JPS61131681A - Carrier wave recovery circuit - Google Patents

Abstract

PURPOSE:To obtain the titled circuit having a stabilized phase jitter and short in time of leading-in by providing two kinds of loop filters which generate controlling voltage and using either of them in accordance with an external control. CONSTITUTION:A converting signal input Si added into a synchronous wave detection circuit 1 is outputted to phase detectors 3a, 3b through an orthogonal distortion compensating filter 2. Each output from the phase detectors 3a, 3b is combined together via LPFs 4a, 4b and then sent to an external control switch 10. The external control switch has a function of switching the said combined output to the first loop filter and the second loop filter 9, and this switching is done by a control signal CONT. For the first loop filter 8, the output which can absorb a frequency offset due to quick synchronous lead-in time is used, and for the second loop filter, the output which can follow up an instantaneous phase change of receiving carrier is used.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a demodulating section of a medium-speed facsimile, and particularly to a carrier regeneration circuit using a synchronous detection method.

[Background of the invention]

For medium-speed facsimile transmission, AyL-PM-VS
The B modulation method is recommended by the CCITT, and its demodulation uses a synchronous detection method.

This synchronous detection demodulation method is performed by extracting phase information from a modulated input signal of a facsimile receiver.

2. Description of the Related Art Conventionally, as a carrier wave recovery circuit using a synchronous detection method, one having a configuration as shown in FIG. 3 is known.

The modulated input signal St is sent to a synchronous detection circuit 1 that performs synchronous detection.
The signal is then input to a running orthogonal distortion correction filter 2 that corrects orthogonal distortion of the modulated input signal Si.

The output of this orthogonal distortion correction filter 2 is input to phase detectors 5a and 3b that detect the phase of the signal, and after passing, high-frequency components are cut through low-pass filters 4a and 4b, and outputs that are orthogonal to each other are obtained. The multiplied and synthesized signals are input to the loop filter 5. The output voltage of the loop filter 5 controls the frequency of the voltage controlled oscillator 60. Note that 7 is a 90° phase shifter. In this way, the phase detectors 5a, 5b, the low-pass filters 4a, 4b, the loop filter 5, the voltage-controlled oscillator 6, and the phase shifter 7 constitute a phase-locked loop.
A phase synchronized signal output from the voltage controlled oscillator 6 is input to the synchronous detection circuit 1. Then, the synchronous detection circuit 1 demodulates the modulated input signal Si according to the phase synchronization signal that is the output signal of the voltage controlled oscillator 6, and outputs it as a demodulated signal So.

However, the AM-PM-VSB method has a problem in that the modulated wave contains orthogonal components, and phase jitter appears due to distortion of the orthogonal components. In order to suppress such phase jitter, it is necessary to narrow the bandwidth of the phase-locked loop, but this increases the pull-in time and makes it almost impossible to absorb phase jitter due to external distortion. . And if you try to improve this point, you will end up with the drawbacks of noise generation and synchronization errors.

[Purpose of the invention]

An object of the present invention is to provide a carrier wave regeneration circuit with less phase jitter and short pull-in time.

[Summary of the invention]

In order to achieve the above object, in the present invention, the oscillation frequency of the voltage-controlled oscillator, which is controlled by a control voltage according to the phase difference between the modulation input signal and the output signal of the voltage-controlled oscillator, is changed to the modulation input signal. In the carrier wave regeneration circuit for synchronization, the loop filter that generates the control voltage is equipped with one that has a fast synchronization pull-in time and can absorb frequency offsets, and one that can only follow instantaneous phase changes of the received carrier. Provided is a carrier regeneration circuit that can select one of them by control.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

Note that the same reference numerals are given to the middle portions of the drawings. 1 is a synchronous detection circuit that performs synchronous detection and receives a modulated input signal St.
is input. This modulated input signal Si is further input to an orthogonal distortion correction filter 2 that corrects distortion of orthogonal components.

The output from the orthogonal distortion correction filter 2 is input to phase detectors 3a and 5b that detect the phase of the signal. This phase detector! The outputs of +a, 5 and :) are input to low-pass filters 4a and 4b, respectively, which cut high-frequency components of the signals. The outputs from the low-pass filters'a*4b are combined and then connected to the external control 5WIO. This external control 5W10 controls switching of the combined output line after passing through the low-pass filters 4a and 4b to the first loop filter 8 and the $2 loop filter 9. Therefore, the control signal C0NT is input. There is. Both the first loop filter 8 and the second loop filter 9 generate a control voltage for controlling the frequency of the voltage controlled oscillator 60, and only one of the outputs is input depending on the operation of the external S control 5W10. It is now possible to do so. The first loop filter 8 is configured so that the synchronization pull-in time is fast and the frequency offset can be absorbed. On the other hand, the second loop filter 9 is configured to be able to follow only the instantaneous phase change of the received carrier.

The phase synchronization signal from the voltage controlled oscillator 6 is input to a 90° phase shifter 7 that converts the phase, and is also input to the phase detector 3a and the synchronous detection circuit 1, respectively. Furthermore, 9
The output of the 0° phase shifter 7 is input to the phase detector 5b.

Thus, the phase detectors 5a, 3b and the low pass filter 4a.

4b, loop filters 8, 9. Voltage controlled oscillators 6 and 9
The 0° phase shifter 7 constitutes a phase locked loop. Then, the synchronous detection circuit 1 demodulates the modulated input signal St according to the phase synchronization signal that is the output signal of the voltage controlled oscillator 6, and outputs it as a demodulated signal So.

Next, details of the operations of the first loop filter 8 and the second loop filter 9 will be explained based on FIG. 2. External control 5W10 is a switch SWI for switching the output line
, SW2 are installed, and synthesis output lines from low-pass filters 4a and 4k are connected to one ends of the switches SW1 and SW2, respectively. Further, the other end of the switch SWi is connected to the first loop filter 8, and the switch SW
The other end of 2 is connected to a second loop filter 9. In each loop filter, the input analog signal is digitally processed and filtered by a multiplication element, an addition/subtraction element, and a delay element Z-1.

In facsimile, high-speed synchronization is required when training signals are transmitted. At this time, the external control signal C0NT selects the switch SW1, and the first loop filter 8 performs an operation of absorbing frequency offset distortion in a short time by complete double integration. Note that the coefficient a1. of the multiplication element provided in the first loop filter 8 is A2 can be set to a large value by setting the signal component to include a 95-chi signal. Next, when the image signal is transmitted, it may contain almost no carrier, that is, it may be an all-black signal, so the external control signal C0NT selects the switch SW2 inside the external control 5W10, and the switch SW2 is turned on. It turns on and the second
The loop filter 9 is selected. The second loop filter 9 has PLL (Phase Loop Log
Since the integration process in the loop of IC) is single, it does not have the ability to absorb frequency offsets, but it has the ability to absorb phase jitter due to line distortion because it follows only instantaneous changes in the carrier. At this time, external control 5W10 is OFF.
Therefore, the initial offset value is absorbed by the integrator of the first loop filter 8. Through these series of operations, image signal reception δ is performed stably.

〔Effect of the invention〕

According to the present invention, since two types of loop filters with different properties are provided, it is possible to obtain a carrier wave regeneration circuit with a simple configuration that has less phase jitter and short pull-in time. Furthermore, since the image signal is made less susceptible to the influence of line distortion, the excellent effect of suppressing the occurrence of noise and synchronization deviation and improving the image quality can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the carrier wave recovery circuit according to the present invention, FIG. 2 is a detailed circuit diagram showing details of a loop filter in the carrier wave recovery circuit according to the present invention, and FIG. 3 is a conventional carrier wave recovery circuit. FIG. 2 is a block diagram showing a circuit. 1... Synchronous detection circuit 2... Orthogonal distortion correction filter 5a, 3b... Phase detector 4a, 4b... Low pass filter 5.8.9... Loop filter 6... Voltage control Oscillator 7...90' Phase shifter 10.
・・External control SW No. 1 2'lfl

Claims (1)

[Claims] 1. A synchronous detection circuit that inputs a modulated signal and detects its synchronization, an orthogonal distortion correction filter that inputs the modulated signal and corrects distortion of orthogonal components, and an output from this orthogonal distortion correction filter. two phase detectors that input and detect the phase, two low-pass filters that input the output from these phase detectors and cut high-frequency components, and a combined output from these low-pass filters. An external control switch that switches the output line based on input and external control, a first loop filter that has a fast synchronization pull-in time and can absorb frequency offsets, and a second loop that can only follow instantaneous phase changes in the received carrier. a filter, a voltage controlled oscillator controlled by the output voltage of the first loop filter or the second loop filter, and inputting the oscillation frequency of this voltage controlled oscillator and transmitting its output to one phase detector. It consists of a 90° phase shifter, and outputs a demodulated signal from the synchronous detection circuit by transmitting the oscillation frequency of the controlled oscillator to one of the phase detectors and the synchronous detection circuit and synchronizing it with the modulation input signal. This is a carrier wave regeneration circuit. 2. The carrier wave regeneration circuit according to claim 1, wherein the external control switch consists of two switches, and the input to the first loop filter or the second loop filter is switched by an external control signal.