JPS6025938B2 - Pseudo pull-in avoidance circuit in reference carrier regeneration circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reference carrier regeneration circuit used in a synchronous detection demodulation device of a communication device that transmits digital signals using a phase shift keying (PSK) method, and particularly to a reference carrier regeneration circuit that detects a pseudo pull-in state and recovers a carrier wave. This invention relates to a circuit that stabilizes the operation of a reproduction system.

In the reference carrier regeneration circuit as described above, if the target frequency of the voltage controlled oscillator and the carrier frequency of the input modulated wave are significantly different, the basic circuit configuration alone is insufficient. has the property of being locked at some frequency different from the carrier wave frequency, that is, exhibiting a pseudo-retraction state.

As a method for avoiding this pseudo-pulling phenomenon, a method is known in which, for example, a signal obtained by amplitude-detecting the phase detection output is used to control the Kusunoki circuit [Reference Document 1]. This method for avoiding pseudo-entrainment is very effective, and can extremely reliably avoid the pseudo-entrainment phenomenon within the range allowed by the signal-to-noise ratio (S/N) of the detection circuit. However, if this method can be improved to operate normally even at lower S/N, a reference carrier recovery circuit equipped with such an improved circuit could be used in a wider range than before. Become. Therefore, an object of the present invention is to provide a pseudo pull-in avoidance circuit that operates stably even when the S/N ratio of an input signal is extremely deteriorated.

In the circuit system of the present invention, when the clock frequency is fc, m and n are positive integers, and m/n is irreducible, the pseudo pull-in frequency differs from the carrier frequency of the input signal by (m/n)·fc. The phase comparator output signal detected in this state has a repetition period of {{m/n)・fc・P}-1, where P represents the number of phases of the PSK wave. This takes advantage of the fact that it is a beat signal.

That is, the pseudo pull-in circuit in the reference carrier regeneration circuit of the present invention is a reference carrier used in a phase shift keying signal coherent detection demodulation device that includes a phase comparator and a voltage controlled oscillator that is controlled based on the output of the phase comparator. In the carrier wave regeneration circuit, a comb-shaped filter which has a characteristic of tuning to a frequency m/n times the clock frequency used for the phase shift keying signal and which inputs the output and reproduces the beat; It includes a detection circuit that detects a signal generated by the filter and generates an output signal, and a sweep circuit that is controlled by the output signal. This is a device that is controlled in conjunction with the output. Next, the present invention will be explained with reference to the drawings.

First, in order to clarify the significance of the present invention, the operation of a typical example of a coherent detection demodulation device for a four-phase PSK modulated wave will be explained. FIG. 1 shows a block diagram of a demodulator having a carrier regeneration circuit of a re-variation comparison type, which is equipped with a follow-on pseudo-pull-in avoidance circuit.

In FIG. 1, 11 is an input terminal for an input four-phase phase modulated wave a, and 12 and 13 are pulse-shaped reproduced data signals b.
and c output terminals, 14 is a 4-phase phase detector, 15 is a signal identification circuit, 16 is a clock extraction circuit, 17 is a voltage controlled oscillator, 18 is a 4-phase phase modulator (hereinafter referred to as a re-modulator), 1
9 is a phase comparator, 20 is a loop fighter, 21 is a detection circuit, and 22 is a sweep circuit. First, the input modulated wave a is 3
One branch goes to the four-phase phase detector 14, the other goes to the clock extraction circuit 16, and the last one goes to the phase comparator 9.
are input into each. The four-phase phase detector 14 inputs the output signal d of the voltage-controlled oscillator 17, synchronously detects the input carrier wave using two orthogonal reference carrier waves, and outputs two detected signals e and f.

The identification circuit 15 inputs the detection signals e and f and the output clock signal g of the clock extraction circuit 16, and detects the detection signals e and f each from a predetermined darkness value at a time position set by the clock signal g. Determine whether it is large or small,
Each determination result is output at a level corresponding to "1" or "0". The reproduced data signals b and c thus obtained are guided to output terminals 12 and 13, respectively, and simultaneously input to a remodulator 18.

This retransformer 18 converts the output signal b of the voltage controlled oscillator 17
is input and subjected to four-phase phase modulation using reproduced data signals b and c. The modulated wave obtained in this way is called a re-modulated wave. The re-modulated wave h is sent to the phase comparator 19. The phase comparator inputs the re-modulated wave h and the input modulator a and generates a voltage signal i proportional to the phase difference between them. The signal i is
It is added to the control state of the voltage controlled oscillator 17 via the loop filter 20. In this way, if this system leads the route of the branching circuit 21 and the diversion circuit 22 out of consideration,
One basic phase control loop is formed, and the output signal d of the voltage controlled oscillator 17 is phase-locked to the carrier wave of the input modulated wave a. By the way, in the above system, when the running frequency of the voltage controlled oscillator 17 is extremely close to the carrier wave frequency of the input modulated wave, the above operation is performed accurately to achieve phase synchronization with the carrier wave. As such, when the frequency difference becomes large, a pseudo-entrainment state occurs at some frequencies.

That is, the voltage controlled oscillator 1 at a frequency different from the intercarrier wave number
7 will be locked. In the vicinity thereof, the oscillation frequency of the voltage controlled oscillator is controlled to follow the fluctuation of the carrier wave frequency while maintaining a constant frequency difference from the carrier wave frequency. This phenomenon is the pseudo-entrainment phenomenon of the phase-locked loop that is causing the problem. Reference document 1 specifically shows a circuit that uses the detection circuit 21 and the sweep circuit 22 to avoid this pseudo-inclusion. There are two possible causes for the pseudo-entrainment phenomenon; one is due to the loop delay time [Reference 2], and the other is related to the modulation clock frequency fc, especially the one in Figure 1. This occurs when a sampling control system is configured in which the loop is controlled by a clock signal as in the configuration [Reference Document 1].

As explained in this document, in general, in a PSK modulated wave demodulator, a pseudo-entrainment phenomenon caused by the latter occurs strongly, and the voltage-controlled oscillator 17 locks at the point where the steel is released by (m/n)·fc. It is known that This document also proposes, as a method for avoiding this pseudo-entrainment phenomenon, a method for controlling the sweep circulation using a signal obtained by amplitude-detecting the output signal of a phase detector. This method is very effective and can avoid the pseudo entrainment phenomenon in the case of boars. However, the input signal S/
In special cases where the line must be maintained until N becomes extremely bad, the noise level and the level of the beat component generated during pseudo pull-in become close to each other, so the beat in the detected output of the amplitude detector It becomes difficult to determine the presence or absence of a component, causing the loop to malfunction. In other words, qualitatively speaking, the amplitude detector that is supposed to detect the beat component mistakenly identifies the noise component as the beat component.
Even when the loop is in the normal retracted state, the bucket pulling circuit ends up being driven. FIG. 2 is a block diagram showing the agglomeration avoidance circuit of the present invention.

The comb filter 31 is a circuit mainly composed of a delay circuit 32 and an adder 33, and the output signal i from the phase comparator 19 is
Apart from the output to the loop filter 20, one of the outputs passes through a delay circuit 32 to an adder 33, and the other output goes directly to the adder 33. In this comb-shaped filter 31, if the delay time of the delay circuit 32 is ,
The frequency response characteristic between input signal i and output signal i is 1/
It has a maximum value and a minimum value with a period of about 10 seconds. Here, the state of the output signal from the phase comparator 19 at the time of pseudo pull-in will be explained.

Voltage controlled oscillator output is m/n・f with respect to carrier frequency
If they have frequencies that differ by c, then referring to Figure 3, the vector S of the carrier signal is located at the center of P division planes obtained by dividing the phase plane into P, and this is divided into P division planes of the voltage controlled oscillator output signal. Vector L crosses the phase plane while rotating. The signal takes one of the states So to SN-. However, the third
The figure shows the case where P is 4. It is clear that the rotation speed of the vector L is 27 (m/n)·fc.

Therefore, the period of crossing the boundary of the P-divided plane is (
27/P)ノ{27(m/n)・fC}=[m/n・f
C・P]-1 ...{1'.

Remodulating means processing the signal so that the vector L falls within the dividing plane to which the vector Si belongs, so the remodulated signal L' has a value of 1 every time it crosses the boundary.
You will be pulled back to the previous boundary.

Therefore, the phase difference between SI and the re-modulated wave L' changes periodically between the two directions, and the period is the period at which L crosses the above-mentioned dividing plane. That is, it is expressed by equation '1}. If the signal corresponding to the phase difference between Si and L' is obtained from the phase comparator and the frequencies of S and L are the same, then the vector L is always at a constant position in the phase plane and does not move. , a DC signal is obtained as the output from the phase comparator. However, in this case, as mentioned above, L
Since it is rotating, the phase difference between Si and Si changes from moment to moment, and correspondingly, a periodically changing output is obtained. This is called a beat signal, and its period is a deviation expressed by formula m, and an example thereof is shown in FIG. 4. This waveform is a combination of a mirror-tooth wave that goes back and forth between the waveform and the waveform, and a transient waveform that occurs when S transitions between possible phases. As mentioned above, at the time of pseudo-retraction {(m/n)
・A beat signal with a repetition period of fc×P}-1 is generated, but as stated in Reference 1, n
As becomes large, the retraction range becomes narrower in inverse proportion to n,
Because of the slight noise component within the loop, the pull-in phenomenon does not actually occur when n increases beyond a certain value.

However, this limit actually differs depending on the circuit.
Therefore, the value of n corresponding to the pseudo attraction point that may occur now is n,
Let n2, ..., ni be, and let ~ be the least common multiple of n, ~ni, then? By setting = {(fc/no)×P}-1 second, the comb-shaped filter can pass the beat signals at all the pseudo pull-in points. On the other hand, since the noise component is reduced to half the power by this filter, the S/N of signal i is improved compared to that of signal j.
That is, by utilizing the regularity of the beat signal at the time of pseudo pull-in, the S/N ratio could be improved. Therefore, the output signal i with improved S/N as described above is applied to the detection circuit 21. That is,
The output signal j is first amplitude-detected by an amplitude detector (not shown) in the detection circuit, then passed through an integrating circuit (not shown), that is, a low-frequency wave generator, and then discriminated based on a predetermined threshold value. The presence or absence of a beat component is determined by a dark value circuit (not shown). The output of this detection circuit is, for example, "1" in response to the presence or absence of a beat component.
A signal k of two levels, ie, "0" and "0", is sent out. These 2
The two signals activate and deactivate the known sweep circuit 22. The output of this sweep circuit is summed with the output m from the loop filter 20 at the input of the voltage controlled oscillator 17 and input. Therefore, when a pseudo pull-in condition is detected, the sweep signal forces the frequency of the voltage controlled oscillator 17 to change and the loop enters a new pull-in process. When this operation continues and the normal retracted state is entered, the lube becomes stationary. Note that the bridge circuit is conventionally well known [Reference Document 1]. In the above explanation, an improvement in S/N was obtained by using a single-stage comb filter as shown at 31 in Figure 2, but if this improvement is insufficient, this It is sufficient to connect several filters in tandem.

Therefore, according to the present invention, it is possible to stably pull the carrier-locked loop beyond the normal pull-in point even for an input carrier wave with an extremely low S/N ratio. In the above embodiment, an amplitude detector, an integrating circuit, a resistive wave detector, and a threshold circuit (none of which are shown) are used as the detection circuit 21, but these are merely examples. Of course, other circuit configurations are also possible.

In short, it may be of a nature that the bridging circuit 22 is activated or stopped when the S/N exceeds a certain value. Furthermore, although the explanation up to now has been made regarding a sampling system, this may also be applied to a non-sampling system.

For example, in FIG. 1, even if the signal discrimination circuit 15 does not receive a clock signal and operates only as an amplitude discriminator, the same machine may be used even if the pull-in range is different because the modulated wave is modulated at a constant modulation speed. exhibits a pseudo-entrainment phenomenon. And in this case as well, generally (m/n)・fc
Since the pull-in point exists at the frequency of , the configuration of the present invention can be applied.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a synchronous detection demodulation circuit using a conventional pseudo pull-in detection circuit, and FIG. 2 is a block diagram showing the pseudo pull-in circuit of the present invention. FIG. 3 is a diagram showing the vectors S and L at the time of pseudo pull-in when P is 4, and FIG. 4 is a diagram showing the waveform of the beat signal output from the phase comparator. Explanation of symbols, 17
19 is a voltage controlled oscillator, 19 is a phase comparator, 20 is a loop filter, 21 is a detection circuit, 22 is a bridge circuit, 31 is a comb filter, 32 is a delay circuit, and 33 is an adder. Reference 1: Ishio, Shimamura et al., Showa 4G Yodo Electronics and Communication Society National Conference, Axis. Satoshi 6 Reference 2: Ishio et al., 1939 Western Yodo Electronics and Communication Society National Conference, M. 7th Round 1st Figure World 2nd Figure 3 Qinmu Map

Claims (1)

[Claims] 1. In a reference carrier recovery circuit used in a coherent detection demodulation device for a phase shift keying signal, which includes a phase comparator and a voltage controlled oscillator controlled based on the output of the phase comparator, a comb filter which inputs the output and filters out the beats, and which has a characteristic of tuning to a frequency m/n times the clock frequency (m and n are positive integers and m/n is irreducible); a detection circuit that detects a signal filtered by a filter and generates an output signal; and a sweep circuit that is controlled by the output signal; A pseudo pull-in avoidance circuit in a reference carrier regeneration circuit which performs continuous control.