Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-06-12
2001-05-29
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Automatic frequency control
C375S340000
Reexamination Certificate
active
06240147
ABSTRACT:
BACKGROUND OF THE INVENTION
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°, 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 oscillatio
Muramatsu Yoshihito
Oga Tadashi
Al-Beshrawi Tony
Chin Stephen
Matsushita Electric - Industrial Co., Ltd.
Pearne & Gordon LLP
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