JP2820143B2 - Automatic frequency control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic frequency control system for a radio transmission apparatus, and more particularly to an orthogonal frequency multiplexing (Orth frequency multiplexing) system.
ogonal Frequency Division Multiplex:
DM ". The present invention relates to an automatic frequency control method for correcting, on the receiver side, a frequency shift and fluctuation between a transmitter and a receiver in a wireless transmission device adopting the method.

[0002]

2. Description of the Related Art When an OFDM system is used, the frequency interval between carrier waves used is very narrow as compared with a system using a single frequency (single carrier). Become. For this reason,
In the OFDM system, a function that can correct the frequency of a carrier or a local signal on the receiver side with higher precision according to the carrier frequency of the transmission signal from the transmitter is required as compared with the system using a single carrier. .

In the conventional method, when converting a high frequency to an intermediate frequency, the frequency deviation between transmission and reception is suppressed as much as possible by using a high-density and high-stability frequency oscillator for the transceiver. .

There has also been proposed a method in which the transmitter transmits a signal serving as a frequency reference in advance so that the receiver can determine the carrier frequency on the transmitter.
For example, in the system described in Japanese Patent Application Laid-Open No. 8-51408, two or more reference carrier frequency locks separated from the transmitter side by a different frequency from the center of the modulation signal within or outside the band of the modulation signal. A method has been proposed in which a carrier signal is arranged and transmitted, and the receiver uses the signal to control a signal generator inside the receiver.

[0005]

A first problem is that in the conventional orthogonal frequency division multiplexing (OFDM) system, it is necessary to use a highly accurate and highly stable frequency oscillator which is difficult to obtain in a transceiver. It is in.

[0006] The reason is that in the OFDM system, as compared with the system using a single carrier, only a frequency shift of several hundreds to one-thousandths can be corrected. It is necessary to be

For example, as a method of correcting the carrier frequency between the transmitter and the receiver on the receiver side, the phase error is detected by comparing the data obtained by the demodulation processing with the original data, and the carrier wave between the transmitter and the receiver is detected. Employing the method of correcting the frequency shift cannot be realized in a situation where the carrier frequency changes by a certain value or more within a period (symbol period) in which the carrier changes.

However, in the OFDM system as described above, the symbol period is longer by the number of carriers (several hundreds to thousands) used than in the single carrier system. Conversely, the number of carriers used is reduced. For this reason, OFDM
In the method, the frequency shift that can be corrected by the demodulation processing is several hundreds to several thousandths.

Specifically, for example, in the case of a single carrier system using a carrier of 7 GHz and a band of 17 MHz, in the case of QPSK modulation, the range in which a frequency shift can be detected is 1 MHz or more. On the other hand, in the OFDM system, when there are about 1000 carriers in the same 17 MHz band, and when the modulation method of each carrier is QPSK, the range in which a frequency shift can be detected is about 2 kHz. Actually, since the range of the frequency shift that can be further corrected becomes narrower, the accuracy of the oscillator actually required in the transceiver is about 1 × 10 ⁻⁷ or less. Such an oscillator can be realized only by a special device such as adding a constant temperature chamber, and is usually difficult to obtain as well as difficult to obtain, so there is a possibility that it will be an obstacle in realizing a device. .

The second problem is that, in a system in which a signal serving as a frequency reference is transmitted on the transmitter side in advance so that the carrier frequency on the transmitter side can be determined on the receiver side, the information transmission capacity is reduced. Or lowering the frequency utilization efficiency.

[0011] The reason is that, in a method in which a signal serving as a frequency reference is transmitted on the transmitter side, when the reference signal is arranged in a band, the information transmission capacity is correspondingly reduced, and the out-of-band signal is reduced. This is because if the frequency band is allocated, the corresponding frequency band is allocated, and the frequency use efficiency is reduced.

(Object of the Invention) An object of the present invention is to provide an OFDM
A relatively inexpensive frequency signal generator without reducing the information transmission capacity and frequency utilization efficiency when using the method, and without requiring a high-density and highly stable frequency signal generator in the transceiver. Is to be able to use.

[0013]

In order to solve the above-mentioned problems, an automatic frequency control system according to the present invention employs an orthogonal frequency division multiplexing method.
Reference frequency signal for frequency conversion to receive double modulated waves
In the automatic frequency control method of the oscillator that generates the signal,
Localizers for synchronous detection synchronized with the output of the oscillator
And a carrier for the orthogonal frequency division multiplex modulation wave.
Two carrier frequencies at the end of the wave or adjacent to the end of the carrier
Signal level of two frequencies where the transmitted carrier is not transmitted
Two synchronous detection means for respectively detecting the
And a level comparator for comparing the outputs of the synchronous detection means.
The output levels of the two synchronous detection means become equal.
So that the output of the level comparator
It is characterized in that the frequency is controlled. Also, the orthogonal frequency
Reference frequency for frequency conversion for receiving division multiplex modulation wave
In the automatic frequency control method of the oscillator that oscillates the wave number signal
For synchronous detection synchronized with the output of the oscillator.
Having a local generator, the orthogonal frequency division multiplex modulation wave
The carrier frequency at the end of the carrier and the
The frequency at which the carrier is not transmitted, which is approximately the carrier frequency
Two synchronous detectors to detect the signal level of each of the numbers
And the level at which the outputs of the two synchronous detectors are compared.
A comparator, and an output level of the two synchronous detection means.
To the output of the level comparator so that the difference between
Controlling the oscillation frequency of the oscillator.
You.

Further, in the present invention, in an automatic frequency control method of a reference frequency signal for receiving an orthogonal frequency division multiplex modulation wave, the carrier frequency of the lowest frequency and the highest frequency is synchronously detected based on the reference frequency signal (for example, , Quadrature demodulation), and the frequency of the reference frequency signal is controlled so that the average value of both detection levels becomes equal.

Further, as the designation of the reception frequency for performing the synchronous detection and the control of the frequency of the reference frequency signal, one of the carrier groups is set to two adjacent carrier frequencies that are not transmitted, and the difference between the average values of the two detection levels is determined. Can be controlled to be the largest. Further, two carrier frequencies adjacent to a carrier frequency that is not transmitted in the carrier group may be used, and the frequency of the reference frequency signal may be controlled so that the difference between the average values of the two detection levels is minimized. it can.

As a more specific configuration of the automatic frequency control system of the present invention, two orthogonal demodulators for orthogonally demodulating an OFDM carrier input with two local signals having different frequencies related to a carrier at its end, A low-pass filter that extracts the low frequency of the output of each quadrature demodulator, an averaging circuit that averages the output from the low-pass filter, a comparator that compares the output of the averaging circuit, and is controlled by the output of the comparator It can be configured to include a voltage-controlled oscillator and a local signal generator that outputs the local signal based on an output of the voltage-controlled oscillator.

[0017]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a basic configuration of an embodiment of the present invention.

This embodiment is shown as an example applied to a system in which a modulated wave input is subjected to frequency conversion by a down converter 2 using a local signal, and a high frequency (RF) is converted to an intermediate frequency (IF). .

A splitter 1 for splitting the modulated wave into three, two specific carrier level detectors 3 and 4 for detecting only the level of a specific carrier from the modulated waves from the splitter 1, A level comparator 5 for comparing the levels of the outputs from the specific carrier level detectors 3 and 4, a local signal generator 6 for generating a local signal according to the output of the level comparator 5, and a local signal generator And a down converter that performs frequency conversion by multiplying the output of the distributor 6 and the modulated wave from the distributor 1.

The basic operation of the frequency control according to the present embodiment will be described.

Two specific carrier level detectors 3, 4
Is set to extract mutually different carriers from among many carriers included in the OFDM modulated wave. For example, it is set so as to extract only the carrier having the highest frequency and the lowest frequency of the OFDM modulated wave, and the average level is detected and output. The level comparator 5 compares the output levels of the specific carrier level detectors and outputs a control voltage corresponding to the output level difference between the specific carrier level detectors 3 and 4 to the local signal generator 6.

The carrier level detector generally outputs the maximum level at the detection center frequency, and the output level decreases in accordance with the frequency shift of the received carrier. Therefore, if the carrier frequency of the OFDM modulated wave shifts, the carrier level detector will According to the setting, while the output level of one carrier level detector decreases toward zero level, the output level of the other carrier level detector maintains the average carrier level of the OFDM modulated wave. Therefore, by controlling the frequency of the local generator 6 based on the comparison result of the outputs of the two carrier level detectors in the level comparator 5, the OF in the intermediate frequency signal (IF output) of the down converter 2 is controlled.
Control can be performed to reduce the influence of the shift in the carrier frequency of the DM modulation wave.

According to an embodiment of the present invention, the two specific carrier level detectors are configured to perform synchronous detection using a received carrier for an OFDM modulated wave, and the carrier is controlled by the output of a comparator 5 to control the specific carrier level. It is preferable that the carrier frequencies (center frequencies) detected by the detectors 3 and 4 be controlled.

As a method of setting the detection frequencies of the two specific carrier level detectors in the above embodiment, the detection frequencies are set at the ends of a plurality of adjacent carrier waves as in the setting corresponding to the highest and lowest frequencies. In addition, a carrier frequency that is not transmitted (not used) may be set in the OFDM modulated wave, and therefore, an end portion of the carrier that is in contact with a gap of a missing carrier generated in a plurality of carriers of the OFDM modulated wave. Can be used. That is, any combination of any two carrier frequencies, including the highest and lowest frequencies and the edge frequencies, can be used. As will be described later, two combinations of the carrier frequency at the end and the like and the unused carrier frequency adjacent thereto can be used.

FIG. 2 employs a synchronous detection type configuration as the two specific carrier level detectors, and the detection center frequency of the specific carrier level detector itself is controlled according to the shift of the carrier frequency of the OFDM modulated wave. 3 is a configuration example of a more specific embodiment of the present invention.

As shown in FIG. 1, a configuration shown in FIG. 1 is used to distribute a modulated wave into two parts, and a main signal, which is one output of the distributor 11, to a first local frequency conversion part. Multiplier 21 for frequency conversion by the output of generator 62
And a band-pass filter 22 for extracting only a signal of a necessary intermediate frequency from the output of the multiplier 21.

A splitter 12 for further splitting the other output of the splitter 11 into two outputs, and two quadrature demodulators 31 and 41 for inputting the split output for frequency control.
And four low-pass filters 32, 3 for extracting only the DC components of the outputs of the quadrature demodulators 31, 41, respectively.
3, 42, 43 and four detection circuits 34, 35, 4 for detecting the output of the low-pass filter and outputting its absolute value.
4, 45; two averaging circuits 36, 46 for averaging the outputs from the two detection circuits, and averaging the outputs over time, and outputting the reception level of each modulated wave signal; and two averaging circuits A level comparator 51 for comparing a reception output level from the circuit and outputting a control signal for controlling the oscillation frequency of the voltage controlled oscillator 61; and changing the oscillation frequency based on the output of the level comparator 51. A voltage controlled oscillator 61, a first local generator 62 for generating a local signal necessary for frequency conversion of the main signal based on the oscillation frequency of the output of the voltage controlled oscillator, and two local oscillators used for frequency control. The control system includes a second local generator 63 and a third local generator 64 for generating local signals required for the quadrature demodulators 31 and 41.

Next, the operation of automatic frequency control and frequency conversion in the block diagram of FIG. 2 will be described with reference to FIG.

The modulated wave input is first split by the splitter 11 and then split by the splitter 12. The modulated wave distributed by the distributor 12 is input to the quadrature demodulator 31 and the quadrature demodulator 41. The quadrature demodulator 31 includes a second local generator 63
A quadrature demodulation of the modulated wave is performed using the local signal output from
The quadrature demodulator 41 quadrature demodulates the modulated wave with the local signal output from the third local generator 64.

FIG. 3 is a diagram showing an example of a circuit of the quadrature demodulators 31 and 41. The configuration of the quadrature demodulator consists of two splitters 3
13, 314 and two multipliers 311, 312.
The modulated wave signal is divided into two, and the local signal is multiplied by a reference signal branched into signals having phases different from each other by 90 ° to obtain quadrature demodulated outputs (I) and (Q). The use of a quadrature demodulator enables frequency selection and demodulation even when the modulated wave input reaches the receiver with the phase rotation from the phase at the output of the transmitter in the process of propagating through the radio transmission path. This is to enable level detection.

The low-pass filters 32 and 33 select only the DC component of the output from the quadrature demodulator 31. Similarly, the low-pass filters 42 and 43 select only the DC component from the output from the quadrature demodulator 41. Detection circuit 34, 3
5, 44 and 45 are low-pass filters 32 and 3 respectively.
The absolute values of the outputs of 3, 42 and 43 are taken.

The averaging circuit 36 includes two detection circuits 34,
A mean square calculation of the 35 outputs is performed. Similarly, the averaging circuit 4
6 performs a root-mean-square calculation of the outputs of the two detection circuits 44 and 45. The level comparator 51 compares the outputs of the two averaging circuits and outputs a voltage corresponding to the frequency shift between the transmitter and the receiver.

The voltage controlled oscillator 61 includes a level comparator 51
, A reference frequency common to the first local generator 62, the second local generator 63, and the third local generator 64 is output. The first, second, and third local generators 62, 63, and 64 receive the output of the voltage controlled oscillator 61 and divide or multiply the output, or perform a phase locked loop (PL).
L), a signal of a required frequency is generated and a multiplier 21, a quadrature demodulator 31,
41. The output of the first local generator 62 is a signal necessary for frequency-converting the modulated wave input,
The outputs of the local generator 63 and the third local generator 64 are signals of a predetermined frequency for detecting a frequency shift of a carrier wave between the transmitter and the receiver.

Next, the operation of the automatic frequency control is shown in FIG.
This will be described more specifically with reference to FIG.

The OFDM modulated wave changes from ft (−n) to ft
It is assumed that 2n + 1 carriers up to (n) are used, and the oscillation frequency of the second local generator is set to the lowest frequency ft (− n) corresponding to the frequency fr (−
n) and the oscillation frequency of the third local generator is OFDM
Frequency fr corresponding to maximum frequency ft (n) of the modulated wave
An example of (-n) will be described.

When the frequency shift between the transmitter and the receiver is zero, ft (i) = fr (i) i = -n, n, so that the output of the averaging circuit 36 forms an OFDM modulated wave. The detection voltage of only the lowest frequency carrier wave is detected, and the output of the averaging circuit 46 is the detection voltage of only the highest frequency carrier wave constituting the OFDM modulated wave.

On the transmitter side, the average level of the carrier wave included in the OFDM modulated wave is usually set to the same value.
The output voltage of the averaging circuit 36 and the output voltage of the averaging circuit 46 have the same value. The level comparator 51 compares the levels of the outputs of the averaging circuits 36 and 37 to generate an output for controlling the voltage controlled oscillator 61.

FIG. 4 is a diagram showing a circuit example of the level comparator 51.

In the circuit of FIG. 4, when the input levels of the two comparator inputs (−) and (+) are equal, the output of the operational amplifier 511 becomes 0 V, and the comparators 513 and 51
4, the output becomes "H", and the switches of both the relays 516 and 517 are released.
Is stored, and the output of the level comparator does not change. Therefore, the oscillation frequency of the voltage controlled oscillator 61 does not change. Therefore, in this case, if the output frequencies of the first, second and third local generators do not change and the carrier frequency on the transmitter side changes to a higher frequency, the output of the averaging circuit 36 becomes almost zero. And the output of the averaging circuit 46 hardly changes with the average level of each carrier constituting the OFDM modulated wave. In this case, the output of the operational amplifier 511 becomes a positive voltage,
4 remains at the “H” level, but the output of the comparator 513 goes to the “L” level. Therefore, the switch of the relay 516 is connected, and the capacitor C511 is charged through the resistor R515. For this reason, the output voltage of the level comparator 512 increases, and the oscillation frequency of the voltage controlled oscillator 61 is controlled to increase. As a result, the carrier frequency on the receiver side can also be increased following the transmitter side, and the carrier frequency deviation between the transmitter and the receiver decreases.

When the carrier frequency on the transmitter side decreases, the output voltage of the level comparator 51 decreases, and the oscillation frequency of the voltage controlled oscillator 61 is controlled to decrease.

The comparator 515 in FIG. 4 is for determining the output voltage of the level comparator 51 when the power is turned on. Only when the power is turned on, the switch of the relay 518 is connected, and the voltage determined by the variable resistor VR514 is output from the level comparator 51 as an initial control voltage.

In the present invention, the OFDM modulated wave is ft
It is assumed that 2n + 1 carriers from (−n) to ft (n) are used, and to detect a frequency shift between the transmitter and the receiver at the receiver side, the oscillation frequency of the second local generator and the third local generator are used. The following combinations of embodiments can be configured in relation to the selection of the frequency of the vessel. That is, (1) the second local signal is a frequency fr (−n) corresponding to the lowest frequency ft (−n) of the OFDM modulated wave,
The oscillation frequency of the local generator is further lower than the lowest frequency of the OFDM modulated wave by the carrier interval of the carrier wave ft.
The frequency fr (-n-1) corresponding to (-n-1) is set. In this case, when the frequency shift between the transmitter and the receiver is zero, the output voltage of the averaging circuit 36 becomes the average level of each carrier of the OFDM modulated wave, and the output voltage of the averaging circuit 46 becomes almost zero. Utilizing this, the comparator 51
In order to control the oscillation frequency of the voltage controlled oscillator 61 so that the value of the input from the averaging circuit 36 becomes the maximum and the difference between the input from the averaging circuit 36 and the input from the averaging circuit 46 becomes the largest. Output voltage.

(2) In addition, the second local signal is
Frequency fr corresponding to maximum frequency ft (n) of the modulated wave
(N) and the oscillation frequency of the third local generator is OFD
The frequency fr (n +) corresponding to ft (n + 1) higher than the highest frequency of the M modulated wave by the carrier interval of the carrier.
1). In this case as well, as in the previous example, the averaging circuit 36 with the frequency shift between the transmitter and the receiver being zero.
Becomes the average level of each carrier of the OFDM modulated wave, and the output voltage of the averaging circuit 46 becomes almost zero.
Utilizing this, the comparator 51 controls the voltage so that the value of the input from the averaging circuit 36 becomes the maximum and the difference between the input from the averaging circuit 36 and the input from the averaging circuit 46 becomes the largest. A voltage for controlling the oscillation frequency of the oscillator 61 is output.

(3) Further, the second local signal is converted to an OFDM signal.
The frequency fr (-n) corresponding to ft (-n-1), which is lower than the highest frequency of the modulated wave by the carrier interval of the carrier.
-1) and the oscillation frequency of the third local generator is OFD
The frequency fr (n +) corresponding to ft (n + 1) higher than the highest frequency of the M modulated wave by the carrier interval of the carrier.
1). In this case, the output voltage of the averaging circuit 36 and the output voltage of the averaging circuit 46 become almost zero in a state where the frequency deviation between the transmitter and the receiver is zero. Taking advantage of this,
The comparator 51 outputs a voltage for controlling the oscillation frequency of the voltage controlled oscillator 61 so that the input voltage from the averaging circuit 36 and the averaging circuit 46 is minimized.

(4) In the case of the OFDM system, a carrier that is not transmitted may be created among many carriers. In such a case, 2 is located at a boundary between a carrier that is not transmitting and a carrier that is transmitting. One carrier frequency can be specified by a second local generator and a third local generator, respectively. In this case, the designation of the carrier frequency in the second and third local generators is based on the combination of the used carrier waves and the combination of the used and unused carrier waves as in the above-mentioned designation methods (1) to (3). In addition, similar combinations can be used, including the aforementioned highest and lowest frequency carriers.

[0047]

[0048]

According to the automatic frequency control system of the present invention,
Reference signal for frequency control of frequency deviation between transceivers
Can be corrected on the receiver side based on the OFDM modulated wave that does not include . Also, the automatic frequency control method of the present invention
Requires a special reference signal to be transmitted at the transmitter
Not applicable to communication using OFDM modulated waves.
Thus, the information transmission capacity and the frequency use efficiency can be maximized.

Further, according to the present invention, the center frequency of the OFDM modulated wave and the frequency of the local signal for frequency conversion can always be kept in a fixed relation, so that the frequency-converted O
Since the center frequency of the FDM modulated wave is always constant,
The performance of the demodulation unit can always be kept in an optimum state, and the performance of the transmission device can be maximized.

Further, according to the present invention, the use of synchronous detection of a quadrature demodulator or the like makes it possible to improve the correction accuracy in correcting a frequency shift. This is because it is easier to manufacture compared to the case of using a narrow-band bandpass filter when correcting the frequency shift, and the influence of temperature is small, so it is necessary to accurately extract the frequency component for detecting the frequency shift. Because it can be.

[0051]

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a basic configuration of an embodiment of the present invention.

FIG. 2 is a diagram illustrating a detailed configuration example of an embodiment of the present invention.

FIG. 3 is a diagram illustrating a circuit example of a quadrature demodulator;

FIG. 4 is a diagram illustrating a circuit example of a level comparator;

FIG. 5 is a diagram illustrating a conventional down converter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Divider 2 Down converter 3, 4 Specific carrier level detector 5, 51 Level comparator 6, 65 Local generator 11, 12, 313 2 Divider 21, 311, 312 Multiplier 22 Bandpass filter 31, 41 Quadrature demodulation Detector 32, 33, 42, 43 Low-pass filter 34, 35, 44, 45 Detection circuit 36, 46 Averaging circuit 61 Voltage controlled oscillator 62 First local generator 63 Second local generator 64 Third local generator 66 Oscillator 314 90 ° 2 divider 511, 512 Operational amplifier 513, 514, 515 Comparator 516, 517, 518 Relay C511, C512 Capacitor R511-R516 Resistor VR511-VR514 Variable resistor

Claims (6)

(57) [Claims] An automatic frequency control method for an oscillator that generates a reference frequency signal for frequency conversion for receiving an orthogonal frequency division multiplex modulation wave, for synchronous detection synchronized with an output of the oscillator. Low
And a carrier for the orthogonal frequency division multiplex modulation wave.
Two carrier frequencies at the end of the transmission or next to the end of the carrier
The signal level of two frequencies where the adjacent carrier is not transmitted
Two synchronous detection means for detecting the bells respectively;
And a level comparator for comparing the outputs of the two synchronous detection means.
The output levels of the two synchronous detection means become equal.
So that the output of the level comparator
An automatic frequency control method characterized by controlling the frequency. 2. An automatic frequency control method for an oscillator that oscillates a reference frequency signal for frequency conversion for receiving an orthogonal frequency division multiplex modulation wave, for synchronous detection synchronized with an output of the oscillator. Low
And a carrier for the orthogonal frequency division multiplex modulation wave.
The carrier frequency at the end of the transmission and the approximate
Carrier frequency
Two synchronous detection means for detecting respective signal levels
And a level ratio for comparing the outputs of the two synchronous detection means.
And an output level of the two synchronous detection means.
Depending on the output of the level comparator, the difference is maximized.
Controlling the oscillation frequency of the oscillator . 3. In an automatic frequency control method of an oscillator for generating a reference frequency signal for frequency conversion for receiving an orthogonal frequency division multiplex modulation wave, a synchronous detection method synchronized with an output of the oscillator is provided. Low
A quadrature frequency division multiplex modulation wave.
Detect signal level of low frequency and highest frequency carrier
Two synchronous detection means, and outputs of the two synchronous detection means.
And a level comparator for comparing the forces.
Comparing the levels so that the output levels of the wave means are equal
An automatic frequency control method, wherein the oscillation frequency of the oscillator is controlled by the output of a device . 4. In an automatic frequency control method of an oscillator for generating a reference frequency signal for frequency conversion for receiving an orthogonal frequency division multiplex modulation wave, a synchronous detection method synchronized with an output of the oscillator is provided. Low
And a carrier for the orthogonal frequency division multiplex modulation wave.
Adjacent to the carrier frequency at which one of the transmission groups is not transmitted
To detect the signal levels of the two carrier frequencies, respectively.
Two synchronous detection means, and outputs of the two synchronous detection means.
A level comparator for inputting a force, and
The difference between the output levels of the detection means is maximized.
The oscillation frequency of the oscillator is controlled by the output of the level comparator.
Automatic frequency control system, characterized in that the control. 5. An automatic frequency control system for an oscillator for generating a reference frequency signal for frequency conversion for receiving an orthogonal frequency division multiplex modulation wave, wherein each of the oscillators for synchronous detection is synchronized with an output of the oscillator. Low
Carrier with orthogonal frequency division multiplex modulation wave
Two adjacent non-transmitted carrier frequencies in the group
Two synchronous detections for detecting the carrier signal level respectively
Means for receiving the outputs of the two synchronous detection means as inputs.
A bell comparator, and the output level of the two synchronous detection means is provided.
The output of the level comparator is minimized so that the bell difference is minimized.
An automatic frequency control method, wherein the oscillation frequency of the oscillator is controlled by a force . 6. A frequency converter synchronized with an output of said oscillator.
And a local generator for the frequency conversion.
Automatic frequency control method according to claim 1, 2, 3, 4 or 5, wherein further comprising a frequency converting means for converting the orthogonal frequency division multiplex modulated wave by the output of Le generator to an intermediate frequency signal.