JP2540982B2 - Digital demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is used in a digital demodulation device used in a microwave digital communication system. In particular, the present invention relates to a digital demodulation device that handles a low code transmission rate and that includes a transversal equalizer of intermediate frequency signal processing type.

[Outline] The present invention provides a fixed oscillator for generating a fixed oscillation signal in a digital demodulation device, and supplies the fixed oscillation signal as a reference signal to a quadrature phase detector for quadrature phase detection to obtain an output signal of a multilevel discriminator. The phase difference signal is obtained based on the phase difference signal, and a transversal equalizer is provided by converting the digital modulator input by the output signal of the voltage controlled oscillator that generates the signal of the controlled oscillation frequency using the phase difference signal as the control voltage. This makes the transversal equalizer more feasible, economical, and has broad pull-in characteristics.

[Conventional technology]

FIG. 2 is a block diagram of a conventional digital demodulator. FIG. 3 is a block diagram of an intermediate frequency signal processing type transversal equalizer.

Conventionally, various microwave digital communication systems have been put to practical use in digital demodulators, and one of them is a conversion system in which the frequency of a microwave is once converted to an intermediate frequency band and modulation / demodulation processing is performed.

In addition, a transversal equalizer is often installed on the receiving side in order to compensate for waveform distortion due to fading characteristic of microwave transmission. Although there are various means for realizing the transversal equalizer, those that process the intermediate frequency signal are in practical use because of the advantage that the circuit configuration is simple.
The intermediate frequency signal processing type transversal equalizer has the advantage that the number of weighting circuits is half that of the baseband signal processing type, but supplies signals to the weighting circuits of each tap. It is difficult to realize a delay circuit of T space (T is the reciprocal of the code transmission rate).

That is, the characteristic required for the delay circuit is that it has a delay amount of the delay of time T, and at the intermediate frequency f ₁ ± 1 / T,
Group delay and amplitude should be flat.

As can be seen from this, when the intermediate frequency f ₁ is high and the code transmission rate is low, it becomes difficult to realize the delay circuit. For example, the intermediate frequency is 70MHz and the code transmission rate is
In the case of 1.5MB, it becomes very severe.

FIG. 2 shows a digital demodulation device equipped with an intermediate processing type transversal equalizer for inputting an input signal having a code transmission rate of several MB, a modulation system of four-phase phase modulation system and an intermediate frequency of 70 MHz.

The input signal enters the transversal equalizer 3 where the intersymbol interference generated in the signal due to fading or the like is compensated. The compensated signal enters the quadrature phase detector 4 composed of the phase detectors 5 and 6 and the π / 2 transfer unit 7, and is quadrature phase detected by the reference signal from the voltage controlled oscillator 8A to become demodulated signals P and Q.

The demodulated signals P and Q are input to 2-bit multi-level discriminators 9 and 10, respectively, where they are multi-level discriminated and output 2-bit data signals of MSB and LSB. Of these, the MSB signal becomes the main signals CH1 and CH2 and becomes the output signal of this device.

On the other hand, an exclusive OR circuit 1 that performs an exclusive OR of the LSB signal output from the multilevel discriminator 9 and the MSB signal output from the multilevel discriminator 10.
The output of 1 and the MSB signal of the output of the multilevel discriminator 9 and the multilevel discriminator
A subtractor 13 subtracts the output of the exclusive OR circuit 12 that performs an exclusive OR with the LSB signal of the output of 10 by a subtracter 13 to obtain a phase error signal for controlling the voltage controlled oscillator 8A. A detailed explanation of the operation is given in "JP-A-57-131151, Carrier wave reproducing circuit".

FIG. 3 is a specific example of the transversal equalizer 3,
14 to 16 are T-space delay circuits, 17 to 22 are weighting circuits, 2
3 and 24 are synthesizing circuits, and 25 is a 90-degree synthesizing device. After the input signal is delayed by T spaces by the T space delay circuits 14 to 16, each output is weighted by a real axis weighting circuit 17,
It is inputted to a pair of 19, 21 and a weighting circuit 18, 20, 22 for the imaginary axis. Coefficients C _{R1 to} C _R3 and C _{M1 to} C _M3 necessary for compensating for intersymbol interference are given to the respective weighting circuits 17 to 22, and the respective outputs are combined with the real axes in the combining circuits 23 and 24. These outputs are combined into an imaginary axis, and those outputs are finally combined by the 90-degree combining circuit 25 to form one output.

Here, the characteristics required for the T-space delay circuits 14 to 16 are determined by the type of the input signal. Now, as described above, the input signal has an intermediate frequency of 70 MHz and a code transmission rate of 1.5 MB.
Then, the required characteristics are flat with a delay time of 167 μs and group delay time characteristics and amplitude characteristics of 70 MHz ± 2 MHz.

[Problems to be Solved by the Invention]

However, in such a conventional digital demodulation device, when an attempt is made to realize a delay circuit having a high pass band frequency and a long delay time, it is necessary to devise a circuit configuration and the circuit scale becomes large and expensive. There was a drawback that became.

One method of eliminating such drawbacks is to adopt a double conversion method. This is a method in which an input signal is converted into a second intermediate frequency by a frequency converter, a transversal equalizer is operated in the second intermediate frequency band, and then a demodulator which operates at the second intermediate frequency is combined.

For example, if 10 MHz is selected as the second intermediate frequency, the group delay time characteristic and the amplitude characteristic out of the characteristics required for the above-mentioned delay circuit can be flat at 10 MHz ± 2 MHz.
The implementation is very simple, the transversal equalizer is very feasible and economical.

However, according to this method, the frequency of the voltage-controlled oscillator 8 used in the demodulator becomes as low as 10 MHz, and the range in which the frequency can be changed cannot be set much larger than that of the conventional 70 MHz band. There is a drawback that it is difficult to obtain a wide pull-in range that should be provided as a demodulator for repair.

The present invention solves the above-mentioned drawbacks, and an object of the present invention is to provide a digital demodulation device that increases the feasibility of a transversal equalizer, is economical, and has a wide pull-in characteristic.

[Means for solving the problem]

The present invention relates to a frequency conversion circuit that outputs a signal of a second intermediate frequency lower than the input frequency using an input digital modulated signal as an output signal of a voltage controlled oscillator as a local oscillation frequency, and a code generated in the signal of the second intermediate frequency. An intermediate frequency signal processing type transversal equalizer that compensates for interference between signals,
An oscillator for generating a reference signal having a fixed frequency in the second intermediate frequency band, a quadrature phase detector for quadrature phase detecting an output signal of the transversal equalizer based on the reference signal, and outputting a demodulation signal; Multi-level discriminator that multi-level discriminates the output signal of the quadrature detector and obtains the main signal from the multi-level discriminated digital signal, and the output of the multi-level discriminator is subjected to exclusive OR operation to control the voltage. And a control signal generating means for generating a control signal for the oscillator.

Further, according to the present invention, the input digital modulation signal is
It is a four-phase phase modulated wave signal in the 70 MHz band, and the fixed oscillator may include means for generating a fixed oscillation signal of 10 MHz.

[Action]

The fixed oscillator generates a fixed oscillation signal and supplies it as a reference signal to the quadrature phase detector. Based on this reference signal, the output signal of the transversal equalizer is quadrature detected by the quadrature detector, and the output demodulated signal is multivalued by the multivalued discriminator. The subtractor obtains the phase difference signal based on the output signal of the multilevel discriminator. The voltage-controlled oscillator generates a signal of a controlled local oscillation frequency by using the output signal of the subtractor as a control voltage and gives it to the frequency conversion circuit. The frequency conversion circuit converts the input digital modulation signal into a signal of the second intermediate frequency by using the output frequency of the voltage controlled oscillator as the local oscillation frequency and gives it to the transversal equalizer.

The digital modulation signal is a 4-phase phase modulation wave signal in the 70 MHz band, and the fixed oscillator generates a fixed oscillation signal of 10 MHz and supplies it to the quadrature phase detector.

As a result of the above, it is possible to increase the feasibility of the transversal equalizer, improve the economy, and obtain a wide pull-in characteristic.

〔Example〕

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a digital demodulator according to an embodiment of the present invention. In FIG. 1, the digital demodulator is
An intermediate frequency signal processing type transversal equalizer 3 for compensating for intersymbol interference generated in the signal of the second intermediate frequency of the input digital modulation signal, an oscillator for generating a reference signal, and a transformer based on this reference signal. Versal equalizer 3
Output signal of quadrature phase is detected to output demodulated signals P and Q, and the output signal of quadrature phase detector 4 is multivalued, and the main signal is selected from the multivalued digital signals. And multi-level discriminators 9 and 10 for obtaining CH1 and CH2.

In addition, the quadrature phase detector 4 includes the phase detectors 5 and 6 and π
Includes a / 2 phaser 7.

A feature of the present invention is that the oscillator is a fixed oscillator 8 for generating a fixed oscillation signal, and a subtracter 13 for obtaining a phase difference signal based on the output signals of the multilevel discriminators 9 and 10 and a subtractor 13 A voltage controlled oscillator 2 for generating a signal having an oscillation frequency controlled by using the output signal of the controller 13 as a control voltage, and a signal of a second intermediate frequency for the input digital modulation signal with the output frequency of the voltage controlled oscillator 2 as a local oscillation frequency. And a frequency conversion circuit 1 which converts the signal into a signal and supplies it to the transversal equalizer 3.

The input digital modulation signal is a 70 MHz band four-phase phase modulated wave signal, and the fixed oscillator 8 includes means for generating a fixed oscillation signal of 10 MHz.

The operation of the digital demodulator having such a configuration will be described. In FIG. 1, the frequency conversion circuit 1 converts the input signal in the 70 MHz band into the second intermediate frequency in the 10 MHz band by the local oscillation signal from the voltage controlled oscillator 2 whose control voltage is controlled so that the output frequency is in the 60 MHz band. Convert to.

The transversal equalizer 3 receives the output signal of the frequency conversion circuit 1 and compensates for intersymbol interference generated in the signal by fading or the like. The quadrature phase detector 4, which is composed of the phase detectors 5 and 6 and the π / 2 phase detector 7, quadrature phase detects the compensated signal by the reference signal of 10 MHz from the fixed oscillator 8 and outputs demodulated signals P and Q. Output.

The 2-bit multi-level discriminators 9 and 10 receive the demodulated signals P and Q.
Is input, multi-level identification is performed, and a 2-bit data signal of MSB and LSB is output. The MSB signals are output as main signals CH1 and CH2.

The exclusive OR circuit 11 outputs the LSB output from the multilevel discriminator 9.
The exclusive OR of the signal and the MSB signal output from the multilevel discriminator 10 is calculated. In addition, the exclusive OR circuit 12 is a multi-value classifier 9
The exclusive OR of the MSB signal output by the multilevel discriminator 10 and the LSB signal output by the multilevel discriminator 10 is calculated.

The subtractor 13 outputs the output signal of the multilevel discriminator 9 and the multilevel discriminator 10.
The phase error signal is output as a control voltage for controlling the output frequency of the voltage controlled oscillator 2 by obtaining the subtraction value from the output signal of the.

Here, since the reference signal is a fixed oscillation signal of 10 MHz, the phase of the output signal of the transversal equalizer 3 in the 10 MHz band is controlled so as to be synchronized with the phase of the output signal of the fixed oscillator 8.

As described above, in the present embodiment, the converted second intermediate frequency signal in the 10 MHz band is input, so that the transversal equalizer 3 may operate in the 10 MHz band and is used in the transversal equalizer 3. The pass characteristics required for the T-space delay circuits 14 to 16 to be satisfied may be 10 MHz ± 2 MHz.
Therefore, the feasibility of the transversal equalizer 3 is increased and the economical efficiency is improved.

Also, since the voltage controlled oscillator 2 having a frequency of 60 MHz which has a great influence on the pull-in characteristic of the digital demodulator can be applied, a wide pull-in characteristic can be obtained.

In the present embodiment, the voltage controlled oscillator 2 used is of the 60 MHz band, but an 80 MHz band can also be used. At this time, it is necessary to invert the polarity of the output phase difference signal of the subtractor 13.

Further, although the 4PSK wave (four-phase modulation wave) has been described as the input signal, the input signal is not limited to this and can be applied to any digital modulation wave. In that case, basically, it is possible to deal with the problem by simply increasing the number of bits of the multilevel discriminators 9 and 10.

〔The invention's effect〕

As described above, the present invention has an excellent effect that the feasibility of a transversal equalizer is increased, the economy is good, and a wide pull-in characteristic can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of a digital demodulator according to an embodiment of the present invention. FIG. 2 is a block diagram of a conventional digital demodulator. FIG. 3 is a block diagram of an intermediate frequency signal processing type transversal equalizer. 1 ... Frequency conversion circuit, 2, 8A ... Voltage controlled oscillator, 3
...... Transversal equalizer, 4 quadrature detector,
5, 6 ... Phase detector, 7 ... π / 2 phase detector, 8 ... Fixed oscillator, 9, 10 ... Multilevel discriminator, 11, 12 ... Exclusive OR circuit, 13 ... Subtractor , 14 to 16 …… T space delay circuit, 17 to 22 …… Weighting circuit, 23,24 …… Synthesis circuit, 25…
… 90 degree synthesis circuit.

Claims (2)

(57) [Claims] 1. A frequency conversion circuit for outputting an input digital modulated signal as a signal of a second intermediate frequency lower than the input frequency with an output signal of a voltage controlled oscillator as a local oscillation frequency, and a frequency conversion circuit generated within the signal of the second intermediate frequency. An intermediate frequency signal processing type transversal equalizer for compensating for intersymbol interference, an oscillator for generating a reference signal of a fixed frequency in the second intermediate frequency band, and an output of the transversal equalizer based on the reference signal. A quadrature phase detector for detecting a quadrature phase of a signal and outputting a demodulated signal, and a multilevel discriminator for discriminating the output signal of the quadrature phase detector from the multilevel and obtaining a main signal from the multilevel discriminated digital signals. And a control signal generating means for performing an exclusive OR operation on the outputs of the multilevel discriminator to generate a control signal of the voltage controlled oscillator. Location. 2. The input digital modulation signal is 70 MHz.
This is a 4-phase phase-modulated wave signal in the band, and the fixed oscillator is 10 MHz.
2. A digital demodulator as claimed in claim 1, including means for generating a fixed oscillation signal of z.