JPH07115445A - Automatic frequency controller - Google Patents

Abstract

PURPOSE:To extend the compensation range of ARC by using the band limiting filter, which has an asymmetrical intra-band phase delay characteristic, to deform the distribution characteristic of the reception phase error of a digital phase modulation signal. CONSTITUTION:A reception signal is inputted to a band limiting filter 2 through an automatic gain controlled amplifier 1. The band limiting filter 2 has the asymmetrical intra-band phase delay characteristic. A demodulator 3 detects the signal phase from the reception signal which has the band limited by the band limiting filter 2. A phase discriminator 4 discriminates the level of the signal from the demodulator 3. The phase error between the input signal and the output signal of the phase discriminator 4 is outputted, and it is inputted to an integrator 5, and the output of the integrator 5 is limited by an integrator output limiter 7.

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 device, and more particularly to an automatic frequency control device used in a phase modulation type data communication device such as a modem or a digital car telephone.

[0002]

2. Description of the Related Art A conventional automatic frequency control device for a data communication device using digital phase modulation (Automatic Frequency Control)
y Control: AFC) detects a phase error of a demodulation phase, estimates a carrier frequency offset (hereinafter, simply referred to as a frequency offset) corresponding to the phase error, and further averages the estimated value to standardize the data communication device. It was controlling the frequency. As an example, in the case of QPSK which is one of the digital phase modulation systems, the relationship between the phase error δφ and the frequency offset δf is expressed as δφ = (2π · δf / b) mod (π / 4) (1). Where b is the modulation speed (symbol frequency)
Is. Here, in the range of | 2πδf / b |> π / 4, δ
φ causes uncertainty. Therefore, the maximum frequency offset δF that can be compensated by the conventional AFC is | δφ
When | = π / 4, δF = | b / 8 | (2)

FIG. 4 shows QPSK (Quadrature Phase Shift Keyin) which is one of the digital phase modulation systems for AFC.
g: four-phase phase shift keying) applied to a data communication device, in which 11 is an automatic gain control amplifier,
Reference numeral 12 is a band limiting filter having a flat phase delay characteristic within the band, 13 is a QPSK demodulator, 14 is a phase discriminator (level determiner), 15 is an integrator, and 16 is a reference oscillator.
The difference between the input and output of the phase discriminator 14 is input to the integrator 15 as a phase error and fed back to the reference oscillator 16. FIG. 5 shows the phase error characteristic caused by the frequency offset. Even in the conventional example, it is −π / in the phase error region.
A frequency offset corresponding to 4 to π / 4 can be compensated.

[0004]

However, in the conventional AFC, as shown by the equations (1) and (2), δf and δφ are in a proportional relationship, and | 2πδf / b |> π / 4
In the range of, since δφ causes uncertainty, the correct frequency offset cannot be estimated. That is, the AFC operates correctly only in the range from -π / 4 to π / 4 in FIG.
At the point x, since the wrong phase error φf is used instead of the correct phase error φt, the AFC malfunctions and has a drawback that it locks at the false frequency f ′ instead of the correct frequency f.

The present invention has been made in view of the above circumstances, and provides an automatic frequency control device capable of easily expanding the AFC operating range without changing the basic configuration of the conventional AFC. The purpose is to do.

[0006]

In order to achieve the above object, the present invention provides a band limiting filter having asymmetry in in-band phase delay characteristics in a data communication device using digital phase modulation, and the band limiting filter. A demodulator that detects the signal phase from the received signal whose band is limited by the limiting filter,
A phase discriminator for judging the level of the signal from the demodulator,
An integrator for integrating the phase error output by the phase discriminator,
And an integrator output limiter that limits the output of the integrator, and is characterized in that the compensation range of the carrier frequency offset is expanded by expanding the identification range of the demodulation phase error distribution.

[0007]

According to the present invention, the band limiting filter having an asymmetry in the phase delay characteristic within the band has a characteristic of rotating the symbol phase in the presence of a frequency offset in the direction of the symbol identification point. As a function, the rotation angle of the phase with respect to a certain frequency offset becomes smaller than that in the conventional method. As a result, the equation (1) is modified as δφ ′ = (2π · α (f) · δf / b) mod (π / 4) (3). Here, α (f) is a function with respect to the frequency indicating the asymmetry of the filter, and is a positive number α (f) <1 in the operating range of the AFC. Therefore, the maximum value δ of the frequency offset that can be compensated by the present invention
F'becomes δF '= | b / (8α (f)) |> δF (4). Therefore, the absolute value of the phase error is compressed only by providing the characteristic of the band limiting filter in the RF band or the IF band with an appropriate asymmetry without significantly changing the configuration of the conventional AFC, and as a result, the AFC can be easily performed. The frequency offset compensation range can be expanded.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings. FIG. 1 is a configuration diagram for explaining an embodiment of an automatic frequency limiting device according to the present invention, and is a configuration diagram showing an embodiment applied to a QPSK modulation digital data communication device. In the figure, 1 is an automatic gain control amplifier, 2 is a band limiting filter having an asymmetric phase delay characteristic in the band, and 3 is a QP.
SK demodulator, 4 is a phase discriminator (level determiner), 5 is an integrator, 6 is a reference oscillator, and 7 is an integrator output limiter.

The received signal is input to the band limiting filter 2 via the automatic gain control amplifier 1. The band limiting filter 2 has asymmetry in the phase delay characteristic within the band. The demodulator 3 detects the signal phase from the received signal band-limited by the band-limiting filter 2. The phase discriminator 4 determines the level of the signal from the demodulator 3. The phase error between the input signal and the output signal of the phase discriminator 4 is output and input to the integrator 5, and the integrator output limiter 7 limits the output of the integrator 5.

Here, if the one as shown in FIG. 2 is used as the in-band phase delay characteristic of the band limiting filter of 2,
The phase error output characteristic can be expanded as shown in FIG. Now, even if the frequency offset gives an x point in phase space that would give a false lock in a conventional AFC, the AFC will lock to the correct frequency f. The integrator output limiter 7 is a device for removing instability in the cutoff frequency region of the band limiting filter, and resets when the value of the integrator largely deviates from the design value.
Here, the output limit value of the integrator output limiter 7 is the desired A
It is a value appropriately selected from the operating range of FC and the form of error phase characteristics.

In this way, the phase-modulated received signal is
The phase delay characteristic in the band has a large delay in the region where the frequency is higher than the center frequency, and a small delay in the region where the frequency is lower. Achieve an extended operating range without changing the AFC configuration.

[0012]

As is apparent from the above description, the present invention operates correctly without making a great change in the AFC configuration itself even in a region where the frequency offset is large and the conventional AFC malfunctions. Broadband AF
C can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of an automatic frequency limiting device according to the present invention.

FIG. 2 is a diagram showing an example of phase delay characteristics of a band limiting filter according to the present invention.

FIG. 3 is a diagram showing a graph of the relationship between frequency offset and phase error in the present invention.

FIG. 4 is a configuration diagram of an AFC according to a conventional example.

FIG. 5 is a diagram showing a graph of a relationship between a frequency offset and a phase error in a conventional example.

[Explanation of symbols]

1 ... Automatic gain control amplifier, 2 ... Band limiting filter, 3 ...
Demodulator, 4 ... Phase discriminator (level determiner), 5 ... Integrator, 6 ... Reference oscillator, 7 ... Integrator output limiter.

Claims (1)

[Claims] 1. In a data communication apparatus using digital phase modulation, a band limiting filter having an asymmetry in a phase delay characteristic within a band, and a demodulation for detecting a signal phase from a reception signal band limited by the band limiting filter. A vessel,
A phase discriminator for judging the level of the signal from the demodulator,
An integrator for integrating the phase error output by the phase discriminator,
An automatic frequency control device comprising an integrator output limiter for limiting the output of the integrator, wherein the carrier frequency offset compensation range is expanded by expanding the identification range of the demodulation phase error distribution.