JPH0444442A - Synchronizing recovery circuit - Google Patents

Abstract

PURPOSE:To avoid an unbalanced operation of phase detection and to reduce a demodulation error by inserting a low pass filter to a VCO output terminal in a loop of a synchronization recovery circuit. CONSTITUTION:A low pass filter(LPF) 29 is inserted to an output terminal of a VCO 25. The LPF 29 leaves a fundamental wave of an output of the VCO 25 and eliminates secondary and succeeding harmonic wave components. Thus, even when an output waveform of the VCO 25 is a square wave or a distorted waveform, a wave close to a sinusoidal wave is obtained at the output of the LPF 29 and sinusoidal waves with nearly equal amplitude and different phase by pi/2 are obtained as subcarriers CW1, CW2 by selecting the gain of a pi/2 phase shifter 26 properly. Since the unbalance in the phase detection by lst and 2nd phase detectors 21,22 is small, the deterioration in the detection characteristic is suppressed and the demodulation error is reduced.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides, for example, 2n-phase
5hift Keying) [n=1.2.・=
] The present invention relates to a synchronous generation circuit for generating synchronous subcarriers used for signal demodulation in a receiver of a PCM communication system.

(Prior art) In a PSK-PCM communication system, a signal is represented by a phase component of a signal carrier wave, and in order to demodulate the signal, a phase detector and a signal demodulator having the same frequency as the signal carrier wave and a specific phase difference are used. subcarrier is required.

As a circuit using a synchronous reproducing circuit for reproducing such a subcarrier for signal demodulation, a signal demodulating circuit as shown in FIG. 5 is described in Japanese Patent Publication No. 2-8502. In this circuit, a 2n-phase PSK-PCM input signal is input to an input terminal 11, and is demodulated by a phase detector 12 into a code output proportional to the phase difference with the subcarrier.

The sign output is led to the output terminal 13 and the switching circuit 1
4 is also input. In the switching circuit 14, the information component of the code output is removed, and an output corresponding to the phase difference between the signal carrier wave and the signal demodulation subcarrier wave is reproduced.

This output is a low pass filter (hereinafter referred to as LPF)
Noise components are removed via the signal 15 and applied as a control signal to a control terminal of a voltage controlled oscillator (hereinafter referred to as CO) 16. As a result, V6O13 generates a stable subcarrier that is synchronized with the signal carrier (has the same frequency as the signal carrier and has a specific phase difference). Using this subcarrier, the phase detector 12 performs stable demodulation.

FIG. 6 shows the above-mentioned synchronization generation circuit, for example, in a 4-phase PSK-P
A circuit configuration when used for demodulating a CM signal is shown.

20 is an input terminal for the 4-phase -PSK-PCM signal, 21.22 is the first. The second phase detector, 23 is a switching circuit, 24 is a noise removal LPF, and 25 is a VCO 126 which is a π/2 phase shifter.

The oscillation output of VCO25, that is, the subcarrier, is 4-phase PSK-
It has approximately the same frequency as the carrier wave of the PCM signal, and its oscillation output is controlled by the output of the LPF 24. 1st
In the phase detector 21, the phase difference between the subcarrier CW1 from the VCO 25 and the input signal is detected, and the phase detection output is guided to the output terminal 27 as a code output. Furthermore, in the second' phase detector 22, the subcarrier C
The phase difference between the input signal and the subcarrier CW2 obtained by phase-shifting W1 by π/2 radians by the π/2 phase shifter 26 is detected, and the phase detection output is guided to the output terminal 28 as a sign output.

In the switching circuit 23, the first and second phase detectors 2
A phase error signal with a period of π/2 radians is formed based on the output of 1.22 radians, and this phase error signal is applied to the control terminal of the VCO 25 via the LPF 24 for removing noise to change the phase of the subcarrier. can be controlled. In this way, the first. A stable code output can be obtained from the second phase detectors 21 and 22.

In such a synchronous regeneration circuit in a signal demodulation circuit, a circuit using an integrator as shown in FIG. 7 or a circuit using a resistor and capacitor may be used as the π/2 phase shifter 26. Now, we will explain the process of generating the subcarrier CW2 from the output of the VCO 25 when the integrator shown in FIG. 7 is used. The integrator is an operational amplifier 41. resistance 4
2 and a capacitor 43.

The Shingin output from VC○25 is sent directly to the subcarrier CW.
1 and the input terminal 40 of the integrator.
is input. The output signal CW2 of this integrator is connected to the resistor 42.
Let the resistance value of the capacitor 43 be R, the capacitance value of the capacitor 43 be C1, and the angular frequency of the subcarrier CW1 be ω, then CW2-Ikamiichi-x c w + ・(1)
In equation (1), where CW2 = CW1, if the values of R and C are appropriately selected, then when a sine wave as shown in Fig. 8(a) is input to the integrator, Figure 8 (
Output signals with the same amplitude and a 90° phase difference as shown in b) can be obtained.

However, when a square wave as shown in FIG. 8(C) is input to the integrator, a triangular wave as shown in FIG. 8(d) is obtained, and the input and output signal waveforms are completely different.

In such a synchronous regeneration circuit for demodulating 4-phase PSK-PCM signals, it is difficult to oscillate a sine wave in the VCO 25, so
Since the output of the VCO 25 is not necessarily a sine wave but often a square wave or a distorted waveform, the waveforms of the subcarrier CW1 and the subcarrier CW2, which is the output of the π/2 phase shifter 26, are different. The disadvantages are that the operation of the phase detectors 21 and 22 becomes unbalanced, the detection characteristics deteriorate, and there are many demodulation errors. In addition, in the 4-phase PSK-PCM signal demodulation synchronization generation circuit, if the loop gain is low, the residual phase error increases as the transmitted carrier frequency and the free run frequency of VC025 differ, so VC○ Since the oscillation response of 25 deteriorates and demodulation errors increase as a result, there is a drawback that the free run frequency of the VCO must be adjusted.

(Problem to be Solved by the Invention) In the synchronous regeneration circuit for demodulating a 2n-phase PSK-PCM signal as described above, the VCO output is not necessarily a sine wave, but is often a square wave or a distorted waveform. , the waveforms of the subcarrier CW2 passed through the π/2 phase shifter and the subcarrier CW1 not passed are different, resulting in unbalanced operation in the first and second phase detectors, deteriorating detection characteristics, and increasing demodulation errors. The drawback was that there were too many. In addition, if the loop gain in the synchronization generation circuit is low, the residual phase error will increase as the transmitted carrier frequency and the free run frequency of the CO differ, so the free run frequency of the CO must be adjusted. There was a drawback that it was not possible.

The present invention has been made in view of the above-mentioned problems. By making the waveforms of the two subcarriers CW1 and CW2 input to the second phase detector into signals with the same amplitude and a 90° phase difference, deterioration of phase detection characteristics is suppressed, and 2n phase-PSK with less demodulation error -P
The object of the present invention is to provide a synchronization generation circuit for demodulating CM signals.

Another object of the present invention is to provide a synchronous regeneration circuit for demodulating a 2n-phase PSK-PCM signal with a high loop gain and a small residual phase error.

[Structure of the Invention] (Means for Solving the Problems) The first invention includes a voltage controlled oscillator, a plurality of phase detectors to which an input signal based on the PSK method is applied to one input terminal, and a plurality of 1. Means for inputting the oscillation output from the voltage controlled oscillator to the other input terminal of the phase detector, after passing the oscillation output from the voltage controlled oscillator through a low pass filter that passes the fundamental wave component, and setting the phases to be different from each other; , a switching circuit that receives the outputs of the plurality of phase detectors and detects a phase error between the respective input signals, and a filter that smoothes the output of the switching circuit and applies it to the control terminal of the voltage controlled oscillator. It is characterized by the following:

A second invention includes a voltage controlled oscillator, a plurality of phase detectors to which an input signal based on the PSK system is applied to one input terminal of each, and a voltage controlled oscillator, and a plurality of phase detectors each having the voltage applied to the other input terminal of the plurality of phase detectors. means for setting and inputting oscillation outputs from a controlled oscillator so that their phases are different from each other; a switching circuit to which the outputs of the plurality of phase detectors are input and detecting a phase error between the respective input signals; The present invention is characterized by comprising means for smoothing the output of the circuit with a filter, amplifying the same, and applying the amplified signal to the control terminal of the voltage controlled oscillator.

The third invention is a combination of the first invention and the second invention, in which the VCO output is supplied to a plurality of phase detectors through an LPF, and the switching circuit output is supplied to the CO through a high gain amplifier. 6 (Operation) In the first invention, the VC
By inserting a low-pass filter at the O output terminal, V
By removing unnecessary components other than the fundamental wave component of the CO output and outputting only a sine wave, the input and output signal amplitude and waveform of the phase shifter are approximately equal and only the phase differs. The amplitudes and waveforms of all subcarriers are made almost equal, and only the phases differ, thereby eliminating operational imbalance in phase detection and reducing demodulation errors.

In addition, in the second invention, by inserting a high gain amplifier at the output end of the switching circuit or by inserting a high gain amplifier at the output end of the noise removal filter, the loop gain is increased, residual phase error is eliminated, and demodulation error is reduced. I tried to reduce it.

Furthermore, in the third invention, by inserting a low-pass filter at the output end of the VCO and a high gain amplifier at the output end of the switching circuit, unbalance of operation in phase detection is eliminated, and residual phase error is eliminated. Demodulation errors can be further reduced.

(Example) Examples will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a synchronous regeneration circuit according to the first invention. The embodiment shown in this figure shows a synchronous regeneration circuit for demodulating a four-phase PSKPCM signal, similar to the conventional example shown in FIG. The same elements as in FIG. 6 are given the same reference numerals.

In FIG. 1, a low pass filter (LPF) 29 is inserted at the output end of the VCO 25 in FIG. 6. The other configurations are the same as in FIG. 6. This LPF 29 is for leaving the fundamental wave of the output of the VCO 25 and removing harmonic components of second order or higher. LPF
29 is a simple one, and the precision of the cutoff frequency does not need to be strict. By inserting the LPF 29, the VCO2
Even if the output waveform of 5 is a square wave or a distorted waveform, an approximately sine wave can be obtained from the output of the LPF 29, and if the gain of the π/2 phase shifter 26 is appropriately selected, the amplitude can be changed as subcarriers CW1 and CW2. Sine waves that are approximately equal but have different phases by π/2 are obtained. Therefore, the first and second phase detectors 21 and 22
Since there is little unbalance in the phase detection operation, deterioration of detection characteristics can be suppressed and demodulation errors can be kept small.

FIG. 2 is a block diagram showing an embodiment of the synchronous regeneration circuit according to the second invention. The embodiment shown in this figure also has four-phase -P
A synchronous reproducing circuit for demodulating an SK-PCM signal is shown.

In FIG. 2, a current gain amplifier 30 is inserted at the output end of the switching circuit 23 of FIG. 6. The other ellipses are similar to those shown in FIG.

With this current gain amplifier 30, the loop gain of the synchronous regeneration circuit can be increased, and the received carrier frequency and VCO2
Even if there is a deviation in the free run frequency of 5, the residual phase error can be reduced in proportion to the loop gain, so the demodulation error can be reduced, and the COC rerun frequency can be eliminated without adjustment.

FIG. 3 is a block diagram showing another embodiment according to the second invention.

In FIG. 3, the noise removal filter 24 of FIG.
A gain amplifier 31 is inserted at the output end of the amplifier. As a result, the loop gain of the synchronous regeneration circuit can be increased as in FIG. 2, and the same effects as in FIG. 2 can be obtained.

Further, as another embodiment of the second invention, the noise removal filter 24 shown in FIG. 6 is constructed using an integrator, so that the gain in the low frequency range is made high, and the same as that shown in FIG. effect can be obtained.

FIG. 4 is a block diagram showing a synchronous regeneration circuit according to the third invention. The embodiment shown in this figure has a circuit configuration that is a combination of the embodiment shown in FIG. 1 and the embodiment shown in FIG. 2 or 3. That is, the circuit configuration shown in FIG. 6 is provided with an LPF 29 and a t-current gain amplifier 30, which suppresses deterioration of phase detection characteristics and residual phase error, and enables high-performance 2n-phase PSK and PCM signal demodulation with less demodulation error. It is possible to realize a synchronous regeneration circuit for It goes without saying that the current gain amplifier 30 may be replaced with a gain amplifier 31 at the output end of the LPF 24, or the LPF 24 may be configured using an integrator.

[Effects of the Invention] As described above, according to the present invention, the subcarrier used for phase detection can be reproduced without distortion, so the operational balance in phase detection can be improved, and deterioration of detection characteristics can be suppressed. 2n phase-PSK with low demodulation error
- A synchronous regeneration circuit for PCM signal demodulation can be realized. In addition, since the loop gain of the synchronous regeneration circuit can be increased, the residual phase error can be reduced even if the transmitted carrier wave frequency and the free run frequency of the VCO are different, eliminating the need to adjust the free run frequency. A synchronization generation circuit with less demodulation errors can be realized.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the synchronous regeneration circuit according to the first invention, FIG. 2 is a block diagram showing an embodiment of the synchronous regeneration circuit according to the second invention, and FIG. FIG. 4 is a block diagram showing an embodiment of the synchronous regeneration circuit according to the third invention, and FIG. 5 is a block diagram showing an example of the synchronous regeneration circuit according to the third invention. A block diagram showing the circuit, FIG. 6 is a block diagram showing a conventional 4-phase PSK-PCM signal demodulation synchronous regeneration circuit, and FIG.
FIG. 8 is a circuit diagram showing an example of the π/2 phase shifter shown in the figure, and FIG. 8 is a waveform diagram explaining the operation of FIG. 7. 20...Input terminal, 21.22...Phase detector, 2
3...Switching circuit, 24.29...Low pass filter, 25...Voltage controlled oscillator, 26...π/2 phase shifter,
27... Output terminal, 30... Current gain amplifier, 31
...gain amplifier.

Claims (3)

[Claims] (1) A voltage controlled oscillator, a plurality of phase detectors to which input signals based on the PSK method are applied to one input terminal, and oscillation from the voltage controlled oscillator to the other input terminal of each of the plurality of phase detectors. means for inputting the outputs after passing through a low-pass filter that passes the fundamental wave component thereof, and setting the outputs to have different phases from each other; A synchronous regeneration circuit comprising: a switching circuit for detecting a phase error of the switching circuit; and a filter that smooths the output of the switching circuit and applies the smoothed output to a control terminal of the voltage controlled oscillator. (2) A voltage controlled oscillator, a plurality of phase detectors to which an input signal based on the PSK method is applied to one input terminal of each, and an input terminal of each of the plurality of phase detectors to which an input signal from the voltage controlled oscillator is applied. means for inputting oscillation outputs set to have different phases from each other; a switching circuit to which the outputs of the plurality of phase detectors are input and detecting a phase error between the respective input signals; and an output of the switching circuit. A synchronization generation circuit characterized by comprising means for smoothing with a filter and amplifying the same and applying the same to a control terminal of the voltage controlled oscillator. (3) A voltage controlled oscillator, a plurality of phase detectors each having one input terminal to which an input signal based on the PSK method is applied, and the other input terminal of each of the plurality of phase detectors receiving the oscillation from the voltage controlled oscillator. means for inputting the outputs after passing through a low-pass filter that passes the fundamental wave component thereof, and setting the outputs to have different phases from each other; A synchronous generation circuit comprising: a switching circuit for detecting a phase error of the switching circuit; and means for smoothing and amplifying the output of the switching circuit with a filter and applying the amplified signal to a control terminal of the voltage controlled oscillator.