JPH08294065A - Rf modulator - Google Patents

Abstract

PURPOSE: To make the RF modulator small by setting a count of a counter circuit with a changeover circuit in a PLL circuit to which a modulation circuit is connected so as to use only one resonance circuit. CONSTITUTION: A PLL circuit is configured by connecting a counter circuit 28, a phase comparator 30, a charge pump circuit 31, a loop filter circuit 35 to an FM modulation circuit 24. An audio subcarrier in use is set by applying a binary voltage to input terminals 32, 33 of a changeover circuit 34. The oscillated frequency from a resonance circuit 37 is divided by a setting value of the circuit 34 at the circuit 28 and the phase of the frequency signal is compared with a phase of a frequency signal from a reference oscillation circuit 29 at a comparator 30. The phase difference is fed to the modulation circuit 24 via the circuits 31, 35, in which the oscillated frequency is varied and the phase is in phase with the oscillated frequency of the circuit 29. Thus, one resonance circuit is enough for the purpose and no switch circuit is required to make the RF modulator small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RF modulator used in the field of video equipment such as a video tape recorder (hereinafter referred to as VTR).

[0002]

2. Description of the Related Art A conventional RF modulator will be described below.

As shown in FIG. 4, a conventional RF modulator has a first input terminal 1 to which a video signal is input, a waveform shaping circuit 2 connected to the first input terminal 1, and the first input terminal 1. Second input terminal 3 to which an audio signal is input separately from the input terminal 1, an FM modulation circuit 4 to which the output of the second input terminal 3 is connected, an output of the FM modulation circuit 4 and the waveform A mixer circuit 6 to which the output of the shaping circuit 2 and the output of the carrier oscillating circuit 5 are respectively input, and an output terminal 7 to which the output of the mixer circuit 6 is connected are provided. The four resonance circuits from the resonance circuit 8 to the resonance circuit 11 are the switch 14, the switch 15, and the switch 1.
It was selected and connected in 6.

The output terminal 7 of the RF modulator is connected to a television receiver so that the television receiver can receive the signal. Therefore, the sound carrier frequency of this RF modulator is basically set in advance by the variable element of the resonance circuit 8 according to the broadcasting system.

In addition, in response to a request for multi-methods of audio subcarriers in a specific area, a plurality of resonance circuits 9, 10, 11 are provided in addition to the resonance circuit 8, and the frequency is changed by the variable elements of each resonance circuit. The resonance circuits 8 to 11 are switched by setting the settings and controlling the switch circuits 14, 15 and 16 with the third and fourth input terminals 12 and 13.

[0006]

However, in recent years R
In response to the demand for miniaturization of the F modulator, in the above conventional technique, four resonance circuits, that is, the resonance circuit 8, the resonance circuit 9, the resonance circuit 10, the resonance circuit 11 and the switch circuit 14,
There is a problem that downsizing is difficult because three switch circuits of the switch circuit 15 and the switch circuit 16 are required.

The present invention solves such a problem, and an object of the present invention is to provide an RF modulator which can be miniaturized.

[0008]

In order to achieve this object, in the RF modulator of the present invention, one input is connected to the other output of the modulation circuit and the variable input of the division ratio is set to the other input. A counter circuit to which the output of a possible switching circuit is connected, a phase comparator in which the output of this counter circuit and the output of the reference oscillation circuit are connected to their respective inputs, and a charge connected to the output of this phase comparator. A configuration in which a pump circuit and an output of the charge pump circuit are connected to the modulation circuit via a loop filter circuit to configure a PLL circuit, and the loop filter circuit includes a charge charging circuit for charging a constant charge It is what

[0009]

With this configuration, the number of counts of the counter circuit constituting the PLL circuit can be set to a predetermined value by the switching circuit, so that only one resonance circuit is required and the switch circuit is provided. Since it is unnecessary, downsizing can be realized.

[0010]

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

As shown in FIG. 1, the RF modulator of the present invention has a first input terminal 21 to which a video signal is input, and a waveform shaping circuit 22 connected to the first input terminal 21.
In addition to the first input terminal 21, an audio signal (about 20
Up to 15 KHz), and a second input terminal 23, and an FM modulation circuit 24 connected to the second input terminal 23.
, A mixer circuit 26 to which one output of the FM modulation circuit 24, the output of the waveform shaping circuit 22 and the output of the carrier oscillation circuit 25 are respectively input, and an output terminal 27 to which the output of the mixer circuit 26 is connected. And the FM modulation circuit 24
Of the counter circuit 28, a switching circuit 34 connected to the count value setting input of the counter circuit 28, and third and fourth input terminals 32 respectively connected to the switching circuit 34. 33 and the counter circuit 28
Is connected to the respective inputs, the charge pump circuit 31 connected to the output of the phase comparator 30, and the output of the charge pump circuit 31. The FM modulation circuit 24 is connected via the loop filter circuit 35 to form a PLL circuit, and the loop filter circuit 35 is provided with a charge charging circuit 36 that charges a constant charge when power is applied. It is configured.

Here, the loop filter circuit 35 is
As shown in FIG. 2, the output of the charge pump circuit 31 is connected to the FM modulation circuit 24 via a resistor 38,
Between the connection point between the FM modulation circuit 24 and the resistor 38 and the ground, a series connection body of the resistance 39 and the capacitor 40 and a capacitor 41 are inserted in parallel with the series connection body. Although a lag-lead filter structure which is generally called is used here, it may be a low-pass filter composed only of a resistor and a capacitor. Also,
In the charge charging circuit 36, as shown in FIG. 2, the collector of the NPN transistor 42 is connected to the power source, and the base is connected to the base of the PNP transistor 43. Further, the collector of the transistor 43 is connected to the ground. Then, each of the transistors 42,
The emitter of 43 is connected, and the resistor 44 is connected from this emitter.
Is connected to the connection point between the resistor 38 and the resistor 39 of the loop filter circuit 35 via. In addition, the resistor 45,
Reference numeral 46 is a base bias setting resistor for the transistors 42 and 43.

In the loop filter circuit 35 of this embodiment,
The resistor 38 uses 25 KΩ, the resistor 39 uses 270 Ω, the capacitor 40 uses 22 μF, and the capacitor 41 uses 0.1 μF.
It is a lag lead filter that blocks passage of frequencies above about 20 Hz.

The operation of the RF modulator configured as described above will be described below.

In FIG. 1, the video signal input from the first input terminal 21 is waveform shaped by the waveform shaping circuit 22 and input to the mixer circuit 26. Here, the output of the carrier oscillating circuit 25 is AM-modulated by the mixer circuit 26 with respect to the waveform-shaped video signal, and becomes a video carrier. on the other hand,
The audio signal input from the second input terminal 23 is converted into an audio wave at the frequency oscillated by the resonance circuit 37 by the FM modulation circuit 24.
Is FM-modulated by and output as an audio carrier, becomes a subcarrier of the AM-modulated video carrier by the mixer circuit 26, and is output from the output terminal 27.

Since the output terminal 27 is connected to the antenna of the television receiver, the output signal can be received by the television receiver. Here, the audio subcarrier frequency of the RF modulator must be set to a frequency according to the broadcasting system of the area where it is used. This frequency is
The voltage can be set by turning on and off the voltage at the third and fourth input terminals 32 and 33, respectively.

For example, the oscillation frequency of the reference oscillation circuit 29 is set to 500 KHz, and the third and fourth input terminals 3
Table 1 sets the relationship of the count numbers of the counter circuit 28 that are switched by the switching circuit 34 depending on the combination of the voltages applied to 2, 33. Although the reference frequency is set to 500 KHz here, it may be set to a step frequency equal to or lower than the switching step frequency.

[0018]

[Table 1]

Now, assuming that a voltage of "H" is applied to the third input terminal 32 and a voltage of "L" is applied to the fourth input terminal 33, the oscillation frequency of the resonance circuit 37 is set to the counter circuit 2
The frequency is divided into 1/11 by 8, and this signal and the oscillation frequency of the reference oscillation circuit 29 of 500 KHz are divided by the phase comparator 3
Compared with 0. Then, the phase difference is passed through the charge pump circuit 31 and the loop filter circuit 35 to the F
It is applied to the M modulation circuit 24. In this way, the FM
The oscillation frequency is changed by the modulation circuit 24 and becomes in phase with the oscillation frequency 500 KHz of the reference oscillation circuit 29. That is, the output signal frequency of the FM modulation circuit 24 is 1/1
1 gives 500 KHz. Therefore, 500 KHz ×
An audio subcarrier frequency of 11 = 5500 KHz is obtained.

Here, when the power of this RF modulator is turned on, the time until the sound carrier oscillates and is locked at the set frequency is the loop filter circuit 35 shown in FIG.
It is determined by the time constants of the resistors 38 and 39 and the capacitors 40 and 41. Therefore, if this time constant is large, the lock time becomes long, and during that period, the FM
Buzz sound is generated without detection. However, the loop filter circuit 35 needs to block a very low frequency (20 Hz to 15 KHz) of the audio signal frequency, and thus requires a very large time constant. Therefore, the RF modulator of the present invention includes a charge charging circuit 36 that charges the loop filter circuit 35 with a constant charge. Therefore, when the power of the RF modulator is turned on, the capacitor of the loop filter circuit 35 is instantly charged with electric charge, and the lockup time of the PLL circuit is shortened.

The operation of the charge charging circuit 36 will be described. Now, the sound carrier frequency is set to 5.5 MHz (“H” is applied to the third input terminal 32 and the fourth input terminal 3 is used).
3 is set so that an “L” voltage is applied), and the output voltage of the charge pump circuit 31 locked by the PLL circuit is 2.5V.

At the moment when the power of the RF modulator is turned on, a bias voltage (2.5 V) is applied to the bases of the transistors 42 and 43 of the charge charging circuit 36. On the other hand, in the loop filter circuit 35, the voltage does not reach the desired potential (2.5V) (state close to 0V).

Therefore, the base of the transistor 42,
A potential is generated between the emitters and a current flows through the transistor 42. During this period, the capacitors 40 and 41 of the loop filter circuit 35 are charged with electric charges. further,
When the PLL circuit starts to operate and a desired voltage (2.5V) is applied to the loop filter circuit 35, the potential between the base and the emitter of the transistor 42 disappears and the transistor 42 disappears.
Turns off and the voice carrier is locked. That is, FIG.
As indicated by the solid line in FIG. 3, the power of the RF modulator is turned on, and the voltage of the loop filter circuit 35 is changed to the charge charging circuit 3.
The capacitors 40 and 41 of the loop filter circuit 35 are charged with electric charge until the voltage reaches a voltage at which the transistor 42 of No. 6 is turned off. As shown in FIG. 3A,
Lockup time = 300 msec. But,
When there is no charge charging circuit, the power is gradually charged by the time constant of the loop filter circuit after the power of the RF modulator is turned on as shown by the broken line in FIG. As shown in B of FIG. 3, lockup time = 500 msec.
Becomes That is, in the RF modulator of the present invention, the charge charging circuit 36 shortens the lockup time of the audio subcarrier (shortening of 200 msec).

Further, the charge charging circuit 36 according to one embodiment of the present invention is provided with the transistor 43, so that when the sound carrier frequency is switched (for example, when the voltage of the loop filter circuit is switched to a lower voltage). ), Discharge of the electric charge accumulated in the capacitors 40 and 41 of the loop filter circuit 35 can be shortened.

[0025]

As described above, according to the RF modulator of the present invention, one input is connected to the other output of the modulation circuit, and the other input has a switching circuit capable of variably setting the division ratio. A counter circuit to which the output is connected, a phase comparator to which the output of this counter circuit and the output of the reference oscillation circuit are connected to the respective inputs, a charge pump circuit connected to the output of this phase comparator, and Since the modulation circuit is connected to the output of the charge pump circuit via the loop filter circuit to form a PLL circuit, only one resonance circuit is required and no switch circuit is required, and miniaturization can be realized.

Further, since the loop filter circuit is provided with a charge charging circuit for charging a constant charge, the time period during which the audio carrier wave is locked at the set frequency when the RF modulator is powered on is shortened, and the counter circuit is provided. The number of counters can be set to a predetermined value by the switching circuit.

Further, since the PLL circuit is configured, it is not necessary to adjust the audio subcarrier frequency.

[Brief description of drawings]

FIG. 1 is a block diagram of an RF modulator according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a loop filter and a charge charging circuit according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing an effect of lockup time of a voice carrier according to an embodiment of the present invention.

FIG. 4 is a block diagram of a conventional RF modulator.

[Explanation of symbols]

 21 1st input terminal 22 Waveform shaping circuit 23 2nd input terminal 24 FM modulation circuit 25 Carrier oscillation circuit 26 Mixer circuit 27 Output terminal 28 Counter circuit 29 Reference oscillation circuit 30 Phase comparator 31 Charge pump circuit 32 Third input Terminal 33 Fourth input terminal 34 Switching circuit 35 Loop filter circuit 36 Charge charging circuit 37 Resonance circuit

Front page continuation (72) Inventor Toshiaki Masagaki 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (1)

[Claims] 1. A first input terminal, a waveform shaping circuit connected to this first input terminal, a second input terminal, a modulation circuit connected to this second input terminal, and this modulation circuit. A mixer circuit to which the output of one of the circuits, the output of the waveform shaping circuit and the output of the carrier oscillation circuit are respectively input, and an output terminal to which the output of the mixer circuit is connected, and the other output of the modulation circuit Is connected to one input and the other input is connected to the output of a switching circuit in which the variable division ratio can be variably set, and the output of this counter circuit and the output of the reference oscillation circuit, respectively. A phase comparator connected to the input of the phase comparator, a charge pump circuit connected to the output of the phase comparator, and the output of the charge pump circuit to the modulation circuit via a loop filter circuit to form a PLL circuit. Constitution At the same time, the loop filter circuit is an RF modulator to which a charge charging circuit for charging a constant charge is connected.