CN103023431A - Method for enlarging tuning range and improving phase noise performance of voltage-controlled oscillator - Google Patents

Abstract

A method for improving the tuning range and phase noise performance of a voltage-controlled oscillator mainly relates to a current-biased capacitive-inductive voltage-controlled oscillator, especially a current-biased capacitive-inductive voltage-controlled oscillator in a current-limited region. It includes: a digital tuning module, which improves the tuning range of the voltage-controlled oscillator by turning on and off the switched capacitor; a switched bias current module, which improves the phase noise of the entire system by turning on the corresponding bias current in each frequency band . In the current-limited region, the output amplitude of the voltage-controlled oscillator will decrease as the frequency decreases, resulting in a reduction in the phase noise of the voltage-controlled oscillator; at the same time, the increase in the frequency range will cause a relatively large change in its phase noise , especially at lower frequencies, its phase noise becomes worse. In this method, a wider tuning range is obtained through the cooperative adjustment of the digital tuning module and the switch bias current module, and better phase noise is obtained in the entire frequency range.

Description

A Method of Improving the Tuning Range and Phase Noise Performance of a Voltage Controlled Oscillator

technical field

The invention relates to a current-biased capacitive-inductive voltage-controlled oscillator, in particular to a current-biased capacitive-inductive voltage-controlled oscillator in a current-limited region.

Background technique

In recent years, with the prosperity of the communication market, wireless communication technology and related applications have developed rapidly. Personal wireless communication systems, wireless local area networks, satellite navigation systems (GPRS, Beidou system), satellite TV, radio frequency tags (RFID), and vehicle positioning systems have experienced rapid growth. At the same time, wireless communication technology has also been widely used in cutting-edge technologies such as electronic countermeasures and space technology. In these applications, the radio frequency front end is the key to the performance of these systems, so it has been the focus of research.

In a wireless communication system, the RF front-end is the interface between the antenna and the baseband processor at the back-end of the wireless transceiver. It not only needs to detect weak received signals (in the order of μV), but also needs to transmit high-power radio frequency signals in the same frequency band. This requires a high-performance RF front-end. The voltage-controlled oscillator is an oscillator whose output frequency varies with the input voltage, and is the most basic and key module of the RF front-end. Voltage-controlled oscillators are mainly used in phase-locked loops as reference frequencies to provide local oscillator frequencies for transceivers. As a key module of the RF front-end, its performance in terms of phase noise directly affects the quality of the entire system for sending and receiving, and the size of the tuning range directly determines whether it is practical.

There are two main types of voltage-controlled oscillators: ring oscillators and capacitive-inductive oscillators. In practical applications, capacitor-inductor voltage-controlled oscillators are generally used, because capacitor-inductor voltage-controlled oscillators have better noise performance. Capacitor-inductance voltage-controlled oscillators generally use capacitive reactance tubes as frequency tuning components, and the maximum tuning range can only reach about 20%. In some applications, such a frequency tuning range cannot meet actual needs. In order to further improve the tuning range, digital tuning technology is often used, which can greatly improve the tuning range of the voltage-controlled oscillator. In the current-limited region, the output amplitude of the voltage-controlled oscillator will decrease as the frequency decreases, which will cause the phase noise of the voltage-controlled oscillator to decrease, and the frequency range will increase, and its phase noise will also undergo relatively large changes. , especially at lower frequencies, its phase noise becomes worse.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method that can overcome the large change in phase noise when the current bias type capacitive-inductive voltage-controlled oscillator obtains a relatively wide tuning range, and the poor phase noise at low frequencies, so that the voltage While the controlled oscillator obtains a wide tuning range, its phase noise remains basically unchanged or increases in the entire channel range, and better phase noise can also be obtained at low frequencies.

The technical scheme adopted by the present invention to solve the technical problem is: to improve the tuning range of the voltage-controlled oscillator, digital tuning technology is adopted to greatly increase the tuning range of the voltage-controlled oscillator by switching on and off the capacitor. The principle is: when the digital signal is low, the capacitor in series is added to the resonant circuit, and when the digital signal is high, the capacitor in series has no effect on the resonant circuit, so the switched capacitor group controlled by the digital signal can realize discrete frequency changes , to realize the coarse adjustment of the frequency, and the fine change of the frequency is completed by the capacitive reactance tube whose capacitance can change continuously, and a wide frequency tuning range can be realized by using digital tuning technology. In order to ensure the continuity of the frequency in the whole frequency range, the discrete frequency change caused by the digital control signal must be smaller than the frequency tuning range of the capacitive reactance tube. The number of digits for digital tuning, that is, the number of groups of switched capacitors is determined by the actual frequency range required and the tuning range of the capacitive reactance tube.

In the current-limited region, the output amplitude of the VCO decreases as the frequency decreases, resulting in a reduction in the phase noise of the VCO. A large tuning range will result in a much lower phase noise at low frequencies than at high frequencies. Since the reduction of phase noise is mainly caused by the reduction of the output voltage amplitude of the voltage-controlled oscillator, the output voltage amplitude of the voltage-controlled oscillator at low frequencies must be increased accordingly. In the current limited region, the VCO output amplitude is:

V _osc = I _bias ωLQ (1)

I _bias is the bias current of the VCO, ω is the angular frequency of the VCO, L is the inductance, and Q is the quality factor of the resonant circuit of the VCO. It can be seen from formula (1) that in order to maintain the same or increase the output amplitude at low frequencies, the bias current must be increased accordingly (inductance and quality factor remain unchanged). The present invention adopts the switch bias current module to realize the improvement of the phase noise of the whole channel. The number of the switch bias current is the same as the number of switch capacitor groups, and has the same digital control port. The principle is as follows:

1. When the switched capacitor is turned off, because the total capacitance of the voltage-controlled oscillator increases, its frequency decreases, resulting in a decrease in the output voltage amplitude relative to high frequencies. At this time, the corresponding bias current is turned on to make the output amplitude of the voltage-controlled oscillator stay the same or get bigger.

2. When the switched capacitor is turned on, since the total capacitance of the voltage-controlled oscillator decreases, its frequency increases, and the corresponding output voltage amplitude increases. At this time, the corresponding bias current is turned off to save power consumption.

3. The magnitude of the switch bias current is set according to the value of the capacitance of the same digital control port, and a larger capacitance corresponds to a larger bias current.

In this way, the voltage-controlled oscillator can achieve better phase noise in the whole frequency band, and because the corresponding switch bias current is only turned on in the corresponding frequency band, the resulting average power consumption is lower.

The beneficial effect of the present invention is that the current-biased voltage-controlled oscillator can not only obtain a wider tuning range, but also obtain better phase noise in the entire frequency range. By properly setting the size of the bias current, at low frequencies Better phase noise than high frequency can be achieved, and the resulting increase in average power consumption is lower.

Description of drawings

The present invention will be described below in conjunction with the accompanying drawings and embodiments.

Figure 1 Schematic diagram of tail current bias type voltage controlled oscillator.

Figure 2 Switched capacitor bank.

Figure 3 Switch Bias Current.

Detailed ways

In Fig. 1, it is a structural diagram of a tail current bias oscillator, and Fig. 1 mainly illustrates the connection relationship between the switched capacitor bank and the switched bias current. The switched capacitor bank is connected between the two output ports Vout+ and Vout- of the oscillator, and the switch bias current is connected between net0 and GND of the oscillator.

Figure 2 shows a switched capacitor group, which consists of three groups of switched capacitors, each group of switched capacitors consists of two identical capacitors, two identical resistors, an NMOS transistor, and an inverter. Among them, the NMOS tube, two resistors, and the inverter constitute a switch, and the opening and closing of the switch is controlled by a digital signal. In order to reduce the number of digits of the digital control signal, a binary weighted capacitor array is used, and the value relationship of the three groups of capacitors is 2 ⁿ times (C01=2 ¹ C11=2 ² C22), and the corresponding NMOS transistor size (aspect ratio, have the same length) is also a 2 ^n- fold relationship.

Figure 3 is a switch bias current diagram, which consists of three groups of bias currents, each group consisting of an inverter and two NMOS transistors. One of the transistors is used as a switch, which is controlled on and off by a digital signal, and the other is used as a bias current source, and its gate is connected to the input reference current. The inverter is mainly to increase the driving capability of the digital control signal. The size of the NMOS transistor used as a switch should be as large as possible, so that the drain-source voltage on it is lower, and the influence on the bias current of the bias transistor is reduced. The size of the NMOS tube as the bias current source is proportional to the capacitance of the control port.

In the embodiment shown in Figure 2, for example, when the control port S2 is low, the NMOS switching transistor is turned on, C21 and C22 are connected to the resonant tank, and the frequency of the voltage-controlled oscillator decreases, resulting in a decrease in the output voltage amplitude, resulting in an increase in phase noise big. But because S2 controls the bias current at the same time, at this time, in the embodiment shown in Figure 3, Ms2 is turned on, and the bias current increases, which compensates for the decrease in the output voltage amplitude of the voltage-controlled oscillator due to frequency reduction, and the bias transistor is set appropriately The M2 size can keep or increase the output voltage range of the voltage-controlled oscillator, and its phase noise performance can be improved. In this way, by increasing the corresponding bias current in each sub-band to compensate the output voltage amplitude of the voltage-controlled oscillator, the voltage-controlled oscillator can obtain a wider tuning range and at the same time obtain better phase noise, and thus The added average power consumption is lower.

Claims (8)

1. A method for improving the tuning range and phase noise performance of a voltage-controlled oscillator, comprising a digital tuning module and a switch bias current module, characterized in that the digital tuning module and the switch bias current module must be coordinated and adjusted simultaneously, and the on-off corresponding The bias current is used to compensate the VCO output amplitude variation caused by digital tuning. 2. The method according to claim 1, wherein the digital tuning module is composed of at least one group of switched capacitors, and each group of switched capacitors is composed of two identical capacitors, two identical resistors, an NMOS transistor, and an inverter , where the NMOS tube, two resistors and an inverter constitute a switch, and the state of the switch is controlled by a digital signal. 3. The digital tuning module according to claim 2, wherein the actual number of groups is determined by the total tuning range and the tuning range of the capacitive reactance tube. 4. The method according to claim 1, characterized in that the switching bias current technique consists of at least one set of bias currents, each set consists of an inverter and two NMOS transistors, one of which is used as a switch, It is turned on and off by a digital signal, and the other is used as a bias current source, and its gate is connected to the input reference current. 5. The switched bias current module according to claim 4, wherein the actual number of bias currents is the same as the number of groups of switched capacitors. 6. The digital tuning module according to claim 2, characterized in that the capacitor adopts a binary weighted capacitor array, the value of adjacent capacitors is twice the relationship, and the corresponding NMOS transistor aspect ratio (the same length) is also twice the relationship . 7. The switch bias current module according to claim 4, characterized in that its digital control port is the same as the digital control port of the digital tuning module, and the aspect ratio of the bias transistor is proportional to the capacitance of the same control port. 8. Digital signal according to claims 2 and 4, which must be the same digital signal.

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