Voltage Controlled Oscillator

Abstract

A ring oscillator based voltage controlled oscillator (VCO) is disclosed. The VCO includes a set of delay cells connected to each other in a ring configuration. Each of the delay cells includes a source-coupled input transistor pair, a current-steering transistor pair and a pair of load resistors. The source-coupled input transistor pair receives a pair of differential voltage inputs. The load resistors, which are connected to the source-coupled input transistor pair, provide a pair of differential voltage outputs. The current-steering transistor pair, which is connected to the source-coupled input transistor pair, receives a pair of differential bias voltage inputs. The output frequency of the VCO is directly proportional to the differential bias voltages at the pair of differential bias voltage inputs.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to digital circuits in general, and in particular to voltage controlled oscillators. Still more particularly, the present invention relates to a voltage controlled oscillator having common-mode feedbacks and differential controls.

2. Description of Related Art

A voltage controlled oscillator (VCO) is an extremely sensitive, high-gain analog circuit. For silicon implementations, the common VCO architectures have been ring oscillators and inductor-capacitor (LC) oscillators.

Ring oscillators are relaxation oscillators that do not require resonant elements such as crystals. Although ring oscillators have many advantages such as wide tuning-ranges, small footprints, etc., they tend to have a relatively high phase noise or timing jitter, which is largely caused by the lack of a high-gain tuned element such as an LC tank.

LC oscillators tend to perform much better in wireless and radio-frequency communication systems that require a local oscillator to generate a pure sinusoid signal. However, for wired serial transceivers that depend on the availability of multiple phases of a serial clock to provide clock recovery/synchronization, ring oscillators are definitely preferred over LC oscillators. This is because a four-stage ring oscillator can, for example, readily generate eight phases of a high-quality, high-frequency clock, but the generation of different phases of a high-frequency clock is a much more difficult task for LC oscillators.

The present disclosure provides an improved ring oscillator based VCO with low jitter for wired serial transceiver applications.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a voltage controlled oscillator (VCO) includes a set of delay cells connected to each other in a ring configuration. Each of the delay cells includes a source-coupled input transistor pair, a current-steering transistor pair and a pair of load resistors. The source-coupled input transistor pair receives a pair of differential voltage inputs. The load resistors, which are connected to the source-coupled input transistor pair, provide a pair of differential voltage outputs. The current-steering transistor pair, which is connected to the source-coupled input transistor pair, receives a pair of differential bias voltage inputs. The output frequency of the VCO is directly proportional to the differential bias voltages at the pair of differential bias voltage inputs.

All features and advantages of the present invention will become apparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram a ring oscillator based voltage controlled oscillator (VCO), in accordance with a preferred embodiment of the present invention;

FIG. 2 is a schematic diagram of a delay cell within the VCO from FIG. 1, in accordance with a preferred embodiment of the present invention;

FIGS. 3 a-3b are conceptual circuits for deriving the concept of introducing negative resistance via a cross-coupled transistor pair within the delay cell from FIG. 2; and

FIG. 4 is a schematic diagram of a differential control voltage circuit for generating bias voltages for the delay cell from FIG. 2, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings and in particular to FIG. 1, there is depicted a block diagram of a ring oscillator based voltage controlled oscillator (VCO), in accordance with a preferred embodiment of the present invention. As shown, a VCO 10 includes four delay cells 11-14 connected to each other in a ring configuration. With delay cells 11-13, the positive/negative outputs are connected to the positive/negative inputs of their adjacent delay cell accordingly. With delay cell 14, the positive output is connected to the negative input of delay cell 11, and the negative output is connected to the positive input of delay cell 11. Eight different phases (e.g., φ₀, φ₄₅, φ₉₀, φ₁₃₅, φ₁₈₀, φ₂₂₅, φ₂₇₀ and φ₃₁₅) of a high-frequency clock can be obtained at corresponding outputs of delay cells 11-14.

Each of delay cells 11-14 is a differential delay (or trans-conductance) stage implemented with fully differential circuits to take advantage of their superior power supply rejection ratio (PSRR) and common-mode rejection ratio (CMRR) performance and inherent noise immunity. The delay time t_d of each of delay cells 11-14 can be controlled by varying the bias current of each corresponding delay cell. The delay time t_d is given by

$t_d=\frac{V_{\mathrm{sw}}C_{\mathrm{load}}}{I_{\mathrm{bias}}}$

where I_bias is the bias current of each delay cell, C_load is the total load capacitance at the output of each delay cell, and V_sw is the voltage swing.

Since delay cells 11-14 are identical to each other, only delay cell 11 will be further described in detail. With reference now to FIG. 2, there is depicted a schematic diagram of delay cell 11, in accordance with a preferred embodiment of the present invention. As shown, a delay cell 11 includes a source-coupled n-channel input transistor pair MN₀-MN₁ along with their respective load resistors R₀ and R₁. P-channel transistors MP₀ and MP₁ operate in their linear region and provide continuous-time common-mode feedback to keep the direct current (DC) level at outputs V_op and V_on constant.

The major drawback of most prior art delay cells is that the DC levels at the outputs of prior art delay cells also vary and can cause common-mode range problems. In order to avoid such common-mode range problems, prior art delay cells either operate with a very limited dynamic range (tail current variation) or require a parallel control path to adjust for the DC level variations. The common-mode feedback feature in delay cell 11 eliminates the common-mode range problems in most of the prior art delay cells.

Another feature incorporated within delay cell 11 is the delay variation with positive feedback. The cross-coupled n-channel transistor pair MN₄-MN₅ form a negative resistance pair that appear in parallel with resistors R₀ and R₁. Suppose that transistor pair MN₄-MN₅ presents a small signal resistance of −R_n, and let R₀=R₁=R_p, then the equivalent resistance at outputs V_op and V_on is given by (R_pR_n)/(R_n−R_p), which is more positive if |R_n|>|R_p|.

The concept of introducing negative resistance via cross-coupled transistor pair MN₄-MN₅ is derived using the conceptual circuit shown in FIG. 3a and its small signal equivalent circuit shown in FIG. 3b. From the equivalent circuits in FIGS. 3a-3b,

$I_x=g_{m4}V_4=-g_{m5}V_5$ $\mathrm{and}$ $V_x=V_5-V_4=-\frac{I_x}{g_{m4}}-\frac{I_x}{g_{m5}}$

The equivalent input resistance can then be calculated as

$\frac{V_x}{I_x}=-\frac{2}{g_m}$

where g_m4=g_m5=g_m is assumed.

The negative resistance is an incremental quantity, indicating that if the voltage applies to delay cell 11 increases, the current drawn by delay cell 11 decreases. As the total tail current in cross-coupled transistor pair MN₄-MN₅ in FIG. 2 (i.e., sum of drain currents of transistors MN₆ and MN₇) increases, the small signal differential resistance −2/g_m becomes less negative, and the equivalent resistance at outputs V_op and V_on, given by R_p/(1−g_mR_p), also increases, thereby lowering the output frequency of delay cell 11.

Delay cell 11 incorporates differential current steering to maintain a fairly constant output swing at outputs V_op and V_on. The differential current steering is achieved by varying bias voltages V_bp and V_bn differentially between transistors MN₂ and MN₆ in FIG. 2. The total tail current in each of source-coupled transistor pairs MN₀-MN₁, and MN₄-MN₅ is given by the sum of a constant current produced by transistors MN₃ and MN₇, respectively, and the controlled currents produced by transistors MN₂ and MN₆, respectfully.

With reference now to FIG. 4, there is depicted a schematic diagram of a differential control voltage circuit for generating bias voltages V_bp and V_bn to supply delay cell 11 from FIG. 2, in accordance with a preferred embodiment of the present invention. As shown, a differential control voltage circuit 40 includes a differential n-channel transistor pair MN₄₁-MN₄₂ with their sources coupled to each other via a resistor R_x. Differential control voltage circuit 40 includes inputs V_cp and V_cn that are the differential outputs of a charge pump and a loop filter.

At quiescent condition when V_cp=V_cn, the current through resistor R_x is zero. Any differential voltage Δv applied between V_cp and V_cn will cause a corresponding differential current Δi to flow through resistor R_x. Such differential current Δi should flow through either transistor MN₄₆ or transistor MN₄₇, and in turn, either gate voltage V_bn or gate voltage V_bp will change accordingly to accommodate the current change in order to establish a differential control voltage for VCO 10 from FIG. 1.

As has been described, the present invention provides a VCO having common-mode feedbacks and differential controls.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims

1. A voltage controlled oscillator (VCO) comprising: a plurality of delay cells connected to each other in a ring configuration, wherein each of said plurality of delay cells includes a source-coupled input transistor pair for receiving a pair of differential voltage inputs;a pair of load resistors connected to said source-coupled input transistor pair for providing a pair of differential voltage outputs; anda current-steering transistor pair connected to said source-coupled input transistor pair for receiving a pair of differential bias voltage inputs, wherein an output frequency of said VCO is directly proportional to differential bias voltages at said pair of differential bias voltage inputs.

2. The VCO of claim 1, wherein said VCO includes an even number of said delay cells.

3. The VCO of claim 1, wherein one of said delay cells has its positive output connected to a negative input of another one of said delay cells, and its negative output connected to a positive input of said another one delay cell.

4. The VCO of claim 3, wherein the remaining of said delay cells have their respective positive/negative outputs connected to positive/negative inputs of their adjacent delay cell accordingly.

5. The VCO of claim 1, wherein said source-coupled input transistor pair is two substantially identical transistors connected in parallel to each other.

6. The VCO of claim 1, wherein said pair of load resistors are substantially identical to each other.

7. The VCO of claim 1, wherein said current-steering transistor pair is two substantially identical transistors cross-coupled to each other.