JP2007288343A - Automatic frequency controller - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an automatic frequency controller capable of controlling a local oscillator even under condition that a frequency error value cannot be estimated because of a large frequency error. <P>SOLUTION: A synchronizing word detection part 5 detects a synchronizing word inserted to a reception signal in advance. A reception field intensity detection part 3 detects a value of reception field intensity of the reception signal. A control part 6 searches a frequency of a VCTCXO 9 within a transition width equal to or smaller than a frequency range allowing the synchronizing word to be stably detected, in the case that a state of being unable to stably detect the synchronizing word by the synchronizing word detection part 5 continues for a prescribed time and the reception field intensity detected by the reception field intensity detection part 3 is in a certain level or higher. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an automatic frequency control device that causes the frequency of a local oscillator to follow the carrier frequency of a received signal in mobile communication and the like, and in particular, the frequency error between the frequency of the local oscillator and the carrier frequency of the received signal is large. This relates to frequency control in the case where estimation of the frequency cannot be performed.

  For example, in mobile radio communication, the carrier frequency of a signal received from a base station is usually different from the frequency of a local oscillator inside the mobile station. For this reason, the mobile station has an automatic frequency control function (AFC function) that estimates the frequency error between the frequency of the received signal and the frequency of the local oscillator inside the mobile station and causes the frequency of the local oscillator to follow the frequency of the received signal. .

  Conventionally, as an automatic frequency control device that realizes such an automatic frequency control function, an output of a frequency divider that divides the second intermediate frequency of the reception input and a frequency divider that divides the reference frequency of the reference oscillator is output. Phase comparison of frequencies was performed to determine the phase error polarity, and the reference frequency was changed stepwise, and when the phase error polarity was reversed, the reference frequency was determined to be within the allowable error range (for example, , See Patent Document 1).

  And an AFC voltage recording circuit that records the control voltage of the reference signal source in the synchronized state as an AFC voltage, and the AFC voltage recorded in the AFC voltage recording circuit when the power is turned on is the initial value of the reference signal source control voltage. There is one in which the AFC circuit is operated as a value (for example, see Patent Document 2).

JP-A-7-264209 JP 2002-353831 A

  However, among the conventional automatic frequency control devices described above, it is assumed that the frequency error can always be estimated when the reference frequency is determined to be within the allowable error range based on the phase error polarity. Therefore, it could not be applied when the frequency error value could not be estimated. In addition, in the case of operating with the value recorded at the time of power-on as the initial value, the frequency is shifted only at the first power-on, so the range in which the local oscillator shifts due to aging / temperature change, etc. However, there is a risk of exceeding the range in which the frequency error value can be estimated.

  As described above, in the conventional automatic frequency control device, when the frequency error between the frequency of the received signal and the frequency of the local oscillator inside the mobile station is large, the reliability of the estimated frequency error value becomes low, and it shifts in the positive direction. It is not possible to judge whether it is in the negative direction. That is, the frequency error value cannot be estimated.

  Since the frequency of the mobile station cannot be controlled in a state where the frequency error value cannot be estimated, the range in which the local oscillator shifts due to aging / temperature change is usually within the range where the frequency error value can be estimated. Select an oscillator. However, in that case, there is a problem that it is difficult to select an inexpensive local oscillator.

  The present invention has been made to solve the above-described problems, and provides an automatic frequency control device capable of controlling a local oscillator even in a situation where a frequency error is large and a frequency error value cannot be estimated. For the purpose.

  An automatic frequency control device according to the present invention includes a synchronization word detection unit that detects a synchronization word inserted in advance in a reception signal, a reception field strength detection unit that detects a value of a reception field strength of the reception signal, and a synchronization word detection unit When the sync word cannot be detected stably at a predetermined time and the received field strength detected by the received field strength detector is above a certain level, the frequency of the local oscillator is detected stably. And a control unit that performs a search with a transition width less than a possible frequency range.

  In the automatic frequency control device of the present invention, the state where the synchronization word cannot be detected stably continues for a predetermined time, and the frequency of the local oscillator is searched when the received electric field strength is above a certain level. Even in a situation where the error is large and the frequency error value cannot be estimated, the local oscillator can be controlled.

Embodiment 1 FIG.
1 is a configuration diagram of a radio receiver to which an automatic frequency control device according to Embodiment 1 of the present invention is applied.
In the figure, a radio receiver includes an antenna 1, a demodulator 2, a received electric field strength detector 3, a frequency error estimator 4, a sync word detector 5, a controller 6, a recorder 7, a D / A converter 8, and a VCTCXO 9 It has.

  The antenna 1 is an antenna for receiving a radio signal in a radio receiver such as a mobile station for mobile radio communication. The demodulator 2 is a functional unit that demodulates a signal received by the antenna 1 using a signal having a local oscillation frequency of the VCTCXO 9. The received electric field strength detection unit 3 is a functional unit that detects the value of the electric field strength of the received signal. The frequency error estimation unit 4 is a functional unit that estimates a frequency error between the carrier frequency of the reception signal demodulated by the demodulation unit 2 and the local oscillation frequency of the VCTCXO 9. The synchronization word detection unit 5 is a functional unit that detects a synchronization word inserted in advance in the received signal. The control unit 6 is a functional unit for controlling each unit of the wireless device. The state in which the synchronization word detection unit 5 cannot stably detect the synchronization word continues for a predetermined time, and the reception field strength detection unit 3 When the received electric field intensity to be detected is equal to or higher than a certain level, the frequency of the VCTCXO 9 is searched with a transition width that is equal to or less than the frequency range in which the synchronization word can be detected stably. The recording unit 7 is a functional unit for recording the control value of the VCTCXO 9 used by the control unit 6. The D / A converter 8 is a functional unit for converting the digital control signal output from the control unit 6 into an analog control signal. The VCTCXO 9 is a voltage controlled temperature compensated crystal oscillator for generating a local oscillation signal having a frequency corresponding to an analog control signal from the D / A converter 8.

Next, the operation of the first embodiment will be described.
First, an outline of frequency control will be described.
The signal received by the antenna 1 is demodulated by the demodulator 2, the received electric field strength detector 3 measures the received electric field strength, the frequency error estimator 4 estimates the frequency error value, and the syncword detector 5 detects the syncword. Detection is performed, and the control unit 6 determines a frequency control value based on the result. The control unit 6 controls the oscillation frequency by changing the control voltage of the VCTCXO 9 via the D / A converter 8 according to the determined frequency control value.

Next, frequency control corresponding to a situation where the frequency error between the frequency of the VCTCXO 9 and the carrier frequency of the received signal is large and the frequency error value cannot be estimated will be described.
FIG. 2 is a flowchart showing the operation of the first embodiment.
When starting the control, the control unit 6 first controls the VCTCXO 9 with the control value recorded in the recording unit 7 (step ST1). Next, based on the output of the sync word detection unit 5, it is confirmed whether or not the sync word can be detected stably (step ST2).

  The determination as to whether or not the synchronization word can be detected stably is determined from the following viewpoint. For example, a synchronization word is inserted into a received signal in mobile radio communication at a certain interval (for example, every 40 ms). The mobile station starts from an asynchronous state at the time of activation, and transitions to a synchronous state when a synchronization word can be detected continuously for a certain number of times (for example, three times). Further, in the synchronous state, when the synchronization word cannot be detected continuously for a certain number of times (for example, 10 times), the state shifts to the asynchronous state. Then, it is determined that the synchronization word is stably detected when in such a synchronization state, and it is determined that the synchronization word is not stably detected when in the asynchronous state. As the synchronization word, a bit string having a low correlation with fixed data such as a preamble or a tail bit in the received signal is used.

  In step ST2, since stable detection means that the reliability of the frequency error estimation value can be determined to be high, the routine proceeds to a normal AFC operation for controlling the VCTCXO 9 to reduce the estimated frequency error value. (Step ST9). If it cannot be detected stably, the process proceeds to step ST3. In step ST3, the control unit 6 confirms whether or not a state in which the synchronization word cannot be stably detected continues for a certain period of time by a held timer (not shown). If it continues, the control unit 6 does not continue to step ST4. In this case, the process returns to step ST2. Note that the determination condition for a predetermined time is appropriately determined according to the reception environment or the like. For example, if it is very rare that the frequency error exceeds the sync word stable detection range, this time is set to be long, and if it frequently occurs, it is set to be short. However, if the time is set short, there is a high possibility that the frequency error is erroneously shifted when the frequency error is not greater than or equal to the synchronization word stable detection range. Therefore, it is desirable to determine this time based on a trade-off between the tracking speed and the certainty.

  In step ST4, the control unit 6 confirms whether or not the received electric field strength is above a certain level based on the output of the received electric field strength detection unit 3, and if it is above the certain level, moves to the next frequency search destination. Therefore, the VCTCXO 9 is controlled (step ST5). That is, in this case, a state where the synchronization word cannot be detected stably continues for a predetermined time and corresponds to a state where the electric field strength is equal to or higher than a certain level. In step ST4, if the received electric field strength is not above a certain level, the process returns to step ST2.

In step ST5, after moving to the next frequency search destination, it is confirmed whether or not the synchronization word is stably detected in the frequency search destination (step ST6). If the synchronization word can be detected stably in step ST6, the control unit 6 shifts to a normal AFC operation for controlling the VCTCXO 9 to reduce the estimated frequency error value (step ST9). Otherwise, the process proceeds to step ST7.
In step ST7, the control unit 6 confirms whether or not a state where the synchronization word cannot be detected stably continues for a certain period of time. If it continues, the control unit 6 returns to step ST8, otherwise returns to step ST6. That is, steps ST6 and ST7 are the same operations as steps ST2 and ST3 after moving to the next frequency search destination.

  In step ST8, the control unit 6 confirms whether or not the search has been completed for one round, and if it has not made one round, controls the VCTCXO 9 to shift to the next frequency search destination (step ST5). If it has made one round, it returns to step ST1 and returns to the initial frequency again. The determination of whether or not the search has made one round is based on whether or not a frequency in a range where a frequency shift of the VCTCXO 9 may occur is searched.

  Further, after the transition to the normal AFC operation (step ST9), the control unit 6 compares the initial control value recorded in the recording unit 7 with the current control value when the frequency pull-in is completed (step ST10). If the difference is greater than or equal to a certain value, the initial control value of the recording unit is updated to the current control value (step ST11). Otherwise, the initial control value is left (step ST12).

  As described above, since the frequency range in which the VCTCXO 9 may be shifted is searched all over, it becomes possible to estimate the frequency error value at any frequency search destination and shift to the normal AFC operation. It becomes possible to follow the carrier frequency of the station.

  FIG. 3 shows a search example when the carrier frequency of the base station is shifted by two frequency transitions in the positive direction with respect to the frequency of the mobile station. Here, since the received signal has a certain amount of bandwidth, it will be described below that the received electric field strength is above a certain level even at the initial frequency.

  First, since the initial frequency has a large frequency error from the carrier frequency of the base station, the synchronization word cannot be detected stably for a certain period of time, so a predetermined transition width (synchronized word is detected stably). A frequency search in the positive direction of the frequency is performed with a transition width equal to or less than the possible frequency range (1). As a result, even at this frequency, the frequency error from the carrier frequency of the base station is large, and the synchronization word cannot be detected stably for a certain period of time. Implement (2). This frequency search is a frequency lower than the initial frequency by a predetermined transition width.

  Similarly, since the frequency error with respect to the carrier frequency of the base station is large, the state in which the synchronization word cannot be detected stably continues for a certain period of time. Therefore, the frequency search in the positive direction of the frequency is performed again (3 ). As a result, since the error with the carrier frequency of the base station is small, the synchronization word can be detected stably. In this case, since it can be determined that the reliability of the frequency error value is high, it is possible to shift to normal AFC control for reducing the estimated frequency error value, and to follow the carrier frequency of the base station as a result.

  In this case, since the difference between the initial control value of the local oscillator recorded in the recording unit 7 and the current control value is large when the frequency pull-in is completed, the control unit 6 is recorded in the recording unit 7. The initial control value is updated with the current control value. Thereby, it is possible to prevent a frequency search from being performed due to a large frequency error when the power is turned on next time.

Next, another example search will be described.
FIG. 4 is an explanatory diagram showing an example of a search when a certain wave that is not the system itself is received and the received electric field strength is above a certain level.
First, at the initial frequency, since a wave that is not its own system is received, the synchronization word cannot be detected stably for a certain period of time, so the frequency search in the positive direction of the frequency with a predetermined transition width (1). As a result, as in the case of the initial frequency, since the synchronization word cannot be detected stably for a certain period of time, a frequency search is performed in the negative frequency direction (2). Similarly, since the synchronization word cannot be detected stably for a certain period of time, a frequency search in the positive direction of the frequency is further performed (3). Similarly, since a state where the synchronization word cannot be detected stably continues for a certain period of time, a frequency search in the negative frequency direction is performed again (4). Similarly, since the state where the synchronization word cannot be detected stably continues for a certain period of time, the search completes once and returns to the original frequency.

As shown in FIG. 4, since the frequency search is performed on the condition that the synchronization word is stably detected, when the frequency search is performed using a wave of another system other than the own system or an intermodulation wave of the own system, the original search is performed. Since it is possible to return to the frequency, it is possible to prevent erroneous tracking.
In addition, since the frequency search is performed only when the received signal level is equal to or higher than a certain level, it is possible to prevent erroneous control outside the service area where the signal from the base station cannot be received.

  In this example, as an example of performing a frequency search in predetermined steps for a range in which the frequency shift of the VCTCXO 9 may occur, an example in which a frequency search is alternately performed in the positive direction and the negative direction is shown. The order does not need to be the same as this example, and it is sufficient that the range in which the VCTCXO 9 can change is covered. Also, the transition width for performing the next frequency search can be appropriately selected as long as the transition width is equal to or smaller than the frequency range in which the synchronization word can be stably detected.

  As described above, according to the automatic frequency control device of the first embodiment, the frequency error between the frequency of the local oscillator and the carrier frequency of the received signal is estimated, and the frequency of the local oscillator follows the carrier frequency of the received signal. In the frequency control device, a synchronization word is detected by a synchronization word detection unit that detects a synchronization word inserted in advance in the reception signal, a reception field strength detection unit that detects a value of the reception field strength of the reception signal, and the synchronization word detection unit stabilizes the synchronization word. If the received electric field intensity detected by the received electric field intensity detection unit is higher than a certain level, the local oscillator frequency is below the frequency range where the synchronization word can be detected stably. It is possible to control the local oscillator even under conditions where the frequency error is large and the frequency error value cannot be estimated. Can, as a result, even an inexpensive local oscillator becomes available, it is possible to reduce the cost of the system.

  Further, according to the automatic frequency control device of the first embodiment, the control unit performs a frequency search within a range in which a frequency shift of the local oscillator may occur in the positive direction and the negative direction with the same transition width. Is performed in predetermined steps, so that a reliable frequency search can be performed and the local oscillator can be controlled more reliably.

  Further, according to the automatic frequency control device of the first embodiment, the control unit determines that the next frequency when the synchronization word cannot be stably detected by the synchronization word detection unit at the frequency search destination for a predetermined time. If the state where the sync word cannot be stably detected in any frequency search destination continues for a predetermined time after the transition to the search destination, control is performed so as to finally return to the original frequency. Even if a frequency search is performed using a signal such as an intermodulation wave of the own system, it is possible to prevent erroneous tracking of these signals.

  In addition, according to the automatic frequency control device of the first embodiment, the frequency error estimation unit that estimates the frequency error between the frequency of the local oscillator and the carrier frequency of the received signal is provided, and the control unit uses the synchronization word at the frequency search destination. When the sync word detection unit can detect stably, the frequency search is stopped and the frequency of the local oscillator is controlled to reduce the frequency error estimated by the frequency error estimation unit. It becomes possible to follow the carrier frequency of the received signal.

  Further, according to the automatic frequency control device of the first embodiment, the recording unit that holds the control value for controlling the local oscillator is provided, and the control unit uses the initial control value recorded in the recording unit to control the local oscillator. When control is started and frequency pull-in is completed, the initial control value is compared with the current control value, and if the difference is greater than a certain value, the initial control value is updated to the current control value. As a result, it is possible to prevent the frequency error from remaining large when the power is turned on next time and to perform a quick AFC operation.

It is a block diagram which shows the radio | wireless receiver to which the automatic frequency control apparatus by Embodiment 1 of this invention is applied. It is a flowchart which shows operation | movement of Embodiment 1 of this invention. It is explanatory drawing which shows control of the frequency search when there exists a received wave of the own system in Embodiment 1 of this invention. It is explanatory drawing which shows control of the frequency search when there is no reception wave of the own system in Embodiment 1 of this invention.

Explanation of symbols

  2 demodulator, 3 received electric field strength detector, 4 frequency error estimator, 5 syncword detector, 6 controller, 7 recorder, 9 VCTCXO (local oscillator).

Claims (5)

In an automatic frequency control device that estimates the frequency error between the frequency of the local oscillator and the carrier frequency of the received signal and causes the frequency of the local oscillator to follow the carrier frequency of the received signal,
A synchronization word detector that detects a synchronization word inserted in advance in the received signal;
A received electric field strength detecting unit for detecting a value of the received electric field strength of the received signal;
When the synchronization word is not detected stably by the synchronization word detection unit for a predetermined time, and when the received electric field strength detected by the received electric field strength detection unit is equal to or higher than a certain level, the frequency of the local oscillator is An automatic frequency control apparatus comprising: a control unit that searches with a transition width that is equal to or less than a frequency range in which the synchronization word can be detected stably.   2. The control unit according to claim 1, wherein the control unit performs a frequency search in a positive direction and a negative direction with the same transition width within a range in which a frequency deviation of the local oscillator may occur, every predetermined step. The automatic frequency control device described.   When the state where the synchronization word cannot be stably detected by the synchronization word detection unit continues for a predetermined time at the frequency search destination, the control unit moves to the next frequency search destination, and the synchronization word is detected at any frequency search destination. 3. The automatic frequency control device according to claim 1, wherein when the state in which the signal cannot be stably detected continues for a predetermined time, control is performed so that the frequency finally returns to the original frequency. A frequency error estimator that estimates the frequency error between the frequency of the local oscillator and the carrier frequency of the received signal,
The control unit stops the frequency search when the synchronization word detection unit can stably detect the synchronization word at the frequency search destination, and reduces the frequency error estimated by the frequency error estimation unit. The automatic frequency control apparatus according to any one of claims 1 to 3, wherein the frequency of the oscillator is controlled. A recording unit for holding a control value for controlling the local oscillator is provided.
The control unit starts controlling the local oscillator according to the initial control value recorded in the recording unit, and compares the initial control value with the current control value when the frequency pull-in is completed, and there is a difference between them. 5. The automatic frequency control device according to claim 4, wherein when the value is equal to or greater than a predetermined value, the initial control value is updated to a current control value.

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