ratio is higher, the carrier is synchronized with the digital modulated signal sooner than in the case where the sweep speed of the sawtooth wave is fixed. While an input of C/N detection circuit 315 is connected to the input of frequency conversion circuit 1 in the example shown in Fig. 6, the connection of the input of C/N detection circuit 315 is not limited to this connection, and the input thereof may be connected to an output of frequency conversion circuit 1 or an output of band-pass filter 201 in demodulation circuit 2. Fig. 7 is a block diagram showing still another embodiment of the present invention. The embodiment shown in Fig. 7 is structured in the same manner as the foregoing embodiment of Fig. 2 except for the following features. In Fig. 7, a frequency conversion circuit 1 only represents a mixer 102, while band-pass filter 101, IF band-pass filter 103 and IF amplifier 104 shown in Fig. 2 are omitted. An AFC circuit 32 includes a switch 316. This switch 316 is switched to one of terminals A and B in response to a control signal 317 supplied as an output from a microprocessor 307. Terminal A is connected to an output of a band-pass filter 201 for band limitation of a demodulation circuit 2, and terminal B is supplied with carrier f L supplied as an output from a carrier reproduction circuit 203. An output of switch 316 is supplied as an input to a frequency demultiplier 310. An operation of AFC circuit 32 shown in Fig. 7 will now be described. First of all, in a case where the oscillation frequency of VCO 309 is swept at high speed when the carrier is not in synchronization with a digital modulated signal, switch 316 is switched to terminal A in response to control signal 317 from microprocessor 307. More specifically, switch 316 selects a band-limited digital modulated signal to apply the same to frequency demultiplier 310. Frequency demultiplier 310 m frequency-demultiplies the applied digital modulated signal, to supply a frequency demultiplication output thereof as an input to counter 311. Microprocessor 307 operates counter 311 for a definite period in response to a control signal LE5. Accordingly, counter 311 counts the frequency-demultiplied digital signal. A count output of counter 311 is accepted via signal line 312 by microprocessor 307. Microprocessor 307 makes the comparison between a count value and a reference value set on the program. Microprocessor 307 then applies data to a D/A converter 308 so as to sweep the oscillation frequency of VCO 309 at high speed in the direction of eliminating the error between the reference value and the count value, thereby varying an AFC voltage of VCO 309. At this time, assuming that the range of an intermediate frequency of the digital modulated signal to be applied to demodulation circuit 2 is 140MHz +/- 5MHz, the resolution power of counting frequencies is 200kHz, and a frequency demultiplying ratio of frequency demultiplier 310 is 40 frequency demultiplying ratio, then the operation time of counter 311 is 0.2msec, and the number of bits is 7 - 10 bits. When microprocessor 307 determines that the count value of the frequency-demultiplied digital modulated signal is close to the synchronization frequency of the carrier, then microprocessor 307 causes switch 316 to be switched to terminal B, i.e., the side of the reproduced carrier f L being an output of carrier reproduction circuit 203, so as to sweep VCO 309 at low speed. When detecting, in response to synchronization detection signal 306, that the carrier f L is synchronized with the digital modulated signal, microprocessor 307 stops the sweeping of VCO 309 to operate counter 311 for a definite period in response to a control signal LE5 so that the counter may count the frequency-demultiplied carrier f L . A count output of counter 311 is accepted by microprocessor 307 via signal line 312 and then compared with the reference value set on the program. Microprocessor 307 applies data to D/A converter 308 in the direction of eliminating an error, so as to subject the oscillation frequency of VCO 309 to fine tuning. Accordingly, the frequency of the frequency-converted digital modulated signal to be applied to demodulation circuit 2 is stabilized. At this time, assuming that the resolution power required for counting the carrier f L is 20kHz, the synchronization frequency range of the carrier f L is 140MHz +/- 1MHz, and the operation time of counter 311 is 2msec, then the count value of counter 311 is 6950-7050 since the frequency demultiplying ratio of frequency demultiplier 310 is 40 frequency demultiplication. This count value is represented by 1101100100110 bits - 1101110001010 bits in binary number, and the number of bits of counter 311 is 8 - 13 bits. Thus, the number of bits of counter 311 should be 8 bits which is the common number of bits in high speed sweeping. As described above, if an appropriate setting is made on the frequency range of VCO 309, the synchronization frequency range of the carrier f L , the resolution power of counting of frequencies in both high speed sweeping and low speed sweeping, the frequency demultiplying ratio of frequency demultiplier 310 and the operation time of counter 311, then the number of bits of counter 311 can be minimized. While the switching of switch 316 is made upon low speed sweeping in the foregoing description, switch 316 may be switched after the carrier f L is synchronized with the digital modulated signal. Fig. 8 is a block diagram showing still another embodiment of the present invention. In the embodiment shown in Fig. 8, the AFC circuit 33 includes a frequency demultiplier 318 for ml frequency demultiplying a digital modulated signal, and a frequency demultiplier 319 for m2 frequency demultiplying carrier f L . Respective outputs of these frequency demultipliers 318 and 319 are selected by switch 316 and then applied to counter 311. The rest of the structure of this embodiment is the same as that of the embodiment shown in Fig. 2. Switch 316 is switched in response to a control signal LE11 supplied as an output from microprocessor 307. An AFC operation in the embodiment shown in Fig. 8 is approximately the same as the operation of Fig. 7. The frequency demultiplying ratio of frequency demultiplier 318 and that of frequency demultiplier 319 may be identical to each other or different from each other. In both cases, the minimal number of bits of counter 311 can be employed if an appropriate setting is made on those frequency demultiplying ratios in accordance with the foregoing description and the various conditions given in the embodiment of Fig. 7. Fig. 9 is a block diagram showing still another embodiment of the present invention. In the embodiment shown in Fig. 9, the AFC circuit includes C/N detection circuit 315 shown in Fig. 6 in addition to the embodiment shown in Fig. 7. Like the embodiment shown in Fig. 6, in the state that C/N detection circuit 315 is employed, the synchronization of carrier can be set earlier when the C/N ratio of a digital modulated signal is higher by a change of the sweep speed of a control voltage of VCO 309 based on the C/N ratio of the digital modulated signal. Referring to Fig. 10, band-pass filter 201 for band limitation of demodulation circuit 2 in the embodiments shown in Figs. 6 - 9 is replaced by low-pass filters 204 and 205 for band limitation for band-limiting a demodulated signal. The characteristics of a demodulation circuit 21 are all the same as those of demodulation circuit 2 in the former embodiments. Even if demodulation circuit 21 for band-limiting the signal, produced by demodulating a digital modulated signal without placing a band limit, is employed, there is no trouble in an AFC operation because the digital modulated signal is utilized in high speed sweeping, and the carrier is directly frequency-demultiplied to count frequencies in low speed sweeping and in the case of synchronization of the carrier, according to one embodiment of the present invention. In this respect, in the conventional AFC circuit, since only a digital modulated signal is utilized, if the signal, produced by demodulating the digital modulated signal without placing a band limit, is band-limited after demodulation, the spectrum of the digital modulated signal to be applied to frequency demultiplier 301 becomes wider than in the case of placing a band limit, and thus the jitter of the frequency-demultiplied signal increases. Moreover, the C/N ratio of the digital modulated signal decreases by the proportion in which the digital modulated signal is not band-limited, resulting in an increase in the error from the carrier. In the embodiments shown in Figs. 6 - 9, like the embodiment of Fig. 4, shift register 313 may be provided between counter 311 and microprocessor 307, to serially apply a count output of counter 311 to microprocessor 307 and thus reduce the number of input terminals of microprocessor 307. Fig. 11 is a block diagram showing still another embodiment of the present invention. In the embodiment of Fig. 11, a PLL circuit 320 is employed as an AFC circuit 34. PLL circuit 320 includes a VCO 309, a prescaler 321, a variable frequency demultiplier 322, a phase comparator 323, a reference oscillator 324 and a loop filter 325. VCO 309 applies a local oscillation signal to frequency conversion circuit 1 and also to prescaler 321. Prescaler 321 frequency-demultiplies the frequency of the local oscillation signal in a predetermined frequency demultiplying ratio, to apply a frequency demultiplication output thereof to variable frequency demultiplier 322. Variable frequency demultiplier 322 frequency-demultiplies the local oscillation signal, which is frequency-demultiplied by prescaler 321, in the frequency demultiplying ratio set by microprocessor 307. A frequency demultiplication output of variable frequency demultiplier 322 is applied to phase comparator 323. Phase comparator 323 compares the phase of the frequency of the local oscillation signal, which is frequency-demultiplied by variable frequency multiplier 322, with that of the frequency of a reference signal supplied as an output from reference oscillator 324, so as to supply an error signal as an output. The applied error signal is applied to loop filter 325, then smoothed and applied as a control voltage to VCO 309. As reference oscillator 324, such a reference oscillator is employed as to be capable of controlling an oscillation frequency in response to an externally applied voltage. A control voltage of reference oscillator 324 is supplied in response to the conversion of digital voltage data applied from microprocessor 307 into an analogue signal by D/A converter 308. The other devices in AFC circuit 34 such as a frequency demultiplier 310, a counter 311, microprocessor 307 and D/A converter 308 are structured in the same manner as in Fig. 2. Fig. 12 is a flow chart for explaining an operation of the embodiment shown in Fig. 11. Fig. 13 is a diagram showing one example of voltage-to-frequency characteristics of the reference oscillator of Fig. 11. A detailed operation of this embodiment will now be described with reference to Figs. 11 - 13. First, microprocessor 307 supplies as an output an initial value for determining an oscillation frequency of reference oscillator 324 of PLL circuit 320 to D/A converter 308. This applied data is converted into an analogue signal by D/A converter 308, and the converted signal is then set as a voltage value in reference oscillator 324. Microprocessor 307 then sets previously stored frequency demultiplying ratio data in variable frequency demultiplier 322. Then, PLL circuit 320 performs a predetermined PLL operation, so that a predetermined local oscillation signal is output from VCO 309, and a digital modulated signal applied to frequency conversion circuit 1 is frequency-converted into an IF signal. Microprocessor 307 makes a determination, in response to a synchronization detection signal 306 supplied as an output from carrier reproduction circuit 203, as to whether or not carrier is synchronized with the frequency of the digital modulated signal. If the carrier is not in synchronization with the frequency of the digital modulation signal, data to be applied to D/A converter 308 is added/subtracted in order to sweep a reference frequency of reference oscillator 324. For example, if the waveform of sweeping is a sawtooth waveform, and the voltage-to-frequency characteristics of reference oscillator 324 is the one 10 shown in Fig. 13, the data to be applied to D/A converter 308 is increased. More specifically, when the minimal value of the data is Nmin and the maximal value thereof is Nmax, the following relation is provided: In the above relation, such a loop should be set that the data is increased from the minimal value up to the maximal value and is then returned to the minimal value. When the waveform of sweeping is a triangular waveform, the following relation is provided likewise: In the above relation, such a loop should be set that the data is increased from the minimal value to the maximal value and then decreased from the maximal value to the minimal value repetitively. Microprocessor 307 determines synchronization detecting signal 306 supplied as an output from carrier reproduction circuit 203 and also repeats sweeping of reference oscillator 324 in the form of steps. When detecting that the carrier is synchronized with the frequency of the digital modulated signal, microprocessor 307 draws out from a loop of sweeping reference oscillator 324 and moves into a loop of counting the frequency of the carrier. In this loop, the reproduced carrier supplied from carrier reproduction circuit 203 is first frequency-demultiplied in a fixed frequency demultiplying ratio by frequency demultiplier 310, so that the carrier which is frequency-demultiplied by counter 311 is counted for a definite period. Microprocessor 307 accepts a count output of counter 311, to compare the same with a reference value set on a program. With such an allowable value as not to affect demodulation characteristics of demodulation circuit 2 set as a reference value, if a count value is within a tolerance, the times of controlling of reference oscillator 324 decrease, and drift on the steps of the IF signal can be reduced, resulting in an improvement in demodulation characteristics. When the count value of counter 311 is within the tolerance, oscillation frequency of reference oscillator 324 does not have to be varied. When the count value is also lower than a lower limit of the allowable value, the data to be applied to D/A converter 308 is increased so as to make the oscillation frequency of reference oscillator 324 higher. Conversely, when the count value is higher than an upper limit of the allowable value, the data should be decreased. Thereafter, microprocessor 307 again determines whether or not the carrier is in synchronization with the frequency of the digital modulated signal. If the carrier is in synchronization with the frequency of the digital modulated signal, microprocessor 307 again returns to the loop of counting the frequency of the carrier. Conversely, if non-synchronization is determined, microprocessor 307 moves to another loop. For setting a frequency demultiplying ratio and data of a reference frequency in such an operation, the next operation can also take place after the frequency becomes stable. If the carrier is not in synchronization with the frequency of the digital modulated signal within the frequency variable range of reference oscillator 324, then the frequency demultiplying ratio of variable frequency demultiplier 322 can be added/subtracted so as to re-sweep the oscillation frequency of reference oscillator 324. As described above, according to the embodiment shown in Fig. 11, the frequency demultiplying ratio is set in variable frequency demultiplier 322 of PLL circuit 320, and the oscillation frequency of reference oscillator 324 is swept. If the carrier reproduced by carrier reproduction circuit 203 is synchronized with the frequency of the digital modulated signal, the sweeping of reference oscillator 324 is stopped, so that the frequency of the directly reproduced carrier is counted. Then, fine tuning of the oscillation frequency of reference oscillator 324 enables stabilization of the frequency of the IF signal to be applied to demodulation circuit 2. Thus, even in the case of a low C/N ratio, the frequency can be accurately counted, and the frequency of the IF signal applied to demodulation circuit 2 can be stable, thereby enabling the frequency of the IF signal to more precisely match a normal frequency. Fig. 14 is a diagram showing one example of reference oscillator 324 shown in Fig. 11. Particularly, Fig. 14 (a) shows reference oscillator 324 constituted by a voltage-controlled crystal oscillator (VCXO) 411 and a frequency demultiplier 412, and Fig. 14 (b) shows reference oscillator 324 constituted by a voltage-controlled oscillator (VCO) 413 and a frequency demultiplier 412. Referring to Fig. 11, if the frequency fluctuation of a digital modulated signal is small, approximately ± 0.01%, then VCXO 411 can be used in reference oscillator 324; however, if the frequency fluctuation becomes larger, then VCO 413 employing an SAW resonator in which a variable range can be taken wider, a ceramic resonator and the like should be employed. As has been described heretofore, in the embodiment shown in Fig. 11, the employment of PLL circuit 320 for constituting AFC circuit 34 enables the stabilization of the frequency of the IF signal of the frequency-converted digital modulated signal to be applied to demodulation circuit 2. Thus, even in the case of a lower C/N ratio, an accurate counting of the frequency can be made, and the frequency of the IF signal applied to demodulation circuit 2 can be stable. Consequently, the frequency of the IF signal can more precisely match a normal frequency.

Claims: An automatic frequency control circuit for stabilizing a frequency of an intermediate frequency signal to be supplied as an input to a demodulation circuit (2), comprising frequency conversion means (1) for frequency-converting a digital modulated signal to output an intermediate frequency signal, voltage-controlled oscillation means (309) for applying to said frequency conversion means a local oscillation signal having an oscillation frequency varying in response to an applied control voltage, carrier reproduction means (203) for generating a reproduced carrier (f L ) required for demodulating said digital modulated signal, said reproduced carrier being mixed with said intermediate frequency signal in said demodulation circuit to demodulate said digital modulated signal, said carrier reproduction means outputting a synchronization detection signal indicating synchronization, between the frequency of the digital modulated signal and the reproduced carrier, which provides valid demodulation of the digital modulated signal, frequency demultiplying means (310) arranged for frequency demultiplying the frequency of the reproduced carrier generated by said carrier reproduction means, counting means (311) for counting the frequency-demultiplied signal output by said frequency demultiplying means for a definite period, and control means (307,308;307,308,321 to 325) for applying a said control voltage to said voltage-controlled oscillation means in response to the count output of said counting means and said synchronization detection signal. The automatic frequency control circuit according to claim 1, wherein said control means includes a microprocessor (307) for supplying said control voltage as a digital signal, and D/A converting means (308) for converting the digital signal supplied as an output from said microprocessor into an analogue signal, to apply the analogue signal to said voltage-controlled oscillation means. The automatic frequency control circuit according to claim 2, further comprising register means (313) for storing the count output of said counting means, to serially apply the stored count output to said microprocessor. The automatic frequency control circuit according to claim 2, further comprising detecting means (315) for detecting that a carrier-to-noise ratio of said digital modulated signal changes, and wherein said microprocessor includes means for generating said control voltage while sweeping said control voltage as a sawtooth wave, to change speed of said sweeping in response to said detecting means detecting that the carrier-to-noise ratio changes. The automatic frequency control circuit according to claim 2, further comprising switch means (316) for selecting one of said digital modulated signal and said reproduced carrier, to apply the selected one of said digital modulated signal and said reproduced carrier to said frequency demultiplying means, and wherein said microprocessor includes means for determining a synchronization state of said digital modulated signal and said reproduced carrier in response to the count output of said counting means and said synchronization detection signal, and controlling said switch means so as to switch said switch means to said digital modulated signal in response to a determination of a non-synchronized state and to switch said switch means to said reproduced carrier in response to a determination of the vicinity of frequency synchronization. The automatic frequency control circuit according to claim 2, wherein said frequency demultiplying means includes first frequency demultiplying means (318) for frequency-demultiplying the frequency of said digital modulated signal, and second frequency demultiplying means (319) for frequency-demultiplying the frequency of said reproduced carrier, said automatic frequency control circuit further comprising switch means (316) for selecting one of the respective outputs of said first and second frequency demultiplying means, to apply the selected output to said counting means, and wherein said microprocessor includes means for determining the synchronization state of said digital modulated signal and said reproduced carrier in response to the count output of said counting means and said synchronization detection signal, and controlling said switch means so as to switch said switch means to the output of said first frequency demultiplying means in response to a determination of a non-synchronized state and to switch said switch means to the output of said second frequency demultiplying means in response to a determination of the vicinity of frequency synchronization. The automatic frequency control circuit according to claim 2, wherein said control means includes variable frequency demultiplying means (322) having a variable frequency demultiplying ratio for frequency-demultiplying the local oscillation signal generated by said voltage-controlled oscillation means, reference oscillation means (324) having an oscillation frequency made variable by a voltage control for generating a reference signal, phase comparison means (323) for making a comparison between the phase of the frequency demultiplication output of said variable frequency demultiplying means and the phase of the reference signal generated by said reference oscillation means, to make the oscillation frequency of said voltage-controlled oscillation means variable in response to the result of the comparison, and a microprocessor (307) for setting voltage data for determining the oscillation frequency of said reference oscillation means and setting frequency demultiplying ratio data for determining the frequency demultiplying ratio of said frequency demultiplying means, to sweep the voltage data applied to said reference oscillation means until a synchronization detection signal is applied from said carrier reproduction means, and in response to the application of said synchronization detection signal, to compare the count output of said counting means and a predetermined reference value, to subject the voltage data applied to said reference oscillation means to fine tuning so that a comparison output thereof may be in a predetermined range. The automatic frequency control circuit according to claim 7, further comprising fixed frequency demultiplying means (321) having a fixed frequency demultiplying ratio for frequency-demultiplying the local oscillation signal generated by said voltage-controlled oscillation means, to apply a frequency demultiplication output thereof to said variable frequency demultiplying means.

Inventors: KITAYAMA TAKAMITSU, MIYAZAKI MASAO, OHTA TOMOZO, KITAYAMA, TAKAMITSU, MIYAZAKI, MASAO, OHTA, TOMOZO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H03J7
IPC codes: H03J7, H04L27