JPH04220813A - Frequency synthesizer - Google Patents

Abstract

PURPOSE:To offer the frequency synthesizer operated stably by eliminating an undesired frequency component of the output of a voltage controlled oscillator inputted to a variable frequency divider of the frequency synthesizer. CONSTITUTION:A narrow band filter 6 whose band width is variable with voltage applied thereto is provided between the output of a voltage controlled oscillator 4 and a variable frequency divider 4 in the PLL frequency synthesizer comprising a reference signal generator 1, a phase comparator 2, a low pass filter 3, the voltage controlled oscillator 4 and the variable frequency divider 5 so as to eliminate an undesired frequency component outputted from the voltage controlled oscillator 4 thereby stabilizing the operation of the synthesizer.

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 in radio equipment.

0002

[Summary of the Invention] In the present invention, in order to prevent unstable operation of the synthesizer due to unnecessary frequency components such as harmonics of the output of a voltage controlled oscillator (hereinafter referred to as VCO) inputted to a conventional variable frequency divider, A filter whose band changes depending on the voltage is inserted between the VCO and the variable frequency divider, and by making the band of this filter narrower than before, the amount of attenuation of unnecessary frequency components is increased, resulting in more stable operation. A frequency synthesizer can be realized.

0003

[Prior Art] The conventional technology will be explained below. FIG. 2 is a block diagram showing the overall configuration of a conventional example. A reference signal generated by a reference signal generator (hereinafter referred to as OSC) 1 is input to a phase comparator (hereinafter referred to as PD) 2, and a phase comparison is performed with a feedback signal inputted from VCO 4 to PD 2 via variable frequency divider 5. be exposed. This phase comparison output signal is passed through a low-pass filter (hereinafter referred to as LPF) 3 to remove unnecessary frequency components, and then output to the VCO.
4, and controls the oscillation frequency of VCO4.

As shown in FIG. 2, a VCO 4 and a variable frequency divider 5
There is a conventional example that has a fixed passband filter 7 that removes harmonic components output from the VCO 4, but the band of the fixed passband filter 7 needs to cover the entire variable range of the VCO 4. A wide band is required, and V
There is a problem that harmonics such as twice or three times the oscillation frequency of CO4 cannot be sufficiently removed.

[Problem to be solved by the invention]

[0005] In the above conventional example, unnecessary harmonic components of the VCO 4 cannot be removed sufficiently and are input to the PD 2, so phase synchronization pull-in may be performed with unnecessary frequency components.
The disadvantage was that the synthesizer would operate unstable.

An object of the present invention is to solve this drawback and provide a frequency synthesizer that operates stably.

[0007]

[Means for Solving the Problems] FIG. 1 is a block diagram showing the overall configuration of the present invention. In order to achieve the above object, the present invention provides a filter 6 whose pass band can be varied by voltage between the output of the VCO 4 and the variable frequency divider 5, and changes the band of the filter 6 according to the oscillation frequency of the VCO 4. This is how it was done.

[0008]

[Operation] To explain the operation of the present invention, the passband variable filter 6 inserted between the VCO 4 and the variable frequency divider 5
can be made into a narrow band filter according to the oscillation frequency of VCO4, and it is possible to increase the amount of attenuation of unnecessary frequency components such as harmonics of the VCO output. It is possible to operate stably without operating at frequency components.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 shows an embodiment of a filter which is a component of the present invention and whose band can be varied by voltage.

In the circuit shown in the figure, the coils L1 and L2 are equal, and the capacitors C1 and C3 are equal and varicap diodes V.
When the characteristics of D1 and VD2 are made equal, the center frequency of the passband is determined by the series capacitance of C1 and VD1 and the resonant frequency f of L1,

[0011]

[Math 1]

##EQU1## The bandpass filter has a characteristic whose bandwidth is determined by C2.

Here, the capacitance CV of the varicap diode is
D1 changes depending on the voltage applied to the cathode, and the higher the voltage, the smaller CVD1 becomes, and according to the resonance frequency equation shown in the above equation (1), the center of the band usually moves to the higher frequency side. Therefore, it is possible to realize a filter in which the center frequency of the passband moves higher as the voltage applied to the control voltage input terminal 8 becomes higher.

Therefore, when the filter shown in FIG. 3 is used, the control voltage of the filter may be increased as the output frequency of the frequency synthesizer becomes higher.

FIGS. 4 and 5 are block diagrams showing an embodiment of the present invention.

FIG. 4 shows an embodiment in which the control voltage input terminal 8 of the variable passband filter 6 is connected to the input of the VCO 4. In FIG.

In FIG. 4, the oscillation frequency of the VCO 4 increases as the input control voltage increases, and as mentioned above, the band of the variable passband filter 6 similarly moves to the higher side, so the filter is adjusted to match the oscillation frequency of the VCO 4. Bandwidth can be varied. Therefore, the bandwidth of the variable passband filter 6 can be made narrower than that of the conventional fixed passband filter 7, and the amount of attenuation of unnecessary harmonic components output from the VCO 4 can be increased.

FIG. 5 shows an embodiment in which the input voltage of the variable passband filter 6 is controlled by a microcomputer. In FIG. 5, a switch 12 (hereinafter referred to as SW) is a switch for switching the oscillation frequency of a frequency synthesizer, and the setting data of the switch is read by the microcomputer 10 via a data input/output element 11 (hereinafter referred to as I/O).

In the microcomputer 10, the frequency division ratio setting of the variable frequency divider 5 is changed via the I/O 11 according to the setting data of the SW 12, and the oscillation frequency of the VCO 4 is switched. In this embodiment, filter band control data is further input from the microcomputer 10 to the D/A converter 9 via the I/O 11 in accordance with the settings of the SW 12.

The D/A converter 9 converts the filter band control data from digital to analog, and by changing the signal voltage input to the control voltage input terminal 8 of the variable passband filter 6, the implementation shown in FIG. Similar to the example, VC
The band of the filter can be varied in accordance with the oscillation frequency of O4.

[0021]

According to the present invention, unnecessary frequency components in the signal input to the variable frequency divider can be easily removed, so that a frequency synthesizer that operates stably can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the overall configuration of the present invention.

FIG. 2 is a block diagram showing the overall configuration of a conventional example.

FIG. 3 is a circuit diagram showing an embodiment of a variable bandpass filter according to the present invention.

FIG. 4 is a block diagram showing an embodiment of the present invention.

FIG. 5 is a block diagram showing an embodiment of the present invention.

[Explanation of symbols]

1 Reference signal generator 2 Phase comparator 3 Low pass filter 4 Voltage controlled oscillator 5 Variable frequency divider 6　Variable passband filter 7 Fixed passband filter 8 Passband control voltage input terminal 9 D/A converter 10 Microcomputer 11 Data input/output element 12 Switch

Claims (3)

[Claims] 1. A reference signal generator, a phase comparator connected to the output of the reference signal generator and having a reference phase comparison input terminal and a feedback phase comparison input terminal, and smoothing the output of the phase comparator. A PLL comprising a low-pass filter, a voltage-controlled oscillator whose oscillation frequency is controlled by the output of the low-pass filter, and a variable frequency divider between the output of the voltage-controlled oscillator and the feedback phase input terminal of the phase comparator. A frequency synthesizer comprising a filter having a control voltage input terminal and whose pass band changes depending on the voltage applied to the control voltage input terminal between the voltage controlled oscillator and the variable frequency divider. synthesizer. 2. The frequency synthesizer according to claim 1, wherein a band of the filter whose pass band changes is controlled by the output voltage of the low-pass filter. [Claim 3] Means for switching and setting a transmission frequency;
A patent claim characterized by comprising means for reading switching setting information and outputting a control signal, and means for converting the control signal into an analog signal, and controlling a band of a filter whose pass band changes with the analog signal. Claim 1
Frequency synthesizer described.