CN102291129A - Phase-locked loop circuit used for inhibiting VCO (voltage-controlled oscillator) voltage ripple - Google Patents

Abstract

The invention discloses a phase-locked loop circuit for suppressing VCO voltage ripple, which includes a frequency and phase detector, a charge pump, an automatic frequency band selector, a loop filter, a voltage-controlled oscillator, a frequency divider and a first Portal. The invention reduces the VCO control voltage ripple caused by the traditional MOS tube when switching between different working modes of the phase-locked loop by using the transmission gate as the mode switching switch, and reduces the charge injection and clock feedthrough, thereby achieving the purpose of suppressing the VCO The purpose of the voltage ripple; at the same time, the present invention embeds the transmission gate in the third-order loop filter, which not only plays the role of switching between different operating modes of the phase-locked loop, but also utilizes the on-resistance of the transmission gate as a third-order loop filter The resistance of the resistor effectively filters out the noise, thereby greatly reducing the VCO voltage ripple caused by the noise.

Description

A Phase Locked Loop Circuit Used to Suppress VCO Voltage Ripple

technical field

The invention belongs to the technical field of radio phase-locking, and in particular relates to a phase-locked loop circuit for suppressing VCO voltage ripple.

Background technique

Charge-pump phase-locked loop (CP-PLL) has the characteristics of high speed, low noise and high precision, and is widely used in various communication circuits, frequency synthesizers and clock recovery circuits. The traditional charge pump phase-locked loop is shown in Figure 1, which includes the following modules: phase frequency detector (Phase Frequency Detector, PFD), charge pump (Charge-Pump, CP), loop filter (Loop Filter ), Voltage Controlled Oscillator (VoltageControlled Oscillator, VCO) and frequency divider (Frequency Divider). Among them, the frequency and phase detector detects the phase difference between the reference signal and the feedback signal output by the frequency divider, and outputs a discrete voltage pulse signal. The charge pump converts the signal into a continuous current signal, and charges and discharges the loop filter. The circuit filter filters the high-frequency noise of the loop, and the output voltage controls the voltage-controlled input terminal of the voltage-controlled oscillator, thereby tuning the output frequency of the voltage-controlled oscillator. The voltage-controlled oscillator adopts an LC oscillation structure. In order to reduce the phase noise while ensuring its tuning range, a multi-band VCO can be used. It can solve this problem by combining discrete tuning and continuous tuning.

The phase-locked loop with automatic frequency band selection function is shown in Figure 2. Compared with the traditional charge pump phase-locked loop structure, it adds an automatic frequency band selector and two switches for switching the phase-locked loop working mode. The automatic frequency band selector controls the working mode of the switched capacitor array and the phase-locked loop by comparing the frequency of the reference signal and the feedback signal, so that the VCO works in the required frequency band. This kind of phase-locked loop has two working modes, which are frequency-locked mode and phase-locked mode, which are realized by frequency-locked loop and phase-locked loop respectively. The working process is as follows: when the loop starts to work, the switch S1 is closed, the switch S2 is opened, the phase-locked loop performs frequency locking first, and selects the appropriate control word of the switched capacitor array. Then the switch S1 is turned off, the switch S2 is closed, and the phase-locked loop performs phase locking, so that the phase-locked loop is locked on an accurate frequency.

The loop filter in the phase-locked loop adopts a third-order loop filter, and the third-order loop filter can filter out high-frequency noise better than the second-order loop filter; at the same time, the phase-locked loop adopts The MOS transistor is used as a switch. In practical applications, the source and drain of the MOS transistor are connected in series to the loop, and the loop is opened or closed by controlling the gate terminal of the MOS transistor.

However, the phase-locked loop with the automatic frequency band selection function will generate charge injection and clock feedthrough phenomena when the switch is turned off and on, which will directly lead to fluctuations in the VCO control voltage. In addition, due to the existence of the parasitic resistance of the MOS tube, the thermal noise generated by it also leads to the fluctuation of the VCO control voltage. Therefore, there is a large voltage ripple in the control voltage of the VCO in the phase-locked loop with automatic frequency band selection function, which greatly deteriorates the phase noise of the phase-locked loop.

Contents of the invention

The invention provides a phase-locked loop circuit for suppressing VCO voltage ripple, which effectively suppresses the voltage ripple of VCO and filters out the phase noise.

A phase-locked loop circuit for suppressing VCO voltage ripple, comprising:

The frequency and phase detector is used to receive the feedback signal and reference signal respectively provided by the frequency divider and the external reference source, and generate a discrete voltage pulse signal;

A charge pump for converting the discrete voltage pulse signal into a continuous current signal;

a loop filter, for converting the continuous current signal into a voltage signal;

The automatic frequency band selector is used to receive the feedback signal and reference signal respectively provided by the frequency divider and the external reference source, and generate a state control word and a positive and negative mode control signal;

A voltage-controlled oscillator, configured to receive the voltage signal and the state control word, and generate a VCO output signal;

a frequency divider, configured to receive the VCO output signal and generate the feedback signal;

The first transmission gate, wherein the input end of the first transmission gate receives a half-amplitude power supply voltage signal, the output end of the first transmission gate is connected to the voltage control input end of the voltage-controlled oscillator, and the first transmission gate The first control terminal and the second control terminal respectively receive the normal phase mode control signal and the negative phase mode control signal.

The loop filter is composed of three capacitors, a resistor and a transmission gate; wherein, one end of the first capacitor is connected to one end of the resistor and the input of the second transmission gate to form the loop filter The input end, the other end of the first capacitor is grounded, the other end of the resistor is connected to one end of the second capacitor, the other end of the second capacitor is grounded, and the output end of the second transmission gate is connected to one end of the third capacitor to form the The output terminal of the loop filter and the other terminal of the third capacitor are grounded, and the first control terminal and the second control terminal of the second transmission gate respectively receive the control signal of the negative phase mode and the control signal of the normal phase mode.

The working voltage of the charge pump is a full-amplitude power supply voltage signal.

In the preferred technical solution, the first transmission gate and the second transmission gate are both composed of PMOS transistors and NMOS transistors with a width-to-length ratio of 3:1, and the linearity of the equivalent resistance of the transmission gates is the best when they are turned on. , can improve the stability of the loop filter and can effectively suppress the voltage ripple at the voltage control input terminal.

In a preferred technical solution, the voltage-controlled oscillator is a multi-band voltage-controlled oscillator, which can ensure the tuning range of the VCO while reducing phase noise.

Working principle of the present invention is:

When the phase-locked loop starts to work, the automatic frequency band selector starts to work, and its state output terminal produces a preset state control word, the positive phase mode control signal output by its mode output terminal is low level, and the negative phase mode control signal is high level , so that the second transmission gate is disconnected, the first transmission gate is closed, the voltage-controlled input terminal of the voltage-controlled oscillator is connected to the reference voltage, and the gate of the MOS transistor of the switched capacitor array in the voltage-controlled oscillator receives the output of the automatic frequency band selector The preset state control word, the loop is in the frequency lock mode at this time; the automatic frequency band selector determines the state control word by comparing the frequency of the reference signal and the feedback signal. If the frequency of the feedback signal is detected to be lower than the frequency of the reference signal, the frequency of the feedback signal is increased by reducing the control word; if the frequency of the feedback signal is detected to be greater than the frequency of the reference signal, the frequency of the feedback signal is reduced by increasing the control word If the frequency of the feedback signal is detected to be equal to the frequency of the reference signal, the control word remains unchanged.

When the loop frequency has been locked, the positive phase mode control signal output by the mode output terminal is high level, and the negative phase mode control signal is low level, so that the second transmission gate is closed, the first transmission gate is disconnected, and the voltage-controlled oscillation The voltage control input terminal of the device is connected to the output terminal of the loop filter, and the loop enters the phase lock mode at this time; the frequency and phase detector detects the phase difference between the reference signal and the feedback signal, and outputs discrete voltage pulses proportional to the phase difference The signal is sent to the input terminal of the charge pump to control the opening and closing of the charge pump, and then charge and discharge the loop filter, thereby changing the voltage of the voltage control input terminal of the voltage controlled oscillator until the loop is locked.

The beneficial technical effect of the present invention is:

(1) The present invention reduces the VCO control voltage ripple caused by the traditional MOS tube when switching between different operating modes of the phase-locked loop by using the transmission gate as the mode switching switch, reduces charge injection and clock feedthrough, thereby The purpose of suppressing the VCO voltage ripple is achieved.

(2) The present invention embeds the transmission gate in the third-order loop filter, which not only plays the role of switching between different operating modes of the phase-locked loop, but also utilizes the on-resistance of the transmission gate as the resistance of the third-order loop filter, Noise is effectively filtered, thereby greatly reducing VCO voltage ripple due to noise.

Description of drawings

FIG. 1 is a schematic structural diagram of a traditional phase-locked loop circuit.

FIG. 2 is a schematic structural diagram of a conventional phase-locked loop circuit with an automatic frequency band selection function.

FIG. 3 is a schematic structural diagram of a phase-locked loop circuit of the present invention.

Fig. 4 is a voltage ripple simulation diagram of a voltage-controlled oscillator in a traditional phase-locked loop circuit with an automatic frequency band selection function.

FIG. 5 is a simulation diagram of the voltage ripple of the voltage-controlled oscillator in the phase-locked loop circuit of the present invention.

Detailed ways

In order to describe the present invention more specifically, the technical solutions and related principles of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 3, a phase-locked loop circuit for suppressing VCO voltage ripple, including: frequency detector, charge pump, automatic frequency band selector, loop filter, voltage controlled oscillator, frequency divider and the first transmission gate.

The reference input terminal of the frequency and phase detector is connected to the external reference source, and the feedback input terminal of the frequency and phase detector is connected to the output terminal of the frequency divider to receive the reference signal and the feedback signal respectively, thereby generating a discrete voltage pulse signal.

The UP input of the charge pump is connected to the UP output of the frequency and phase detector, and the DN input of the charge pump is connected to the DN output of the frequency and phase detector to receive the discrete voltage pulse signal and convert it into a continuous current signal.

The reference input terminal of the automatic frequency band selector is connected with the external reference source, and the feedback input terminal of the automatic frequency band selector is connected with the output terminal of the frequency divider to receive the reference signal and the feedback signal respectively, so as to generate the state control word and the positive and negative phase mode control Signal.

The input of the loop filter is connected to the output of the charge pump to receive the continuous current signal and convert it into a voltage signal;

The loop filter is composed of three capacitors, a resistor and a transmission gate; wherein, one end of the first capacitor C1 is connected to one end of the resistor R and the input end of the second transmission gate S2 to form the input end of the loop filter, The other end of the first capacitor C1 is grounded, the other end of the resistor R is connected to one end of the second capacitor C2, the other end of the second capacitor C2 is grounded, and the output end of the second transmission gate S2 is connected to one end of the third capacitor C3 to form a The output end of the loop filter, the other end of the third capacitor C3 is grounded, the first control end and the second control end of the second transmission gate S2 are connected to the mode output end of the automatic frequency band selector, and respectively receive the inverting mode control signal /SW and the normal phase mode control signal SW.

The voltage control input terminal of the voltage controlled oscillator is connected with the output terminal of the loop filter to receive the voltage signal; the gate of the MOS transistor of the switched capacitor array in the voltage controlled oscillator is connected with the state output terminal of the automatic frequency band selector to Receives the status control word, thereby produces the VCO output signal.

The input end of the frequency divider is connected with the output end of the voltage-controlled oscillator to receive the VCO output signal and generate a feedback signal.

The input terminal of the first transmission gate receives the half-amplitude power supply voltage signal VDD/2, the output terminal of the first transmission gate is connected with the voltage control input terminal of the voltage-controlled oscillator, the first control terminal of the first transmission gate and the second The control terminal is connected to the mode output terminal of the automatic frequency band selector, and receives the normal phase mode control signal SW and the negative phase mode control signal /SW respectively.

Both the first transmission gate and the second transmission gate are composed of PMOS transistors and NMOS transistors with a width-to-length ratio of 3:1. Pole and body end. The body ends of the PMOS transistors all receive the power supply voltage signal VDD to reduce current leakage, and the body ends of the NMOS transistors are all grounded to reduce current leakage.

When the phase-locked loop of this embodiment starts to work, the automatic frequency band selector starts to work, and its state output terminal produces the preset state control word, and the positive phase mode control signal SW output by its mode output terminal is low level, and the negative phase mode control signal SW /SW is high level, so that the second transmission gate S2 is disconnected, the first transmission gate S1 is closed, the voltage control input terminal of the voltage controlled oscillator is connected to the reference voltage VDD/2, and the switched capacitor array in the voltage controlled oscillator The gate of the MOS transistor receives the preset state control word output by the automatic frequency band selector. At this time, the loop is in the frequency lock mode; the automatic frequency band selector determines the state control word by comparing the frequency of the reference signal and the feedback signal. If the frequency of the feedback signal is detected to be lower than the frequency of the reference signal, the frequency of the feedback signal is increased by reducing the control word; if the frequency of the feedback signal is detected to be greater than the frequency of the reference signal, the frequency of the feedback signal is reduced by increasing the control word If the frequency of the feedback signal is detected to be equal to the frequency of the reference signal, the control word remains unchanged.

When the loop frequency is locked, the positive phase mode control signal SW output from the mode output terminal is at high level, and the negative phase mode control signal /SW is at low level, so that the second transmission gate S2 is closed and the first transmission gate S1 is off. Open, the voltage control input terminal of the voltage controlled oscillator is connected to the output terminal of the loop filter, and the loop enters the phase lock mode at this time; the frequency and phase detector detects the phase difference between the reference signal and the feedback signal, and the output is proportional to the phase difference The proportional discrete voltage pulse signal UP, DN is sent to the input terminal of the charge pump to control the opening and closing of the charge pump, and then charge and discharge the loop filter, thereby changing the voltage of the voltage control input terminal of the voltage controlled oscillator until the loop Road locked.

Figure 4 is a schematic diagram of the Specter simulation results of a traditional phase-locked loop circuit with an automatic frequency band selection function, where the abscissa represents the simulation time, and the ordinate represents the voltage at the voltage control input terminal. It can be seen from the figure that when the phase-locked loop is locked, the voltage ripple at the voltage-controlled input terminal of the voltage-controlled oscillator is 173mv.

FIG. 5 is a schematic diagram of Specter simulation results of a phase-locked loop circuit with an automatic frequency band selection function in this embodiment. It can be seen from the enlarged figure that when the phase-locked loop is locked, the voltage ripple at the voltage-controlled input terminal of the voltage-controlled oscillator is 1.85mv. It can be seen from the comparison that this embodiment can reduce the voltage ripple at the voltage-controlled input terminal of the voltage-controlled oscillator from the original 173mv to 1.85mv, suppressing 98.93% of the ripple.

Claims (4)

1. A phase-locked loop circuit for suppressing VCO voltage ripple, is characterized in that: comprising: The frequency and phase detector is used to receive the feedback signal and reference signal respectively provided by the frequency divider and the external reference source, and generate a discrete voltage pulse signal; A charge pump for converting the discrete voltage pulse signal into a continuous current signal; a loop filter, for converting the continuous current signal into a voltage signal; The automatic frequency band selector is used to receive the feedback signal and reference signal respectively provided by the frequency divider and the external reference source, and generate a state control word and a positive and negative mode control signal; A voltage-controlled oscillator, configured to receive the voltage signal and the state control word, and generate a VCO output signal; a frequency divider, configured to receive the VCO output signal and generate the feedback signal; The first transmission gate, wherein the input end of the first transmission gate receives a half-amplitude power supply voltage signal, the output end of the first transmission gate is connected to the voltage control input end of the voltage-controlled oscillator, and the first transmission gate The first control terminal and the second control terminal respectively receive the normal phase mode control signal and the negative phase mode control signal. 2. the phase-locked loop circuit for suppressing VCO voltage ripple according to claim 1 is characterized in that: described loop filter is made of three electric capacity, a resistor and a transmission gate; Wherein, the first One end of the capacitor is connected to one end of the resistor and the input end of the second transmission gate to form the input end of the loop filter, the other end of the first capacitor is grounded, and the other end of the resistor is connected to one end of the second capacitor. The other end of the second capacitor is grounded, the output end of the second transmission gate is connected to one end of the third capacitor to form the output end of the loop filter, the other end of the third capacitor is grounded, and the first control of the second transmission gate The terminal and the second control terminal respectively receive the reverse mode control signal and the normal phase mode control signal. 3. The phase-locked loop circuit for suppressing VCO voltage ripple according to claim 1 or 2, characterized in that: the first transmission gate and the second transmission gate are composed of a width-to-length ratio of 3:1 Composed of PMOS tubes and NMOS tubes. 4. The phase-locked loop circuit for suppressing VCO voltage ripple according to claim 1, characterized in that: said voltage-controlled oscillator is a multi-band voltage-controlled oscillator.

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