Voltage-controlled oscillator

Details Voltage-controlled oscillator Voltage-controlled oscillator

2000-09-13

2001-09-25

Mis, David (Department: 2817)

Oscillators

Ring oscillators

C331S017000, C331S175000, C331S186000, C327S157000

Reexamination Certificate

active

06294963

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a voltage-controlled oscillator (VCO), especially to a voltage-controlled oscillator with improved power supply rejection ratio (PSRR) effects.
BACKGROUND OF THE INVENTION
A conventional voltage-controlled oscillator (VCO) is shown in FIG.
1
. Such a VCO is used in a phase lock loop (PLL). The power supply rejection ratio (PSRR) feature of the VCO is decisive of its jitter. As shown in
FIG. 1
, a VCO circuit comprises a charge pump circuit
10
and a ring oscillator
20
. The charge pump circuit
10
comprises two charge pumps, PUMP-up and PUMP-down. With the ON and OFF operations of two switches UP and DOWN in sequence, the voltage at PUMP may be controlled. VDD is power supplied to the circuit. In such a circuit, the voltage at PUMP changes along with the change of the voltage of VDD. The voltage of VDD-PUMT is transformed into current by transistor
11
and into a bias by transistor
12
. The bias is supplied to the ring oscillator
20
.
The ring oscillator
20
comprises several oscillator circuits
21
,
22
and
23
which are connected as a ring. Bias supplied by transistor
12
drives the ring oscillator
20
to generate oscillation. With this circuit, oscillation at a pre-determined frequency may be generated by the ring oscillator
20
according to the difference of the voltage at PUMP and the VDD voltage.
The above-mentioned conventional voltage-controlled oscillator is capable of generating oscillations under a pre-determined frequency according to the voltage of the input voltage. However, when the voltage of VDD has a sudden change, the oscillation frequency of the oscillator will raise in response to the raise of the VDD voltage. As a result, an undesired change will be caused to the oscillation frequency of the ring oscillator.
It is thus a need in the industry to provide an improved voltage-controlled oscillator in which the influence of the voltage of the power supply to the oscillator frequency of the oscillator may be moderated.
OBJECTIVES OF THE INVENTION
The objective of this invention is to provide a novel voltage-controlled oscillator wherein influence brought by the voltage of the power supply to the oscillator frequency of the ring oscillator may be reduced.
Another objective of this invention is to provide a voltage-controlled oscillator with an automatic compensation effect to the variation of the voltage of the power supply.
SUMMARY OF THE INVENTION
According to the voltage-controlled oscillator of this invention, a voltage compensation transistor is provided in the circuit of the voltage-controlled oscillator. The voltage compensation transistor is able to provide an equivalent resistance. When a sudden raise in the voltage of the power supply takes place, the equivalent resistance of the compensation transistor will drop, such that the bias voltage supplied to the oscillator may be pulled down, whereby the oscillation frequency of the ring oscillator may be adjusted.
The above and other objectives and features of the voltage-controlled oscillator of this invention may be clearly understood from the detailed description by referring to the drawings.


REFERENCES:
patent: 5896068 (1999-04-01), Moyal

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