JPH09247220A - Automatic frequency control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the bit error rate of a reproduced signal small by further reducing the frequency shift between a carrier frequency and a local oscillation frequency when a frequency-modulated received wave is demodulated and detected. SOLUTION: A comparing circuit 3 compares the demodulation output of a demodulating circuit 1 with the output of a decision circuit 2 and outputs its positive/negative sign. When the same positive/negative sign is outputted more than once successively, a digital filter 4 outputs an up signal or down signal to vary the value in a counter 5, and the value is converted by a D/A converter 6 into an analog value, which is used as the control voltage of a VCO 7. A frequency synthesizer 8 supplies a local oscillation signal based upon the output of the VCO 7 to the demodulating circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a demodulation circuit of a frequency modulation type radio receiver and has an automatic frequency control (AFC: automatic) having a frequency offset correcting function.
frequency control) circuit.

[0002]

2. Description of the Related Art FIG. 2 shows a configuration example of a conventional AFC. In the figure, 11 is a local oscillation circuit that oscillates and outputs a local oscillation frequency signal, 12 is a detection circuit that receives the received signal and the oscillation output of the local oscillation circuit 11, and obtains a phase detection output signal, and 13 is the timing of the phase detection output signal. Digital PL that outputs received symbol timing that is phase-synchronized with
L (DPLL: Digital Phase Locked Loop), 14 is a determination circuit that determines a signal using a phase detection output signal and obtains a detection output signal, and 15 is an adder that calculates an error between the input of the determination circuit and the determination output , 16 is an averaging circuit for obtaining the average value of the errors, 17 is an integrating circuit for obtaining the phase difference error by integrating the average value of the errors, 18 is the phase difference error subtracted from the phase detection output from the detection circuit 12, and its value Is an adder for inputting to the determination circuit 14. In the above circuit, as long as the judgment output shows a substantially correct result, the judgment circuit 1 is finally
The control automatically advances toward the direction where the input / output difference of 4 becomes zero, and the frequency offset correction function AFC is realized.

[0003]

However, in this AFC configuration, since the phase error component of the output signal after detection is corrected, the frequency is still between the carrier frequency and the local oscillation frequency. However, if the frequency shift continues to occur and a large frequency drift occurs, the AFC function cannot be achieved.

An object of the present invention is to receive a frequency-modulated signal, detect the direction of frequency drift from the phase error component of the output signal after detection, and directly control the VCO (voltage controlled oscillator) of the local station. Accordingly, it is an object of the present invention to provide an AFC circuit that sequentially corrects the deviation between the carrier frequency and the local oscillation frequency.

[0005]

SUMMARY OF THE INVENTION An automatic frequency control circuit of the present invention comprises a demodulation circuit for demodulating and detecting a frequency-modulated received signal by a local oscillation signal to reproduce a baseband signal, and the reproduced baseband signal. A determination circuit that determines a signal by a threshold value to obtain demodulation data, a baseband signal from the demodulation circuit, and a signal obtained by converting the determined demodulation data to ideal data are compared, and the positive / negative sign is output. A comparator circuit, a digital filter that outputs an up signal or a down signal when the positive and negative signs input from the comparator circuit are the same for a plurality of times in succession, and a counter in the counter based on the up signal or the down signal input from the digital filter. An up / down counter that changes the value and outputs the counter value as a digital value, and the digital value to an analog value. A D / A converter, a voltage controlled oscillator that converts the analog value into a voltage value, and a frequency synthesizer that gives an oscillation output of a local oscillation frequency based on the voltage value from the voltage controlled oscillator to the demodulation circuit. It is characterized by that.

[0006]

FIG. 1 shows an example of the configuration of an AFC circuit according to the present invention. In the figure, 1 is a demodulation circuit that demodulates and detects a received signal and reproduces transmission data (baseband signal), 2 is a determination circuit that determines the reproduced data by a threshold value, and 3 is a reproduction that is transmitted from the demodulation circuit Data and
The comparison circuit 4 compares the determined data with the ideal data and outputs the positive / negative sign of the comparison data.
When the positive and negative signs input from are consecutively the same, the digital filter 5 sends out the up signal or the down signal, and 5 changes the value in the counter based on the up signal or the down signal input from the digital filter 4. , And the counter value is sent as a digital value
Down counter (up / down counter), 6 is a D / A converter that converts a digital value into an analog value, 7 is a VCO that converts an analog value into a voltage value, 8 is a local oscillation frequency based on the voltage value from the VCO Is a frequency synthesizer that oscillates.

[0007]

The demodulation circuit 1 reproduces baseband data from the received signal and the local oscillation frequency signal input from the frequency synthesizer 8. The determination circuit 2 determines the data reproduced by the demodulation circuit 1 based on a predetermined threshold value,
Obtain demodulated data. The comparison circuit 3 converts the demodulation data into demodulation data when there is no frequency offset, that is, ideal data, compares the magnitude of the baseband data output from the demodulation circuit 1 with the ideal data, and compares the result. Output as a plus / minus sign.

The digital filter 4 takes in a positive / negative code for each bit, and outputs an up signal or a down signal when the same positive / negative code continues a plurality of times. In this case, when the number of consecutive times is set small, it responds to the occurrence of offset sensitively, and when the number of consecutive times is set to be large, it becomes insensitive to the occurrence of offset, but is less susceptible to sudden noise.

An initial value is set in advance as a counter value in the up / down counter 5. This initial value is an ideal output value of the up / down counter 5 when there is no frequency offset. The up / down counter 5 is
The counter value is updated each time based on the up signal or the down signal input from the digital filter 4, and the counter value is sent out as a digital value. The D / A converter 6 converts the digital value input from the up / down counter 5 into an analog value. The VCO 7 converts an analog value into a voltage value. The frequency synthesizer 8 oscillates and outputs a local oscillation frequency based on the voltage value from the VCO 7. In the circuit according to the present invention, the control of the local oscillation frequency automatically progresses in the direction in which the phase error component of the output signal after the detection becomes stepwise, and the frequency offset correction function AF is performed.
C can be realized.

By detecting the direction of frequency drift from the phase error component of the output signal after detection and directly controlling the VCO of the local station, the local oscillation frequency sequentially follows the carrier frequency with respect to variations in the carrier frequency. However, the AFC method in which the frequency offset is corrected has been realized.

FIG. 3 shows BER (Bit Errer Rate).
This is a static characteristic, and a circuit was manufactured by an actual machine based on the configuration of the present invention, and the AFC effect was measured. Legend in the figure,
"NON AFC" means that there is no AFC circuit, "method 1" means that the conventional AFC circuit is used, and "method 2" means that the AFC circuit of the present invention is used. The vertical axis of the figure compares the transmission data with the demodulation data after reception, and shows the error degree as a percentage. The horizontal axis shows the frequency offset width (frequency shift between carrier frequency and local oscillation frequency)
It is.

[0012]

It has been confirmed that, by carrying out the present invention, a remarkably stable AFC effect can be obtained as compared with the method according to the prior art.

[Brief description of drawings]

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

FIG. 2 is a diagram illustrating a configuration example of a conventional AFC circuit.

FIG. 3 is a BER static characteristic example comparing the AFC effect between the conventional method and the method according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Demodulation circuit 2 Judgment circuit 3 Comparison circuit 4 Digital filter 5 Up / down counter 6 D / A converter 7 VCO 8 Frequency synthesizer 11 Local oscillation circuit 12 Detection circuit 13 DPLL 14 Judgment circuit 15 Adder 16 Averaging circuit 17 Integration circuit 18 adder

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenzo Urabe 3-14-20 Higashi-Nakano, Nakano-ku, Tokyo Kokusai Electric Co., Ltd.

Claims (1)

[Claims] 1. A demodulation circuit for demodulating and detecting a frequency-modulated received signal by a local oscillation signal to reproduce a baseband signal, and a judgment for judging the reproduced baseband signal by a threshold value to obtain demodulated data. A circuit, comparing the baseband signal from the demodulation circuit with the determined demodulated data converted to ideal data,
A comparison circuit that outputs the positive / negative sign, a digital filter that outputs an up signal or a down signal when the positive / negative signs input from the comparison circuit are the same for a plurality of times consecutively, and an up signal or a down signal input from the digital filter. An up / down counter that changes a value in the counter based on a signal and outputs the counter value as a digital value, a D / A converter that converts the digital value into an analog value, and the analog value into a voltage value. An automatic frequency control circuit comprising: a voltage controlled oscillator for converting; and a frequency synthesizer for giving an oscillation output of a local oscillation frequency based on a voltage value from the voltage controlled oscillator to the demodulation circuit.