Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Converting input current or voltage to output frequency
Reexamination Certificate
1998-04-28
2001-02-13
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Converting input current or voltage to output frequency
C327S102000, C327S158000, C331S034000, C331S00100A
Reexamination Certificate
active
06188252
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having an oscillation circuit and, more particularly, to a horizontal oscillation circuit which can change the frequency of a horizontal sync signal over a wide range and is applied to a monitor or a television receiver.
2. Description of the Related Art
A PLL (Phase Locked Loop) circuit is used as a horizontal oscillation circuit in a monitor used for, e.g., a computer, or a television receiver. This PLL circuit has a voltage-controlled oscillator (to be referred to as a VCO hereinafter) for generating a clock signal having a frequency n (n is an integer) times higher than the frequency of a horizontal sync signal.
Conventionally, the VCO is only required to generate clock signals of the same frequency. A recently developed monitor called a multi-scanning monitor can switch scanning speeds. The frequency of a horizontal sync signal supplied from a computer to a monitor of this type changes, for example, in the range of 20 kHz to 100 kHz. For this reason, a VCO applied to a horizontal oscillation circuit is required to be capable of oscillating over a wide range of the frequencies of horizontal sync signals.
If one VCO is made to oscillate over a wide frequency range, the VCO requires a high oscillation gain. If a high gain is set, the frequency response speed increases, resulting in a deterioration in jitter characteristics.
In order to solve this problem, a plurality of VCOs with different oscillation frequency ranges may be arranged such that the VCOs are switched to operate in accordance with the frequency of a horizontal sync signal. In this case, however, in addition to a plurality of VCOs, the oscillation circuit requires a frequency/voltage converter for generating a voltage corresponding to the frequency of a horizontal sync signal, resulting in an-undesirable increase in circuit size.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a horizontal oscillation circuit which can prevent a deterioration in jitter characteristics by suppressing the oscillation gain of a VCO, and prevent an increase in circuit size.
In order to achieve the above object, there is provided a semiconductor integrated circuit having an oscillation circuit, comprising:
frequency detection means for detecting a frequency of a horizontal sync signal, the frequency detection means generating a switching signal in accordance with the detected frequency; and
a voltage-controlled oscillator which has a plurality of oscillation modes obtained by dividing a frequency equal to an integer multiple of the frequency of the horizontal sync signal into a plurality of frequency ranges, and oscillates signals in the respective frequency ranges in accordance with control voltages, the oscillation modes of the voltage-controlled oscillator being switched in accordance with a switching signal output from the frequency detection means.
According to the present invention, signals in a wide frequency range can be oscillated by one voltage-controlled oscillator. In addition, since the frequency range in each oscillation mode of the voltage-controlled oscillator is narrow, the oscillation gain of the voltage-controlled oscillator can be decreased, and jitter can be reduced. Since the frequency detection circuit can be realized by a simple circuit, an increase in the overall circuit size can be suppressed.
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Callahan Timothy P.
Kabushiki Kaisha Toshiba
Luu An T.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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