JPH0738434A - Frequency synthesizer - Google Patents

Abstract

PURPOSE:To reduce a noise component from a charge pump and to prevent a time to be settled to a desired frequency when a large phase difference takes place from being increased in the frequency synthesizer used for a portable telephone set or the like. CONSTITUTION:When a phase difference detected by a phase comparator 14 is large, a control circuit 16 controls the synthesizer so that the conversion gain of a charge pump 15 is increased and the conversion gain of a loop filter 17 is decreased thereby increasing a variable range in a control voltage and decreasing a time to be settled to a desired frequency. Furthermore, when the phase difference is small, the control circuit 16 controls the synthesizer so that the conversion gain of the charge pump 15 is increased and the conversion gain of the loop filter 17 is decreased thereby reducing the noise component from the charge pump 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency synthesizer used for portable telephones, wireless devices and the like.

[0002]

2. Description of the Related Art The configuration of a conventional frequency synthesizer is shown in FIG.
Is shown in the functional block diagram of. This frequency synthesizer
Reference signal generator 10 that always generates a constant frequency signal
A reference signal divider 11 that divides the reference signal output from the reference signal generator 10, a voltage controlled oscillator 12 that outputs a frequency signal according to a predetermined control voltage, and the frequency signal is divided. A variable frequency divider 13, a phase comparator 14 for detecting a phase difference between the signal output by the reference signal frequency divider 11 and the signal output by the variable frequency divider 13, and the phase comparator 14 outputs the phase difference. The charge pump 15 that outputs a voltage corresponding to the phase difference signal, the loop filter 17 that converts the voltage output from the charge pump 15 and outputs the control voltage of the voltage controlled oscillator 12, and the reference signal frequency divider 11. And a control circuit 16 for outputting a frequency division ratio setting signal 28 to the.

The signal output from the reference signal generator 10 is divided by the reference signal frequency divider 11 and output to the phase comparator 14. The signal output from the voltage controlled oscillator 12 is frequency-divided by the variable frequency divider 13 and output to the phase comparator 14. The phase comparator 14 detects the phase difference between the signal divided by the reference signal divider 11 and the signal divided by the variable divider 13, and outputs the phase difference signal to the charge pump 15. The charge pump 15 generates a phase difference voltage according to the phase difference signal. The loop filter 17 converts the phase difference voltage into a control voltage and outputs it to the voltage controlled oscillator 12. The voltage controlled oscillator 12 oscillates a frequency according to the control voltage, and outputs a signal of a stable frequency to a transmitter or receiver (not shown) so that the phase difference is always small.

[0004]

By the way, the phase difference voltage output from the charge pump 15 has a noise component (a minute phase difference or the like) due to some influence. It is not possible to detect a minute phase difference. FIG. 5 shows the conversion characteristics when the conversion gain of the charge pump is small, where the horizontal axis represents the phase difference and the vertical axis represents the voltage value. As shown in FIG.
When the conversion gain of the charge pump is small, the change in the phase difference in the phase difference voltage cannot be detected.
The noise component included in the phase difference voltage is the loop filter 1
After being converted in 7, the voltage-controlled oscillator 12 is input and is output as a noise component of the oscillation signal of the voltage-controlled oscillator 12.

On the other hand, if the conversion gain of the voltage output from the charge pump 15 is increased and the conversion gain of the loop filter 17 is decreased according to the phase difference signal of the phase comparator 14 as shown in FIG. The generated noise component can be reduced. However, for a large phase difference, since the output voltage of the charge pump 15 is saturated and the conversion gain of the loop filter 17 is small, the control voltage becomes small when the phase difference is large as shown in FIG. However, there is a problem that it takes a long time to absorb and stabilize a large phase difference.

As described above, in the conventional frequency synthesizer, if the conversion gain of the charge pump and the conversion gain of the loop filter are set so that the noise component generated from the charge pump becomes small, the control voltage for a large phase difference becomes small, There is an inconvenience that it takes a long time to stabilize at a desired frequency. Further, if the conversion gain of the charge pump is reduced in order to prevent the control voltage from decreasing due to a large phase difference, the conversion gain of the loop filter will be increased, so that the noise component generated from the charge pump cannot be reduced. There was a problem.

An object of the present invention is to provide a frequency synthesizer in which the noise component from the charge pump is reduced and the time required to stabilize at a desired frequency does not increase when a large phase difference occurs. To do.

[0008]

To solve the above problems, in the frequency synthesizer according to the present invention, the charge pump has conversion gain switching means for switching the conversion gain of the voltage to be output according to the phase difference signal. However, the loop filter has conversion gain switching means for switching the conversion gain of the control voltage to be output corresponding to the input voltage, and the charge pump of the charge pump is switched in accordance with the level of the phase difference detected by the phase comparator. A control means for outputting a predetermined gain switching signal to the conversion gain switching means or the conversion gain switching means of the loop filter is provided, and the magnitude of the phase difference is input from the phase comparator to the control means. It is characterized in that each switching means is controlled in accordance with the phase difference.

[0009]

By the control of the switching means described above, the relationship between the phase difference detected by the phase comparator and the control voltage input to the voltage controlled oscillator is that the conversion gain of the charge pump is reduced when there is a large phase difference. , By increasing the conversion gain of the loop filter,
The characteristics are as shown in. Further, when the phase difference is small (stable state), the conversion gain of the charge pump is increased and the conversion gain of the loop filter is decreased, so that the characteristic shown in FIG. 8 is obtained.

That is, when the phase difference is large, the variable range of the control voltage can be increased, so that the time required to reach the desired frequency can be shortened. When the generated phase difference is small, the conversion gain of the charge pump can be increased and the conversion gain of the loop filter can be decreased. Therefore, the noise component from the charge pump can be reduced and the oscillation signal with a small noise component can be reduced. Obtainable.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the frequency synthesizer according to the present invention will be described below with reference to the drawings.

FIG. 1 is a functional block diagram showing an embodiment of a frequency synthesizer according to the present invention. The basic configuration and function of the frequency synthesizer shown in FIG. 1 are the same as those of the frequency synthesizer of FIG. 2 described above, and the same parts are denoted by the same reference numerals. The characteristic configuration of the frequency synthesizer of this embodiment is as follows.

The charge pump 15 has a function of outputting a phase difference voltage 23 corresponding to the phase difference signal 22 output from the phase comparator 14, and a charge pump gain switching signal 25 output from the control circuit 16 Phase difference signal 2
It has a conversion gain switching function for switching the conversion gain from 2 to the phase difference voltage 23.

FIG. 3 shows an example of the circuit configuration of the charge pump 15. This circuit includes an amplifier 31 that amplifies the phase difference signal 22 from the phase difference comparator 14, FETs 32 and 33 that control the current from the power supply VCC, a switch 34 that turns on / off by the charge pump gain switching signal 25, and a resistor R.
1, R2, etc. In FIG. 3, when the charge pump gain switching signal 25 from the control circuit 16 is not input, the switch 34 is turned off.
1 + R2 are connected in series, and the conversion gain from the phase difference signal 22 to the phase difference voltage 23 becomes small. On the other hand, when the charge pump gain switching signal 25 is input, the switch 34 is turned on, so that the resistor R1 is short-circuited to only R2, and the conversion gain from the phase difference signal 22 to the phase difference voltage 23 increases.

The loop filter 17 is the charge pump 1
The phase difference voltage 23 output from the signal 5 is converted into the control voltage 2
In addition to the function of outputting 4, the loop filter gain switching signal 26 output from the control circuit 16 has a conversion gain switching function of switching the conversion gain from the phase difference voltage 23 to the control voltage 24.

A circuit configuration example of the loop filter 17 is shown in FIG. This circuit is composed of an amplifier 41 which is connected to the power supply VCC and amplifies the phase difference voltage 23 from the charge pump 15, a switch 42 which is turned on / off by the loop filter gain switching signal 26, resistors R1 to R3, and the like.
In FIG. 4, when the loop filter gain switching signal 26 from the control circuit 16 is not input, the switch 42 is turned off, so that (R2 + R3) / R1 and the conversion gain from the phase difference voltage 23 to the control voltage 24 increases. . On the other hand, when the loop filter gain switching signal 26 is input, the switch 42 is turned on, the resistor R2 is short-circuited and becomes R3 / R1, and the conversion gain from the phase difference voltage 23 to the control voltage 24 becomes small.

The control circuit 16 is adapted to obtain the magnitude of the phase difference detected by the phase comparator 14 from the phase difference detection signal 27. Then, the phase difference detection signal 27
It has a function of outputting a charge pump gain switching signal 25 and a loop filter gain switching signal 26 to a switch for switching the conversion gains of the charge pump 15 and the loop filter 17 according to the level of.

Next, the operation of the frequency synthesizer shown in FIG. 1 will be described. First, the reference signal generated by the reference signal generator 10 is divided according to the division ratio of the reference signal frequency divider 11, and is output to the phase comparator 14 as the reference frequency division signal 20. Further, the signal generated by the voltage controlled oscillator 12 is divided according to the division ratio of the variable frequency divider 13, and the variable frequency divided signal 21
Is output to the phase comparator 14. The phase comparator 14 detects a phase difference between the reference frequency-divided signal 20 and the variable frequency-divided signal 21, generates a phase-difference signal 22 according to the phase difference, and outputs the phase-difference signal 22 to the charge pump 15. The charge pump 15 generates a phase difference voltage 23 according to the phase difference signal 22 and outputs it to the loop filter 17. The loop filter 17 converts the phase difference voltage 23 into a control voltage 24 and outputs it to the voltage controlled oscillator 12. The voltage controlled oscillator 12 oscillates at a frequency according to the control voltage 24. The control circuit 16 inputs the phase difference detection signal 27 from the phase comparator 14, and when the phase difference detected by the phase comparator 14 is large, the charge pump gain switching signal so that the conversion gain of the charge pump 15 becomes small. And the loop filter gain switching signal is controlled so that the conversion gain of the loop filter becomes large. When the phase difference detected by the phase comparator 14 is small, the charge pump gain switching signal is controlled so that the conversion gain of the charge pump 15 increases.
Further, the loop filter gain switching signal is controlled so that the conversion gain of the loop filter becomes small. That is, when the phase difference detected by the phase comparator 14 is large, that is, the frequency set by the division ratio of the reference signal frequency divider 11 and the variable frequency divider 13 and the oscillation frequency of the voltage controlled oscillator 12 are significantly different. In this case, the conversion characteristics are as shown in Fig. 7,
The control voltage can be largely controlled. For this reason, it is possible to shorten the time until the desired frequency is stabilized. When the phase difference detected by the phase comparator 14 is small, that is, when the set frequency and the oscillation frequency of the voltage controlled oscillator 12 are almost the same (stable state or the frequency difference is small), the conversion as shown in FIG. As a result, the conversion gain of the loop filter can be reduced. Therefore, the noise component included in the oscillation signal of the voltage controlled oscillator 12 can be reduced.

[0019]

As described above, in the frequency synthesizer according to the present invention, when the phase difference detected by the phase comparator is large, the conversion gain of the charge pump is reduced and the conversion gain of the loop filter is increased. By doing so, the variable range of the control voltage is increased, so that the time required to reach the desired frequency can be shortened. When the phase difference is small, the conversion gain of the charge pump is increased and the conversion gain of the loop filter is decreased, so that the noise component from the charge pump can be reduced. Therefore, without increasing the time to stabilize at the desired frequency,
Moreover, it is possible to realize a frequency synthesizer with a small noise component.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing a frequency synthesizer according to an embodiment.

FIG. 2 is a functional block diagram showing a conventional frequency synthesizer.

FIG. 3 is a diagram showing a circuit configuration example of a charge pump.

FIG. 4 is a diagram showing a circuit configuration example of a loop filter.

FIG. 5 is a characteristic diagram showing conversion characteristics when the conversion gain of the charge pump is small.

FIG. 6 is a characteristic diagram showing conversion characteristics when the conversion gain of the charge pump is large.

FIG. 7 is a characteristic diagram showing conversion characteristics of the charge pump and the loop filter combined when the conversion gain of the charge pump is small and the conversion gain of the loop filter is large.

FIG. 8 is a characteristic diagram showing conversion characteristics of the charge pump and the loop filter combined when the conversion pump has a large conversion gain and the loop filter has a small conversion gain.

[Explanation of symbols]

 10 ... Reference signal generator 11 ... Reference signal frequency divider 12 ... Voltage controlled oscillator 13 ... Variable frequency divider 14 ... Phase comparator 15 ... Charge pump 16 ... Control circuit 17 ... Loop filter

Claims (1)

[Claims] 1. A reference signal generator for generating a constant frequency signal, a reference signal divider for dividing the reference signal output by the reference signal generator, and a frequency signal according to a predetermined control voltage. A voltage controlled oscillator, a variable frequency divider that divides the frequency signal, and a phase comparator that detects a phase difference between the signal output by the reference signal frequency divider and the signal output by the variable frequency divider. By including a charge pump that outputs a voltage according to the phase difference signal output from the phase comparator and a loop filter that converts the voltage output from the charge pump and outputs a control voltage, In the frequency synthesizer for obtaining a frequency, the charge pump has conversion gain switching means for switching the conversion gain of the voltage to be output according to the phase difference signal, and the loop filter is The conversion gain of the charge pump and the conversion of the loop filter according to the phase difference detected by the phase comparator. A frequency synthesizer comprising a control means for outputting a predetermined gain switching signal to the gain switching means.