JPH0923249A - Digital afc circuit and fsk receiver using the circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the phase error not completely corrected by executing suitable AFC even when C/N is low to extend the frequency correction range. SOLUTION: An arc tangent arithmetic circuit 24 obtains a phase θof quadrature signal (I, Q) obtained by quadrature detection circuit 10 and a regression line f×t+Φ with respect to the phase θ is obtained as to a time (t). Since the phase θ is the information relating to a difference between a phase of a carrier frequency and a phase of a local oscillating frequency, the carrier frequency (f) and the phase Φ are estimated by obtaining the regression line. The AGC relating to a local oscillating signal is realized without causing a phase error over a wide frequency range principally by controlling the oscillating frequency and phase of a DDS(digital direct synthesizer) 32 based on the result even when the C/N is low.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital AFC (frequency automatic adjustment) circuit used in an FSK (frequency shift keying) receiver.

[0002]

AFC used in FSK receivers
As the circuit, for example, there is a circuit having the configuration shown in FIG. The circuit shown in this figure has a configuration in which, when the FSK modulated wave is converted into a quadrature signal by the quadrature detection circuit 10, the output of the polar coordinate conversion circuit 12 is used to control the oscillation frequency and phase of the local oscillator 16 via the digital filter 14. Have

First, the local oscillator 16 generally oscillates at a frequency equal to the carrier frequency of the FSK modulated wave, and supplies the resulting local oscillation signal to the quadrature detection circuit 10. In the quadrature detection circuit 10, the in-phase component I and the local oscillation signal are π by mixing the local oscillation signal and the FSK modulated wave.
The quadrature component Q is generated by mixing the / 2 phase-shifted signal and the FSK modulated wave. Polar coordinate conversion circuit 12
Converts an FSK modulated wave given in the rectangular coordinate format, that is, a rectangular signal (I, Q) into a signal (r, θ) in the polar coordinate format. The phase signal θ indicating the phase of the quadrature signal among the obtained signals is band-pass filtered by the digital filter 14. The digital filter 14 has, for example, a transfer function corresponding to a lag lead filter used in an analog PLL (phase locked loop) circuit or the like. Then, by controlling the oscillation frequency and phase of the local oscillator 16 using the phase signal θ filtered by the digital filter 14, the frequency and phase of the local oscillation signal are almost synchronized with the frequency and phase of the carrier wave of the FSK modulated wave. Can be made.

[0004]

The conventional structure is FSK.
When the C / N (carrier-to-noise ratio) of the modulated wave is high, it can function effectively, but when the C / N is low, it may not function sufficiently. That is, if the pass band of a digital filter that filters a phase signal is set to be sufficiently narrow, AFC can be performed even for an FSK modulated wave having a low C / N, but local oscillation that can be corrected by this AFC The range of frequency error becomes small. In addition, in the above method, the local oscillation frequency and phase are controlled based on the phase signal including the phase error due to noise in addition to the phase error of the local oscillation signal with respect to the carrier wave, so that the phase error is set to 0 in principle. It is not possible.

The present invention has been made to solve the above problems, and the C / N is improved by improving the means for setting the control target for the frequency and phase of the local oscillation signal. An object of the present invention is to realize an AFC circuit capable of performing correction over a wide frequency range even with a low FSK modulated wave and theoretically reducing the phase error to zero. It is another object of the present invention to make it possible to realize an AFC circuit having such a function as a digital circuit over its entirety, and to easily realize miniaturization and no adjustment of the device.

[0006]

In order to achieve such an object, the digital AFC circuit according to the first configuration of the present invention is converted into a quadrature signal using a local oscillation signal. Regarding the FSK modulated wave, a means for obtaining the phase θ of the quadrature signal based on the in-phase component I and the quadrature component Q in the quadrature signal according to the equation θ = tan ⁻¹ (Q / I), and a regression line of the phase θ with respect to time t It is characterized by comprising means for estimating the carrier frequency f and phase φ of the FSK modulated wave by obtaining f × t + φ, and means for controlling the frequency and phase of the local oscillation signal according to the estimation result.

In this configuration, the control target regarding the frequency and phase of the local oscillation signal is set by using the regression line of the phase θ of the quadrature signal. Here, even when the C / N is low, the noise component included in the FSK modulated wave can be regarded as having an average value of 0. Therefore, by using the above regression line, the frequency and phase of the local oscillation signal are You can get information about the error of. Therefore, if the carrier frequency f and the phase φ of the FSK modulated wave are estimated using the regression line and the frequency and phase of the local oscillation signal are controlled based on this, in the case where the C / N of the FSK modulated wave is low, Even if there is, it is possible to correct the frequency of the local oscillation signal over a wide frequency range. Also, unlike the conventional configuration in which the phase θ of the quadrature signal is fed back to the local oscillator, the phase φ of the carrier wave of the FSK modulated wave is estimated from the regression line of the phase θ of the quadrature signal, so noise can be eliminated, and in principle It is also possible to reduce the phase control error of the local oscillation signal to zero.

An FSK receiver according to a second configuration of the present invention includes a local oscillator that generates a local oscillation signal, a quadrature detection circuit that converts the FSK modulated wave into a quadrature signal by using the local oscillation signal, and the present invention. AF according to the first configuration of
It is characterized by comprising a C circuit, means for detecting a frequency deviation related to FSK modulation by differentiating the phase θ, and means for reproducing data based on the detected frequency deviation. According to this configuration, it is possible to obtain the FSK receiver capable of realizing the same operation and effect as the first configuration described above.

The FSK receiver according to the third aspect of the present invention is the FSK receiver according to the second aspect, wherein
It is characterized in that the K-modulated wave is a wave having a low frequency modulation degree and is modulated according to an FSK modulation method such as MSK (minimum shift keying) or GMSK (Gaussian filtered MSK). According to this configuration, these FSK
It is possible to realize an FSK receiver that is suitably applicable to the modulation method.

An FSK receiver according to a fourth configuration of the present invention is the FSK receiver according to the second configuration of the present invention,
When the obtained phase θ exceeds the range of −π / 2 <θ <π / 2, the value of the phase θ is −π / prior to the differentiation.
In order to always change continuously within the range of 2 <θ <π / 2,
It is characterized in that it comprises means for converting the range of the phase θ, and that the FSK modulated wave is a wave modulated according to the FSK modulation method having a high frequency modulation degree. According to this configuration, since the obtained phase θ does not have a discontinuity such as jumping from π / 2 to −π / 2, the range in which the frequency deviation can be detected by the differentiation of the phase θ is widened. An FSK receiver suitable for the FSK modulation method having a high frequency modulation degree can be obtained.

[0011]

Preferred embodiments of the present invention will be described below with reference to the drawings. The same components as those in the conventional example shown in FIG.

FIG. 1 shows an F according to the first embodiment of the present invention.
The configuration of the SK receiver is shown. In this FSK receiver, the FSK modulated wave received by the antenna 18 is subjected to processing such as amplification, filtering and frequency conversion in the RF / IF circuit 20 in the subsequent stage. RF / IF
The FSK modulated wave that has been subjected to the processing related to the circuit 20 is A / D
Converted into a digital signal by the converter circuit 22,
It is supplied to the quadrature detection circuit 10. The quadrature signal (I, Q) obtained by the quadrature detection circuit 10 is supplied to the arctangent calculation circuit 24 at the subsequent stage. The arctangent calculation circuit 24 calculates the phase θ = tan ⁻¹ (Q / I) based on the quadrature signal (I, Q), and the calculated phase θ is used by the differentiation circuit 26 on the one hand and the regression line calculation circuit on the other hand. 28
Respectively. The differentiating circuit 26 differentiates the phase θ. As a result, the change in the phase θ is obtained. The determination circuit 30 reproduces the modulation data relating to the FSK modulation based on the output of the differentiating circuit 26, and outputs this to the circuit in the subsequent stage.

A feature of the present invention is that a regression line calculation circuit 28 provided in a subsequent stage of the arctangent calculation circuit 24.
Therefore, the regression line of phase θ f × t + φ (where t is the time)
Then, the carrier frequency f of the FSK modulated wave is estimated from the slope of the obtained regression line, and the phase φ of the FSK modulated wave is estimated from the intercept of the regression line, and the digital direct synthesizer (DDS) is used by using the estimation result. To control 32. The DDS 32 oscillates at the frequency and phase according to the estimation result obtained by the regression line calculation circuit 28, that is, the correction value, and supplies the oscillation output to the quadrature detection circuit 10 as a local oscillation signal.

Here, the frequency and phase of the carrier can be estimated by statistical processing called regression line calculation based on the phase θ, that the average value of the noise component superposed on the FSK modulated wave is 0. Can be regarded as. Since this relationship holds even when the C / N is low, the frequency of the carrier of the FSK modulated wave and the local oscillation signal are controlled by controlling the oscillation frequency and phase of the DDS 32 using the information obtained by the estimation. Even if there is a relatively large difference from the frequency of the FSK modulated wave, the oscillation frequency of the local oscillation signal can be synchronized with the frequency of the carrier wave of the FSK modulated wave, and the phase φ As for the above, it becomes possible to execute the correction under the low C / N. In particular, the configuration shown in this figure is MSK, GMSK
And the like, which is significant in the FSK modulation method in which the modulation degree is relatively low.

FIG. 2 shows a second embodiment of the present invention. In this embodiment, the demodulation range expansion circuit 3 is provided between the arctangent calculation circuit 24 and the differentiation circuit 26.
4 is provided, and instead of the determination circuit 30, L
A PF 36 is provided. The demodulation range expansion circuit 34
When the phase θ obtained by the arc tangent calculation circuit 24 exceeds the range of −π / 2 <θ <π / 2, the range of the phase θ is converted so that, for example, the value of the phase θ changes from + π / 2 to −. It prevents the situation of jumping to π / 2. Therefore, even when the FSK modulation method having a relatively large modulation degree is adopted, the discontinuous change of the phase θ is suppressed, and as a result, the demodulatable range of the phase θ is expanded. Further, the LPF 36 removes unnecessary waves from the output of the differentiating circuit 26. As a result of adopting such a configuration, high-quality demodulation with few unnecessary waves becomes possible.

Further, in any of these embodiments, it is possible to easily realize downsizing and no adjustment of the device. That is, since the circuit configurations after the A / D converter circuit 22 are circuits that execute digital processing, there is less need to make adjustments in use like circuits that handle analog signals. That leads to cost reduction of the device.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an FSK receiver according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of an FSK receiver according to a second embodiment of the present invention.

FIG. 3 is an FSK receiver according to a conventional example, especially its AF
It is a block diagram showing a configuration of a C circuit.

[Explanation of symbols]

10 Quadrature detection circuit, 24 Arctangent calculation circuit, 26 Differentiation circuit, 28 Regression straight line calculation circuit, 30
Judgment circuit, 32 DDS (digital direct synthesizer), 34 demodulation range expansion circuit, 36 LPF.

Claims (4)

[Claims] 1. Regarding an FSK modulated wave converted into a quadrature signal by using a local oscillation signal, a phase θ of the quadrature signal is
Based on the in-phase component I and the quadrature component Q in the quadrature signal, the equation θ =
tan ⁻¹ (Q / I), a means for estimating the carrier frequency f and the phase φ of the FSK modulated wave by obtaining a regression line f × t + φ of the phase θ with respect to the time t, and a local oscillation according to the estimation result. A digital AFC circuit comprising: means for controlling the frequency and phase of a signal. 2. The digital AFC receiver according to claim 1, wherein the FSK receiver includes a local oscillator that generates a local oscillation signal, and a quadrature detection circuit that converts the FSK modulated wave into a quadrature signal using the local oscillation signal. An FSK receiver comprising: a circuit; a means for detecting a frequency deviation related to FSK modulation by differentiating the phase θ; and a means for reproducing data based on the detected frequency deviation. 3. The FSK receiver according to claim 2, wherein the FSK modulated wave is a wave modulated according to an FSK modulation method having a low frequency modulation degree. 4. The FSK receiver according to claim 2, wherein when the obtained phase θ exceeds the range of −π / 2 <θ <π / 2, the value of the phase θ is − prior to the differentiation. π /
In order to always change continuously within the range of 2 <θ <π / 2,
An FSK receiver comprising means for converting the range of the phase θ, wherein the FSK modulated wave is a wave modulated according to an FSK modulation method having a high frequency modulation degree.