The present invention relates to a frequency-shift keying signal type auto frequency control apparatus for use in digital wireless communication. More particularly, the invention relates to a structure for varying the frequency by changing the division ratio of the comparison signal with an AFC control voltage so as to make coincide the frequency of an output signal from a local oscillation portion and that of a carrier-wave signal for the frequency-shift keying signal with each other.
In recent years, there is a trend in digital wireless communication toward reduction in the size of a radio wave receiver terminal equipment adapted to a frequency-shift keying method in order to improve the portability.
The radio-wave receiver is adapted to a receiving method including a heterodyne method with which the frequency of a carrier-wave signal is mainly mixed with the frequency of a local oscillation portion so as to perform demodulation from the extracted intermediate frequency. Another method is a direct conversion method wherein the frequency of the carrier-wave signal and that of the local oscillation portion coincide with each other so that the modulation signal is directly extracted.
However, the heterodyne method encounters deterioration in the sensitivity characteristic if the difference between the frequency of the carrier-wave signal and that of the local oscillation portion is deviated from a predetermined intermediate frequency. Also the direct conversion method encounters deterioration if the frequency of the carrier-wave signal is deviated from that of the local oscillation portion.
Hitherto, a direct-conversion receiver has been disclosed in Unexamined Japanese Patent Publication 7-154435 in which an automatic frequency control (AFC) loop is formed by using a frequency detector. The structure of the frequency receiver will now be described with reference to FIGS. 7, 8 and 9.
FIG. 7 shows a conventional AFC loop in the direct conversion type receiver. Received wave Fsig subjected to frequency-shift keying modulation with the digital signal is amplified by an amplifier 701. The output from the amplifier 701 is divided, and then supplied to mixers 702 and 703. A signal transmitted from a local oscillator 706 is divided. The phase of one of the divided portions is delayed by a phase shifter 705 by an angular degree of 90.degree., and then supplied to the mixer 702. Another portion of the divided portions is supplied to the mixer 703.
An in-phase base band signal (an I signal) obtained from the output of the mixer 703 and a quadrature base band signal (a Q signal) obtained from the output of the mixer 702 and time-functionally quadrature with respect to the output of the mixer 703 are detected by a demodulating portion 707. Thus, a demodulation signal is obtained.
The demodulating portion of the direct conversion type receiver performs the detection operation required in the AFC will now be described. The I signal and the Q signal has the relationship that the phase shifts are the same when the frequency of the voltage control oscillator coincides with that of the received wave. When the frequencies are shifted from each other, the phase shifts are made to be different from each other. The change in the phase shift is detected by a frequency detector 710. A signal which is logical in a direction in which the shifted frequency is restored is transmitted to a next control means 711. The output of the control means 711 is allowed to pass through a low pass filter 708 so as to vary the frequency of the local oscillation portion 706 with the control voltage from which noise has been removed. Thus, the AFC loop is constituted.
The control voltage which is the output of the control means 711 shown in FIG. 7 is directly applied to the oscillator 706 in the local oscillation portion 704. Another AFC loop arranged to perform a similar operation is shown in FIG. 8. The structure shown in FIG. 8 is arranged in such a manner that the operations of a mixer 802 and a demodulating portion 806 disposed in a passage with which the received wave is demodulated are adapted to the above-mentioned direct conversion receiving method. The heterodyne receiving method may be employed in which the frequency of the carrier-wave signal is converted into the intermediate frequency with the frequency of the output signal from a local oscillation portion 803 after which a demodulating portion 806 demodulates the signal.
The heterodyne method is arranged in such a manner that the amount of deviation between the difference between the frequency of the carrier-wave signal and the frequency of the output signal from the local oscillation portion and a predetermined reference intermediate frequency is detected by a frequency detector 808. To cancel the amount of the difference, the AFC loop is constituted such that an output of control voltage is produced by a control means 809 so as to vary the frequency of the local oscillation portion 803.
The structure shown in FIG. 8 is the same as that shown in FIG. 7 in the structure of the receiver and the circuit structures of the frequency detector and the control means. However, the structure is different from that shown in FIG. 8 in that a phase synchronizing means 811 is provided which makes coincide the phase of a division signal of a reference-signal generating portion 810 and that of the division signal of the voltage control oscillator 804 with each other.
FIG. 9 shows an example of the structure of a circuit for use in the phase synchronizing means 811. Data of the division ratio transmitted from a division-data setting means 812 is received by a data control portion 905. Then, data of the division ratio for each system which must be divided is transmitted to a shift register 906. The shift register 906 temporarily stores data of the division ratio so as to produce an output to storage means 904 and 907 of the corresponding systems in accordance with the length of data.
The storage means 904 is provided for the purpose of temporarily storing data of the division ratio which must be supplied to the signal system which must be subjected to a comparison. The signal which must be subjected to a comparison is the output signal from the local oscillation portion 803. The storage means 907 is provided for the purpose of temporarily storing data of the division ratio which must be supplied to the reference signal system. The reference signal is the output signal from the reference-signal generating portion 810.
The amplitude of the reference signal is limited by a limit amplifier 908, and then supplied to a program counter 909. The frequency of the supplied signal is divided with the division ratio determined by reading data of the division ratio stored on the storage means 907, and the divided frequency is transmitted to a phase comparator 910.
On the other hand, the amplitude of the signal which must be subjected to a comparison is limited by a limit amplifier 901, and then supplied to a pre-scaler 902 so as to be divided. An output signal from the pre-scaler 902 is supplied to a program counter 903. The frequency of the supplied signal is divided with the division ratio determined by reading data of the division ratio stored on the storage means 904, and then the divided frequency is transmitted to the phase comparator 910.
The phase comparator 910 subjects, to a comparison, the phase difference between the reference signal which is the output of the program counter 909 and the signal which is the output of the program counter 903 and which must be subjected to the comparison. Then, the phase comparator 910 transmits, to a charge pump 911, a difference signal for making coincide the phases with each other. The charge pump 911 produces an output of the control voltage corresponding to the difference signal, the control voltage being transmitted as information for controlling the frequency of the local oscillation portion 803.
Since the phase synchronizing means 811 is provided as described above, a characteristic is realized in that the frequency can easily be varied when the set division ratio is changed. Thus, the foregoing structure is known as an effective means when a plurality of channels are received. Specifically, the instruction to set the division ratio is transmitted by the setting means 813 to the division-data setting means 812. The division-data setting means 812 writes data of the division ratio on the phase synchronizing means 811.
The above-mentioned means has a characteristic that when the frequency of the reference-signal generating portion has been changed, the frequency of the voltage control oscillator is changed by a quantity corresponding to the changed deviation. To use the above-mentioned characteristic to form the AFC loop, the structure in the reference-signal generating portion 810 is arranged such that, for example, a voltage control type crystal oscillator (VCXO) is provided. Moreover, AFC control voltage which is the output of the control means 809 is transmitted to the VCXO so that the oscillation frequency is varied. The changed deviation is allowed to act on the voltage control oscillator, as a result of which the frequency of the local oscillation portion is varied. Thus, the AFC loop can be constituted.
In recent years, there arises a necessity for the digital wireless communication system to demodulate frequency-shift keying signal having a low modulation index in order to make effective use of frequency resources. Therefore, the wireless receiver adapted to the heterodyne method must be structured such that the difference between the center of the spectrum of the received signal and the frequency of the local oscillation portion must coincide with a predetermined intermediate frequency as accurate as possible. In a case of the direct conversion method, the difference must be made to be zero.
In the conventional wireless receiver having the AFC loop shown in FIG. 8, the control means 809 produces an output of control voltage which is in proportion to the frequency detected by the frequency detector 808 so as to transmit the output to the reference-signal generating portion 810. However, in general, the voltage-capacity characteristic of a device constituting the oscillation circuit in the reference-signal generating portion and arranged to convert the voltage into a capacity is not a linear characteristic. Therefore, the influence of the characteristic of the device is directly exerted on the variable range of the control voltage. Thus, there arises a problem in that the variable width of the frequency per unit voltage is undesirably changed and correct coincidence cannot be achieved.
Therefore, the change in the voltage-capacity characteristic occurring because of the dispersion of the device and the temperature characteristic sometimes changes the variable width of the frequency per unit voltage.
Since the reference-signal generating portion 810 shown in FIG. 8 generates the reference signal for use in the phase synchronizing means 811, a crystal oscillator is usually employed because high Q can be obtained and stability at high frequencies can be realized. In an example case of the voltage control type crystal oscillator (VCXO), influence of external noise on the control voltage sometimes arises a possibility that the S/N ratio of the oscillation spectrum easily deteriorates and the stability of the frequency is lost.
When the direct conversion type receiver or the heterodyne receiver is formed into a portable unit, the structure must be simplified for reducing the size by, for example, integrating circuits.