JPH10164444A - Pll circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate special oscillator for control, to reduce the number of external components and elements for an IC and to facilitate the formation of IC by automatically controlling the central frequency of frequency modulator at the PLL circuit of RF modulator while using an oscillator to be used for the frequency conversion of chrominance signal. SOLUTION: A 1/35 frequency divider 13 divides a chrominance subordinate carrier wave frequency oscillated by an OSC 12 inside a chroma signal processing circuit 11 and outputs the result to a phase comparator 7 of PLL circuit 9. The phase comparator 7 compares the phase of output signal from a 1/44 frequency divider 14, for dividing the central frequency of frequency modulator VCO 1 with the phase of output signal from the 1/35 frequency divider 13, a voltage for controlling the oscillation frequency of VCO 1 is generated, and the VCO 1 is controlled through an LPF 8 and an adder 3. At such a time, the frequency oscillated by the VCO 1 has sufficiently allowable frequency difference from the required central frequency. Thus, a crystal oscillator for regulating the central frequency of VCO 1 is dispensed with IC formation is facilitated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PLL circuit of an RF modulator for RF modulating a video signal or an audio signal used in a home VTR or the like, and more particularly to a PLL circuit used for RF modulating an audio signal. Circuit.

[0002]

2. Description of the Related Art For example, when a video signal and an audio signal reproduced by a home VTR are reproduced by a TV receiver, they are first converted into RF (radio frequency) signals and then applied to an antenna of the TV receiver. I am trying to do it. In the case of an audio signal, the baseband audio signal is converted to a 4.5 MHz VCO
(FM modulator), and further modulates the frequency in channel 2 (9
7.25MHZ)
1.75 MHZ.

In this case, a PLL circuit may be used to frequency-modulate a baseband audio signal with a 4.5 MHz VCO. Such a PLL circuit is shown in FIG. The VCO (1) in FIG. 2 oscillates with a center frequency of 4.5 MHz. A baseband audio signal is applied to the input terminal (2) and applied to the adder (3). The signal from the adder (3) is applied to the control terminal of the VCO (1), and the VCO oscillates with a center frequency of 4.5 MHz.
The frequency modulation of (1) is performed.

The output signal of 4.5 MHZ of the VCO (1) is divided by 1/36 by the 1/36 frequency divider (4) to obtain 125000H.
Z frequency. An oscillator (5) that oscillates at a constant frequency of 4.0 MHZ and a 1/32 divider (6) that divides 1/32 are used to generate a signal having a frequency equal to the frequency of 125000 Hz. By dividing the frequency of the 4.0 MHZ signal by 1/32, a signal of 125000 HZ can be created.

For this reason, 12,500 is added to the phase comparator (7).
Two signals having the same frequency of 0HZ are applied, phase comparison is performed, and a signal corresponding to the difference is applied to the LPF (8). The frequency and phase of the VCO (1) are adjusted according to the output signal of the LPF (8), and the PLL circuit (9) locks on the phase of the output signal of the oscillator (5).

Since the oscillation frequency of the oscillator (5) is accurate, the VCO (1) oscillates with a center frequency of exactly 4.5 MHz. Accordingly, a signal obtained by FM-modulating a baseband audio signal with a carrier signal having a frequency of 4.5 MHz is obtained from the output terminal (10).

[0007]

However, the PLL circuit of FIG. 2 has a problem that a dedicated oscillator (5) is required to generate a signal of 125000 HZ. Since the oscillation frequency of the oscillator (5) needs to be accurate, a crystal oscillator is required. Generally, the PLL circuit shown in FIG.
Was unsuitable for use in an IC.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has an oscillator which oscillates at a color subcarrier frequency of 3579545HZ of the NTSC system, and an oscillation output signal of the oscillator is reduced by 1/35. A first frequency divider that oscillates, a VCO that oscillates with a center frequency of 4500000 HZ that is an audio intercarrier frequency, a second frequency divider that divides the oscillation output signal of the VCO by 1/44, and the first and second frequency dividers. A phase comparator for comparing the phases of the frequency-divided output signals of the two-frequency divider,
And an LPF for smoothing an output signal of the phase comparator and applying the smoothed signal to the VCO as a control voltage of an oscillation frequency.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a PLL circuit according to the present invention. (11) is a chroma signal processing circuit for recording and reproducing chroma (color) signals, and (12) is a chroma signal processing circuit ( An oscillator which is built in 11) and oscillates at 3579545HZ which is a color subcarrier frequency fSC of the NTSC system and is used for frequency conversion of a chroma signal, and (13) divides the oscillation output signal of the oscillator (12) by 1/35. The first frequency divider (14) outputs the oscillation output signal of the VCO (1) by 1/44
A second frequency divider for frequency division, (15) an input terminal to which a baseband video signal is applied, (16) an input terminal to which a carrier having a frequency of a channel 2 (97.25 MHz) of a TV receiver is applied, (17) is a frequency-modulated audio intercarrier frequency signal from the VCO (1) and the input terminal (1).
The signal obtained by frequency-converting the video signal from 5) with the carrier from the input terminal (16) is added to the AM-modulated signal according to the carrier from the input terminal (16), and is derived to the output terminal (18). RF mixing circuit.

In FIG. 1, the same circuit elements and circuit blocks as those in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. In the present invention, the VTR includes an oscillator (12) that oscillates at 3579545 HZ, which is the color subcarrier frequency of the NTSC system, and that is used for frequency conversion of the chroma signal, in the chroma signal processing circuit that performs the recording and reproduction processing of the chroma signal. And the VCO (1) using the oscillator (12).
The frequency of is adjusted.

35795 which is the frequency of the oscillator (12)
45HZ is 450 which is the voice intercarrier frequency.
Since there is no greatest common divisor as the reference frequency between 0000HZ and 0000HZ, it is considered that phase synchronization cannot be normally performed. However, the greatest common divisor can be obtained by allowing a certain amount of error margin.

The color subcarrier fSC = 35795545HZ, the audio intercarrier frequency fVC = 4500000HZ (nominal). The frequency division number of the first frequency divider (13) is n, the frequency division number of the second frequency divider (14) is m, the output signal frequency of the first frequency divider (13) is frn, and the VCO (1) The oscillation frequency is fVCO
Assuming that 1, frn = fSC / nm ≦ fVCO1 / frn <m + 1. At this time, when the PLL operates, the phase comparator (7)
Feedback is performed so that the frequencies match at fVCO1 = mfrn.

At this time, the frequency difference Δf becomes Δf = fVCO1−fVC. The minimum value of n when Δf ≦ 1000HZ is n
= 35, and m at this time is 44. At this time, the frequency difference Δf is 0.5HZ7, which is a sufficiently allowable frequency difference.

The frequency division number n of the first frequency divider (13), the frequency division number m of the second frequency divider (14), the output signal frequency of the first frequency divider (13) is frn, and the frequency difference Δf Is shown in FIG. As shown in FIG. 3, except for N = 35, the frequency difference Δf is
It becomes a very large value and cannot be put to practical use. However, fVC is limited to the combination of n = 35 and m = 44.
O1 will be very close to the nominal value.

For this reason, the frequency of the VCO (1) in FIG. 1 is automatically adjusted based on the accurate frequency of the oscillator (12). As a result, a signal obtained by FM-modulating a 4.5 MHZ carrier from the VCO (1) with a baseband audio signal is obtained and applied to the RF mixing circuit (17).

[0016]

As described above, according to the present invention, the RF
4500000 used for modulator PLL circuit
The center frequency of a VCO that oscillates with HZ as the center frequency can be automatically adjusted using an oscillator that oscillates at 3579545 HZ and is used for frequency conversion of color signals. Therefore, a special oscillator for adjustment becomes unnecessary, and the number of external components and elements of the IC can be reduced, which is suitable for the IC.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a PLL circuit of the present invention.

FIG. 2 is a circuit diagram showing a conventional PLL circuit.

FIG. 3 is a numerical table for explanation of FIG. 1;

[Explanation of symbols]

 (1) VCO (7) Phase comparator (8) LPF (12) Oscillator (13) First frequency divider (14) Second frequency divider

Claims (2)

[Claims] 1. An oscillator which oscillates at 3579545HZ which is a color subcarrier frequency of the NTSC system, a first frequency divider which divides an oscillation output signal of the oscillator by 1/35, and a voice intercarrier frequency which is 4500000HZ
VCO that oscillates at a center frequency, a second divider that divides the oscillation output signal of the VCO by 1/44, and a phase comparator that compares the phases of the divided output signals of the first and second dividers And a LPF for smoothing an output signal of the phase comparator and applying the same to the VCO as a control voltage of an oscillation frequency. 2. An oscillator which oscillates at 3579545HZ which is a color subcarrier frequency of the NTSC system and is used for frequency conversion of a chrominance signal, a first frequency divider which divides an oscillation output signal of the oscillator by 1/35, 4500000HZ which is the voice intercarrier frequency
A VCO that oscillates at a center frequency, a second divider that divides the oscillation output signal of the VCO by 1/44, a third divider that further divides the output signal of the first divider, A fourth frequency divider for further dividing the output signal of the second frequency divider, a phase comparator for comparing the phases of the divided output signals of the third and fourth frequency dividers, and an output of the phase comparator And a LPF for smoothing a signal and applying the signal to the VCO as a control voltage of an oscillation frequency.