JPH0969986A - Rf modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a SAW(surface acoustic wave) resonator, and decrease the number of components and lower the cost by employing a PLL frequency synthesizer system for the RF modulator. SOLUTION: The RF modulator consists of a PLL circuit composed of a 1/N frequency divider circuit 1, a phase detector circuit 2, a low-pass filter 3, a voltage-controlled oscillator circuit 4, a 1/P frequency divider circuit 5, and a variable frequency divider circuit (counter) 6. Then the frequency dividing rate of the variable frequency divider circuit 6 is switched between a USA channel and a Japan channel at a selection terminal S1 of a frequency dividing ratio selecting circuit 7 and between an Lo channel and a Hi channel at a selection terminal S2.

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 a video tape recorder (VTR) or the like.

[0002]

2. Description of the Related Art A VTR reproduction signal or a reception signal output from a VTR tuner is output as an RF signal from a VTR RF modulator, and the output signal can be received by a television tuner to watch a video.

Therefore, the output signal of the RF modulator defines a specific channel frequency. For example, in the case of NTSC, 3 channels (61.25 MHz) of Lo channel and 4c of Hi channel in USA channel are specified.
In the Japanese channel, the Lo channel is 1ch (91.25MHz) and the Hi channel is 2ch (97.25MHz).

Therefore, as shown in FIG. 2, in the USA channel specification RF modulator, 3ch or 4ch,
In the RF modulator of the Japanese channel specification, the element that resonates at the frequency of 1ch or 2ch is composed of an oscillation circuit using two surface acoustic wave resonators (SAW resonator 8), and each element in the SAW resonator is ch. The changeover switch 9 is used selectively.

[0005]

However, although the oscillation circuit using the SAW resonator is excellent in frequency accuracy and temperature characteristics, the cost of the SAW resonator is high, which prevents the cost of the RF modulator from being reduced.

Therefore, it is conceivable to construct a voltage controlled oscillation circuit by using an inexpensive air-core coil, a capacitor and a varactor diode without using a costly SAW resonator. Such a so-called LC oscillation In the circuit,
Frequency drift due to frequency precision and temperature is large, and standard value of frequency precision of general RF modulator: ± 80M
There is a problem that Hz cannot be satisfied.

[0007]

In order to solve the above problems, the present invention provides a reference input signal, a voltage control type oscillation circuit which oscillates at a predetermined frequency, and an oscillation frequency signal from the oscillation circuit which is variable. A variable frequency dividing means for performing frequency division, a phase comparing means for comparing the phases of the variable frequency dividing signal frequency-divided by the variable frequency dividing means and the reference input signal, and a phase difference signal detected by the phase comparing means. A PLL (Phase) including a low-pass filter that integrates and supplies to the oscillation circuit.
In an RF modulator having a Locked Loop circuit, an RF modulator provided with a frequency division ratio switching means for switching the frequency division ratio of the variable frequency division means is provided.

Further, in the present invention, the dividing ratio switching means is
First selecting means for selecting a country in which the RF modulator is used, and Hi in the country selected by the first selecting means
Second selecting means for selecting either the channel or the Lo channel.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the drawings.

In the present invention, the RF modulator is a PLL.
By using the (Phase Locked Loop) frequency synthesizer method, a simple and inexpensive RF modulator can be provided while maintaining high frequency accuracy.

FIG. 1 shows the circuit configuration of the essential parts of the RF modulator of the present invention. In the figure, the reference input signal (x'ta
l) is an oscillation frequency signal of 4 MHz from a crystal oscillator, 1
Is a 1 / N divider circuit (N = 51) that divides this reference input signal to 1/512 to generate a reference frequency (fr) signal.
2), 4 is a voltage control type oscillation circuit that oscillates at a predetermined frequency, 5 is a 1 / P frequency divider, and 6 is the voltage control type oscillation circuit 4
A variable frequency dividing circuit for further variably frequency dividing the oscillation frequency signal from 1 through the 1 / P frequency divider 5 includes a variable frequency dividing signal frequency-divided by the variable frequency dividing circuit 6 and the reference input signal. Phase detection circuit for comparing phases, 3 is the phase detection circuit 2
A low-pass filter for integrating the phase difference signal detected by the above and supplying it to the voltage controlled oscillation circuit 4, a frequency division ratio selection circuit 7 for switching the frequency division ratio of the variable frequency division circuit 6, S1
And S2 are selection terminals for setting the DC potential.

The frequency division ratio selection circuit 7 selectively switches the frequency division ratio of the variable frequency division circuit 6 by closing (Lo) and opening (Hi) the respective selection terminals, so that the desired output frequency fo can be obtained. It is configured to set the frequency of the channel.

That is, the output frequency (fo) is obtained by the following equation,

[0014]

[Equation 1]

In the above equation, fr = 4 MHz / 512
= 7.8125 KHz, P = 32, the variable frequency dividing circuit 6
By selecting the frequency division ratio X of X in accordance with the states of the selection terminal S1 and the selection terminal S2 as shown in the table below,

[0016]

[Table 1]

When the selection terminal S1 is closed (Lo), the USA channel is selected, and when the selection terminal S1 is opened (Hi), the Japanese channel is selected. Further, the selection terminal S2 is selected.
When closed (Lo), Hi channel (USA:
4ch, Japan: 2ch), select terminal S2 open (H
If set to i), Lo channel (USA: 3ch, Japan:
1ch) is selected.

With this configuration, in one RF modulator, it becomes possible to easily use different destinations, for example, by setting the state of each selection terminal by the presence or absence of a jumper chip.

To the selection terminal S2 for switching between the Hi channel and the Lo channel, an electric signal from the VTR main body or an OV (Lo) or open (Hi) potential is applied by a mechanical switch or the like.

The circuit of the present invention handles 100 frequencies.
Since it is as low as MHz or less, it does not require a special high frequency technology which is often found in the UHF band. If a variable frequency dividing circuit that can handle about 100 MHz is used, the 1 / P frequency dividing circuit 5
Can be eliminated, and a simpler circuit can be obtained.

The reference input signal (x '
It is also possible to share the crystal oscillator that supplies tal) with the crystal oscillator of another circuit block. For example, a 4 MHz oscillation frequency signal for PLL of a television tuner,
By utilizing the oscillation frequency signal of 3.58 MHz for processing the video signal on the VTR main body side, the crystal oscillator for generating the oscillation frequency signal of 4 MHz can be eliminated.

Further, when the crystal oscillator for the PLL of the television tuner is used, when the reference frequency signal (7.8125 MHz) after being divided into 1/512 by the PLL circuit of the television tuner is quoted: Not only is the / N frequency dividing circuit 1 unnecessary, but even if the distance between the tuner and the RF modulator is long, mutual interference due to the signals is extremely unlikely to occur.

Further, since the frequency difference between the Lo channel and the Hi channel is 6 MHz for both the USA channel and the Japan channel, the phase detection circuit 2 has a wide pull-in range and a circuit configuration capable of pulling in the frequency difference of 6 MHz. Then, it is not necessary to use a varactor diode in the LC oscillation circuit in the voltage controlled oscillation circuit 4, which can further contribute to simplification of the circuit and cost reduction.

[0024]

As described above in detail, according to the present invention, the reference input signal, the voltage control type oscillation circuit which oscillates at a predetermined frequency, and the oscillation frequency signal from the oscillation circuit are frequency-divided. Variable frequency dividing means, phase comparing means for comparing the phases of the variable frequency dividing signal frequency-divided by the variable frequency dividing means and the reference input signal, and the phase difference signal detected by the phase comparing means is integrated. And a low-pass filter for supplying the oscillation circuit to the oscillation circuit (Phase Locked L).
In the RF modulator having the loop circuit, since the frequency division ratio switching means for switching the frequency division ratio of the variable frequency division means is provided, it is possible to provide U without the expensive SAW resonator.
It can be used as an RF modulator corresponding to the SA channel and the Japanese channel, which is extremely useful for reducing the number of parts and cost.

Further, according to the present invention, the division ratio switching means includes first selecting means for selecting a country in which the RF modulator is used, and Hi channel in the country selected by the first selecting means. Further, since the second selection means for selecting any one of the Lo channels is provided, it is possible to easily switch between the USA channel and the Japanese channel and the Hi channel and the Lo channel.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing a main configuration of an RF modulator of the present invention.

FIG. 2 is a circuit block diagram showing a configuration of a conventional RF modulator.

[Explanation of symbols]

 1 1 / N division ratio circuit 2 phase detection circuit 3 low pass filter 4 voltage control type oscillation circuit 5 1 / P division circuit 6 variable division circuit 7 division ratio selection circuit S1 selection terminal S2 selection terminal

Claims (2)

[Claims] 1. A reference input signal, a voltage-controlled oscillating circuit that oscillates at a predetermined frequency, a variable frequency dividing means that variably divides an oscillation frequency signal from the oscillating circuit, and a variable frequency dividing means. A PLL comprising phase comparison means for comparing the phases of the divided variable frequency divided signal and the reference input signal, and a low-pass filter for integrating the phase difference signal detected by the phase comparison means and supplying it to the oscillation circuit. (Phase
An RF modulator having a Locked Loop circuit, characterized in that frequency division ratio switching means for switching the frequency division ratio of the variable frequency division means is provided. 2. The frequency division ratio switching means includes first selection means for selecting a country in which the RF modulator is used, and the first selection means.
The RF modulator according to claim 1, further comprising a second selection unit that selects either the Hi channel or the Lo channel in the country selected by the selection unit.