JP2907152B2 - PLL circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PLL circuit using a charge pump circuit, and more particularly to a PLL circuit capable of reducing a spurious signal of a phase comparison frequency component accompanying an output signal of the PLL circuit.

[0002]

2. Description of the Related Art PL using a conventional charge pump circuit
As shown in the block diagram of FIG.
A voltage-controlled oscillator 3 capable of varying an output frequency by a control voltage, a phase comparator 1 for comparing the phase of the output signal with a reference signal, and a charge pump circuit for outputting a current signal according to the comparison result of the phase comparator 1 And a loop filter circuit 4 for transmitting an output of the charge pump circuit 2 to generate a control signal, and the output voltage of the loop filter circuit 4 is fed back to the voltage controlled oscillator 3 as a control signal, thereby providing a PLL circuit. 10.

In the loop filter circuit 4, a capacitor 15 is normally grounded to an input 8 as shown in FIG. 5 (b), and a resistor 16 and a capacitor 17 are connected in parallel.
Are connected to each other in series, and a passive filter that obtains an output 9 by grounding a series circuit, or an operational amplifier 51 as shown in FIG.
An active filter in which a parallel circuit of a resistor 15 and a series circuit in which a resistor 16 and a capacitor 17 are connected in series is often used.

When the PLL circuit 10 is in phase synchronization, no signal is output to the output of the charge pump circuit 2, and the output terminal 11 of the charge pump circuit is in a high impedance state to control the control voltage of the voltage controlled oscillator. Is kept constant. In an actual PLL circuit, the charge pump circuit 2 is not operated even in the phase-locked state because the dead zone of the phase comparator is eliminated or the high impedance state in which the output terminal 11 of the charge pump circuit 2 can be realized is limited. Output signals of the phase comparison frequency component in the sucking direction and the discharging direction are output.

Ideally, these signals are output so as to cancel each other. However, in practice, a slight difference occurs, and the signal of the phase comparison frequency component is generated by the ripple of the control voltage of the voltage controlled oscillator 3. Voltage, and the output signal of the voltage controlled oscillator is frequency-modulated by the ripple voltage,
At the output 14 of the voltage controlled oscillator 3, a spurious of a phase comparison frequency component accompanying the desired output signal is generated.

In general, the ripple voltage generated for the above-described reason can be controlled by a loop filter 4 or by adding a low-pass filter or a rejection filter to the loop filter 4 so that a desired signal-to-spurious ratio can be obtained. Can be attenuated.

When the components used in the reduction filter and the like have a loss current, the PLL circuit operates so as to compensate for the loss current, so that the charge pump circuit 11 always operates at the cycle of the phase comparison frequency. As a result, the ripple of the phase comparison frequency component increases in the control voltage of the voltage controlled oscillator 3, the spurious of the frequency modulation component in the output of the voltage controlled oscillator 3 increases, and the purity of the desired signal is reduced. This is a problem of deterioration.

In the technique described in Japanese Utility Model Application Laid-Open No. 63-033221, a method of compensating for this loss current by providing a constant current source at the output of the charge pump circuit to reduce the spurious of the phase comparison frequency component is disclosed. Has been stated.

[0009]

The problem with the prior art described above is that in a PLL circuit in which a desired signal-to-spurious ratio is obtained in a normal operating state, a change in temperature or humidity causes a change. When the insulation of the signal line from the output of the charge pump circuit 2 to the voltage controlled oscillator 3 is deteriorated and an impedance is generated between the signal line and the surrounding ground pattern or power supply wiring, the voltage controlled oscillator 3 of the PLL circuit is used. In this case, the signal ratio between the output signal 14 and the spurious of the phase comparison frequency component accompanying the output signal is deteriorated.

The first reason is that leak current flows in from outside the signal line or leaks out to the outside via the impedance between the signal line and the surrounding ground pattern or power supply wiring. .
Since the PLL circuit operates to compensate for the leakage current, the charge pump circuit 2 always outputs a signal for compensating the leakage current at the cycle of the phase comparison frequency. The ripple of the frequency component increases, the spurious of the frequency modulation component at the output of the voltage controlled oscillator 3 increases, and the purity of the desired signal deteriorates. In the configuration in which the constant current circuit section is provided at the output of the charge pump circuit as described in the related art, the purpose is to compensate for a fixed component called component loss current. , Can not be reduced.

Another reason is that the balance between the output signal in the discharge direction and the output signal in the suction direction of the charge pump circuit, which is output so as to almost cancel each other out in the normal phase synchronization state, depends on the load of the charge pump circuit. For this reason, it is deteriorated by changing.
Also in this case, the ripple of the phase comparison frequency component increases in the control voltage of the voltage controlled oscillator 3, the spurious of the frequency modulation component in the output of the voltage controlled oscillator 3 increases, and the purity of the desired signal deteriorates. .

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems described above and to prevent spurious degradation of a phase comparison frequency component caused by insulation degradation due to a change in temperature and humidity.

[0013]

The PLL circuit of the present invention comprises:
A voltage-controlled oscillator whose output frequency is varied by a control voltage; a phase comparator for comparing the phase of an output signal of the voltage-controlled oscillator with a reference signal; and a charge pump circuit for outputting a signal corresponding to the comparison result of the phase comparator And a PLL circuit comprising a loop filter circuit that generates the control voltage based on the output of the charge pump circuit, wherein all of the signal patterns from the output of the charge pump circuit to the input of the voltage controlled oscillator and the components of the loop filter circuit Alternatively, a part of the guard pattern is provided, and a guard pattern having a predetermined potential is provided.

Further, the predetermined potential is a voltage at the center of a change range of the control voltage.

Further, the predetermined potential is supplied by connecting a DC voltage source to the guard pattern when the loop filter is of a passive type.

Here, the predetermined potential is supplied by connecting the control voltage to the guard pattern via a buffer amplifier when the loop filter is of a passive type.

The predetermined potential is, when the loop filter is an active filter using an inverting amplifier of an operational amplifier, an output of the charge pump circuit, a signal line to an inverting input terminal of the operational amplifier, and the operational amplifier. And a non-inverting input terminal of the operational amplifier is connected to the second guard pattern.

As described above, according to the present invention, even when the insulation of the circuit signal pattern connected to the output of the charge pump circuit is deteriorated, substantially the same voltage as the signal pattern is applied to the periphery of the signal pattern. It is surrounded by another pattern, and there is no potential difference between them, so there is no leakage current, and the load potential of the charge pump circuit does not fluctuate. It has the effect of removing.

[0019]

Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of the present invention. This figure is a diagram schematically showing components constituting a PLL circuit and a pattern. A phase comparator 1, a charge pump circuit 2, a loop filter circuit 4, and a voltage controlled oscillator 3 are mounted on a substrate, and are connected in a pattern. The passive type loop filter circuit 4 connected to the output terminal 11 of the charge pump circuit 2
2 and further connected to a frequency control terminal 13 of the voltage controlled oscillator 3 by a signal pattern 12. The pattern from the output terminal 11 of the charge pump circuit to the signal line 12 and the frequency control terminal 13 is surrounded by a pattern 6, and a voltage is applied to the pattern 6 by a voltage source 5. The potential of the voltage source 5 is set substantially equal to a frequency control voltage corresponding to a desired output frequency of the voltage controlled oscillator 3. Also,
The potential of the voltage source 5 is set to a voltage substantially at the center of the variable range of the frequency control voltage at which a desired output frequency can be obtained, or set to a load potential that does not degrade the balance between the suction and discharge currents of the charge pump circuit. .

In FIG. 1, the connection between the voltage controlled oscillator 3 and the phase comparator 1 and the input of the reference oscillation frequency to the phase comparator 1 are omitted.

The operation of the embodiment of the present invention shown in FIG. 1 will be specifically described with reference to FIG. In FIG. 2 (a),
1 shows a diagram in which a conventional charge pump circuit 2, a loop filter circuit 4, and a voltage controlled oscillator 3 are mounted on a printed circuit board.

It is assumed that the insulation of the substrate deteriorates due to changes in temperature and humidity, and that an impedance 21 of Z is generated between the signal line 12 and the ground 31 around the signal line 12.
The control voltage terminal 13 of the voltage controlled oscillator 3 in the phase locked state
Is VC, the leak current I flowing from the signal line 12 to the ground in this case is represented by I = VC / Z (1). As a result, the charge pump circuit 2 always outputs a signal for compensating the leak current at the cycle of the phase comparison frequency, and the ripple of the phase comparison frequency component increases in the control voltage of the voltage controlled oscillator 3, that is, the voltage controlled oscillator 3 The spurious of the frequency modulation component at the output 14 increases, and the purity of the desired signal is degraded. Further, the signal line 12 and the power supply line 3 having the potential VD around the signal line 12
When the impedance 22 is generated between the two, the leak current I at that time becomes I = (VD−VC) / Z (2) As a result, the purity of the desired signal is degraded as in the case where the leak current flows to the ground.

Next, the operation of the embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG. It is assumed that the insulation of the substrate is deteriorated due to a change in temperature and humidity, and an impedance 21 or 21 is generated between the signal line 12 and the pattern 6 surrounding the signal line 12. The potential V of the pattern 6 is controlled by the voltage source 5 in the voltage controlled oscillator 3 in a phase locked state.
Is set equal to the voltage VC of the control voltage terminal 13 of FIG. Here, the leak current I flowing between the signal line 12 and the pattern 6 is I = (VC−V) / Z = 0 (3), which is smaller than the conventional case of FIG. No current flows and therefore the signal purity does not degrade.

The leak current will be described with specific numerical values.

In FIG. 2A, for example, assuming that the voltage VC of the control voltage terminal 13 in the phase synchronization state is +2.5 V, the impedance Z caused by insulation deterioration is 1 MΩ, and the potential of the power supply line 32 is +5 V, the power supply line The leak current due to the poor insulation between the signal line 32 and the signal line 12 is I = (+ 5V − (+ 2.5V)) / 1MΩ = 2.5 μA (4) Similarly, the leak current due to the poor insulation between the ground 31 and the signal line 12 The current is (0V − (+ 2.5V)) / 1MΩ = −2.5 μA (5) Next, in FIG. 2B, the potential V
Is assumed to be set to +2.6 V, the leak current is (+2.6 V − (+ 2.5 V)) / 1 MΩ = 0.1 μA (6), and the leak current is compared with the case of FIG. Can be reduced by a factor of 25. Of course, it goes without saying that the closer the potential V of the voltage source 5 is to the potential VC of the control voltage terminal 13, the greater the reduction effect.

For this reason, it is desirable to use a configuration in which the potential V of the voltage source 5 is adjusted to be equal to the potential VC by using a variable voltage source.

Further, in the case where the frequency control voltage VC has a certain variable range, for example, +2.5 V ± 0.5 V, in the configuration of FIG. Will flow from 2.0 to 2.5 μA. On the other hand, in the configuration of the present invention, when calculated based on the equation (6), the leak current is 0.6 μA at the maximum, and it is possible to reduce the leak current by about 70% as compared with the conventional case.

In the above description, all of the components of the charge pump output terminal 11, the control input terminal 13 of the voltage controlled oscillator, and the loop filter 4 are surrounded by the pattern 6, but the leakage current is reduced little but not all. It is not necessary to surround it, and by surrounding the portion from the terminal 11 to the terminal 13 via the signal line 12, a considerable leak current can be reduced.

Next, FIG. 3A shows a second embodiment of the present invention. The configuration is the same as that of FIG. 1 except that the voltage source 5 in FIG.

The amplifier 40 outputs the same voltage as the voltage applied to the input terminal 41 from the output terminal 42. For example, as shown in FIG. 3B, a buffer amplifier circuit using an operational amplifier 43 It consists of. In this embodiment, the potential of the pattern 6 is always set to the signal line 12.
Therefore, even when the control voltage range of the voltage controlled oscillator 3 has a certain variable range, no leak current flows.

Next, a third embodiment of the present invention is shown in FIG. The configuration is the same as that of FIG. 1 except that the passive loop filter 4 in FIG. The active filter 50 includes an operational amplifier 51,
A voltage source 52 is provided to keep the load potential of the output 11 of the charge pump circuit 2 constant.

The output signal of the charge pump circuit 2 is transmitted by the loop filter 50, and generates a control voltage of the voltage controlled oscillator 3. The pattern from the output terminal 11 of the charge pump circuit 2 to the inverting input terminal 53 of the operational amplifier 51 through the signal line 12 is surrounded by another pattern 6, and a voltage from a voltage source 52 is applied to the pattern 6. Therefore, the potentials of the signal line 12 and the pattern 6 are kept equal.

Here, considering the case where the insulation of the substrate is deteriorated, the periphery from the output terminal 11 of the charge pump circuit 2 through the signal line 12 to the inverting input terminal 53 of the operational amplifier 51 is surrounded by the pattern 6 having the same potential as those. Therefore, no leak current flows. In addition, since the load of the charge pump circuit 2 does not have an impedance with respect to the surrounding power supply or ground, the load is always kept at a desired potential, and the balance between sinking and discharging of the output current of the charge pump circuit 2 is prevented from deteriorating. Is possible.

[0035]

As described above, the effect of the PLL circuit of the present invention is that the increase in spurious of the phase comparison frequency component due to deterioration of the insulation of the substrate due to a change in temperature and humidity can be suppressed.

This is because the entire signal line from the output of the charge pump circuit to the voltage controlled oscillator or a part thereof is surrounded by another pattern having a certain potential.
This is because leakage current due to deterioration of insulation can be suppressed. Another reason is that the load of the charge pump circuit does not change due to deterioration of insulation.

[Brief description of the drawings]

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

FIG. 2A is a circuit diagram for explaining an operation of a conventional PLL circuit. FIG. 2B is a circuit diagram for explaining the operation of FIG. 1.

FIG. 3A is a circuit diagram showing a second embodiment of the present invention. FIG. 3B is a diagram illustrating a circuit of the amplifier unit 40 in FIG.

FIG. 4 is a circuit diagram showing a third embodiment of the present invention.

FIG. 5A is a configuration diagram illustrating an example of a conventional PLL circuit. FIG. 6B is a circuit diagram showing an example of a passive loop filter for the loop filter 4 of FIG. FIG. 6C is a circuit diagram showing an example of an active loop filter for the loop filter 4 of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Phase comparator 2 Charge pump circuit 3 Voltage controlled oscillator 4 Loop filter circuit 5 Voltage source 6 Pattern 7 Reference signal 8 Input terminal 9 Output terminal 10 PLL circuit 11 Charge pump output terminal 12 Signal line 13 Control voltage terminal 14 Output 15 Capacitor 16 Resistance 17 Capacitor 21 Impedance 22 Impedance 31 Ground 32 Power supply wiring 40 Amplifier 41 Input terminal 42 Output terminal 43 Operational amplifier 50 Active filter 51 Operational amplifier 52 Voltage source 53 Inverting input terminal 54 Non-inverting input terminal

Claims (5)

(57) [Claims] 1. A voltage-controlled oscillator whose output frequency is varied by a control voltage, a phase comparator that compares a phase of an output signal of the voltage-controlled oscillator with a reference signal, and a signal corresponding to a comparison result of the phase comparator. In a PLL circuit comprising a charge pump circuit for outputting and a loop filter circuit for generating the control voltage based on an output of the charge pump circuit, a signal pattern from an output of the charge pump circuit to an input of the voltage controlled oscillator and the loop filter A PLL circuit surrounding all or a part of a circuit component and having a guard pattern having a predetermined potential. 2. The PLL circuit according to claim 1, wherein the predetermined potential is a voltage at the center of a change range of the control voltage. 3. The guard pattern according to claim 1, wherein the predetermined potential is supplied by connecting a DC voltage source to the guard pattern when the loop filter is of a passive type.
The PLL circuit as described in the above. 4. The control circuit according to claim 1, wherein the predetermined potential is supplied by connecting the control voltage to the guard pattern via a buffer amplifier when the loop filter is of a passive type. The PLL circuit as described in the above. 5. When the loop filter is an active filter using an inverting amplifier of an operational amplifier, the predetermined potential is an output of the charge pump circuit, a signal line extending to an inverting input terminal of the operational amplifier, and the operational amplifier. 2. The PLL circuit according to claim 1, wherein a second guard pattern surrounding the inverting input terminal is provided, and a non-inverting input terminal of the operational amplifier is connected to the second guard pattern.