Frequency adjusting method for use with digital receiver and frequency adjusting circuit thereof

A frequency adjusting circuit for use with a digital receiver is disclosed, the circuit comprising a means (mixers 3 and 4) for converting the frequency of a received digital signal into an intermediate frequency signal, a local oscillator (VCOs 7 and 8 and PLLs 9 and 10) for supplying a local oscillation signal to the frequency converting means, and a frequency adjusting means (20) for calculating a frequency deviation of the intermediate frequency signal (D1) for each sample, performing a predetermined calculation corresponding to the measured value, and controlling the oscillation frequency of the local oscillator corresponding to the obtained calculated value, wherein the frequency adjusting means compensates the calculated value corresponding to a receiving electric field intensity of the intermediate frequency signal. The frequency adjusting means includes a means (calculating portion 23) for detecting a variation value of the receiving electric field intensity, comparing the deviation value with a threshold value, and when the variation value is larger than the threshold value, removing the frequency deviation at that time from those that are input from the frequency adjusting means.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a digital receiver for use with a mobile 
station or the like of a digital mobile communication, in particular, to a 
frequency adjusting (automatic frequency control AFC) method and a 
frequency adjusting circuit that cause a reference frequency of a mobile 
station to follow a received frequency so as to stabilize the operation of 
the receiver. 
2. Description of the Related Art 
As disclosed in Japanese Patent Laid-Open Publication No. 6-6180, a 
conventional mobile station uses a super heterodyne type receiver. The 
receiver has a local oscillator that converts the received frequency into 
an intermediate frequency. The local oscillator comprises a reference 
oscillator such as a voltage control temperature compensated quartz 
oscillator (hereinafter referred to as TCXO) and a means (such as PLL 
synthesizer) that converts the oscillation frequency of the reference 
oscillator into an intermediate frequency. If the oscillation frequency of 
the local oscillator deviates from the reference frequency, the 
intermediate frequency also deviates from the predetermined frequency. 
Thus, the received signal cannot be accurately demodulated. To cause the 
reference frequency of the mobile station to follow the received frequency 
and stabilize the operation of the receiver, the oscillation frequency of 
the local oscillator should be prevented from deviating. To compensate the 
deviation of the oscillation frequency of the local oscillator, a 
frequency adjusting circuit (automatic frequency control circuit AFC) is 
used. 
FIG. 1 is a block diagram showing an example of the structure of a double 
super heterodyne receiver having such an AFC circuit. In FIG. 1, reference 
numeral 1 is a receiving antenna. Reference numeral 2 is an amplifier. 
Reference numeral 3 is a first mixer. Reference numeral 4 is a second 
mixer. Reference numeral 5 is an intermediate frequency amplifier. 
Reference numeral 6 is a demodulator. Reference numerals 7 and 8 are 
voltage control oscillators. Reference numerals 9 and 10 are PLL 
synthesizers. These devices compose a pair of local oscillators. Reference 
numeral 11 is an A/D converter. Reference numeral 20 is an AFC circuit. 
Reference numeral 21 is a TCXO that supplies a reference frequency to the 
PLL synthesizers 9 and 10. The AFC circuit 20 comprises a calculating 
portion 23 and a D/A converter 22. 
In this structure, a RF signal received from the receiving antenna 1 is 
supplied to the amplifier 2. The amplifier 2 amplifies the RF signal. The 
amplified signal is supplied to the first mixer 3. The first mixer 3 mixes 
the amplified signal with a first local oscillation signal F1 generated by 
the voltage control oscillator 7 and the PLL synthesizer 9 and outputs a 
first intermediate frequency signal IF1. Next, the first intermediate 
frequency signal IF1 is supplied to the second mixer 4. The second mixer 4 
mixes the first intermediate frequency signal IF1 with a second local 
oscillation signal F2 generated by the voltage control oscillator 8 and 
the PLL synthesizer 10 and outputs a second intermediate frequency signal 
IF2. The second intermediate frequency signal IF2 is supplied to the 
intermediate frequency amplifier 5. The intermediate frequency amplifier 5 
amplifies the second intermediate frequency signal IF2. The amplified 
signal is supplied to the demodulator 6. The demodulator 6 demodulates the 
amplified signal and outputs a demodulated signal. The intermediate 
frequency amplifier 5 monitors the level of the electric field intensity 
of the received signal and outputs a receiving electric field intensity 
(RSSI) as a DC voltage. The output signal of the intermediate frequency 
amplifier 5 is supplied to the A/D converter 11. The A/D converter 11 
converts the DC voltage into a digital value and supplies the digital 
value as a receiving electric field intensity signal D2 to the calculating 
portion 23. In such a manner, the receiver monitors the receiving electric 
field intensity. 
The AFC circuit 20 compensates the deviation of the oscillation frequency 
of the local oscillator so as to stabilize the intermediate frequencies of 
the intermediate frequency signals IF1 and IF2 corresponding to the 
frequency of the received signal. In other words, the calculating portion 
23 that is composed of a counter and so forth calculates a compensation 
value of the deviation of the frequency with the intermediate frequency 
signal D1. FIG. 5 is a flow chart showing a calculating process performed 
by the calculating portion 23. In FIG. 5, the frequency deviation is 
measured with the intermediate frequency signal D1 by each received symbol 
unit. The measured frequency deviations are successively added for each 
received symbol unit that represents signal levels "0" and "1" of digital 
signals (at step S21). The number of received symbol units are monitored 
so as to obtain the average value of the frequency deviation of the 
intermediate frequency signal D1 in the receiving unit time as the 
received symbol units (at step S22). When the predetermined number of 
symbol units have been detected, the flow advances to step S23. When the 
predetermined number of symbols have not been detected, the flow returns 
to step S21. At step S23, the added value of the frequency deviations is 
divided by the number of samples in the receiving unit time and thereby 
the average value is obtained. The average value is referred to as a 
compensation value D3 of the local oscillator. The compensation value D3 
that has been obtained in the just preceding receiving unit time is 
substituted with the compensation value D3 that has been newly obtained 
(at step S24). 
The D/A converter 22 converts the updated compensation value D3 into an 
analog DC voltage. The analog DC voltage is supplied to the TCXO 21 so as 
to control the oscillation frequency f0 of the TCXO 21. An output signal 
of the TCXO 21 is supplied to the PLL synthesizers 9 and 10. The PLL 
synthesizers 9 and 10 control the oscillation frequencies of the voltage 
control oscillators 7 and 8 so as to feed back the oscillation frequencies 
of the local oscillators. Thus, the deviation of the oscillation frequency 
f0 of the TCXO 21 and the received frequency is decreased so that the 
oscillation frequency of the TCXO 21 follows the received frequency RF. In 
addition, since the oscillation frequencies of the local oscillators are 
based on one reference oscillator TCXO 21, the structure can be 
simplified. 
In the conventional AFC circuit, regardless of the receiving condition of 
the received signal, the frequency deviation of the intermediate frequency 
is always measured. With the frequency deviation, the compensation value 
of the oscillation frequency is calculated. Thus, when the receiving 
electric field intensity is low due to fading and thereby the reliability 
of the frequency of the received frequency degrades, the oscillation 
frequency is compensated with the received frequency. Thus, the 
reliability of the compensation of the oscillation frequency degrades and 
thereby the oscillation frequency cannot be precisely compensated. 
This problem becomes critical when the AFC is used for a general purpose 
European standard digital cellular phone (GSM). In European countries, the 
operating environments of the digital cellular phones largely vary country 
by country. In addition, fading takes place due to geographical 
conditions. Thus, the frequency deviation may be measured with an error of 
the intermediate frequency signal. Thus, even if the average value of the 
frequency deviations is obtained, it is affected by the error. The error 
affects the voltage control oscillator. Consequently, the oscillation 
frequency cannot be accurately compensated. Thus, the conventional AFC 
circuit and AFC method have problems as mentioned above description. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an AFC circuit and an AFC 
method that accurately compensate an oscillation frequency without 
influence of fading and strong noise components. 
A first aspect of the present invention is a frequency adjusting method for 
use with a digital receiver, the method comprising the steps of receiving 
a digital signal, converting the frequency of the received signal into an 
intermediate frequency signal, measuring and calculating a frequency 
deviation of the intermediate frequency signal for each sample, 
controlling a local oscillator corresponding to the calculated value, 
detecting the receiving condition of the digital signal, and compensating 
the calculated value corresponding to the receiving condition. 
A second aspect of the present invention is a frequency adjusting circuit 
for use with a digital receiver, the circuit comprising a means for 
converting the frequency of a received digital signal into an intermediate 
frequency signal, a local oscillator for supplying a local oscillation 
signal to the frequency converting means, and a frequency adjusting means 
for calculating a frequency deviation of the intermediate frequency signal 
for each sample, performing a predetermined calculation corresponding to 
the measured value, and controlling the oscillation frequency of the local 
oscillator corresponding to the obtained calculated value, wherein the 
frequency adjusting means compensates the calculated value corresponding 
to a receiving electric field intensity of the intermediate frequency 
signal. 
The frequency adjusting means includes means for detecting a variation 
value of the receiving electric field intensity, comparing the deviation 
value with a threshold value, and when the variation value is larger than 
the threshold value, removing the frequency deviation at that time from 
those that are input from the frequency adjusting means. 
The frequency adjusting means includes means for calculating an average 
value of the receiving electric field intensity in a receiving unit time, 
calculating the absolute value of the difference between the average value 
and the receiving electric field intensity for each receiving sample, 
comparing the absolute value with a threshold value, and when the absolute 
value is larger than the threshold value, removing the frequency deviation 
at the time from those that are input of the frequency adjusting means. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the following detailed description 
of best mode embodiments thereof, as illustrated in the accompanying 
drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Next, with reference to the accompanying drawings, an embodiment of the 
present invention will be described. In a first embodiment of the present 
invention, the deviation value of the receiving electric field intensity 
is treated as an absolute difference value of symbols in a receiving unit 
time. The structure of a receiver according to the first embodiment is the 
same as the structure of the receiver shown in FIG. 1. In other words, a 
signal RF received from a receiving antenna 1 is supplied to an amplifier 
2. The amplifier 2 amplifies the received signal. The amplified signal is 
supplied to a first mixer 3. The first mixer 3 mixes the amplified signal 
with a first local oscillation signal F1 generated by a voltage control 
oscillator 7 and a PLL synthesizer 9 and outputs a first intermediate 
frequency signal IF1. Next, a first intermediate frequency signal IF1 is 
supplied to a second mixer 4. The second mixer 4 mixes the first 
intermediate frequency signal IF1 with a second local oscillation signal 
F2 generated by a voltage control oscillator 8 and a PLL synthesizer 10 
and outputs a second intermediate frequency signal IF2. The second local 
oscillation signal IF2 is supplied to an intermediate frequency amplifier 
5. The intermediate frequency amplifier 5 amplifies the second 
intermediate frequency signal IF2. The amplified signal is supplied to a 
demodulator 6. The demodulator 6 demodulates the amplified signal and 
outputs a demodulated signal. The intermediate frequency amplifier 5 
monitors the level of the electric field intensity of the received signal 
and outputs a receiving electric field intensity (RSSI) as a DC voltage. 
The DC voltage is supplied to an A/D converter 11. The A/D converter 11 
converts the DC voltage into a digital value D2. The digital value D2 is 
supplied to a calculating portion 23 of an AFC circuit 20. Thus, the 
receiver monitors the receiving electric field intensity. The AFC circuit 
20 supplies a frequency deviation to the calculating portion 23 so that 
the intermediate frequencies of the intermediate frequency signals IF1 and 
IF2 follow the frequency of the received RF signal and stabilizes them. 
The calculating portion 23 outputs a compensation value D3 for 
compensating the deviation of the oscillation frequency of the local 
oscillator received from the calculating portion 23. 
In the calculating portion 23 according to the first embodiment, the 
process protocol for obtaining the compensation value D3 with the 
frequency deviation and the receiving electric field intensity D2 is 
different from a conventional process protocol. FIG. 2 is a flow chart of 
the process protocol of the calculating portion 23. The receiving electric 
field intensity D2 that is a digital value of the receiving electric field 
intensity (RSSI) is stored for the receiving unit time (at step S01). The 
deviation value of the receiving electric field intensity is obtained for 
the receiving unit time with the receiving electric field intensity D2 
stored at step S01 (at step S02). Assuming that the receiving unit time is 
a time for which one burst (data of N symbols) is received, the deviation 
value of the receiving electric field intensity is the absolute value of 
the difference between the (N-1)th value of the receiving electric field 
intensity D2 and the N-th value of the receiving electric field intensity 
D2. In other words, the difference absolute value is obtained as 
.vertline.(N-1)th receiving electric field intensity - N-th receiving 
electric field intensity.vertline.. In addition, the absolute value 
obtained at step S03 is compared with a predetermined threshold value (at 
step S03). As the compared result at step S03, when the absolute value is 
larger than the threshold value, the flow advances to step S05. When the 
absolute value is smaller than the threshold value, the flow advances to 
step S06 (at step S04). 
The frequency deviation of the intermediate frequency signal D1 whose 
intensity absolute value is larger than the threshold value is not used 
for the calculation of the average value (at step S05). For example, when 
the deviation of the receiving electric field intensity due to fading is 
larger than 14 dB, since the reliability of the received signal becomes 
low, assuming that the threshold value is 14 dB, if the obtained absolute 
value A is larger than 14 dB (namely, the receiving condition is bad due 
to fading), the frequency deviation of the intermediate frequency signal 
D1 is not included in the added value used for the calculation of the 
average value (namely, the frequency deviation of the intermediate 
frequency signal D1 whose reliability is low is removed), the error due to 
fading can be alleviated. In this example, although it is assumed that the 
threshold value is 14 dB, the threshold value is not limited to 14 dB. The 
average value of the frequency deviations of the intermediate frequency 
signal D1 is obtained in the receiving unit time (namely, the number of 
received symbols is counted) (at step S06). As the determined result at 
step S06, when the predetermined number of symbols have been received, the 
flow advances to step S07. When the predetermined number of symbols have 
not been received, the flow returns to step S02. Since the added value of 
which the frequency deviation of the intermediate frequency signal D1 
whose reliability is low is removed at step S05 is divided by the number 
of symbols. Thereby, the average value D3 of the frequency deviations of 
the intermediate frequency signal D1 from which the error has been removed 
is obtained (at step S07). The compensation value that has been obtained 
in the just preceding receiving unit time is substituted with the average 
value D3 obtained at step S07 as a new compensation value and updated. The 
updated compensation value D3 is supplied to the D/A converter 22. The D/A 
converter 22 converts the compensation value D3 into an analog value. With 
the analog value, the oscillation value of the TCXO 21 is controlled. 
Thus, the error due to fading is decreased and thereby the AFC can be 
accurately compensated. 
Thus, in the first embodiment, when the receiving electric field intensity 
sharply varies due to fading (namely, the deviation of the receiving 
electric field intensity is larger than the threshold value), it is 
determined that the reliability of the frequency deviation of the 
intermediate frequency signal at the time is low. Thus, the frequency 
deviation is removed and thereby the average value of the frequency 
deviations of the remaining intermediate frequency signal D1 is obtained. 
Consequently, a compensation value is obtained. Thus, the influence due to 
fading can be alleviated. As a result, the AFC can be more accurately 
controlled than before. 
FIG. 3 is a flow chart for explaining a second embodiment of the present 
invention. As with the first embodiment, FIG. 3 shows a process protocol 
of the calculating portion 23 of the AFC circuit of the receiver shown in 
FIG. 1. In the second embodiment, in addition to determining the sharp 
deviation of the receiving electric field intensity, the receiving 
electric field intensity of each symbol is evaluated. Thus, the receiving 
condition can be more accurately determined. A receiving electric field 
intensity D2 is stored for a receiving unit time (at step S11). With the 
receiving electric field intensity D2 stored at step S11, the average 
value of the receiving electric field intensity for the receiving unit 
time is obtained (at step S12). For example, assuming that the receiving 
unit time is a time for which one burst (data of N symbols) is received, 
the average value of the receiving electric field intensity is obtained by 
adding the receiving electric field intensity D2 for N symbols and 
dividing the result by N symbols. The absolute value of the difference 
between the average value of D2 obtained at step S12 and the receiving 
electric field intensity D2 of each received symbol is obtained (at step 
S13). The absolute value of the difference of the average value is 
obtained as .vertline.(average value of receiving electric field intensity 
in receiving unit time)--(receiving electric field intensity of each 
received symbol.vertline.. The absolute value obtained at step S13 is 
compared with the threshold value (at step S14). As the compared result at 
step S14, when the absolute value is larger than the threshold value, the 
flow advances to step S16. When the absolute value is smaller than the 
threshold value, the flow advances to step S17 (at step S15). 
The frequency deviation of the intermediate frequency signal D1 whose 
absolute value is larger than the threshold value is not used for the 
calculation of the average value (at step S16). For example, when the 
deviation width of the receiving electric field intensity is larger than 
14 dB due to fading, since the reliability of the received signal becomes 
low, assuming that the threshold value is 14 dB, if the absolute value is 
larger than 14 dB (namely, the receiving condition is bad due to fading), 
the frequency deviation of the intermediate frequency signal D1 at the 
time is not included in the added value used for the calculation of the 
average value (namely, the frequency deviation is removed as a frequency 
deviation of the intermediate frequency signal D1 whose reliability is 
low) and, the error due to fading can be alleviated. In this example, the 
threshold value is 14 dB. However, according to the present invention, the 
threshold value is not limited to 14 dB. 
Next, the number of received symbols is counted so that the average value 
of the frequency deviations of the intermediate frequency signal D1 is 
obtained (at step S17). When the predetermined number of symbols have been 
received, the flow advances to step S18. When the predetermined number of 
symbols have not been received, the flow returns to step S13. The 
frequency deviation D1 of the intermediate frequency signal D1 whose 
reliability is low has been removed and then the added value is divided by 
the number of symbols. Thus, the average value D3 of the frequency 
deviations of the intermediate frequency signal from which the error has 
been removed is obtained (at step S18). The compensation value that has 
been obtained in the just preceding receiving unit time is substituted 
with the average value D3 of the frequency deviations of the intermediate 
frequency signal D1 obtained at step S18 as a new compensation value and 
thereby updated. The compensation value D3 is supplied to the D/A 
converter 22. The D/A converter 22 converts the compensation value D3 into 
an analog value. With the analog value, the oscillation frequency of the 
TCXO 21 is controlled. Thus, the error due to fading is decreased and 
thereby the AFO circuit is accurately compensated. 
Next, with reference to FIG. 4, the determination of the receiving 
condition and the selection of the frequency deviations corresponding to 
the receiving electric field intensity according to the second embodiment 
will be described. The average value (straight line 1) of the receiving 
electric field intensity D2 (polygonal line) in the receiving unit time 
(one burst) is obtained. The absolute value (polygonal line 2) of the 
difference between the obtained average value and the receiving electric 
field intensity of each symbol is calculated. The absolute value 
represents the deviation from the average value. With the threshold value 
(straight line 2), the receiving condition can be determined. When the 
receiving electric field intensity D2 largely deviates from the average 
value (namely, the receiving condition is bad), the absolute value largely 
deviates. When the receiving electric field intensity D2 is larger than 
the threshold value, since the receiving condition is bad, since the 
reliability of the frequency deviation measured with the intermediate 
frequency signal D1 at the time is low, it is removed from the calculation 
of the average value. With the remaining frequency deviations, the 
calculation of the average value is calculated and thereby the 
compensation value D3 is obtained. The obtained compensation value D3 is 
supplied to the D/A converter 22. The D/A converter 22 converts the 
compensation value D3 into an analog value. With the analog value, the 
oscillation frequency of the TCXO 21 is controlled. 
As described above, according to the present invention, the receiving 
condition is detected with the receiving electric field intensity of the 
received digital signal. Corresponding to the receiving condition, the 
calculated value for controlling the local oscillator corresponding to the 
frequency deviations is compensated. Thus, even if the receiving electric 
field intensity sharply varies due to fading (namely, the receiving 
electric field intensity largely varies than the threshold value) or 
strong impulse noises like mobile spark noises, it is determined that the 
reliability of the frequency deviation at the time is low. Thus, the 
frequency deviation at the time is deleted. The average value of the 
remaining frequency deviations is obtained. Consequently, the influence of 
fading is alleviated and the local oscillator can be more accurately 
controlled. 
Although the present invention has been shown and described with respect to 
best mode embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the present invention.