JPH06169254A - Pll circuit - Google Patents

Abstract

PURPOSE:To obtain the PLL circuit with high spectrum impurity by using the combination of two operational amplifier circuits and attenuators for a loop filter, thereby suppressing external noise invaded in an input of a voltage controlled oscillator. CONSTITUTION:A loop filter 2 has an input circuit 2A, an operational amplifier 2C inputting its output to an inverting input terminal, and an attenuator 2D receiving its output, and also an operational amplifier 2E receiving an output of the circuit 2A to the inverting input terminal, and a feedback circuit 2B feeding back the output of the amplifier 2E to an output of the circuit 2A. Then the PLL circuit is formed by using the combination of the amplifier 2C, the attenuator 2D and the amplifier 2E. The amplifier 2C and the amplifier 2E are combined to obtain a large gain band width product, and the characteristic of the amplifier with a large DC gain is obtained simultaneously to suppress external noise invaded in the input of the voltage controlled oscillator 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PLL circuit, and more particularly to a PLL circuit which suppresses spectrum deterioration due to external noise mixed in the input of a voltage controlled oscillator.

[0002]

2. Description of the Related Art The configuration of a PLL circuit according to the prior art is shown in FIG.
Shown in. 2, 1 is a phase comparator, 3 is a voltage controlled oscillator,
Reference numeral 4 is a frequency divider, and 5 is a loop filter. In FIG. 2, the loop filter 5 comprises an input circuit 5A, an operational amplifier 5E, and a feedback circuit 5B, and constitutes a low pass filter.
In FIG. 2, the output of the voltage controlled oscillator 3 is divided into two, one of which is connected to the input of the frequency divider 4. The phase comparator 1 detects a phase difference between a signal externally input to the reference signal input terminal 1A and an output signal of the frequency divider 4 input to the comparison signal input terminal 1B, and outputs a voltage according to the phase difference. To do. The output of this phase detector 1 is connected to the input of the loop filter 5. The loop filter 5 determines the loop characteristic of the PLL circuit, removes a high frequency component contained in the output signal of the phase comparator 1, and causes the voltage controlled oscillator 3 to switch.

[0003]

In the PLL circuit using the loop filter 5 shown in FIG. 2, when the band of the loop is widened, it is necessary to use a large gain bandwidth product as the operational amplifier forming the loop filter. was there. But,
Since an operational amplifier having a large gain bandwidth product generally has a low DC gain, there is a problem that external noise mixed in the input of the voltage controlled oscillator 3 cannot be suppressed sufficiently. An object of the present invention is to suppress the external noise mixed in the input of the voltage controlled oscillator 3 and realize a PLL circuit with high spectral purity.

[0004]

In order to achieve this object, the present invention detects a phase difference between a signal input to a reference signal input terminal 1A and a signal input to a comparison signal input terminal 1B, and detects the phase difference according to the phase difference. And a phase comparator 1 that outputs a voltage,
The loop filter 2 that inputs the output signal from the phase comparator 1, the voltage-controlled oscillator 3 that outputs a signal having a frequency corresponding to the output voltage from the loop filter 2, and the output signal from the voltage-controlled oscillator 3 are frequency-divided. Then, in the PLL circuit having the frequency divider 4 which outputs the frequency-divided signal to the comparison signal input terminal of the phase comparator 1, the loop filter 2 has an input circuit 2A whose input is the output of the phase comparator 1. , Input circuit 2A
Operational amplifier 2C for inputting the output of the input to the inverting input terminal, attenuator 2D for inputting the output of the operational amplifier 2C, and output of the input circuit 2A for inputting the inverting input terminal, and output for the attenuator 2D is non-inverting It includes an operational amplifier 2E which is an input of the terminal, and a feedback circuit 2B which is an input to the output of the operational amplifier 2E and is connected to the output of the input circuit 2A.

[0005]

Next, the configuration of the PLL circuit according to the present invention is shown in FIG.
Shown in. Reference numeral 2 in FIG. 1 is a loop filter, and the others are the same as those in FIG. The loop filter 2 includes an input circuit 2A, an operational amplifier 2C for inputting the output of the input circuit 2A to an inverting input terminal, and an attenuator 2D for inputting the output of the operational amplifier 2C.
And an operational amplifier 2E having the output of the attenuator 2D as the input of the non-inverting input terminal and the output of the input circuit 2A as the input of the inverting input terminal, and a feedback circuit 2B for feeding back the output of the operational amplifier to the output of the input circuit 2A. With.

In the loop filter 2, the input circuit 2A, the operational amplifier 2E and the feedback circuit 2B are the loop filter 5 of FIG.
The input circuit 5A, operational amplifier 5E, and feedback circuit 5B are the same. In the present invention, instead of the operational amplifier 5E in the circuit shown in FIG. 2, an operational amplifier 2C, an attenuator 2D, and
A PLL circuit is configured using a combination with the operational amplifier 2E.

By combining the operational amplifier 2C and the operational amplifier 2E, the characteristics of the amplifier having a large gain bandwidth product and a large direct current gain can be obtained, and the voltage controlled oscillator 3 can be obtained.
It greatly suppresses the external noise mixed in the input of.

[0008]

FIG. 3 shows the comparison result of the fluctuation amount of the output phase of the PLL circuit when the external noise is mixed in the input of the voltage controlled oscillator in the case of the present invention and the case of the conventional technique. Here, the loop band of the PLL is 400 kHz, and the input circuit 2A uses a resistor of 150Ω. The feedback circuit 2B has a 1.1kΩ resistor and a 1000pF capacitor connected in series, and the DC gain when the operational amplifier 2C is open loop is 1
The gain bandwidth product of the operational amplifier 2C is 20 dB, the direct current gain of the operational amplifier 2E is 75 dB, and the gain bandwidth product of the operational amplifier 2E is 50 MHz.

The open-loop DC gain of the operational amplifier 5E is 75 dB, and the gain bandwidth product of the operational amplifier 5E is 50 MH.
z, the attenuation amount of the attenuator 2D is 60 dB, and the frequency division ratio of the frequency divider 4 is
The circuit constant of 1/64 is used. In FIG. 3, a shows the result in the case of the present invention and b shows the result in the case of the conventional technique.
It is improved by 60 dB.

[0010]

According to the present invention, by using a combination of two operational amplifiers and an attenuator for the loop filter, the external noise mixed in the input of the voltage controlled oscillator is suppressed, and the PLL circuit having high spectral purity. Can be realized.

[Brief description of drawings]

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

FIG. 2 is a configuration diagram of a PLL circuit according to a conventional technique.

FIG. 3 is a comparison result of the fluctuation amount of the output phase of the PLL circuit when the external noise is mixed in the input of the voltage controlled oscillator in the case of the present invention and the case of the conventional technique.

[Explanation of symbols]

 1 phase comparator 2 loop filter 3 voltage controlled oscillator 4 frequency divider 5 loop filter 2A input circuit 2B feedback circuit 2C operational amplifier 2D attenuator 2E operational amplifier

Claims (1)

[Claims] 1. A phase comparator (1) for detecting a phase difference between a signal input to a reference signal input terminal (1A) and a signal input to a comparison signal input terminal (1B) and converting the phase difference into a voltage amount. , Loop filter (2) that inputs the output signal of the phase comparator (1)
And the voltage control oscillator (3) that outputs a signal with a frequency according to the output voltage of the loop filter (2) and the output signal of the voltage control oscillator (3) are frequency-divided, and the signals after frequency division are compared in phase. vessel
In the PLL circuit including the frequency divider (4) that outputs to the comparison signal input terminal of (1), the loop filter (2) has an input circuit (2A) that receives the output of the phase comparator (1), A first operational amplifier (2C) that inputs the output of the input circuit (2A) to the inverting input terminal, and an attenuator (2D) that inputs the output of the first operational amplifier (2C)
And the output of the input circuit (2A) as the input of the inverting input terminal.
The second operational amplifier (2E), which uses the output of (2D) as the input of the non-inverting input terminal, and the output and input of the second operational amplifier (2E), and the input circuit (2A)
And a feedback circuit (2B) connected to the output of the PLL circuit.