JPH01171369A - Standard frequency generating circuit - Google Patents

Abstract

PURPOSE:To realize the circuit of a small jitter by executing the PLL control of first and second crystal oscillators respectively with first and third control signals at a normal time and executing the control of the first and second crystal oscillators so as to execute a free-run respectively with first and second control circuits at an abnormal time in a television broadcasting device. CONSTITUTION:A color burst extracting circuit 11 inputs a video signal S1 and extracts a color burst signal S2 to be included in the video input S1. A crystal oscillator 15 is controlled by control voltages S4 and S5 and outputs an output S6 which has a same frequency as a color sub-carrier. A phase comparator 13 impresses the control voltage S4 through a loop filter 14 to the crystal oscillator 15 so that a phase difference can be eliminated. A phase comparator 22 impresses a control voltage S8 through a loop filter 23 to a crystal oscillator 25 so that the phase difference can be eliminated. When a deciding output S7 of the phase comparator 22 is not normal, a control circuit 24 causes the crystal oscillator 25 to execute the free-run with the frequency of 10MHz by a control voltage S9.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a standard frequency generation circuit for a television broadcasting device or a television relay broadcasting device.

[Conventional technology]

Conventionally, in order to stabilize the transmission frequency of a television broadcasting device or a television relay broadcasting device, a frequency refinement device with one type of synthesizer is used to adjust the transmission frequency to, for example, IOMII.
It is said that the method is to synchronize with the standard frequency of z. In the case of a master station, a highly stable oscillator such as a rubidium oscillator is used as this standard frequency, but in the case of a relay station, among the information contained in the radio waves emitted from the master station, the video carrier frequency and color subcarrier frequency are used. A method has been considered in which a standard frequency is generated by extracting the frequency and multiplying it by the same jlJ].

Also, even if there is a master station, instead of placing a rubidium oscillator at the transmitting station, the color subcarrier frequency included in the video signal sent from the performance venue is taken out and cycled to this.
Methods of generating standard frequencies are being considered.

FIG. 2 is a block diagram showing a standard frequency generation circuit for a television broadcasting device or a television relay broadcasting device that uses conventional color subcarrier frequencies.

The color subcarrier frequency contained in the video input SI is extracted by the color burst extraction circuit 31, and the output S2 is divided into 1/M by the frequency divider 33 and then applied to the phase comparator 34. On the other hand, the IOMtlz output S16 of the lOMHz crystal oscillator 36 is divided into 1 by the frequency divider 37.
After dividing the frequency into 7N, it is applied to the phase comparator 34. The phase comparator 34 compares the phases of the two applied signals; the tiIJ control voltage S13 is applied to the crystal oscillator 36 through the loop filter 35. In the phase-locked loop configured in this way, equation (1) holds true.

l 0M1lz = -x color subcarrier frequency...
・(1) The control circuit 32 controls the color subcarrier, video input S1, and automatic phase control (hereinafter referred to as AP).
(denoted as C) is output as output S2. When detected from SI5, the crystal oscillator 36 is set to jt by the control voltage S16 of the control circuit 32 so that the crystal oscillator 36 does not receive the :b control voltage SI4 from the phase comparator 34 and runs freely.
i (I control.

Here, in the case of a master station, the video input SI is a video signal sent from a studio, and in the case of a relay station, it is a video signal obtained by demodulating received radio waves. In either case, since jitter is included in the color subcarrier, the time constant of the loop filter 35 should be made sufficiently large to prevent the influence of jitter from appearing on the l0M1lz output.

[Problem that the invention seeks to solve]

In the conventional standard frequency generation circuit for television relay broadcasting described above, when there is no color subcarrier or when there is no video signal S1, the control circuit 32 operates so that the crystal oscillator 36 is free-run. When the APC is restored, the time constant of the loop filter 35 is large as described above, so there is a drawback that the APC pull-in time is long, so it is also possible to provide a switching function such as shortening the time constant at the time of pull-in. However, the effect of this switching is 10MHz
,. It also has the disadvantage that it appears as a phase fluctuation.

[Means for solving problems]

The standard frequency generation circuit of the present invention includes a color burst extraction circuit that inputs a video signal and extracts a color burst signal, and an oscillation output of the same frequency as the color subcarrier under the control of a first or second control signal. Compares the phase of the first crystal oscillator to output, the output of the color burst extraction circuit, and the output of the first crystal oscillator, outputs a phase comparison output so that the phases match, and determines whether its own operation is normal. a first phase comparator that outputs a determination output indicating whether the output is the same; and a first loop filter that filters the phase comparison output of the first phase comparator and applies it as a first control signal to the first crystal oscillator. , the color burst signal output from the color burst extraction circuit and the judgment output output from the first phase comparator are input, and when it is detected that the phase comparator is not normal, the second control signal is sent to the first crystal oscillator. a first control circuit that controls the first crystal oscillator to emit at a frequency equal to the color subcarrier based on the second control signal and not on the first control signal; a first frequency divider that divides the output of the crystal oscillator by a first frequency division ratio; a second crystal oscillator that outputs an oscillation output of a standard frequency under control of a third or fourth control signal; a second frequency divider that divides the oscillation output of the second crystal oscillator by a second frequency division ratio;
A second phase comparison that compares the phases of the outputs of the first and second frequency dividers, outputs a phase comparison output so that the phases match, and outputs a judgment output indicating whether or not its own operation is normal. filters the phase comparison output of the second phase comparator, and filters the phase comparison output of the second phase comparator.
A second loop filter having a larger time constant than the first loop filter, which is applied to the second crystal oscillator as a 1t control signal, and the judgment output of the second phase comparator are input, and the input judgment output is If it detects that something is not normal, it outputs a fourth control signal to the second crystal oscillator, and the second crystal oscillator oscillates at the standard frequency based on the fourth control signal and not on the third control signal. and a second control circuit for controlling.

(Function) In this way, since the time constant of the first loop filter is small, the first crystal oscillator is ;t+lJ control can be quickly switched to the first control signal from the first loop filter, eliminating the pull-in delay of automatic phase control;
Since the time constant of the first loop filter is set to be large, the jitter generated due to the small time constant of the first loop filter is absorbed by the second loop filter.

[Example] Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the standard frequency generating circuit of the present invention.

In this embodiment, a surface stage oscillation circuit 10 which inputs a video input Sl and outputs an output S6 having almost the same frequency as the color subcarrier, and an output SF, generates a 10M1lz output S.
It is composed of a second-stage oscillation circuit 20 that outputs 1°.

The navigation stage oscillation circuit 10 includes a color burst extraction circuit 11, a control circuit 12, a phase comparator I3, a loop filter 14, and a crystal oscillator 15, and the subsequent stage oscillation circuit 20 includes a frequency divider 21, a phase comparator 22 , a loop filter 23, a control circuit 24, a crystal oscillator 25, and a frequency divider 26.

The color burst extraction circuit I+ receives the video input S, and extracts the color burst signal S contained in the video input S1.
Extract 2. The control voltage S, , S,
It outputs an output S6 of the same frequency as the color subcarrier. The phase comparison 1AY13 compares the phases of the color paper toy No. 1-S and the output S6 of the crystal oscillator, and applies a control rm pressure S4 to the crystal oscillator 15 via the loop filter 4 so that there is no phase difference.

When the color burst signal S2 and the judgment output S of the phase comparison 1y13 are no longer normal, the control circuit I2 performs crystal oscillation at the control voltage S5? 515 to run freely at the color subcarrier frequency.
Control with 5.

The frequency divider 21 inputs the output S6 and divides the frequency into -.

The crystal oscillator 25 receives a control voltage S+1. Output S for S9
, ◇ is output. The frequency divider 26 divides the output S1◇ into −. The phase comparator 22 compares the phases of the outputs of the frequency division av2] and 26, and applies the control voltage S II to the crystal oscillator 25 via the loop filter 23 so that the phase difference is eliminated.

The control circuit 24 is the judgment output S7 of the phase comparator 22.
When the crystal oscillator 25 becomes abnormal, the control voltage S9 causes the crystal oscillator 25 to run freely at the frequency of ]OMtlz.

In this embodiment, the frequency division numbers M and N of the frequency dividers 21 and 2B are set so as to satisfy equation (1) in the description of the conventional example.

Therefore, when there is no color subcarrier, or when there is no video input itself, the crystal oscillator FA unit 15 of the front-stage oscillation circuit IO becomes free-run due to the operation of the control circuit 12. Automatic phase control (hereinafter referred to as APC)
In the case of an error, the control circuit 12 controls the same. In any case, the reference (
Since g itself continues, the APC of the subsequent stage oscillation circuit 20 is maintained. Furthermore, the time constant of the loop filter 14 is set to be short to speed up the response, but the time constant of the loop filter 230 is set to be sufficiently long, so that there is no jitter or phase fluctuation in the oscillator circuit IO. is also absorbed, and the output S, o of the crystal oscillator 25 of the subsequent stage oscillation circuit 20
does not appear in If there is an abnormality in the phase matching loop of the second-stage oscillation circuit 20 and the APC is disconnected, the control circuit 24 causes the crystal oscillator 25 to free run.

[Effects of the Invention] As explained above, the present invention reduces the time constant of the first loop filter that outputs the first control signal, and decreases the time constant of the second loop filter that outputs the third control signal. During normal operation, the first and second crystal oscillators are controlled by PLL control using the first and third control signals, respectively, and when abnormal, the first and second crystal oscillators are controlled by the first and second control circuits, respectively. By controlling the machine to free run, the following effects can be achieved.

(1) When the first control circuit free-runs the first crystal oscillator in the absence of Color Burst Toy No. 5 or in the event of an abnormality, when a return to normality occurs, the time constant of the first loop filter changes. Since it is small, the pull-in time of automatic phase control can be shortened.

(2) The second loop filter is a color burst (rls
%, and since the time constant is set sufficiently large, jitter in the output of the first crystal oscillator that occurs due to the small time constant of the first loop filter can be absorbed.

(3) From the above, in terms of long JtJI, it is possible to maintain the stability of the color subcarrier of the parent station and realize a standard frequency generation circuit with less jitter.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the standard frequency generating circuit of the present invention, and FIG. 2 is a block diagram showing a conventional example. 10......Pre-stage oscillation circuit, 11...
・・・・・・・・・Color burst extraction circuit, 12.
24... Control circuit, 13.22...
...Phase comparator, IIl, 23... Loop filter, 15.25
......Crystal oscillator, 20......Late stage oscillation circuit, 21.2
6...Divide: Bi;. Figure 1

Claims (1)

[Claims] A color burst extraction circuit that inputs a video signal and extracts a color burst signal, and a color burst extraction circuit that outputs an oscillation output having the same frequency as the color subcarrier under the control of a first or second control signal. Compares the phases of the output of the first crystal oscillator, the output of the color burst extraction circuit, and the output of the first crystal oscillator, outputs a phase comparison output so that the phases match, and indicates whether or not its own operation is normal. a first phase comparator that outputs a determination output; a first loop filter that filters the phase comparison output of the first phase comparator and applies it as a first control signal to the first crystal oscillator; and a color burst. inputting the color burst signal output from the extraction circuit and the determination output output from the first phase comparator;
If it detects that one of them is not normal, it outputs a second control signal to the first crystal oscillator, and the first crystal oscillator controls the color subcarrier based on the second control signal, not on the first control signal. a first control circuit that controls the output of the first crystal oscillator to transmit at a frequency equal to the frequency of the first crystal oscillator;
a frequency divider, a second crystal oscillator that outputs an oscillation output of a standard frequency under control of a third or fourth control signal, and the oscillation output of the second crystal oscillator is divided by a second frequency division ratio. Compares the phases of the outputs of the second frequency divider and the outputs of the first and second frequency dividers, and outputs a phase comparison output so that the phases match, and also indicates whether or not its own operation is normal. a second phase comparator that outputs a determination output; and a first loop filter that filters the phase comparison output of the second phase comparator and applies it as a third control signal to the second crystal oscillator. The judgment output of the second loop filter having a time constant and the second phase comparator is inputted, and when it is detected that the input judgment output is not normal, the fourth control signal is sent to the second loop filter.
and a second control circuit that outputs the output to the crystal oscillator and controls the second crystal oscillator to oscillate at the standard frequency based on the fourth control signal but not on the third control signal. circuit.