This application claims priority to Japanese Patent Application No. 2008-325242 filed on Dec. 22, 2008, the disclosure of which including the specification, the drawings, and the claims is hereby incorporated by reference in its entirety.
The present disclosure relates to a voltage controlled oscillator and specifically to a voltage controlled oscillator used for a semiconductor integrated circuit.
Wireless radios as represented by mobile phones have receivers and transmitters. The receiver includes a down converter and a frequency synthesizer to convert a received signal into a baseband signal having a low frequency. The transmitter includes an up converter and the frequency synthesizer to convert the baseband signal into a transmitting signal having a high frequency. The frequency synthesizer has a voltage controlled oscillator or a digital control oscillator (a type of the voltage controlled oscillator). Recently, efforts are being made to achieve higher performance, lower power consumption, and down-sizing of wireless radios. Similarly, lower phase noise, lower power consumption, and down-sizing of frequency synthesizers, which are used in the wireless radios, are demanded, too. Performance of frequency synthesizers is mainly dependent on performance of voltage controlled oscillators or digital control oscillators, and therefore, lower phase noise, lower power consumption, and down-sizing of the voltage controlled oscillators or digital control oscillators are crucial. Various circuit types are used for the voltage controlled oscillators, such as a ring oscillator type, an LC resonance type which uses inductors and capacitors, and a standing-wave oscillator (SWO) type. In recent years, rotary traveling-wave (RTW) type oscillators are receiving attention among others. The RTW type oscillator has a transmission line like a Moebius strip which is twisted so as to form two circles, and oscillation is caused by an active circuit interposed between two signal lines which constitute the transmission line (see, for example, U.S. Pat. No. 6,556,089). The RTW type oscillator can be implemented compactly on a semiconductor substrate, and is thus expected to greatly contribute to the down-sizing of the frequency synthesizers.
However, conventional voltage controlled oscillators have the following problems. In the case where a frequency synthesizer is configured using a voltage controlled oscillator, the transient response and noise band characteristics of the frequency synthesizer are dependent on a frequency sensitivity to a control voltage. Thus, in the case where the frequency does not change linearly according to the change of the voltage, the characteristics of the frequency synthesizer are varied according to the frequency. Moreover, in the area where the frequency sensitivity to the control voltage is high, the frequency is varied by a small noise received at a frequency control terminal, and thus, a phase noise characteristic is degraded.
To implement the voltage controlled oscillator on a semiconductor substrate, a MOS varactor (a metal oxide semiconductor (MOS) transistor) is used as a variable capacitor element. The MOS varactor has a characteristic that its capacitance value significantly changes in the area where a voltage is close to a threshold voltage of the MOS varactor. As such, the oscillation frequency of the voltage controlled oscillator using the MOS varactor significantly changes in the area where the voltage is close to the threshold voltage of the MOS varactor.
Using a variable capacitor element having high linearity may improve the noise characteristic. However, using a variable capacitor element having high linearity so as to achieve the voltage controlled oscillator on the semiconductor substrate requires a large cost, and thus, is difficult.
The present invention is advantageous in solving the above problems and achieving an RTW type voltage controlled oscillator with a superior phase noise characteristic while using a widely-used variable capacitor element without additional fabrication costs.
To achieve the above, an example voltage controlled oscillator has a structure in which different reference voltages are applied to variable capacitor elements.
Specifically, an example voltage controlled oscillator includes: a loop-shaped transmission line having an odd number of parallel portions in each of which signal lines are arranged in parallel to each other with a space therebetween, and an odd number of intersection portions in each of which the signal lines intersect spatially; active circuits connected to the signal lines; and variable capacitor blocks connected to the signal lines and including a plurality of variable capacitor units, wherein each of the plurality of variable capacitor units includes a variable capacitor element, a control terminal for applying a control voltage to the variable capacitor element, and a reference voltage terminal for applying a reference voltage to the variable capacitor element, and wherein at least two of the plurality of variable capacitor units receive the reference voltages having different values.
In the example voltage controlled oscillator, at least two variable capacitor units receive reference voltages having different values. Thus, values of the control voltage at which the capacitance values of the variable capacitor units are greatly varied cannot be the same between all of the variable capacitor units. Therefore, the rate of change in the total capacitance of the variable capacitor blocks with respect to the control voltage can be lowered, and the frequency sensitivity to the control voltage can thus be lowered. As a result, an RTW type voltage controlled oscillator with a superior phase noise characteristic can be achieved even if a widely-used MOS varactor is used.
The transmission line can be considered as a circuit in which a plurality of inductances and capacitances are connected. In this case, a phase rotational speed Vp can be represented by
wherein L0 represents an inductance per unit length and C0 represents a capacitance per unit length. One round of the transmission line 15 corresponds to one circle of the phase. Thus, the oscillation frequency f0 of the transmission line 15 can be represented by
wherein λ represents a wavelength; L1 represents the sum of inductances corresponding to half a round of the transmission line 15; and C1 represents the sum of capacitances to the ground which correspond to half a round of the transmission line 15. In the case where capacitor elements are deliberately added, C1 represents the sum of the capacitance of the added capacitor elements and parasitic capacitances. In the case of the circuit of
Now, if the reference voltage applied to each variable capacitor unit 23 is a ground voltage (0 V) and constant, the relationship between the capacitance value of each variable capacitor unit 23 and the control voltage applied to the control terminal 41 is as shown in
On the other hand, if different reference voltages are applied to the variable capacitor units 23, the relationship between the capacitance value of each variable capacitor unit 23 and the control voltage is as shown in
By adjusting the value of the reference voltage applied to each variable capacitor unit 23, the total capacitance of the variable capacitor blocks 21 can be reduced gradually, and can be almost linear with respect to the control voltage as shown in
An example in which the frequency sensitivity is maintained almost constant by applying different reference voltages to the variable capacitor units 23 is described. However, the frequency sensitivity does not necessarily have to be almost constant. In some cases, a frequency sensitivity that can achieve a predetermined phase noise characteristic may be enough. In such cases, for example, changing the reference voltage applied to one variable capacitor unit 23 to a value that is different from the reference voltages applied to the other variable capacitor units 23 may be enough.
The larger the operating current supplied to the active circuit 17 is, the higher the voltage amplitude of the voltage controlled oscillator becomes. Thus, to improve the phase noise characteristic of the voltage controlled oscillator, it is preferable that the operating current supplied to the active circuit 17 is large. However, if a large operating current is supplied to the active circuit 17, it increases the power consumption of the voltage controlled oscillator. On the other hand, according to the voltage controlled oscillator of the present embodiment, the frequency sensitivity to the control voltage can be kept low, and therefore, the phase noise characteristic is not degraded. Thus, in the case where the voltage controlled oscillator exhibits a sufficient phase noise characteristic, the operating current supplied to the active circuit 17 can be reduced.
By using an operating current variable circuit 53 for varying the operating current supplied to the active circuit 17 as shown in
The operating current variable circuit 53 may be structured as shown, for example, in
In the case where a plurality of active circuits 17 are provided, the operating current for a specific active circuit 17 may be controlled, or the operating currents for all the active circuits 17 may be controlled. In the case where the operating currents for the plurality of active currents 17 are controlled, the operating currents may be controlled separately for each active current 17, or may be controlled simultaneously for the plurality of active circuits 17.
As shown in
There are cases in which the active circuits 17 have slightly different characteristics if provided at a plurality of locations on a substrate. In such cases, using only an active circuit 17 which has a superior characteristic is possible. For example, the output of the voltage controlled oscillator may be monitored and an active circuit 17 used when the voltage controlled oscillator exhibits the highest amplitude may be selected. Parameters other than the amplitude can be used, too. Monitoring may be done beforehand in the inspection step to select an active circuit 17 to be operated, or monitoring may be done as appropriate during the operation of the voltage controlled oscillator to switch between active circuits 17 to be operated.
To increase a range of the oscillation frequency of the voltage controlled oscillator, a fixed capacitor block 19 as shown in
The oscillation frequency and the capacitance value of the voltage controlled oscillator have the relationship such as defined by the equation (2). According to the equation, an attempt to cover a very wide range of the oscillation frequency while maintaining a linear change in the capacitance, results in a significant increase in frequency sensitivity to the control voltage, and hence degradation of the phase noise characteristic, in the area where the oscillation frequency is high as shown in
Values of the reference voltage supplied to a predetermined variable capacitor unit 23 can be changed by the reference voltage variable circuit 71. For example, in the case where the reference voltage variable circuit 71 is connected to a first variable capacitor unit 23 (1) to allow the reference voltage applied to the first variable capacitor unit 23 (1) to be varied between Vref1a and Vref1b, the relationship between the control voltage and the capacitance value of the variable capacitor blocks 21 is as shown in
The variable capacitor unit 23 to which the reference voltage variable circuit 71 is connected may be determined according to the amount of change in required capacitance values. The reference voltage variable circuit 71 may be connected to one variable capacitor unit 23, and may also be connected to each of a plurality of variable capacitor units 23. An example in which the reference voltage is varied according to the oscillation frequency is described in the above, but the reference voltage can be varied according to a communication mode, too.
To vary the rate of change in the total capacitance value of the variable capacitor blocks 21 with respect to the control voltage, a control voltage fixing circuit 81 may be provided, as shown in
If the voltage applied to the control terminal of the first variable capacitor unit 23 (1) is held constant, the capacitance value of the first variable capacitor unit 23 (1) stays constant irrespective of the control voltage. Thus, the rate of change in the total capacitance value of the variable capacitor blocks 21 with respect to the control voltage becomes lower compared to when the control voltage is applied to the first variable capacitor unit 23 (1). Accordingly, the rate of change in the capacitance value with respect to the control voltage can be within an appropriate range according to the oscillation frequency, a communication mode, etc.
The variable capacitor unit 23 to which the control voltage fixing circuit 81 is connected may be determined according to the amount of change in required capacitance values. The control voltage fixing circuit 81 may be connected to one variable capacitor unit 23, and also may be connected to each of a plurality of variable capacitor units 23. Further, control of the control voltage fixing circuit 81 and control of switching bands in the fixed capacitor block 19 may be linked to each other. Moreover, providing both of the control voltage fixing circuit 81 and the reference voltage variable circuit 71 is possible.
The fixed capacitor block 19 may be configured by a plurality of parts which are located at a plurality of different places, or may be located collectively at one place. The capacitance values of all the fixed capacitor units 65 included in a fixed capacitor block 19 may be equal to each other, or a fixed capacitor block 19 may include multiple types of fixed capacitor units 65 whose capacitance values are different from one another. The number of the fixed capacitor units 65 included in a fixed capacitor block 19 may be determined based on the number of bands needed. It is preferable that the capacitance value of each fixed capacitance unit 65 is set such that ranges of variable capacitance values overlap with nearby bands.
At least one active circuit 17 is enough, but if a plurality of active circuits 17 are provided, the active circuits 17 may be located at a plurality of different places, or may be collectively located at the same place.
An example in which the transmission line 15 includes one parallel portion 15A and one intersection portion 15B is shown in
As described in the above, an example voltage controlled oscillator can realize an RTW type voltage controlled oscillator with a superior phase noise characteristic while utilizing a widely-used variable capacitor element without additional costs, and is particularly useful as a voltage controlled oscillator or the like used in a semiconductor integrated circuit.
The description of one embodiment of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.
Number | Date | Country | Kind |
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2008-325242 | Dec 2008 | JP | national |