Differential crystal oscillator with large voltage swing
By introducing a cross-coupled regeneration network and a capacitor feedback structure into the differential crystal oscillator, the swing of the oscillation signal is enhanced, the problem of limited oscillation signal is solved, and the effect of low phase noise is achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- REALTEK SEMICON CORP
- Filing Date
- 2022-01-06
- Publication Date
- 2026-06-16
AI Technical Summary
The voltage swing of the oscillation signal in a differential crystal oscillator is limited, resulting in high phase noise.
By employing a regenerative network and a capacitive feedback network with cross-coupling structure, combined with a DC-coupled resistor and a resonant network, the swing of the oscillation signal is enhanced, and effective signal regeneration and feedback are achieved through an NMOS transistor.
It achieves a large swing of the oscillation signal, reduces phase noise, and improves the performance of the differential crystal oscillator.
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Figure CN115701683B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to crystal oscillators, and more particularly to differential crystal oscillators with large voltage swing and low phase noise. Background Technology
[0002] As is well known, crystal oscillators are important circuits in many applications. Most crystal oscillators are based on single-ended circuit structures. However, single-ended crystal oscillators are prone to generating a large amount of harmonic spurs, which can cause harmful interference to other circuits. Because differential crystal oscillators can effectively mitigate harmonic spurs, they are a preferred choice. However, differential crystal oscillators can be prone to latching and failure to oscillate. As described in the paper "A48-MHz Differential Crystal Oscillator With 168-fs Jitter in 28-nm CMOS" published in IEEE Journal of Solid-State Circuits, VOL.52, NO.10, OCT 2017, Rajavi et al. proposed a differential crystal oscillator 100 (e.g., Figure 1 (As shown).
[0003] The differential crystal oscillator 100 includes: a resonant network 110, comprising a crystal 113 and two shunt capacitors 111 and 112, used to determine the oscillation signal V. osc When implemented as a differential signal, the frequency of the oscillation signal V is... osc Includes two voltages V at nodes 101 and 102 respectively. osc+ and V osc- The regenerative network 120 includes two NMOS (n-channel metal-oxide-semiconductor) transistors 121 and 122, configured as a cross-coupled circuit across nodes 101 and 102 to maintain the oscillation signal V. osc The oscillation; a first active inductor 130, including a PMOS (p-channel metal-oxide-semiconductor) transistor 131 and a resistor 132 coupled to node 101; and a second active inductor 140, including a PMOS transistor 141 and a resistor 142 coupled to node 102. In this disclosure, "V" DD "" indicates a power supply node. The working principle of the differential crystal oscillator 100 has been explained in detail in the aforementioned paper and will not be repeated here.
[0004] One of the disadvantages of the differential crystal oscillator 100 is the voltage V. osc+ and V osc- The amplitude of the swing is limited. Although V osc+ (Vosc- It can be effectively pulled down by NMOS transistor 122 (121) and pulled up by PMOS transistor 131 (141), but since PMOS transistor 131 (141) is configured as a diode, the pull-up has no effect, and in V osc+ (V osc- When pulled up, the pull-up capability of PMOS transistors 131 (141) decreases. Oscillation signal V osc Phase noise and voltage V osc+ and V osc- The swing amplitude is highly correlated. This is because the allowable voltage V... opsc+ and V osc- The effective pull-up capability with a large swing is insufficient, so the phase noise-related effect that the differential crystal oscillator 100 can achieve is inherently limited.
[0005] Therefore, a differential crystal oscillator is needed that allows the oscillation signal to have a large swing while thus having low phase noise. Summary of the Invention
[0006] An embodiment of the present invention provides a differential crystal oscillator, comprising: a first source follower including a first metal-oxide-semiconductor (MOS) transistor and a second MOS transistor of a first type, for receiving an oscillation signal including a first voltage and a second voltage respectively at a first node and a second node, and outputting a regenerated signal including a third voltage and a fourth voltage respectively at a third node and a fourth node; a regeneration network including a third MOS transistor and a fourth MOS transistor of the first type configured in a cross-coupled structure between the third node and the fourth node to regenerate the regenerated signal; a capacitor feedback network including a first capacitor and a second capacitor for coupling the first node and the second node to the third node and the fourth node respectively, and a third capacitor and a fourth capacitor for shunting the third node and the fourth node to ground; a first DC coupling resistor and a second DC coupling resistor for coupling a first bias voltage to the first node and the second node respectively; and a resonant network including a crystal disposed between the first node and the second node.
[0007] Another embodiment of the present invention provides a differential crystal oscillator, comprising: a source follower for receiving an oscillation signal and outputting a regenerated signal; a resonant network including a crystal and used as a node of the oscillation signal and for determining an oscillation frequency of the oscillation signal; a regeneration network for regenerating the regenerated signal; and a capacitor feedback network for providing feedback from the regenerated signal to the oscillation signal. Attached Figure Description
[0008] Figure 1 A schematic diagram showing a prior art differential crystal oscillator;
[0009] Figure 2 A schematic diagram showing an embodiment of the differential crystal oscillator disclosed herein; and
[0010] Figure 3 show Figure 2 The simulation results of the waveform of the differential crystal oscillator.
[0011] Symbol Explanation
[0012] 100, 200: Differential crystal oscillators
[0013] 101, 102: Nodes
[0014] 110: Resonant Network
[0015] 111, 112: Shunt capacitors
[0016] 113: Crystal
[0017] 120: Regenerative Network
[0018] 121, 122, M1, M2, M3, M4: NMOS transistors
[0019] 130: First Active Inductor
[0020] 131, 141, M5, M6: PMOS transistors
[0021] 132, 142: Resistors
[0022] 140: Second active inductor
[0023] 210: First source follower
[0024] 220: Regenerative Network
[0025] 240: Capacitive Feedback Network
[0026] 250: Resonant Network
[0027] 251: Crystal
[0028] 252: Tuning capacitor
[0029] 260: Second source follower
[0030] C1, C2, C3, C4, C5, C6: Capacitors
[0031] R1, R2, R3, R4: Resistors
[0032] N1, N2, N3, N4, N5, N6: Nodes
[0033] V DD Power Node
[0034] V A+ V A- Oscillation signal
[0035] V B+ V B+ :Regeneration signal
[0036] V C+ V C- Auxiliary oscillation signal Detailed Implementation
[0037] This disclosure relates to differential crystal oscillators. While the specification describes several embodiments of this disclosure, and these embodiments are considered preferred modes for implementing the invention, it should be understood that the invention can be implemented in many ways and is not limited to the specific examples described below or any particular way of implementing those examples. For the sake of focusing on various aspects of this disclosure, well-known details will not be shown or described in other instances.
[0038] Those skilled in the art will understand the microelectronics-related terms and basic concepts used in this disclosure, such as "crystal," "voltage," "signal," "differential signal," "bias," "DC (direct current)," "AC (alternating current)," "capacitor," "resistor," "first source follower," "parallel," "circuit node," "ground," "power supply," "metal oxide semiconductor (MOS) transistor," "complementary metal oxide semiconductor (CMOS) process technology," "n-channel metal oxide semiconductor (NMOS) transistor," and "p-channel metal oxide semiconductor (PMOS) transistor." Such terms and basic concepts are readily apparent to those skilled in the art in the field of microelectronics and therefore will not be explained in detail herein.
[0039] Those skilled in the art can understand units such as pF (pico-Farad), nm (nanometer), μm (micron), and KOhm (kilo-Ohm) without explanation.
[0040] Those skilled in the art can read circuit diagrams containing electronic components such as capacitors, resistors, NMOS transistors, and PMOS transistors without needing to describe in detail how one component is connected to another. Those skilled in the art can also identify ground symbols, capacitor symbols, resistor symbols, and the symbols for PMOS and NMOS transistors, and recognize their "source terminal," "gate terminal," and "drain terminal." For the sake of brevity, regarding MOS transistors, the "source terminal" will be simply referred to as "source," the "gate terminal" as "gate," and the "drain terminal" as "drain."
[0041] MOS transistors (PMOS or NMOS) have a threshold voltage. A MOS transistor turns on when its gate-source voltage is greater than its threshold voltage (in absolute value). The difference between its gate-source voltage and its threshold voltage when the MOS transistor is on is called the "over-drive voltage" (in absolute value). When a MOS transistor is on and its over-drive voltage is less than its drain-source voltage (in absolute value), the MOS transistor is in the "saturation region." A MOS transistor is only an effective gain element in the "saturation region"; when this occurs, we say that the MOS transistor is biased in the saturation region.
[0042] A source follower includes a MOS transistor configured to receive an input voltage from its gate and output a voltage via its source. A MOS transistor can only effectively function as a source follower when it is biased in the saturation region.
[0043] In this disclosure, one of the NMOS transistors and the PMOS transistor is referred to as a first type of MOS transistor, and the other as a second type of MOS transistor. In one embodiment, the NMOS transistor is referred to as a first type of MOS transistor, and the PMOS transistor is referred to as a second type of MOS transistor. In other embodiments, the PMOS transistor is referred to as a first type of MOS transistor, and the NMOS transistor is referred to as a second type of MOS transistor.
[0044] In this disclosure, a circuit is a collection of transistors, capacitors, resistors and / or other electronic components interconnected in a certain way to achieve a certain function.
[0045] In this disclosure, a network is a circuit or a collection of circuits.
[0046] In this disclosure, when the meaning of "circuit node" is clear in the context, "circuit node" is often simply referred to as "node".
[0047] In this disclosure, a signal is a voltage with a variable level that carries specific information and can change over time. The level of a signal at a given moment represents the state of the signal at that moment. In this disclosure, "signal" and "voltage signal" refer to the same thing and are therefore interchangeable.
[0048] Differential signaling schemes are widely used throughout the scheme. When a signal is implemented using a differential signaling scheme, the signal comprises two voltages (indicated by the subscripts "+" and "-" respectively), and the value of the signal is represented by the difference between the two voltages. For example, a signal V A (V B V C When implemented as a differential signal, it includes two voltages V. A+ (V B+ V C+ ) and V A- (V B- V C- ), and signal V A (V B V C The value of ) is determined by V A+ (V B+ V C+ ) and V A- (V B- V C- The difference between ) represents V; A+ (V B+ V C+ It is called V A (V B V c The first end of ) and V A- (V B- V C- It is called V A (V B V c The first terminal is also called the positive terminal; the second terminal is also called the negative terminal. In differential signal implementations, the average value of the first and second terminals of a signal is called the "common mode" of the signal.
[0049] ("common-mode") voltage.
[0050] Figure 2 This diagram illustrates an embodiment of the differential crystal oscillator 200 disclosed herein. The differential crystal oscillator 200 includes: a first source follower 210 comprising two NMOS transistors, namely a first NMOS transistor M1 and a second NMOS transistor M2, for receiving an oscillation signal V. A(When implemented as a differential signal, it includes two voltages: a first voltage V at the first node N1 and a second node N2 respectively) A+ With the second voltage V A- And output regenerated signal V b (When implemented as a differential signal, it contains two voltages: a third voltage V at the third node N3 and the fourth node N4 respectively.) B+ With the fourth voltage V B- The regeneration network 220 includes two NMOS transistors, namely a third NMOS transistor M3 and a fourth NMOS transistor M4, and is configured in a cross-coupled structure between the third node N3 and the fourth node N4 (the gate of the third NMOS transistor M3 is not electrically connected to the gate of the fourth NMOS transistor M4) to regenerate the regeneration signal V. B The capacitive feedback network 240 includes four capacitors: a first capacitor C1, a second capacitor C2, a third capacitor C3, and a fourth capacitor C4, and is used to provide feedback from the regenerated signal V. B To the oscillation signal V A The feedback includes a third capacitor C3 and a fourth capacitor C4 configured to shunt the third node N3 and the fourth node N4 to ground, respectively, while a first capacitor C1 and a second capacitor C2 are configured to couple the first node N1 and the second node N2 to the third node N3 and the fourth node N4, respectively; and a resonant network 250 comprising a crystal 251 and a tuning capacitor 252, which are connected in parallel and placed between the two nodes N1 and N2. DD "This indicates a power node."
[0051] The differential crystal oscillator 200 also includes two DC-coupled resistors, namely a first DC-coupled resistor R1 and a second DC-coupled resistor R2, for converting the first bias voltage V. B1 The bias condition for the first source follower 210 is established by coupling the gates of two NMOS transistors M1 and M2. The first source follower 210, the regeneration network 220, and the capacitor feedback network 240 form a self-sustaining network to maintain the oscillation signal V. A The oscillation, and the resonant network 250 determines the oscillation signal V. A The oscillation frequency.
[0052] In a further embodiment, the differential crystal oscillator 200 also includes two AC coupling capacitors, namely a first AC coupling capacitor C5 and a second AC coupling capacitor C6, used to convert the oscillation signal V... A Coupled to auxiliary oscillation signal V C(When implemented with a differential signal, V) C It contains two voltages: the fifth voltage V at the fifth node N5 and the sixth node N6 respectively. C+ and the sixth voltage V C- The system includes a second source follower 260, which comprises two PMOS transistors, namely a first PMOS transistor M5 and a second PMOS transistor M6, for receiving an auxiliary oscillation signal V. C And together with the first source follower 210, they regenerate the regenerated signal V. B The differential crystal oscillator 200 also includes two additional DC-coupled resistors, namely a third DC-coupled resistor R3 and a fourth DC-coupled resistor R4, used to convert the second bias voltage V. B2 The gates of the two PMOS transistors M5 and M6 are coupled to establish the bias conditions for the second source follower 260.
[0053] Compared to the existing differential crystal oscillator 100, the advantage of the differential crystal oscillator 200 lies in the oscillation signal V. A The amplitude can be larger because the oscillation signal V A It is based on the gates of NMOS transistors M1 and M2, and therefore is not limited by the pull-up or pull-down capabilities of the transistors.
[0054] For example (but not as a limitation), in one embodiment: except for crystal 252, differential crystal oscillator 200 is integrated on a silicon substrate using a 55nm CMOS process; the resonant frequency of crystal 251 is approximately 40MHz; capacitor 251 is 5pF; the W / L (width / length) of NMOS transistors M1 and M2 is 56μm / 260nm; the W / L of NMOS transistors M3 and M4 is 28μm / 260nm; the W / L of PMOS transistors M5 and M6 is 70μm / 260nm; capacitors C1, C2, C3, and C4 are all 10pF; capacitors C5 and C6 are 2pF; V DD It is 1.2V; V B1 It is 1.2V; V B2 It is 0V; resistors R1, R2, R3, and R4 are all 200KΩ. A+ and V A- The simulated waveform of one oscillation period is shown in Figure 3 As shown in the figure, V A+ It can swing from approximately 298mV (M_low) to 1.9V (M_high), with a peak-to-peak value of approximately 1.3V. Using the same 1.2V supply voltage, the V of a prior art differential crystal oscillator 100... osc+The swing amplitude does not exceed the 1.2V power supply voltage, and is only about 1V.
[0055] Resistors (e.g., R1, R2, R3, and R4) and capacitors (e.g., C1, C2, C3, C4, C5, C6, and tuning capacitor 252) can be implemented in various ways, and those skilled in the art are free to choose.
[0056] Although the resonant network 250 is shown in the accompanying drawings as a parallel connection of crystal 251 and tuning capacitor 252, this is an example and not a limitation of this disclosure. The resonant network 250 can be connected in conjunction with... Figure 1 The same structure as the resonant network 110 is used; that is, two capacitors replace the tuning capacitor 252, which respectively shunt the first node N1 and the second node N2 to ground. This is clear to those skilled in the art and is therefore not shown in the figures. Furthermore, if the accuracy of the oscillation frequency of the differential crystal oscillator 200 is already satisfactory, then the tuning capacitor 252 is not initially required.
[0057] It is known in the prior art that for any circuit containing one or more NMOS and PMOS transistors operating across a power node and a ground node, there exists a complementary circuit that can have the same function. This complementary circuit can be constructed by replacing each NMOS transistor with a PMOS transistor, replacing each PMOS transistor with an NMOS transistor, and swapping the power node with the ground node. Using this technique, in another embodiment (not shown but well known to those skilled in the art), the differential crystal oscillator 200 is converted into a complementary circuit in which each NMOS transistor is replaced by a PMOS transistor, each PMOS transistor is replaced by an NMOS transistor, and the power node V... DD Swap with the grounding node. Note that V needs to be adjusted. B1 and V B2 The voltage level is determined to establish appropriate bias conditions for the complementary circuit, which is something the circuit designer can decide for themselves.
[0058] Those skilled in the art will readily recognize that various modifications and alterations can be made to the apparatus and method while retaining the teachings of this disclosure. Therefore, the foregoing discussion should not be construed as limiting it solely to the claims.
Claims
1. A differential crystal oscillator, comprising: A first source follower includes a first metal-oxide-semiconductor transistor and a second metal-oxide-semiconductor transistor of a first type, for receiving an oscillation signal including a first voltage and a second voltage respectively at a first node and a second node, and outputting a regenerated signal including a third voltage and a fourth voltage respectively at a third node and a fourth node. A regeneration network includes a third metal-oxide-semiconductor transistor and a fourth metal-oxide-semiconductor transistor of the first type, configured in a cross-coupled structure between the third node and the fourth node to regenerate the regeneration signal. A capacitor feedback network includes a first capacitor and a second capacitor for coupling the first node and the second node to the third node and the fourth node respectively, and a third capacitor and a fourth capacitor for shunting the third node and the fourth node to ground. A first DC coupling resistor and a second DC coupling resistor are used to couple a first bias voltage to the first node and the second node, respectively. as well as A resonant network comprising a crystal placed between the first node and the second node.
2. The differential crystal oscillator of claim 1, further comprising: a first AC coupling capacitor and a second AC coupling capacitor for coupling the first node and the second node to a fifth node and a sixth node, respectively; a second source follower comprising a fifth metal-oxide-semiconductor transistor and a sixth metal-oxide-semiconductor transistor of a second type for receiving an auxiliary oscillation signal comprising a fifth voltage and a sixth voltage respectively at the fifth node and the sixth node, and for outputting, together with the first source follower, the regenerated signal comprising the third voltage and the fourth voltage respectively at the third node and the fourth node; and a third DC coupling resistor and a fourth DC coupling resistor for coupling a second bias voltage to the fifth node and the sixth node, respectively.
3. The differential crystal oscillator as described in claim 1, wherein, The resonant network also includes a tuning capacitor placed between the first node and the second node.
4. A differential crystal oscillator, comprising: A first source follower is used to receive an oscillation signal and output a regenerated signal; A resonant network includes a crystal and is used as a node for the oscillation signal and to determine an oscillation frequency of the oscillation signal; A regenerating network is used to regenerate the regenerated signal; and A capacitor feedback network is used to provide feedback from the regenerated signal to the oscillation signal; in, The regeneration network includes a first metal-oxide-semiconductor transistor and a second metal-oxide-semiconductor transistor of a first type configured in a cross-coupled structure to regenerate the regenerated signal; The gates of the first metal-oxide-semiconductor transistor and the second metal-oxide-semiconductor transistor are not electrically connected.
5. The differential crystal oscillator as described in claim 4, wherein, The first source follower includes a third metal-oxide-semiconductor transistor and a fourth metal-oxide-semiconductor transistor of a first type, for receiving a first terminal and a second terminal of the oscillation signal at a first node and a second node respectively, and outputting a first terminal and a second terminal of the regenerated signal at a third node and a fourth node respectively.
6. The differential crystal oscillator as described in claim 5, wherein, The capacitive feedback network includes a first capacitor and a second capacitor that couple the first node and the second node to the third node and the fourth node, respectively, and a third capacitor and a fourth capacitor that shunt the third node and the fourth node to ground, respectively.
7. The differential crystal oscillator of claim 6 further comprises: an AC coupling network for coupling the oscillation signal to an auxiliary oscillation signal; and a second source follower for receiving the auxiliary oscillation signal and outputting the regenerated signal together with the first source follower.
8. The differential crystal oscillator as claimed in claim 7, wherein, The auxiliary oscillation signal includes a first terminal and a second terminal on a fifth node and a sixth node, respectively. The AC coupling network includes a first AC coupling capacitor and a second AC coupling capacitor, used to couple the first node and the second node to the fifth node and the sixth node, respectively.
9. The differential crystal oscillator as claimed in claim 8, wherein, The second source follower includes: a fifth metal-oxide-semiconductor transistor of the second type, used to receive the first terminal of the auxiliary oscillation signal, and together with the third metal-oxide-semiconductor transistor of the first type, to output the first terminal of the regenerated signal; and a sixth metal-oxide-semiconductor transistor of the second type, used to receive the second terminal of the auxiliary oscillation signal, and together with the fourth metal-oxide-semiconductor transistor of the first type, to output the second terminal of the regenerated signal.
10. The differential crystal oscillator as claimed in claim 9, wherein, The first type of metal-oxide-semiconductor transistor, the second type of metal-oxide-semiconductor transistor, the third type of metal-oxide-semiconductor transistor, and the fourth type of metal-oxide-semiconductor transistor are n-channel metal-oxide-semiconductor transistors, and the second type of fifth type of metal-oxide-semiconductor transistor and the sixth type of metal-oxide-semiconductor transistor are p-channel metal-oxide-semiconductor transistors; or, the first type of metal-oxide-semiconductor transistor, the second type of metal-oxide-semiconductor transistor, the third type of metal-oxide-semiconductor transistor, and the fourth type of metal-oxide-semiconductor transistor are p-channel metal-oxide-semiconductor transistors, and the second type of fifth type of metal-oxide-semiconductor transistor and the sixth type of metal-oxide-semiconductor transistor are n-channel metal-oxide-semiconductor transistors.