Automatic frequency control circuit for control of reference oscillator in communication apparatus

An automatic frequency control device in which a burst frequency is sampled and held on the output side of an IF amplifier, the held frequency is measured and compared with a preset reference frequency data for obtaining a difference therebetween, and a reference oscillator is controlled so as to keep the difference below a predetermined value, by which the stabilization of an IF frequency and a transmitter carrier wave frequency, both of which are controlled by a reference frequency from said reference oscillator, can be accomplished.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an automatic frequency control device which is 
applied to radio communication apparatus operating in a burst mode, for 
example, an automobile telephone or the like. 
2. Description of the Prior Art 
FIG. 1 is a block diagram showing the component configuration of a 
receiving unit of a conventional radio communication apparatus. In FIG. 1, 
reference numeral 1 is an antenna, 2 a high frequency amplifier which 
amplifies a received signal input to the antenna 1, 3 a frequency 
converter which mixes the amplified received signal from the high 
frequency amplifier 2 with a local oscillation signal to convert the 
received signal into an intermediate frequency signal, 4 an intermediate 
frequency amplifier which amplifies the intermediate frequency signal, 5 a 
demodulator which demodulates an output signal of the intermediate 
frequency amplifier 4 to obtain a baseband signal, 6 a phase-locked loop 
which operates as a sample and hold means and which comprises a phase 
comparator 7, a loop filter 8, a through/hold circuit 9, and a voltage 
controlled oscillator 10 (hereinafter abbreviated as VCO). 
Reference numeral 11 is a synchronous circuit which operates by receiving a 
received burst signal contained in the demodulated data;. 12 is a control 
circuit which receives an output signal of the synchronous circuit 11 and 
sets the through/hold circuit 9 in a through state in a carrier 
reproducing portion of the receiving burst and sets the through/hold 
circuit 9 in a hold state in a data portion of the received burst; 13 is a 
reference oscillator; 14 is a receiving local oscillator which outputs a 
local oscillation signal in response to an oscillation signal of the 
reference oscillator 13 and 15 is a transmitting carrier wave oscillator 
which uses the frequency of oscillation of the reference oscillator 13 as 
a transmitting frequency. 
Next, the operation will be described for the case where a carrier 
reproducing portion 102 of the received burst is continuously formed as 
shown in FIG. 2 and alternates with a data portion 101. A signal received 
by the antenna 1 is amplified by the high frequency amplifier 2 and then 
mixed with the local oscillation signal from local oscillator 14 to be 
converted into an intermediate frequency signal in the frequency converter 
3. The intermediate frequency signal is amplified by the intermediate 
frequency amplifier 4 and the demodulated data is taken out of the 
demodulator 5. 
The synchronous circuit 11 operates upon receiving the demodulated data, 
and the output signal of the control circuit 12 which has received the 
output signal of the synchronous circuit 11 sets the through/hold circuit 
9 into a through state during the carrier wave reproducing portion 102 of 
the receiving burst. In this state, the frequencies and phases of the 
output signal of the VCO 10 and the intermediate frequency amplifier 4 are 
compared, the error output is outputted from the phase comparator 7, and 
the VCO 10 is controlled via the loop filter 8 and the through/hold 
circuit 9 by this error output to synchronize with the output of IF 
amplifier 4. 
Next, in a data portion 101 of the received burst, the output signal of the 
control circuit 12 sets the through/hold circuit 9 into a hold state, by 
which the output frequency and phase of the VCO 10 after control is 
retained, and the output signal of the VCO 10 is supplied to the 
demodulator 5 to be used in the demodulation of the IF signal. 
Since the receiving unit of a typical conventional radio communication 
apparatus is arranged as described above, that is, reference oscillator 13 
is not subjected to phase-locked loop correction, the output frequencies 
of the receiving local oscillator 14 and the transmitting carrier wave 
oscillator 15 are varied by output variations of the reference oscillator 
13. Therefore, in particular, when the allowable error in the transmitting 
carrier wave frequency is very small, the reference oscillator must be 
highly stable, which results in high cost. 
A circuit arrangement similar to that shown in FIG. 1 is described in the 
form of a color synchronous circuit in the TV Engineering Handbook, first 
edition, edited by a corporate Juridical person, TV Society, published by 
OHM Co., Japan, pp. 12-79. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an automatic frequency control 
system in which a reference oscillator is controlled after receipt of a 
receiving frequency to stabilize the intermediate frequency and 
transmitting carrier frequency used in the reception and transmission of 
communication signals. 
In order to accomplish the above-mentioned object, the automatic frequency 
control system of this invention is provided with a frequency sample and 
holding means which samples a receiving frequency in the carrier 
reproducing portion of the received burst and holds the frequency 
concerned during the data portion of the received burst, a frequency 
measuring means which measures the held receiving frequency, and a 
comparison and decision means in which the measured frequency data and 
reference frequency data from a reference oscillator are compared to 
control the reference oscillator to reduce the error to within a certain 
value, and is constructed so as to control the output frequency of the 
reference oscillator to follow the received frequency. 
The present invention takes advantage of the fact that the local oscillator 
of a mobile telephone base station is highly stabilized in comparison to 
the local reference oscillator of a mobile station and thus can be 
advantageously used to correct the frequency of the mobile station 
reference oscillator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of this invention are hereinafter described in detail 
with reference to accompanying drawings. 
In FIG. 3, in which the same parts as those in FIG. 1 are identified by the 
same numerals to avoid duplication, reference numeral 16 is an 
intermediate frequency (IF) bandpass filter connected between the 
frequency converter 3 and the IF amplifier 4, 17 is a frequency measuring 
means composed of counters the input of which is the output of 
phase-locked loop 6, 18 is a frequency comparison means which comprises a 
sequence circuit composed of adders, latch circuits, and a reference 
frequency generator or a microprocessor and outputs a frequency error 
correction data in accordance with an output signal of the frequency 
measuring means 17, 19 is a digital-to-analog converter (hereinafter 
abbreviated as D/A converter), and 20 is a field intensity level detection 
circuit which detects the field intensity of the IF signal. 
FIG. 4 is a wave diagram of a received burst applied to the embodiments 
described above. The wave form includes a carrier reproducing portion 101 
and a data portion 102 which are separated from each other. 
FIG. 5 is a block diagram showing examples of arrangements of the 
synchronous circuit 11, the control circuit 12, the frequency measuring 
means 17, and the frequency comparison means 18 in the embodiment 
described above. 
The synchronous circuit 11 comprises a binary correlator 117 and a 
magnitude comparator 118, the former having a sampler 111 for inputting 
demodulated data output from the demodulator 5, a comparator 112 for 
comparing an output signal of the sampler 111 with a reference signal, a 
reference register 113, a shift register 114, an exclusive OR circuit 115, 
and an adder 116. 
The control circuit 12 comprises an AND gate 121 which obtains the logical 
AND of the synchronous signal output from the magnitude comparator 118 and 
the output signal from the electric field level detection unit 20, a 
counter 122 which receives the output signal from the AND circuit 121, 
begins counting of the reference oscillation signal from the reference 
oscillator 13 until the time T.sub.1 shown in FIG. 4 elapses and a 
predetermined count number is reached, a counter 123 which receives the 
count up signal from the counter 122, begins counting of the reference 
signal from the reference oscillator 13 until the time T.sub.2 shown in 
FIG. 4 elapses and a predetermined count number is reached, and a 
flip-flop 124 which is reset by the count up signal of the counter 122 and 
set by the count up signal of the counter 122 to output a control signal. 
In short, the count up signal of the counter 122 brings the through/hold 
circuit 9 into a through state, and at the same time, makes the frequency 
measuring means 17 stop the measuring operation, and the count up signal 
of the counter 123 brings the through/hold circuit 9 into a hold state, 
and at the same time, makes the frequency measuring means 17 start the 
measuring operation. 
The frequency measuring means 17 and the frequency comparison means 18 
comprise a gate time generation circuit 171 which has been supplied with 
the reference oscillation signal from the reference oscillator 13 and 
starts its operation upon the reception of the control signal from control 
circuit 12 and a down counter 172 which starts its operation upon the 
reception of the control signal, subtracts the held frequency signal from 
the phase-locked loop until it receives gate signal from the gate time 
generation circuit 171, and outputs frequency correction data. 
Next, the operation of the embodiment will be described. Since the 
operation of reception, and that of acquisition and holding of a frequency 
are same as those in FIG. 1 described above, the duplicated description is 
omitted. The operation of an automatic frequency control device in this 
invention will be described in the following. 
As described above, upon initiation of holding of a receiving frequency in 
the carrier reproducing portion 102 of the received burst, the frequency 
is measured by the frequency measuring means 17. The result of frequency 
measurement thus obtained is sent to the frequency comparison means 18 and 
compared with an IF reference data preset in a memory in the frequency 
comparison means 18 which, as one example, may be 10.7 MHz, by which a 
correction data corresponding to the frequency difference between them is 
output. 
As shown in the flowchart of FIG. 6, the frequency comparison means 18 
outputs an initial value of a frequency correction data at step ST1. Next, 
the means 18 waits until an oscillation frequency of the receiving local 
oscillator 14 becomes stable (Step ST2) and decides whether holding starts 
or not (Step ST3). In the case of YES, the means 18 measures the held 
frequency (Step ST4) and decides whether the difference between the 
measured frequency data and the reference frequency data is within a fixed 
value or not (Step ST5), and in the case of NO, the means 18 outputs a 
frequency correction data and returns to the Step ST2 (Step ST6). 
After the frequency correction data output from the frequency comparison 
means 18 is converted into an analog signal by the D/A converter 19, it is 
added to the reference oscillator 13. As a result, the reference 
oscillator 13 is controlled to follow the received frequency, and 
consequently, an IF frequency becomes constant, and at the same time, the 
oscillation frequency of the transmitting carrier wave oscillator 15 
becomes stable. 
An electric field detector 20 monitors a received electric field level, and 
disables the control circuit 12 When the received field level is low by 
sending a logical "0" to AND circuit 121 shown in FIG. 5. As a result, 
when the received electric field level is low, the reference oscillator 13 
cannot be controlled and the previous reference oscillation frequency is 
used as it is. 
FIG. 7 shows an example in which a narrow bandpass filter 21 is provided on 
the input side of the phase-locked loop 6 in the constitution in FIG. 3 
described above. This constitution allows the sensitivity in a weak 
electric field to be raised, and, the automatic frequency control to be 
effectively done even in the case of a weak antenna input. 
FIG. 8 shows an example in which a frequency divider 22 is provided on the 
input side of the phase-locked loop 6 in the constitution in FIG. 3 
described above. This constitution divides the IF down to a frequency 
which is easy to process to sample the IF frequency, allowing it to be 
input to the phase-locked loop 6. 
Each embodiment described above shows a constitution of a single 
superheterodyne receiving unit having one IF frequency. FIG. 9 shows a 
constitution of a double superheterodyne receiving unit having two IFs, 
which is provided with a first IF generating unit 23-1 comprising a first 
frequency converter 3-1, a first IF amplifier 4-1, a first IF BPF 16-1, 
and a first receiving local oscillator 14-1 and a second IF generating 
unit 23-2 comprising a second frequency converter 3-2, a second IF BPF 
16-1, a second IF amplifier 4-2, and a second receiving local oscillator 
14-2. The double superheterodyne receiving unit can make the two IF 
frequencies constant by controlling the first and second receiving local 
oscillators 14-1 and 14-2 by using the output of the reference oscillator 
13. 
The case in which a frequency in the carrier reproducing portion 102 of the 
received burst becomes constant is assumed. Even in the case where signals 
are modulated by an arbitrary data pattern using such a modulation method 
as a GMSK method, the so-called Costas loop in which an error component 
between the frequency of the received carrier, obtained by the result of 
the multiplication of the in-phase component I and the orthogonal 
component Q, and the reproduced clock and the oscillation frequency of the 
VCO 10 is detected. The error component is fed back to the VCO 10, and the 
PLL 6 is brought into a hold state, by which an automatic frequency 
control device which performs the same action as each of the embodiments 
described above can be constituted. 
FIG. 10 is a block diagram showing a constitution of a receiving unit 
provided with an automatic frequency control device using a costas loop, 
and the same parts as those in FIG. 2 are identified by the same reference 
numerals to omit duplicated description. 
In FIG. 10, reference numeral 24-1 denotes a first multiplier, 24-2 a 
second multiplier, 25-1 a first lowpass filter (LPF), 25-2 a second 
lowpass filter (LPF), 24-3 a third multiplier, 24-4 a fourth multiplier, 
26 a clock regenerative circuit, and 27 a 90.degree. phase shifter. 
FIG. 11 is a block diagram showing a concrete constitution of the clock 
reproducing circuit 26 of FIG. 10. The clock reproducing circuit 26 
comprises a variable frequency divider 261 which divides the frequency of 
the reference oscillation signal from the reference oscillator 13, a fixed 
frequency divider 262 which divides tile frequency of an output signal of 
the variable frequency divider 261 and outputs a reproduced clock, a phase 
comparator 263 which compares a phase of an output signal of a multiplier 
265 with the phase of the reproduced clock. Multiplier 265 outputs a 
signal equal to the product of the in-phase component I and the orthogonal 
component Q of the demodulated data. An up/down counter 264 receives an 
output signal of tile multiplier 265(?) at its clock terminal and an 
output signal of tile phase comparator 263 is inputted to its up/down 
terminal to perform up/down counting and the counted result is fed back to 
the variable frequency divider 261, thereby performing control so as to 
make the phase of tile output signal of the multiplier 24-1 coincide with 
that of tile reproduced clock. 
The first multiplier 24-1 is supplied with an output signal from the IF 
amplifier 4 and an output signal of the VCO 10. Assuming that an angular 
frequency slip .DELTA..omega. exists in an angular frequency .omega..sub.c 
in the expression S(t)=I(t) cos .omega..sub.c t+Q(t) sin .omega..sub.c t, 
the following operation is performed. 
##EQU1## 
Then, the first lowpass filter 25-1 filters the output of the first 
multiplier 24-1 and outputs the following signal. 
EQU 1/2.multidot.I(t)cos .DELTA..omega.t-1/2.multidot.Q(t)sin .DELTA..omega.t 
On the other hand, the second multiplier 24-2 receives the output signal of 
the IF amplifier 4 and the output signal of the 90.degree. phase shifter 
27, which represents a 90.degree. phase shifted VCO signal, and performs 
the following operation. 
##EQU2## 
Then, the second lowpass filter 25-2 filters the output signal of the 
second multiplier 24-2 and outputs the following signal. 
EQU 1/2I(t)sin .DELTA..omega.t+1/2Q(t)cos .DELTA..omega.t 
Next, the third multiplier 24-3 multiplies the output of the first lowpass 
filter 25-1, and the output of the second lowpass filter 25-2. The output 
signals of multipliers 25-1 and 25-2 can be represented by the following. 
##EQU3## 
and the product of these expressions is calculated to be 
##EQU4## 
where .rho.(t)=.pi./2T t: mark, and -.pi./2T t: space 
Then the fourth multiplier 24-4 obtains 
EQU 1/8.multidot.sin.sup.2 .DELTA..omega.t 
as the product of the output signal of the third multiplier 24-3 and the 
output signal of the clock reproducing circuit 26, that is, the reproduced 
clock timing signal V(t)=cos(.pi./T.multidot.t). 
The output signal of the fourth multiplier 24-4 is fed back via the loop 
filter 8 and the through/hold circuit 9 in a through state to the VCO 10, 
and the received carrier wave frequency retained in the VCO 10 is measured 
by the frequency measuring means 17 when the through/hold circuit 9 is in 
a hold state. 
The result of this frequency measurement is compared with the preset 
frequency reference data by the frequency comparison means 18, and the 
frequency correction data corresponding to the difference frequency is 
converted into an analog signal by the D/A converter 19. Then, the analog 
signal is supplied to the reference oscillator 13 to control the reference 
oscillator 13 to follow the received carrier frequency. 
Incidentally, in each embodiment described above, the case where the 
received burst in which the data portion 101 and the carrier reproducing 
portion 102 are not contiguous with each other is applied as shown in FIG. 
4 has been described. But, as shown in FIG. 2, the received burst in which 
both portions are contiguous is similarly applicable. 
As described above, according to this invention, a receiving frequency is 
stored and held, the held receiving frequency is measured to compare it 
with the preset frequency reference, and the reference oscillator is 
controlled in response to the frequency correction data corresponding to 
the difference between the preset frequency and the held receiving 
frequency. Accordingly, the IF can be made constant without the use of an 
expensive reference oscillator, and the oscillation frequency of the 
transmitting carrier oscillator is stabilized.