CN100407570C - Voltage controlled oscillator with wide frequency band - Google Patents

Abstract

The present invention discloses a voltage controlled oscillator with a wide frequency band, which comprises an LC resonance loop and a circuit used for providing negative conductance, and the current of the circuit of negative conductance can be adjusted. The present invention uses the circuit structure of changeable negative conductance. The circuit uses a switch for controlling the connection or the disconnection of the network of a plurality of active circuits of negative conductance so as to change the total equivalent negative conductance of the voltage controlled oscillator. Thus, the voltage controlled oscillator can optimally counteract the conductance loss in the circuit in each frequency band to obtain the oscillating condition of the oscillator. Meanwhile, the phase noise and amplitude of an output signal can reach optimal values.

Description

Wide band voltage controlled oscillator

Technical field

The present invention relates to a kind of RF Voltage-Controlled Oscillator, relate in particular to a kind of wide band voltage controlled oscillator.

Background technology

In more than ten years in the past, personal wireless communications system such as beep-pager, cordless telephone, simulation and digital cellular telephone and Digital Television, broadcasting have obtained fast development, and radio communication becomes most important industry after the PC industry.Volume is little, low in energy consumption, cost is low because integrated circuit has, flourish the developing rapidly of facilitating radio frequency integrated circuit of wireless communication technology.At present, radio frequency integrated circuit has become one of microelectronics important field of research.Design and study high performance Key Circuit module and become the vital motive force of promotion radio frequency integrated circuit technological progress.Voltage controlled oscillator promptly is one of wherein crucial circuit module.

Typical voltage-controlled oscillator circuit as shown in Figure 1, it comprises LC resonant tank and active circuit part.C wherein _V1And C _V2Be the electric capacity of varactor, and C _V1=C _V2Total capacitance C is C _V1And C _V2Value after the series connection, V is the control voltage of varactor, L is an inductance.Other MOS device provides keeps the required energy of LC resonant tank persistent oscillation.Tail current source I _BiasProvide circuit working needed bias current.Voltage controlled oscillator changes the capacitance of whole LC resonant tank by the voltage V that regulates the varactor control end, thereby changes the frequency of oscillation of output signal.This process can be explained with following formula:

$f_{\mathrm{out}}=1/\sqrt{2\mathrm{\π}\·L\·C}---\left(1\right)$

C=C in the formula (V).Be output signal frequency f _OutBe the function of capacitance C, and capacitance C is the function of control voltage V.

When practical application, the inductance among Fig. 1 is not desirable element.Consider the loss of inductance itself, can come the inductance L in the presentation graphs 1 usually with simplified model shown in Figure 2.L wherein _sThe inductance value of expression equivalence, R _sThe series resistance of expression equivalence.

Voltage controlled oscillator shown in Figure 1 itself is a positive feedback system.Guarantee that circuit can keep lasting vibration, must satisfy following oscillating condition:

$\frac{g_{\mathrm{mn}}+{\mathrm{<figure-callout\; id="2"\; label="g\; mp"\; filenames="C20041000995700082.png"\; state="\{\{state\}\}">g</figure-callout>}}_{\mathrm{mp}}}{2}\&GreaterEqual;\mathrm{\&alpha;}\&CenterDot;g_{\tan k}---\left(2\right)$

G wherein _MnAnd g _MpBe respectively the mutual conductance of nmos device (MN1 among Fig. 1 and MN2) and PMOS device (MP1 among Fig. 1 and MP2), α is the constant (getting 1.5～3 usually) greater than 1.g _MnAnd g _MpApproximate formula as follows:

$g_{\mathrm{mn}}=\sqrt{2{\mathrm{\&mu;}}_n\&CenterDot;C_{\mathrm{OX}}\&CenterDot;{\left(\frac{w}{l}\right)}_n\&CenterDot;I_n}---\left(3\right)$

$g_{\mathrm{mp}}=\sqrt{2{\mathrm{\&mu;}}_p\&CenterDot;C_{\mathrm{OX}}\&CenterDot;{\left(\frac{w}{l}\right)}_p\&CenterDot;I_p}---\left(4\right)$

Wherein, μ _n, μ _pAnd C _OXBe constant, w and l are the MOS size of devices, I _nAnd I _pIt is respectively the electric current that flows through nmos device and PMOS device.

In addition, g _TankThe equivalent parallel electricity that is the LC resonant tank is led, and its approximate formula is as follows:

$g_{\tan k}\&ap;\frac{R_s}{{\mathrm{\&omega;}}^2\&CenterDot;{L_s}^2}---\left(5\right)$

R wherein _sAnd L _sAs shown in Figure 2, ω is a frequency of oscillation, i.e. ω=2 π f _Out

The amplitude V of voltage controlled oscillator output signal _SwingDirectly and g _TankRelevant, generally speaking, satisfy: V _Swing=I _Bias/ g _Tank(I _BiasAs shown in Figure 1).The size of signal amplitude can influence the most important index of voltage controlled oscillator: phase noise.Generally speaking, signal amplitude is big more, and phase noise is low more, and the quality of output signal is also high more.

g _TankExpression is that electricity is led the energy loss that is caused in the resonant tank in fact, and g _MnAnd g _MpEquivalence is that a negative electricity is led in fact, balances out g _TankCaused energy loss.The condition of oscillator vibration promptly is a process of guaranteeing that this energy loss is cancelled, thereby keeps the persistent oscillation of oscillator.Therefore, voltage controlled oscillator shown in Figure 1 can be expressed as the form of Fig. 3 again.

For wide band voltage controlled oscillator, the scope that ω changes greatly, therefore at different Frequency points, pairing g _TankBigger fluctuation numerically also takes place.Existing wideband voltage controlled oscillator has following shortcoming:

(1) since the scope that ω changes greatly, so at different Frequency points, the starting condition for oscillation of voltage controlled oscillator is significantly different, promptly needed g _MnOr g _MpSignificant difference is arranged.(2) since the scope that ω changes greatly, at different Frequency points, the amplitude of output signal is significantly different, therefore, bigger fluctuation also takes place in phase noise.

Summary of the invention

At the existing problem and shortage of above-mentioned existing broadband RF Voltage-Controlled Oscillator, the purpose of this invention is to provide a kind of phase noise of output signal and the wide band voltage controlled oscillator that amplitude is more optimized.

The present invention is achieved in that a kind of wide band voltage controlled oscillator, comprises LC resonant tank and the circuit that is used to provide negative conductance, and the electric current of described negative conductance circuit is adjustable.

Further, described variable negative conductance circuit is a plurality of, and by switch control folding separately, back parallel with one another is as total negative conductance circuit of described voltage controlled oscillator.

The present invention adopts the circuit structure of variable negative conductance.This circuit is controlled the connection or the disconnection of a plurality of negative conductance active circuit networks by switch, thereby change the total equivalent negative conductance of voltage controlled oscillator, make its electricity in the bucking circuit of each frequency band energy the best lead loss, reach the oscillating condition of oscillator, make the phase noise of output signal and amplitude reach optimal value simultaneously.

Description of drawings

Below in conjunction with accompanying drawing, the present invention is made detailed description.

Fig. 1 is typical voltage-controlled oscillator circuit figure;

Fig. 2 is the simple equivalent circuit model schematic diagram of inductance among Fig. 1;

Fig. 3 is a voltage controlled oscillator energy compensating schematic diagram;

Fig. 4 is a structural representation of the present invention;

Fig. 5 is the topology example figure of another embodiment of the present invention;

Embodiment

As shown in Figure 4, the present invention is improved to form on the basis of typical voltage-controlled oscillator circuit (as shown in Figure 1).Compared to Figure 1, added another negative conductance circuit among Fig. 4.Promptly two negative conductance circuit networks are altogether controlled by switch separately respectively.When two negative conductance circuit all are access in, be equivalent to change the bias current I of MOS size of devices (being w and the l in formula (3) and (4)) in the active circuit and entire circuit _Bias(be I _Bias=I _Bias+ I _Bias2).By formula (3) and (4) as can be known, the present invention comes down to change g by the electric current that changes the MOS size of devices or flow through the MOS device _MnAnd g _MpValue.When the negative conductance of needs is big (for example low-frequency range), can simultaneously two negative conductance circuit networks all be inserted; When the negative conductance of needs hour (for example high band), only need to insert a negative conductance circuit network.The bias current of each negative conductance circuit network and MOS device size can independently be adjusted.Therefore, this scheme has the ability of flexible control negative conductance, can make g accordingly at the designing requirement of wideband voltage controlled oscillator in the different frequency section _MnAnd g _MpReach the optimal design value, the amplitude of voltage controlled oscillator output signal and phase noise are optimized.

The present invention has optimized the voltage controlled oscillator design, makes it have different g at different frequency bands _MnAnd g _MpThereby, guarantee that the circuit performance in whole frequency range all reaches optimal value.

As shown in Figure 5, as required, negative conductance of the present invention can be made of a plurality of active circuits.

Claims (1)

1. wide band voltage controlled oscillator, comprise LC resonant tank and the circuit that is used to provide negative conductance, it is characterized in that, the electricity of described negative conductance circuit is led variable, the annexation of described negative conductance circuit is: a switch is arranged between negative conductance circuit and the ground termination points, another switch is arranged between negative conductance circuit and the power supply, and by the connecting and disconnecting of above-mentioned two switches control negative conductance circuit, a plurality of described negative conductance circuit back parallel with one another is as total negative conductance circuit of voltage controlled oscillator.

Images