Abstract:
A portable radio terminal for realizing automatic frequency control (AFC) for automatically controlling the oscillation frequency of an oscillator includes a unit for intermittently performing AFC operation, and a unit for shortening an AFC operation stop period when the frequency shift of the oscillation frequency is large. An AFC control method is also disclosed.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application is a division of application No. 10/052,777, filed Jan. 23, 2002, now pending, and related to a concurrently filed application entitled PORTABLE RADIO TERMINAL AND AFC CONTROL METHOD (Attorney Docket No. 040425-0155) and based on Japanese Patent Application No. 2001-016345, filed Jan. 24, 2001, by Mariko Matsumoto and Shigeru Ono. This application claims only subject matter disclosed in the parent application and therefore presents no new matter.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a portable radio terminal and AFC control method which realize AFC (Automatic Frequency Control) for automatically controlling the oscillation frequency of an oscillator.  
         [0004]     2. Description of the Prior Art  
         [0005]     Conventional portable radio terminals applied to systems such as QPSK and WCDMA incorporate low-cost, low-precision oscillators (to be referred to as mobile station oscillators hereinafter) in order to reduce the cost of the portable radio terminal. The portable radio terminal performs AFC (Automatic Frequency Control) for detecting a frequency shift in the mobile station oscillator on the basis of a received wave sent from a higher-frequency-precision base station, feeding back the detected shift, and adjusting the frequency of the mobile station oscillator.  
         [0006]     An example of conventional AFC control methods is disclosed in Japanese Unexamined Patent Publication No. 10-229491. In this prior art, correction data for correcting an AFC signal to be fed back to a mobile station oscillator is stored in a memory. The correction data is read out from the memory in accordance with the lapse of time so as to keep the output frequency of one mobile station oscillator constant with respect to the lapse of time after power-on operation.  
         [0007]     In this prior art, however, large power is consumed because AFC operation continues after power-on operation. If the oscillation frequency of the oscillator greatly shifts, the signal timing shifts in an idle time, and reception may fail. High-precision frequency control is not achieved, and a malfunction may occur. Further, it is difficult to correct a frequency shift or rapidly follow phasing.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made in consideration of the above situations in the prior art, and has as its first object to provide a portable radio terminal and AFC control method which intermittently operate and control an AFC control system, shorten the intermittent operation period to frequently perform AFC operation for a large frequency shift, and extend the intermittent operation period and AFC operation stop period for a small frequency shift, thereby realizing high-precision AFC operation while reducing power.  
         [0009]     It is the second object of the present invention to provide a portable radio terminal and AFC control method which change intermittent operation into the intermittent operation of the whole radio mobile station in an idle time, thereby preventing a signal timing shift and a reception failure in an idle time when the oscillation frequency of an oscillator greatly shifts.  
         [0010]     It is the third object of the present invention to provide a portable radio terminal and AFC control method which can update a frequency shift value to the oscillator to realize high frequency precision without any malfunction when shifts in the same direction are detected N times (N: arbitrary number) because the frequency shift value includes many errors for a sufficiently small frequency shift.  
         [0011]     It is the fourth object of the present invention to provide a portable radio terminal and AFC control method which monitor the reception quality or sync state, determine whether to input an AFC signal to the oscillator, and can avoid an AFC malfunction caused by low reliability such as a low reception quality or poor sync state.  
         [0012]     It is the fifth object of the present invention to provide a portable radio terminal and AFC control method which monitor the reception quality, when the transmission channel state is poor, shorten the AFC intermittent operation period because the frequency may greatly shift or phasing is to be followed, and can correct a frequency shift or rapidly follow phasing.  
         [0013]     To achieve the above objects, according to the first aspect of the present invention, there is provided a portable radio terminal for realizing automatic frequency control (AFC) for automatically controlling an oscillation frequency of an oscillator, comprising means for intermittently performing AFC operation, and means for shortening an AFC operation stop period when a frequency shift of the oscillation frequency is large.  
         [0014]     According to the second aspect of the present invention, there is provided a portable radio terminal, further comprising means for extending the stop period in intermittent operation of the AFC operation when the frequency shift of the oscillation frequency is small.  
         [0015]     According to the third aspect of the present invention, there is provided a radio terminal, wherein the intermittent operation includes not only the AFC operation but also operation stop of the portable radio terminal.  
         [0016]     According to the fourth aspect of the present invention, there is provided a portable radio terminal for realizing automatic frequency control (AFC) for automatically controlling an oscillation frequency of an oscillator, comprising means for updating a frequency shift to the oscillator when the frequency shift of the oscillation frequency is smaller than a predetermined value and frequency shifts in the same direction are detected a predetermined number of times.  
         [0017]     According to the fifth aspect of the present invention, there is provided a portable radio terminal for realizing automatic frequency control (AFC) for automatically controlling an oscillation frequency of an oscillator, comprising means for monitoring a reception quality or sync state and determining in accordance with a result whether to input a frequency shift value to the oscillator.  
         [0018]     According to the sixth aspect of the present invention, there is provided a portable radio terminal, further comprising means for performing the AFC operation at a predetermined short period when the portable radio terminal fails in decoding, does not detect any pilot signal, or detects a step out state.  
         [0019]     According to the seventh aspect of the present invention, there is provided an AFC control method of realizing automatic frequency control (AFC) for automatically controlling an oscillation frequency of an oscillator, comprising intermittently performing AFC operation, and when a frequency shift of the oscillation frequency is large, shortening an AFC operation stop period.  
         [0020]     According to the eighth aspect of the present invention, there is provided an AFC control method, wherein when the frequency shift of the oscillation frequency is small, the stop period in intermittent operation of the AFC operation is extended.  
         [0021]     According to the ninth aspect of the present invention, there is provided an AFC control method, wherein the intermittent operation includes not only the AFC operation but also operation stop of a portable radio terminal.  
         [0022]     According to the 10th aspect of the present invention, there is provided an AFC control method of realizing automatic frequency control (AFC) for automatically controlling an oscillation frequency of an oscillator, comprising updating a frequency shift to the oscillator when the frequency shift of the oscillation frequency is smaller than a predetermined value and frequency shifts in the same direction are detected a predetermined number of times.  
         [0023]     According to the 11th aspect of the present invention, there is provided an AFC control method of realizing automatic frequency control (AFC) for automatically controlling an oscillation frequency of an oscillator, comprising monitoring a reception quality or sync state and determining in accordance with a result whether to input a frequency shift value to the oscillator.  
         [0024]     According to the 12th aspect of the present invention, there is provided an AFC control method, wherein the AFC operation is performed at a predetermined short period when decoding fails, no pilot signal is detected, or a step-out state is detected.  
         [0025]     As is apparent from the above aspects, according to the present invention, an AFC control system is intermittently operated and controlled. When the frequency shift is large, the intermittent operation period is shortened, and AFC operation is frequently performed. When the frequency shift is small, the intermittent operation period and AFC operation stop period are extended. Accordingly, high-precision AFC operation can be realized while power is reduced.  
         [0026]     According to the present invention, intermittent operation is changed into the intermittent operation of the whole radio mobile station in an idle time. When the oscillation frequency of an oscillator greatly shifts, a signal timing shift and a reception failure can be prevented in an idle time.  
         [0027]     According to the present invention, the frequency shift value includes many errors when the frequency shift is sufficiently small. Thus, when shifts in the same direction are detected N times (N: arbitrary number), a frequency shift value to the oscillator can be updated to realize high frequency precision without any malfunction.  
         [0028]     According to the present invention, the reception quality or sync state is monitored, and whether to input an AFC signal to the oscillator is determined. An AFC malfunction caused by low reliability such as a low reception quality or poor sync state can be avoided.  
         [0029]     According to the present invention, the reception quality is monitored. When the transmission channel state is poor, the AFC intermittent operation period is shortened because the frequency may greatly shift or phasing is to be followed. This enables correcting a frequency shift or rapidly following phasing.  
         [0030]     The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a block diagram for explaining an embodiment of an AFC control method for a portable radio apparatus according to the present invention;  
         [0032]      FIG. 2  is a flow chart showing an AFC control flow in the first embodiment of the present invention;  
         [0033]      FIG. 3  is a timing chart for explaining intermittent operation and a related operation in the present invention; and  
         [0034]     FIGS.  4  to  7  are flow charts showing the AFC control flow of a portable radio terminal in each of the second to fifth embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     Several preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.  
         [0036]      FIG. 1  is a block diagram for explaining an embodiment of an AFC control method for a portable radio apparatus according to the present invention.  FIG. 1  shows a base station  101  and a radio mobile station (hereinafter called mobile station)  104  applied to the embodiment. The schematic arrangement of the mobile station  104  is shown by the block diagram.  
         [0037]     The mobile station  104  is constituted by a mobile station radio unit  105 , an A/D converter  106 , a signal processing unit  107  having a DSP, gate array, and standard cell, a control unit  116  having a CPU, an output unit  108  such as a loudspeaker, a mobile station oscillator  109 , a mobile station PLL unit  110 , an LPF (Low Pass Filter)  111 , and an AFC D/A  112 .  
         [0038]     The signal processing unit  107  is made up of a mobile station data processing unit  113 , frequency shift detection unit  114 , TCXO AFC unit  115 , and control unit  116 . The mobile station data processing unit  113  is comprised of a sync detection unit  117 , demodulation unit  118 , deformat unit  119 , decoding unit  120 , and power detection unit  121 .  
         [0039]     The operation of the portable radio terminal  104  in the embodiment will be described with reference to  FIG. 1 . A digital signal  122  modulated in the base station  101  is transmitted from a base station antenna  102 . A radio wave  123  transmitted from the base station antenna  102  is received by a portable radio terminal antenna  103 , and sent as a signal  124  to the mobile station radio unit  105 .  
         [0040]     An analog signal which is obtained by down-conversion and quadrature demodulation of the channel frequency is converted into a digital signal  126  by the A/D converter  106 . The digital signal  126  is input to the sync detection unit  117  of the signal processing unit  107  having a DSP, gate array, and standard cell. The sync detection unit  117  sends a sync detection signal  127  to the control unit  116  having a CPU.  
         [0041]     The digital signal  126  sent from the A/D converter  106  is demodulated by the demodulation unit  118 . A demodulated signal  128  is sent to the deformat unit  119  where the signal  128  is deformatted. Deformatted data  129  is decoded by the decoding unit  120 . A decoded signal  131  is sent to the output unit  108 . The decoding unit  120  outputs CRC information  132  to the control unit  116 .  
         [0042]     The deformat unit  119  transfers a power detection signal  130  to the power detection unit  121 . The power detection unit  121  sends power detection information (RSSI)  133  to the control unit  116 . The control unit  116  outputs a control signal  144  for controlling the frequency shift detection unit  114  and TCXO AFC unit  115  on the basis of the sync detection signal  127 , CRC information  132 , and RSSI  133 .  
         [0043]     The deformat unit  119  transfers to the frequency shift detection unit  114  an AFC detection signal  134  prepared by gathering pilot signals to the same frequency. The frequency shift detection unit  114  calculates a frequency shift from the AFC detection signal  134 , and transfers a frequency shift value (Δf)  135  to the TCXO AFC unit  115 . The frequency shift detection unit  114  intermittently operates under the control of the control unit  116 .  
         [0044]     The TCXO AFC unit  115  adds a frequency shift value and updates the TCXO AFC value (Δf VCXO ) under the control of the control unit  116 . The TCXO AFC value is transferred as a digital AFC signal  136  to the AFC D/A  112 . An AFC signal  137  D/A-converted by the AFC D/A  112  passes through the LPF  111 , and is input as an AFC signal  138  to the AFC terminal of the TCXO  109 . Then, the oscillation frequency of the TCXO  109  is changed.  
         [0045]     A signal  139  oscillated from the TCXO  109  is converted by the mobile station PLL unit  110  into a plurality of signals having different frequencies. The mobile station PLL unit  110  supplies a signal  140  to the mobile station radio unit  105 , a signal  141  to the A/D converter  106 , a signal  142  to the signal processing unit  107 , and a signal  143  to the AFC D/A  112 .  
         [0046]     In an idle time, intermittent operation is performed over a wide range in the whole mobile station  104  including the signal processing unit  107  and mobile station radio unit  105 . In an idle time, the intermittent operation of the TCXO AFC unit  115  determines the intermittent operation of the overall mobile station  104 .  
       First Embodiment  
       [0047]      FIG. 2  is a flow chart showing an AFC control flow in the first embodiment of the present invention. AFC control operation generally starts when a portable radio terminal is powered on. The oscillation frequency of a mobile station oscillator  109  shifts due to degradation in temperature characteristic over time or the like.  
         [0048]     A control unit  116  sets a minimum value T MIN  as an intermittent operation period T in a frequency shift detection unit  114  in advance, and sets 0 as a TCXO AFC value (Δf VCXO ) in a TCXO AFC unit  115  (step S 201 ). If the frequency shift detection unit  114  detects Δf from an AFC detection signal  134  (step S 202 ), the control unit  116  checks whether the detected Δf is larger than a predetermined value Δf tk1  (positive value) (step S 203 ).  
         [0049]     If the control unit  116  determines that Δf is Δf tk1  or less (NO in step S 203 ), it checks whether the intermittent operation period T (minimum value T MIN  at this time) set in the frequency shift detection unit  114  is larger than a predetermined maximum value T MAX  or more (step S 204 ). Since the intermittent operation period set in the frequency shift detection unit  114  is the minimum value T MIN , the control unit  116  determines that the intermittent operation period T is smaller than T MAX  (NO in step S 204 ), and sets an intermittent operation period twice the intermittent operation period T MIN  in the frequency shift detection unit  114  (step S 205 ).  
         [0050]     If Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds a predetermined value Δf FIX  (positive value) to Δf VCXO  (0 at this time) to update the TCXO AFC value; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds a predetermined value −Δf FIX  to Δf VCXO  (0 at this time) to update the TCXO AFC value (step S 208 ).  
         [0051]     If the control unit  116  determines in step S 204  that the intermittent operation period T set in the frequency shift detection unit  114  is T MAX  or more (YES in step S 204 ), it does not change the intermittent operation period T. After that, if Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds the predetermined value Δf FIX  (positive value) to Δf VCXO  to update the TCXO AFC value; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds the predetermined value −Δf FIX  to Δf VCXO  to update the TCXO AFC value (step S 208 ).  
         [0052]     If the control unit  116  determines in step S 203  that Δf is larger than Δf tk1  (YES in step S 203 ), it checks whether the intermittent operation period T is the minimum value T MIN  or less (step S 206 ). If the control unit  116  determines that the intermittent operation period T is larger than T MIN  (NO in step S 206 ), it sets an intermittent operation period ½ the current intermittent operation period in the frequency shift detection unit  114  (step S 207 ).  
         [0053]     If Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds the predetermined value Δf FIX  (positive value) to Δf VCXO  to update the TCXO AFC value; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds the predetermined value −Δf FIX  to Δf VCXO  to update the TCXO AFC value (step S 208 ).  
         [0054]     If the control unit  116  determines in step S 206  that the intermittent operation period T is T MIN  or less (YES in step S 206 ), it does not change the current intermittent operation period T. Then, if Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds the predetermined value Δf FIX  (positive value) to the TCXO AFC value (Δf VCXO ) of the TCXO AFC unit  115 ; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds the predetermined value −Δf FIX  to the TCXO AFC value (Δf VCXO ) of the TCXO AFC unit  115  (step S 208 ).  
         [0055]     (a) in  FIG. 3  shows a control signal output from the control unit  116  to the frequency shift detection unit  114 , a period τ during which the frequency shift detection unit  114  is ON, and the intermittent operation period T of the frequency shift detection unit  114 . (b) shows the state of the control signal when the intermittent operation period T is set ½ in step S 207  of  FIG. 2 . (c) shows the state of the control signal when the intermittent operation period T is set twice in step S 205  of  FIG. 2 .  
         [0056]     (d-1) to (d-3) show an example of a control signal output from the control unit  116  in the operation in the flow chart of  FIG. 2 . (d-1) shows that the frequency shift detection unit  114  is operated while the intermittent operation period T is changed. (d-2) shows Δf detected by the frequency shift detection unit  114  at that time. (d-3) shows the value Δf VCXO  output from the TCXO AFC unit  115 . At this time, T MIN  is ½ T MAX .  
       Second Embodiment  
       [0057]      FIG. 4  is a flow chart showing an AFC control flow in the second embodiment of the present invention. AFC control operation generally starts when a portable radio terminal is powered on. The oscillation frequency of a mobile station oscillator  109  shifts due to degradation in temperature characteristic over time or the like.  
         [0058]     A control unit  116  sets N- 1  (N: predetermined repeat number) in flag, and sets 0 as a TCXO AFC value (Δf VCXO ) in a TCXO AFC unit  115  (step S 401 ). If a frequency shift detection unit  114  detects Δf (step S 402 ), the control unit  116  checks whether the detected Δf is a predetermined value Δf tk2  (positive value) or more (step S 403 ).  
         [0059]     If the control unit  116  determines that Δf is Δf tk2  or more (YES in step S 403 ), it registers in the TCXO AFC unit  115  a new TCXO AFC value obtained by adding Δf to Δf VCXO , and registers N- 1  in flag again (step S 404 ).  
         [0060]     If the control unit  116  determines that Δf is smaller than Δf tk2  (NO in step S 403 ), it checks whether flag is 0 (step S 405 ). Since flag is N- 1 , the control unit  116  determines that flag is not 0 (NO in step S 405 ), and checks whether the currently detected Δf and the previously detected Δf, i.e., Δfp (flag) have the same sign (step S 407 ).  
         [0061]     If the control unit  116  determines in step S 407  that the currently detected Δf and the previously detected Δf have the same sign (YES in step S 407 ), it decrements flag by 1, and registers the currently detected Δf as Δfp (flag) (step S 408 ).  
         [0062]     If the control unit  116  determines in step S 407  that the currently detected Δf and the previously detected Δf do not have the same sign (NO in step S 407 ), it registers N- 1  in flag, and registers the currently detected Δf as Δfp (flag) (step S 409 ).  
         [0063]     If the control unit  116  determines in step S 405  that flag is 0 (YES in step S 405 ), it registers as the TCXO AFC value a value obtained by adding to Δf VCXO  the average of Δf detected N times by the frequency shift detection unit  114 , registers N- 1  in flag, and registers the currently detected Δf as Δfp (flag) (step S 406 ).  
         [0064]     In the second embodiment, if the detected frequency shift Δf is smaller than the predetermined value Δf tk2 , the value Δf is reflected on Δf VCXO  only when frequency shifts Δf of the same sign are successively detected N times. Although noise attains a larger influence for a smaller frequency shift, a malfunction by inputting an erroneous value Δf to the TCXO AFC unit  115  can be avoided.  
       Third Embodiment  
       [0065]      FIG. 5  is a flow chart showing an AFC control flow in the third embodiment of the present invention. AFC control operation generally starts when a portable radio terminal is powered on. The oscillation frequency of a mobile station oscillator  109  shifts due to degradation in temperature characteristic over time or the like.  
         [0066]     A control unit  116  sets 0 as a TCXO AFC value (Δf VCXO ) in a TCXO AFC unit  115  (step S 501 ). If a frequency shift detection unit  114  detects Δf (step S 502 ), the control unit  116  checks on the basis of sync information from a sync detection unit  117  whether the communication state has stepped out (step S 503 ). If the control unit  116  determines that the communication state has stepped out when the frequency shift detection unit  114  detects Δf (YES in step S 503 ), the control unit  116  counts a timer for a predetermined period to establish synchronization (step S 504 ), and executes processing from step S 501 .  
         [0067]     If the control unit  116  determines in step S 503  that the communication state is in sync (NO in step S 503 ), it checks whether RSSI output from a power detection unit  121  is larger than a predetermined value RSSI tk  (step S 505 ). If the control unit  116  determines that RSSI is larger than RSSI tk  (YES in step S 505 ), it registers the sum of Δf VCXO  and Δf as the TCXO AFC value in the TCXO AFC unit  115  (step S 506 ). If the control unit  116  determines in step S 505  that RSSI is RSSI tk  or less (NO in step S 505 ), it determines that the obtained value Δf is low in reliability because of a small power of a received signal, and does not update Δf VCXO .  
       Fourth Embodiment  
       [0068]      FIG. 6  is a flow chart showing an AFC control flow in the fourth embodiment of the present invention. AFC control operation generally starts when a portable radio terminal is powered on. The oscillation frequency of a mobile station oscillator  109  shifts due to degradation in temperature characteristic over time or the like.  
         [0069]     A control unit  116  sets 0 as a TCXO AFC value (Δf VCXO ) in a TCXO AFC unit  115  (step S 601 ). If a frequency shift detection unit  114  detects Δf (step S 602 ), the control unit  116  checks on the basis of sync information from a sync detection unit  117  whether the communication state has stepped out (step S 603 ). If the control unit  116  determines that the communication state has stepped out when the frequency shift detection unit  114  detects Δf (YES in step S 603 ), the control unit  116  counts a timer for a predetermined period to establish synchronization (step S 604 ), and executes processing from step S 601 .  
         [0070]     If the control unit  116  determines in step S 603  that the communication state is in sync (NO in step S 603 ), and determines from CRC (Cyclic Redundancy Check) information obtained by a decoding unit  120  that the transmission frame does not contain any error (YES in step S 605 ), the control unit  116  registers the sum of Δf VCXO  and Δf as the TCXO AFC value in the TCXO AFC unit  115  (step S 606 ). If the control unit  116  determines from CRC information that the transmission frame contains an error (NO in step S 605 ), it determines that the obtained value Δf is low in reliability because of a poor transmission channel state between a base station  101  and a portable radio apparatus, and does not update Δf VCXO .  
       Fifth Embodiment  
       [0071]      FIG. 7  is a flow chart showing an AFC control flow in the fifth embodiment of the present invention. AFC control operation generally starts when a portable radio terminal is powered on. The oscillation frequency of a mobile station oscillator  109  shifts due to degradation in temperature characteristic over time or the like.  
         [0072]     A control unit  116  sets a minimum value T MIN  as an intermittent operation period T in a frequency shift detection unit  114  in advance, and sets 0 as a TCXO AFC value in frequency shift detection unit  114  (step S 701 ). The control unit  116  checks whether RSSI output from a power detection unit  121  is larger than a predetermined value RSSI tk2  (step S 702 ).  
         [0073]     If the control unit  116  determines in step S 702  that RSSI is RSSI tk2  or less (NO in step S 702 ), it determines that the oscillation frequency of the TCXO  109  may have greatly shifted, sets the minimum value T MIN  as an intermittent operation period (step S 703 ), and performs AFC at a short period. If the frequency shift detection unit  114  detects Δf (step S 704 ), the control unit  116  checks whether the detected value Δf is larger than a predetermined value Δf tk1  (positive value) (step S 705 ).  
         [0074]     If the control unit  116  determines that Δf is Δf tk1  or less (NO in step S 705 ), it checks whether the intermittent operation period T (minimum value T MIN  at this time) set in the frequency shift detection unit  114  is a predetermined maximum value T MAX  or more (step S 706 ). Since the intermittent operation period set in the frequency shift detection unit  114  is the minimum value T MIN , the control unit  116  determines that the intermittent operation period T is smaller than T MAX  (NO in step S 706 ), and sets an intermittent operation period twice the intermittent operation period T MIN  in the frequency shift detection unit  114  (step S 707 ).  
         [0075]     If Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds a predetermined value Δf FIX  (positive value) to Δf VCXO  (0 at this time) to update the TCXO AFC value; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds a predetermined value −Δf FIX  to Δf VCXO  (0 at this time) to update the TCXO AFC value (step S 710 ).  
         [0076]     If the control unit  116  determines in step S 706  that the intermittent operation period T set in the frequency shift detection unit  114  is T MAX  or more (YES in step S 706 ), it does not change the intermittent operation period T. Then, if Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds the predetermined value Δf FIX  (positive value) to Δf VCXO  to update the TCXO AFC value; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds the predetermined value −Δf FIX  to Δf VCXO  to update the TCXO AFC value (step S 710 ).  
         [0077]     If the control unit  116  determines in step S 705  that Δf is larger than Δf tk1  (YES in step S 705 ), it checks whether the intermittent operation period T is the minimum value T MIN  or less (step S 708 ). If the control unit  116  determines that the intermittent operation period T is larger than T MIN  (NO in step S 708 ), it sets an intermittent operation period ½ the current intermittent operation period in the frequency shift detection unit  114  (step S 709 ).  
         [0078]     If Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds the predetermined value Δf FIX  (positive value) to Δf VCXO  to update the TCXO AFC value; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds the predetermined value −Δf FIX  to Δf VCXO  to update the TCXO AFC value (step S 710 ).  
         [0079]     If the control unit  116  determines in step S 708  that the intermittent operation period T is T MIN  or less (YES in step S 708 ), it does not change the current intermittent operation period T. Then, if Δf detected by the frequency shift detection unit  114  is a positive value, the control unit  116  adds the predetermined value Δf FIX  (positive value) to the TCXO AFC value (Δf VCXO ) of the TCXO AFC unit  115 ; or if Δf detected by the frequency shift detection unit  114  is a negative value, the control unit  116  adds the predetermined value −Δf FIX  to the TCXO AFC value (Δf VCXO ) of the TCXO AFC unit  115  (step S 710 ).