JP2001274681A - Pll circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize stability in an oscillation frequency and to shorten a lock-up time by changing a supply current in accordance with a phase deviation while using a simple loop filter as a conventional one in a PLL circuit. SOLUTION: A current supply circuit 14 for outputting a current Id which is increased with a time indicating a phase difference between the delay amount and the advance amount of a comparison output in a phaser comparator 13 is arranged between the phase comparator 13 and the loop filter 15 in the PLL circuit. Then the control voltage of a voltage control oscillator 16, which is the output voltage of the loop filter 15, is operated to be large when the phase is largely deviated and, then, the phase difference is made to be small in a short time. That is, the lock-up time is shortened. When the phase deviation is the small one, the control voltage of the voltage control oscillator is kept small so that excessive control is prevented to stabilize the oscillation frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase locked loop circuit (hereinafter referred to as a PLL circuit) used as a frequency source, and more particularly to a phase locked loop (PLL) circuit having a reduced lock-up time.
It relates to an L circuit.

[0002]

2. Description of the Related Art PLL circuits are incorporated in ICs and LSIs and used as frequency sources for various circuits. As a final product, it is mounted on many products such as FDD, CD, MD, and DVD.

FIG. 4 shows a P which is generally used in the prior art.
FIG. 5 is a diagram illustrating a configuration of the LL circuit, and FIG. 5 is a diagram illustrating a supply current characteristic accompanying the phase comparison.

In FIG. 4, a first comparison input a obtained by dividing an input signal frequency Fin by a frequency dividing circuit 41 and a second comparison input b obtained by dividing an output signal frequency Fout by a frequency dividing circuit 42.
Is compared by the phase comparator 43, and the phase comparison output c is supplied to the loop filter 44 only during the period of the phase difference between the two inputs. The loop filter 44 is usually composed of a capacitor, a resistor and the like. Then, the oscillation frequency of the voltage controlled oscillator 45 is controlled such that the frequency and the phase of the second comparison input b are equal to those of the first comparison input a.

The shorter the oscillation frequency of the voltage controlled oscillator 45 becomes stable, that is, the shorter the lock-up time, the better the performance of the PLL circuit. To shorten the time, a method of adjusting the characteristics of the loop filter is generally adopted. Have been. However, since the oscillation frequency needs to be stable at the same time as the lock-up time is shortened, there is naturally a limit in adjusting the characteristics of the loop filter to shorten the lock-up time.

On the other hand, a constant current source is often used as a current supply source from the phase comparator 43. In this case, as shown in FIG. The comparison output c is generated, that is, the constant current Id starts to flow, and the phase comparison output c stops at the rising of the second comparison input b. Thus, the constant current Id which does not change with time is supplied to the loop filter 44 only during the phase shift between the comparison inputs a and b. Accordingly, the hatched portion in FIG.

As a current supply source from the phase comparator 43, a power supply voltage may be used instead of a constant current source. In this case, the supplied current is controlled by a capacitor in the loop filter 44. As the battery is charged, it gradually becomes saturated with time. In this case, even if the initial current is the same as the constant current Id, the current supply amount is further smaller than in the case where a constant current source is used.

[0008]

As described above, in the conventional PLL circuit, even if a constant current source is used as the current supply source from the phase comparator 43, the phase comparison is performed in accordance with the phase shift. The current supply from the heater 43 (current × time) changes only proportionally (primarily).
, The change in the control voltage is small, resulting in a longer lock-up time.

Therefore, the present invention realizes the stability of the oscillation frequency and the shortened lock-up time by changing the supply current according to the phase shift while using the same simple loop filter as the conventional one. It is an object to provide a PLL circuit.

[0010]

According to a first aspect of the present invention, a PLL circuit compares a phase of a first comparison input with a phase of a second comparison input, and outputs a phase comparison result. A current supply circuit that outputs a current that increases with time indicating a phase difference between the amount of delay and the amount of advance of the comparison output, a loop filter having a capacitor that is charged or discharged by receiving a current from the current supply circuit; A voltage-controlled oscillator that receives an output of the filter as a control voltage, outputs an oscillation signal, and feeds back the oscillation signal as the second comparison input.

According to the PLL circuit of the first aspect of the present invention,
A current supply circuit is provided between the phase comparator and the loop filter for outputting a current that increases with time indicating the phase difference between the amount of delay and the amount of advance of the comparison output of the phase comparator.

As a result, the control voltage of the voltage-controlled oscillator, which is the output voltage of the loop filter, operates so as to increase when the phase is largely shifted, and the phase difference can be reduced in a short time. That is, the lockup time can be reduced. Further, when the phase shift is small, the control voltage of the voltage controlled oscillator remains small, so that excessive control is prevented and the oscillation frequency can be stabilized.

The loop filter may have a simple passive type having a capacitor, and does not require the use of an advanced filter such as an active filter.
The cost of the circuit can be reduced.

[0014]

FIG. 1 is a diagram showing an embodiment of a PLL circuit according to the present invention, and FIGS. 2A and 2B are diagrams showing examples of supply current characteristics accompanying the phase comparison. .

In FIG. 1, a first comparison input a whose input signal frequency Fin is frequency-divided by a frequency divider 11 and a second comparison input b whose output signal frequency Fout is frequency-divided by a frequency divider 12 are phase-shifted. The comparator 13 compares the phases, and the phase comparison output c is supplied to the current supply circuit 14 only during the period of the phase difference between the two inputs.

The current supply circuit 14, which is a feature of the present invention, outputs a current Id that increases with time indicating the phase difference between the amount of delay and the amount of advance of the comparison output c of the phase comparator 13.

The loop filter 15 is usually composed of a capacitor and a resistor. During the period in which the phase comparison output c shown in FIG. 2A is generated, the capacitor is charged by the supply current Id which continues to flow while increasing, a potential difference is generated at the output terminal, and the oscillation frequency of the voltage controlled oscillator 16 is controlled. Control voltage Vvco1. Then, the oscillation frequency of the voltage controlled oscillator 16 is controlled such that the frequency and the phase of the second comparison input b are equal to the frequency and the phase of the first comparison input a.

Now, the operation of the PLL circuit of the present invention will be described with reference to an example of the supply current characteristic accompanying the phase comparison in FIG. First, assuming that the phase of the first comparison input “a” is ahead of the phase of the second comparison input “b” as shown in FIG. 2A, the phase comparison output “c” becomes high at the rising time t1 of the first comparison input “a”. This phase comparison output c continues until the rising point t2 of the second comparison input b. The phase comparison output c generated at time t1 is input to the current supply circuit 14.

The current supply circuit 14 starts supplying the supply current Id to the loop filter 15 simultaneously with the input of the phase comparison output c, and the value of the supply current Id increases with the passage of time of the phase comparison output c. The charge of the capacitor of the loop filter 15 is charged by the supply current Id that increases with time as described above, and the charged voltage of the capacitor is supplied as the control voltage Vvco1 of the voltage controlled oscillator 16. The hatched portion in the figure indicates the current supply amount. In addition to the control voltage Vvco1, a voltage that determines a reference oscillation frequency is applied to the voltage controlled oscillator 16.

FIG. 2B shows a case where the phase of the second comparison input b is ahead of the phase of the first comparison input a, contrary to the case of FIG. 2A. The phase comparison output c is generated at the time when the comparison input b rises, and the phase comparison output c
Continue until the first comparison input a rises. The phase comparison output c generated at time t1 is input to the current supply circuit 14.

The current supply circuit 14 starts supplying the supply current Id to the loop filter 15 simultaneously with the input of the phase comparison output c, and the value of the supply current Id increases with the passage of time. However, the supply current Id in this case depends on the phase relationship between the first comparison input a and the second comparison input b, as shown in FIG.
It is a current flowing in the opposite direction to the case. Therefore, the charge of the capacitor of the loop filter 15 is discharged by the supply current Id that increases with time, and the voltage of the capacitor resulting from the discharge is supplied as the control voltage of the voltage controlled oscillator 16.

In the examples shown in FIGS. 2A and 2B, the current-time characteristic of the supply current Id from the current supply circuit 14 shows a characteristic that increases in the form of a triangular wave. A characteristic that increases in the form of a multi-order function higher than the order, or a characteristic that increases in the form of an exponential function or the like can be obtained.

FIG. 3A shows a current supply circuit 14 according to the present invention.
FIG. 9 is a diagram for explaining a control voltage Vvco1 of the voltage controlled oscillator 16 when it has a characteristic of increasing in the shape of a triangular wave.

When the current supply circuit 14 outputs a triangular wave at the peak k, Vvco1 = k (t2−t1) ² / (2 ·
C). Here, if the unit time is Δt,
Since (t2−t1) = n · Δt, Vvco1 =
k · n ² · (Δt) ² / (2 · C) ∝k · n ² / (2 ·
C) Note that n is the number of unit times Δt.

As is apparent from this equation, in this embodiment of the present invention, the current supply circuit 14 that outputs a current that increases with time indicating a phase difference is provided, so that the phase shift amount n is squared. Control voltage Vvco proportional to
It can be seen that 1 has occurred.

On the other hand, FIG. 3B shows the control voltage V of the voltage controlled oscillator 45 of the conventional example shown for reference.
It is a figure explaining vco2. Since the supply current Id is constant, Vvco2 = Id (t2-t1) /C=Id.n..DELTA.t
/ C∝Id · n. That is, in the conventional example, the control voltage Vvco2 which is linearly proportional to the phase shift amount n
Only occurs.

As described above, when a current supply circuit 14 having a characteristic of increasing in the form of a triangular wave is used, when the phase shift increases by a factor of two or three, the supply from the current supply circuit 14 is increased. Since the amount of current changes according to the squared characteristic of 4 times and 9 times, the control voltage increases when the phase shift is large, and as a result of this operation, the lock-up time is shortened. When the phase shift is small, the current supply circuit 1
4 supplies the same amount of current as the conventional phase comparator 13, so that the stability of the oscillation frequency can be ensured.

The current supply circuit 14 may have a characteristic that increases by a quadratic function or a higher-order function, or a characteristic that increases by an exponential function. In this case, the amount of current supplied from the current supply circuit 14 with respect to the phase shift between the two inputs is smaller than that in the case of a triangular waveform.
The lock-up time is further reduced because the change is larger and a larger control voltage is obtained.

It should be noted that the current supply circuit 14 only needs to have a characteristic of outputting a current that increases with time indicating a phase difference, and may be configured by combining various characteristics.

[0030]

According to the PLL circuit of the first aspect of the present invention, a current supply circuit that outputs a current that increases with time indicating a phase difference between a delay amount and a lead amount of a comparison output of a phase comparator is provided by a phase comparison circuit. The control voltage of the voltage-controlled oscillator, which is the output voltage of the loop filter, operates so as to increase when the phase is greatly deviated. Can be reduced. That is, the lockup time can be reduced. Further, when the phase shift is small, the control voltage of the voltage controlled oscillator remains small, so that excessive control is prevented and the oscillation frequency can be stabilized.

Further, the loop filter may be of a simple passive type having a capacitor, and it is not necessary to use an advanced filter such as an active filter.
The cost of the circuit can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration according to an embodiment of a PLL circuit of the present invention.

FIG. 2 is a diagram showing an example of a supply current characteristic accompanying a phase comparison of the PLL circuit of the present invention.

FIG. 3 is a diagram illustrating a control voltage of a voltage controlled oscillator.

FIG. 4 is a diagram showing a configuration of a conventional PLL circuit.

FIG. 5 is a diagram showing an example of a supply current characteristic accompanying a phase comparison of a conventional PLL circuit.

[Explanation of symbols]

 11 frequency divider circuit 12 frequency divider circuit 13 phase comparator 14 current supply circuit 15 loop filter 16 voltage controlled oscillator Fin input signal frequency Fout output signal frequency Id supply current

Claims (1)

[Claims] 1. A phase comparator for comparing a phase of a first comparison input with a phase of a second comparison input and outputting a phase comparison result, and a phase difference of a delay amount or a lead amount of a comparison output of the phase comparator. A current supply circuit that outputs a current that increases with time, a loop filter that has a capacitor that receives or charges or discharges the current from the current supply circuit, an oscillation signal that receives the output of the loop filter as a control voltage, And a voltage-controlled oscillator that feeds back the oscillation signal as the second comparison input.