JPH08102770A - Carrier wave recovery system - Google Patents

Abstract

PURPOSE: To provide a carrier wave recovery system used in a multi-value MODEM for digital signal transmission which is excellent even when frequency fluctuation generated at the time of frequency conversion is present. CONSTITUTION: Reference signals generated from an oscillator 13 are synthesized with modulation waves on a modulation side, the reference signals are extracted by a band-pass filter 15 on a demodulation side and signals synchronzied with the reference signals are generated in a PLL circuit composed of a phase comparator 16, a low-pass filter 17 and a voltage controlled oscillator 18. The band of the low-pass filter 17 is sufficiently narrowed against the frequency fluctuation, the output of the phase comparator 16 is added to the control voltage of the voltage controlled oscillator 17 for generating the recovery carrier waves of a demodulator and carrier wave recovery following up the frequency fluctuation is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a carrier recovery system,
In particular, it relates to a carrier recovery system used in a multilevel modulator / demodulator for digital signal transmission.

[0002]

2. Description of the Related Art Optical SCM is one example of digital signal transmission.
FIG. 2 shows an example of a conventional carrier recovery system used in a digital signal transmission system.

In FIG. 2, the intermediate frequency signal which is multi-valued modulated by the modulator 1 is input to the up converter 2 and frequency-converted into a required transmission frequency. The frequency-converted modulated wave is converted into an optical modulated wave by the electric / optical converter 3,
It is sent to the optical transmission line 100. The optical modulated wave that has passed through the optical transmission path 100 is converted into an electric signal by the optical / electrical converter 4 and input to the down converter 5.

The modulated wave converted into the intermediate frequency signal again by the down converter 5 passes through the band pass filter 6 for extracting the modulated wave, and then is multiplied by the regenerated carrier wave output from the voltage controlled oscillator 7 by the multiplier 8. To be done. And
The output signal of the multiplier 8 is a demodulated baseband signal after unnecessary harmonics are removed by the low-pass filter 9.
Thereafter, the analog-digital converter 10 converts the sampled and quantized digital signal. From this digital signal, the phase error detector 11 detects the phase error between the carrier wave and the reproduced carrier wave, and outputs a control signal according to the error. The control signal is smoothed by the low pass filter 12 and becomes a carrier wave phase control signal of the voltage controlled oscillator 7.

The carrier recovery method described above uses a phase locked loop (PLL), and is disclosed in, for example, Japanese Patent Laid-Open No.
It is also described in JP-A-205939.

[0006]

However, the above-described conventional modulation / demodulation system has a problem that it becomes difficult to reproduce the carrier when the frequency fluctuation (phase jitter) generated during frequency conversion in the up converter and the down converter is large. It was

The object of the present invention is to follow a large frequency fluctuation occurring during frequency conversion and signal transmission,
An object of the present invention is to provide a carrier wave reproduction method for performing excellent carrier wave reproduction.

[0008]

A carrier recovery system according to the present invention is an carrier generation system used in a multilevel modulator / demodulator for digital signal transmission, and an oscillator for generating a reference signal to be added to a modulated wave at a modulating side. A band-pass filter for extracting the reference signal on the demodulation side, a first voltage-controlled oscillator for generating a reproduction reference signal, an output of the band-pass filter and an output of the first voltage-controlled oscillator as inputs A phase comparator that outputs a phase difference signal between the two, a low-pass filter that outputs the control signal of the first voltage controlled oscillator using the output of the phase comparator as an input, and a second carrier that generates a regenerated carrier wave. Of the voltage controlled oscillator, the output of the phase comparator and the carrier phase control signal of the carrier recovery circuit of the demodulator, and an adder for outputting the output as the control signal of the second voltage controlled oscillator. To.

[0009]

In the carrier recovery method according to the present invention, in the multi-level modulator / demodulator used in digital signal transmission, the reference signal sent together with the modulation signal on the modulation side receives the phase comparator, the low-pass filter, and the voltage on the demodulation side. P consisting of controlled oscillator 1
It is detected by the LL circuit and the frequency fluctuation component is extracted. By synthesizing the frequency fluctuation information with the control voltage of the reproduced carrier oscillator of the demodulator, the reproduced carrier follows the frequency fluctuation common to the modulated wave and the reference signal to provide good carrier reproduction.

[0010]

Next, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a block diagram showing an embodiment of a carrier recovery system according to the present invention. In this figure, components 1 to 12 are the same as those shown in FIG. That is, 1 is a modulator, 2 is an up converter, 3 is an electric / optical converter, 4 is an optical / electrical converter, 5 is a down converter, 6 is a band pass filter,
Reference numeral 7 is a voltage controlled oscillator, 8 is a multiplier, 9 is a low-pass filter, 10 is an analog-digital converter, 11 is a phase error detector, and 12 is a low-pass filter. The difference between this embodiment and the above-mentioned carrier recovery system of the prior art is that the oscillator 13,
A combiner 14, a bandpass filter 15, a phase comparator 16, a low-pass filter 17, a voltage controlled oscillator 18, and an adder 19 are newly provided. Then, the reference signal (pilot signal) consisting of the unmodulated single frequency f _R (Hz) output from the oscillator 13 is combined by the combiner 14 with the modulated wave output (carrier frequency f _C (Hz)) of the modulator 1. It This combined signal is sent to the optical / transmission line 100 through the up converter 2 and the electric / optical converter 3. Optical transmission line 100
The optical signal that has passed through is converted into an electrical signal by the optical / electrical converter 4, and then frequency-converted by the down converter 5 into a modulated wave and a reference signal component by band pass filters 6 and 15 having different frequency bandwidths. To be separated. The reference signal separated by passing through the band pass filter 15 is input to the phase comparator 16 and compared in phase with the output of the voltage controlled oscillator 18. The output of the phase comparator 16 is smoothed by the low pass filter 17 and becomes a control signal of the voltage controlled oscillator 18. At the same time, the phase comparator 16
Is added to the output signal of the low pass filter 12 by the adder 19 and output as the control voltage of the voltage controlled oscillator 7.

According to the configuration of FIG. 1, the modulator 1 and the oscillator 1
Since the frequency fluctuation of 3 itself is a low frequency, it is almost negligible. On the other hand, the combined signal of the modulator 1 and the oscillator 13 is subjected to frequency fluctuation by passing through the up converter 2, the electric / optical converter 3, the optical transmission line 100, the optical / electrical converter 4, and the down converter 5. Becomes This frequency fluctuation has a large value because it is a high frequency band signal. It should be noted that this frequency fluctuation causes the reference signal and the modulated wave to undergo the same frequency fluctuation.
FIG. 2 is a diagram showing these frequency fluctuations.
In the figure, (A) is a diagram showing a combined signal of a reference signal of frequency f _R and a modulation signal of center frequency f _C.

(B) shows the output signal of the up-converter 2. In the present embodiment, the reference signal and the modulated signal are frequency-converted by the transmission station-generated frequency f _L0 of the up-converter 2, and the frequency f _R + f respectively. It has been converted to _L0 , f _C + f _L0 . Now, assuming that the frequency fluctuation of the up-converter 2 is ± Δf _U , the reference signal and the modulation signal are f _R + f
The frequency changes _L0 ± Δf _U and f _C + f _L0 ± Δf _U.

(C) shows the output signal of the down converter 5. When the frequency of the receiving station of the down converter 5 is f _{L0, which} is the same frequency as the frequency of the transmitting station f _L0 , the reference is used when there is no frequency fluctuation. The signal and the modulated signal are frequency-converted to obtain the frequencies f _R and f _C , respectively. When the up-converter 2 receives a frequency fluctuation ± Δf _U , the reference signal frequency f _R is f _R + Δf _U + Δf _D or f _R −Δ.
f _U −Δf _D , the output frequency f _C of the modulator is f _C + Δf _U
+ Δf _D or f _R −Δf _U + Δf _D. That is, the frequency fluctuation Δf _U due to the up converter 2 and the frequency fluctuation Δf _D due to the down converter 5 are commonly added to the reference signal and the modulated wave, and the frequency fluctuation amount on the demodulation side is both Δf _U
It becomes + Δf _D. Here, on the demodulation side, the reference signal is separated from the modulated wave by the bandpass filter 15, and the phase comparator 1
PL consisting of 6, low-pass filter 17 and voltage controlled oscillator 18
It serves as a reference signal for the L circuit. When this PLL circuit is synchronized with the reference signal, and the band of the low-pass filter 17 is sufficiently narrower than the frequency fluctuation component, the output of the phase comparator 16 has a voltage corresponding to the frequency fluctuation of the reference signal. appear.

This voltage is passed through the phase shifter 22 to the adder 19
To enter. The phase shifter 22 is configured so that the outputs of the low-pass filter 12 and the phase shifter 22 are added to the same phase when both the modulated wave PLL circuit and the reference signal PLL circuit are in phase synchronization and have no phase error. It is provided for adjustment. Then, the adder 19 adds the reproduced carrier wave of the demodulator to the control voltage of the voltage-controlled oscillator 7 to obtain a reproduced carrier wave that follows the frequency fluctuation of the modulated wave, and good demodulation is performed.

FIG. 3 shows a second carrier recovery system according to the present invention.
It is a block diagram showing an example of. In this embodiment, in addition to the first embodiment, a high pass filter 20 is inserted between the phase comparator 16 and the adder 19.

According to the configuration of FIG. 3, the phase jitter component due to the normal frequency fluctuation is often higher than the cutoff frequency of the high pass filter 20, and the low frequency component is followed by the carrier recovery loop of the demodulator. Therefore, by transmitting only the high frequency component, the same effect can be obtained without affecting the operating point of the carrier recovery loop of the demodulator.

For example, in the 64QAM system, 4.4 MBPS
In order to transmit the above signal, the cutoff frequency of the low-pass filter 12 of the carrier recovery loop of the demodulator is about 25 KHz. Therefore, the cutoff frequency of the high pass filter 20 is equal to the cutoff frequency of the low pass filter 12, or
It should be more than that.

FIG. 4 shows a third carrier recovery system according to the present invention.
It is a block diagram showing an example of. In this embodiment, in addition to the first embodiment, a low pass filter 21 is inserted between the adder 19 and the voltage controlled oscillator 7.

The cut-off frequency of the low-pass filter 21 may be selected to be higher than the cut-off frequency of the low-pass filter 12 serving as a loop filter. For example, in the above example, the low-pass filter is cut off. The cutoff frequency of the wave filter 21 may be sufficiently higher than the cutoff frequency of the low-pass filter 12, and is 50 KHz in this embodiment.

As a result, when the output of the phase comparator 16 contains a harmonic component such as a digital waveform, it has an effect of removing an unnecessary harmonic component, and also operates as a part of the loop filter, so that the frequency fluctuation occurs. There is also the effect that the following speed can be adjusted.

Of course, it is possible to simultaneously construct the second and third embodiments. Further, in the above embodiment, the carrier recovery loop obtains the control signal from the digital signal after analog-digital conversion, but the present invention is not limited to this, and is a demodulator which obtains the reproduced carrier from the control signal. It is clear that if there is any, it can be applied regardless of the control method and the modulation method.

Further, although the transmission line is constituted by the optical transmission line in the above-mentioned embodiments, the present invention is not limited to this, and the same effect as that of the present embodiment can be obtained by a radio transmission line, for example.

[0023]

As described above, according to the present invention, even if the frequency fluctuation occurs in the path from the up converter to the down converter, it is possible to follow the fluctuation and perform a good demodulation operation.

[Brief description of drawings]

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

FIG. 2 is a diagram showing frequency fluctuations during frequency conversion.

FIG. 3 is a block diagram of a second embodiment of the present invention.

FIG. 4 is a block diagram of a third embodiment of the present invention.

FIG. 5 is a block diagram of a conventional demodulator.

[Explanation of symbols]

 1 modulator 2 up converter 3 electric / optical converter 4 optical / electrical converter 5 down converter 6 band pass filter 7 voltage controlled oscillator 8 multiplier 9 low-pass filter 10 analog-digital converter 11 phase error detection Device 12 Low-pass filter 13 Oscillator 14 Combiner 15 Band-pass filter 16 Phase comparator 17 Low-pass filter 18 Voltage-controlled oscillator 19 Adder 20 High-pass filter 21 Low-pass filter 22 Phaser 100 Optical transmission line

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location 9297-5K H04L 27/00 H

Claims (6)

[Claims] 1. A carrier wave reproducing system for reproducing a carrier wave of a multilevel modulation signal for digital signal transmission, wherein the modulation signal is converted to a high frequency by superimposing a non-modulation signal reference signal of a predetermined frequency on a modulation wave. Receiving the modulated signal, converting the frequency into a predetermined reception frequency, receiving the reception frequency, extracting the modulated wave with a first bandpass filter, and extracting the modulated wave with a second bandpass filter. A signal is extracted, the extracted modulated wave is input to a first phase-locked loop (PLL) circuit, and the extracted reference signal is extracted by a second phase-locked loop (PLL) circuit. And the second P
The output voltage of the phase error component of the LL circuit is set to the first PLL.
A carrier reproducing system characterized in that the carrier of the modulated wave is reproduced by being superposed on a circuit. 2. The multiplier circuit, wherein the first PLL circuit receives the output voltage of the first bandpass filter and multiplies the output voltage by the output voltage of the first voltage controlled oscillator; Low-pass filter that suppresses high-frequency components of the output voltage of the filter, an analog-digital converter that performs analog / digital conversion on the output of the first low-pass filter, and the analog-digital converter A phase error detector for detecting the phase error by comparing the phase of the output of the phase error with a predetermined frequency, a second low pass filter for band limiting the output of the phase error detector, and a second low band An adder for superimposing an output signal of the filter and a phase error signal of the second PLL, and an output signal of the adder for controlling an output frequency of the first voltage controlled oscillator. The carrier recovery method according to item 1. 3. The phase comparator for comparing the phases of the output voltage of the second bandpass filter and the output voltage of the second voltage controlled oscillator in the second PLL circuit, and the phase comparator of the phase comparator. A third low-pass filter that band-limits the output, an output frequency of the second voltage-controlled oscillator controlled by the output voltage of the third low-pass filter, and an output of the phase comparator The carrier recovery system according to claim 1, wherein a phase shifter for adjusting a phase and an output of the phase shifter are input to the adder. 4. The first PLL circuit is provided with a fourth low-pass filter having a cutoff frequency higher than a cutoff frequency of the second low-pass filter at the output of the adder. The carrier recovery system according to claim 2, wherein 5. The high frequency filter having a cutoff frequency equal to or higher than the cutoff frequency of the second low pass filter is provided at the output of the phase comparator in the second PLL circuit. The carrier recovery system according to claim 3, wherein the carrier recovery system is provided. 6. The phase shifter includes a second low-pass filter of the first PLL circuit when the first PLL circuit and the second PLL circuit are in phase synchronization and have no phase error. 4. The carrier recovery system according to claim 2, wherein the output signal of the wave filter and the output signal of the phase comparator of the second PLL circuit are adjusted in phase so as to have the same phase.