# Patent

## Title
Current driven crystal oscillator

## Abstract

An oscillator circuit with an oscillator stage and a first current source arranged to drive the oscillator stage is presented. The oscillator stage has an oscillator stage input terminal, an oscillator stage output terminal, an oscillator arranged to provide an oscillating signal between the oscillator stage input terminal and the oscillator stage output terminal. The oscillator circuit has an operational amplifier with an inverting input, a non-inverting input and an operational amplifier output. The oscillator stage input terminal and the oscillator stage output terminal are coupled to the inverting input and non-inverting input. The operational amplifier output is coupled to the oscillator stage input terminal such that the oscillator stage input terminal and the oscillator stage output terminal are controlled to have a same DC voltage level.

## Background
TECHNICAL FIELD
The invention relates to oscillator circuits, and in particular, to crystal oscillator circuits.
BACKGROUND
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create a signal with a very precise frequency.
This frequency is commonly used to keep track of time, to provide a stable clock signal required by a digital system, and/or to stabilize frequencies for radio transmitters and receivers.
The most common type of piezoelectric material used in crystal oscillators is quartz crystal, but other materials like polycrystalline ceramics are also used.
Typically, quartz crystals are cut and mounted to vibrate best at a desired resonance frequency or a multiple of the desired resonant frequency. When the crystal is vibrating, it can be modeled as an RLC circuit producing a rapidly changing reactance with frequency, wherein the RLC circuit provides positive feedback and gain at the resonant frequency therefore producing sustained oscillations.
FIG. 1 illustrates a current-controlled CMOS-inverter oscillator circuit as known from E. Vittoz, “Low-Power Crystal and MEMS Oscillators: The Experience of Watch Developments”, Integrated Circuits and Systems, FIG. 5.25, page 129, DOI 10.1007/978-90-481-9394-3. The circuit of FIG. 1 comprises a transistor T1 having a gate G1, a source S1, and a drain D1, a transistor T2 having a gate G2, a source S2, and a drain D2, a transistor T3 having a gate G3, a source S3, and a drain D3, a capacitor C1 having a first end 10 and a second end 11, a capacitor C2 having a first end 12 and a second end 13, a capacitor C3 having a first end 14 and a second end 15, a resistor R1 having a first end 16 and a second end 17 and a crystal oscillator 18 having a first end 19 and a second end 20. The source S1 is connected to the source S2 and to the second end 15 of the capacitor C3. The drain D2 is connected to the second end 17 of the resistor R1, to the second end 20 of the crystal oscillator 18, to the drain D3 and to the second end 13 of the capacitor C2. The gate G2 is connected to the first end 16 of the resistor R1, to the first end 19 of the crystal oscillator 18, to the gate G3 and to the second end 11 of the capacitor C1. The first end 10 of the capacitor C1 is connected to the source S3, to the first end 12 of the capacitor C2 and to the first end 14 of the capacitor C3.
In the known circuit according to FIG. 1, the resistor R1 is a feedback resistor from the drains D2 and D3, respectively, of the transistors T2 and T3, respectively, to the gates G2 and G3, of the transistors T2 and T3, respectively, to ensure that the DC voltage levels of these drains D2 and D3 and these gates G2 and G3 of the transistors T2 and T3, are the same. Therefore, the DC voltage level at both terminals 19, 20 of the oscillator crystal 18 is the same. Feedback resistor R1 should have a very high resistance value in case of low power requirements, as it is consuming electrical power all the time.
SUMMARY
In a first aspect, the invention provides an oscillator circuit comprising an oscillator stage (OSC) and a first current source (Iddx) arranged to drive said oscillator stage (OSC), the oscillator stage (OSC) comprising an oscillator stage input terminal, an oscillator stage output terminal, an oscillator (X1) arranged to provide an oscillating signal between said oscillator stage input terminal and said oscillator stage output terminal, said oscillator circuit comprising an operational amplifier with an inverting input, a non-inverting input and an operational amplifier output, said oscillator stage input terminal and said oscillator stage output terminal being coupled to said inverting input and non-inverting input, and said operational amplifier output being coupled to the oscillator stage input terminal such that said oscillator stage input terminal and said oscillator stage output terminal are controlled to have a same DC voltage level.
By doing so, all elements in the oscillator circuit are current driven. This provides several features to the oscillator circuit, including much less spread over processing corners, the oscillator circuit is self-adjusting to a suitable supply voltage which will be as low as possible but not lower than a minimum value necessary for the oscillator circuit to operate properly and the individual components can be easily designed, especially scaling is simple.
The dependent claims are focusing on advantageous embodiments.
In an embodiment, as claimed in claims 15 and 16, the oscillator circuit may work in a transceiver mode wherein stabilized frequencies for radio transmitters and/or receivers can be generated. In this mode, rising/falling edges of the generated oscillating signal as eventually produced should be as accurate as possible such that they define moments in time at which certain actions may start/end are as accurate as possible. This is achieved by injecting relatively more current in the oscillator circuit by current source Iddx such that lower phase noise is obtained.
In another embodiment, as claimed in claims 17 and 18, the oscillator circuit illustrated may work in a timer mode in order to provide a stable clock signal. In this embodiment, requirements as to phase noise are less strict because the timer will count an average number of oscillation cycles by counting the number of rising/falling edges only. So, here, the current as injected into the oscillator circuit by current source Iddx can be much less than in the transceiver mode.
The person skilled in the art will understand that the features described above may be combined in any way deemed useful.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following, aspects of the invention will be elucidated by means of examples, with reference to the drawings. The drawings are diagrammatic and may not be drawn to scale.
The features and effects of the present invention will be explained in more detail below with reference to drawings in which preferred and illustrative embodiments of the invention are shown. The person skilled in the art will realize that other alternatives and equivalent embodiments of the invention can be conceived and reduced to practice without departing from the scope of the present invention.
FIG. 1 illustrates a current-controlled CMOS-inverter oscillator circuit.
FIG. 2 illustrates a current driven crystal oscillator circuit according to one embodiment of the invention.
FIG. 3 shows a higher level block diagram of the circuit according to FIG. 2.
FIG. 4 shows an alternative bias circuit BC′ to the one shown in FIG. 2 as well as an example of the operational amplifier OA1 of the oscillator circuit.
FIG. 5, FIG. 6, FIG. 7, FIG. 8 and FIG. 9 illustrate simulation results of the current driven crystal oscillator circuit of FIG. 2 working in a transceiver mode.
FIG. 10, FIG. 11, FIG. 12 and FIG. 13, FIG. 14, FIG. 15, FIG. 16, FIG. 17, FIG. 18 and FIG. 19 illustrate simulation results of the current driven crystal oscillator circuit of FIG. 2 working in a timer mode.



DESCRIPTION
The examples and embodiments described herein serve to illustrate rather than to limit the invention. The person skilled in the art will be able to design alternative embodiments without departing from the scope of the claims. Reference signs placed in parentheses in the claims shall not be interpreted to limit the scope of the claims. Items described as separate entities in the claims or the description may be implemented as a single or multiple hardware items combining the features of the items described.
FIG. 2 illustrates a current driven crystal oscillator circuit according to one embodiment of the invention.
The current driven crystal oscillator circuit of FIG. 2, seen from left to right, comprises a series circuit of the following sub-circuits: a bias circuit BC, a regulator OA1 and Low Pass Filter circuit R/LPF, an oscillator stage OS, a current-mode comparator CMC, and an output circuit OC. A level shifter L receives an output voltage Vout of output circuit OC.
The current driven crystal oscillator circuit of FIG. 2 comprises a plurality of transistors Ti (i=4, 5, . . . , 12). Each one of these transistors Ti has a respective gate Gi, a source Si, and a drain Di. For the sake of simplicity, a transistor Ti will be called an ith transistor in the specification hereinafter. It is observed that the reference number concerned may be different in the claims, as they may appear in another order in the claims.
The current driven crystal oscillator circuit of FIG. 2 includes a resonating crystal X1 and a current source I1.
FIG. 2, also shows the currents received by each of the sub-circuits: the bias circuit BC receives a current Ibias, the regulator and Low Pass Filter circuit R/LPF receives a current Ireg, the oscillator stage OS receives a current Iosc, the current-mode comparator CMC receives a current Icp1, and the output circuit OC receives a current Icp2.
The bias circuit BC of the current driven crystal oscillator circuit includes a fourth P-type transistor T4 and a fifth N-type transistor T5 having their respective drain terminals D4 and D5 connected to each other, and their gate terminals G4 and G5 respectively connected to their drain terminals D4 and D5. The source terminal S5 of the N-type transistor T5 is connected to ground and the source terminal S4 of the P-type transistor T4 is connected to the current source I1.
The R/LPF circuit of the current driven crystal oscillator circuit of FIG. 2 includes an operational amplifier OA1, a capacitor C4 having a first end c41 and a second end c42, a capacitor C5 having a first end c51 and a second end c52, a resistor R2 having a first end r21 and a second end r22, a resistor R3 having a first end r31 and a second end r32, a resistor R4 having a first end r41 and a second end r42, and a sixth N-type transistor T6 having a source S6, a drain D6 and a gate G6. The second end c42 of the capacitor C4 is connected to the inverting input of the operational amplifier OA1, the second end c52 of the capacitor C5 is connected to the non-inverting input of the operational amplifier OA1 and the first end c41 of the capacitor C4 is connected to the first end c51 of the capacitor C5 and to the ground. The gate G6 of the transistor T6 is connected to the gate G5 of the transistor T5, the drain D6 of the transistor T6 is connected to the negative power supply of the operational amplifier OA1, and the source S6 of the transistor T6 is connected to the ground. The output of the operational amplifier OA1 is connected to the first end r31 of the resistor R3, the second end r32 of the resistor R3 is connected to the second end r22 of the resistor R2 and to the second end r42 of the resistor R4, the first end r21 of the resistor R2 is connected to the inverting input of the operational amplifier OA1 and the first end r41 of the resistor R4 is connected to the non-inverting input of the operational amplifier OA1.
Furthermore, the oscillator stage OS of the current driven crystal oscillator circuit of FIG. 2 includes an eighth N-type transistor T8 having a source S8, a drain D8 and a gate G8, a seventh P-type transistor T7 having a source S7, a drain D7 and a gate G7, a capacitor C7 having a first end c71 and a second end c72, a resistor R5 having a first end r51 and a second end r52, a resistor R6 having a first end r61 and a second end r62. The first end r51 of the resistor R5 is connected to the first end c71 of the capacitor C7 and to the current source I1. The second end c72 of the capacitor C7 is connected to the ground and the second end r52 of the resistor R5 is connected to the source S7 of the transistor T7. The gate G7 of the transistor T7 is connected to the second end r22 of the resistor R2 and the drain D7 of the transistor T7 is connected to the drain D8 of the transistor T8. The gate G8 of the transistor T8 is connected to the second end r22 of the resistor R2 and the source S8 of the transistor T8 is connected to the first end r61 of the resistor R6. Furthermore, the oscillator stage OS of the current driven crystal oscillator circuit of FIG. 2 includes a resistor R7 having a first end r71 and a second end r72 and a switch S2 wherein the first end r71 of the resistor R7 is connected to the second end c62 of the capacitor C6, the second end r72 of the resistor R7 is connected to one end of the switch S2 and the other end of the switch S2 is connected to the first end c81 of the capacitor C8.
The current-mode comparator CMC of the current driven crystal oscillator circuit of FIG. 2 includes a tenth N-type transistor T10 having a source S10, a drain D10 and a gate G10 and a ninth P-type transistor T9 having a source S9, a drain D9 and a gate G9. The gate G9 of the ninth transistor T9 is connected to the gate G10 of the tenth transistor T10 and to second end r22 of the resistor R2. The drain D9 of the ninth transistor T9 is connected to the drain D10 of the tenth transistor T10. The source S10 of the tenth transistor T10 is connected to the ground and the source S9 of the ninth transistor T9 is connected to the current source I1.
The output circuit OC of the current driven crystal oscillator circuit of FIG. 2 includes a twelfth N-type transistor T12 having a source S12, a drain D12 and a gate G12 and an eleventh P-type transistor T11 having a source S11, a drain D11 and a gate G11. The gate G11 of the eleventh transistor T11 is connected to the gate G12 of the twelfth transistor T12 which are together connected to the drains D9, D10 of the ninth and tenth transistors T9, T10, respectively. The drain D11 of the eleventh transistor T11 is connected to the drain D12 of the twelfth transistor T12, The source S12 of the twelfth transistor T12 is connected to the ground and the source S11 of eleventh the transistor T11 is connected to the current source I1.
Furthermore, the current driven crystal oscillator circuit of FIG. 2 includes a capacitor C6 having a first end c61 and a second end c62, a capacitor C8 having a first end c81 and a second end c82, a resistor R7 having a first end r71 and a second end r72, and crystal oscillator X1 having a first end x11 and a second end x12. The first end c61 of the capacitor C6 is connected to the ground, the second end c82 of the capacitor C8 is connected to the ground, the second end c62 of the capacitor C6 is connected to the first end r71 of the resistor R7, to the first end x11 of the crystal oscillator X1, and to the second end r22 of the resistor R2. The second end r72 of the resistor R7 is connected, via a switch SW2 to first end c81 of the capacitor C8, to the second end x12 of the crystal oscillator X1, and to the second end r42 of the resistor R4.
Finally, the circuit of FIG. 2 comprises a further current source IS having an output connected to an output of the current source I1 via a switch SW1. So, all elements in the oscillator circuit shown in FIG. 2 are current driven. This provides several features to the oscillator circuit, including:

    
    
        Much less spread over processing corners
        The oscillator circuit is now self-adjusting to a suitable supply voltage which will be as low as possible but not lower than a minimum value necessary for the oscillator circuit to operate properly
        The individual components can be easily designed, especially scaling is simple.
    
    


The basic operation of the circuit of FIG. 2 will now be described.
The person skilled in the art will recognize that the oscillator stage OS has a similar construction as the oscillator shown in FIG. 1. The main differences between the oscillator stage OS and the oscillator of FIG. 1 are that the feedback resistor R1 is replaced by the regulator OA1, that source S8 of eighth transistor T8 is connected to ground via resistor R6, that the source S7 of seventh transistor T7 is connected to resistor R5, and that resistor R7 (which is comparable to resistor R1 in FIG. 1) is arranged in series with switch SW2. Moreover all peripheral circuits of the oscillator are supplied by current source Iddx.
FIG. 3 shows a higher level block diagram of the circuit according to FIG. 3 only showing the current source Iddx, the bias circuit BC, the regulator/Low Pass Filter circuit R/LPF, the oscillator stage OSC, the current mode comparator CMC, and output stage OS, and their mutual connections.
The bias circuit BC provides an output voltage VBC,out.
The regulator/Low Pass Filter circuit R/LPF receives the output voltage VBC, out of the bias circuit BC as its input voltage VR/LPF,in. Moreover, the regulator/Low Pass Filter circuit R/LPF provides an output voltage VR/LPF,out to the input of the oscillator stage OSC. Furthermore, the operational amplifier OA1 in the regulator/Low Pass Filter circuit R/LPF receives a feedback signal which is derived from the voltage difference between the input and output of the oscillator stage OSC, which, in the shown embodiment is equal to the voltage across the oscillator X1.
The regulator/Low Pass Filter circuit R/LPF is arranged such that DC input voltage of the oscillator stage OSC is equal to the DC output voltage of the oscillator stage OSC. In the shown embodiment this is taken care of by the operational amplifier OA1 in regulator R/LPF which charges/discharges capacitor C6 so that the average input voltage of the oscillator stage OSC is equal to the average output voltage of the oscillator stage OSC. It is observed that a feedback of the voltage difference between the input and output of the oscillator stage OSC to any type of operational amplifier of which the output is coupled to the input of the oscillator stage OSC can be used for this purpose.
In contrast to most other oscillators, in the preferred embodiment, the comparator is not a ‘voltage-mode’ comparator. Such a ‘voltage-mode’ comparator would have its input connected to the output of the oscillator stage OSC. Here, the input of the current-mode comparator CMC is connected to the input of the oscillator stage OSC. The current-mode comparator CMC flags if the absolute current through transistor T7 is larger or smaller than the absolute current through transistor T6. A voltage-mode comparator could be used as well, though would result in more phase noise.
The oscillator and all its peripheral circuits run at self-biasing voltage Vddx. Vddx is lower than the supply voltage Vdd of other circuitry to save power in the oscillator. Inside the oscillator, apart from the level shifter, the logic level is the internal supply voltage Vddx. The output stage of the oscillator is a level-shifter L which converts the logic level of the signal as received by the output circuit OC to the supply voltage Vdd of the circuitry to which the resulting oscillating signal is to be provided.
The bias circuit BC of FIG. 3 receives a current from the current source Iddx at the source S4 of the transistor T4. See also FIG. 2. This current generates a voltage Vgs4 between the gate G4/drain D4 and the source S4 of the transistor T4 such that T4 is in saturation mode. The transistor T4 working in saturation provides a current Ibias to the transistor T5. As a consequence, a voltage VP is present between the source S4 and the gate G4/drain D4 of the transistor T4, and a voltage VN is present between the drain D5/gate G5 and the source S5 of the transistor T5. Defining the output voltage of current source Iddx as Vddx the following equation holds:

Vddx−VP=VN 

Transistor T5 is connected in a current mirror arrangement with transistor T6. I.e., the voltage across the gate-source of transistor T6 is equal to the voltage across the gate-source of transistor T5. Since all transistors have been produced in the same manufacturing step on the same die, the current that flows through transistor T6 has a fixed ratio to the one flowing through transistor T5 as determined by their relative surface areas. The drain current through T6 is the bias current of the operational amplifier OA1 in regulator R/LPF (see e.g., the embodiment of FIG. 4). The reference voltage at the non-inverting input of the operational amplifier OA1 is the low-pass filtered output voltage of oscillator stage OSC (high frequency components in the output voltage of oscillator stage OSC are short-circuited to ground via capacitor C5). The feedback voltage of the regulator R/LPF at the inverting input of the operational amplifier OA1 is the low-pass filtered input voltage of oscillator stage OSC (high frequency components in the output voltage of oscillator stage OSC are short-circuited to ground via capacitor C4). The output of the regulator R/LPF is connected to the input of the oscillator stage OSC and charges/discharges capacitor C6 until the voltage at both inputs of the operational amplifier OA1 in the regulator R/LPF are the same. As the Wi/Li ratio's of T7 and T8 are similar to T4 and T5 (wherein Wi represents the channel width of transistor Ti and Li represents the channel length of transistor Ti) the average of input and output voltage of the oscillator stage OSC is equal to VN. The same applies for the Wi/Li ratio's of the transistors of stages CMC and OC and therefore their turn-over point is close to a voltage level equal to VN.
Therefore, due to the configuration of the circuit of FIG. 2, the same voltage difference VP is present between the output voltage Vddx of the current source Iddx and the gate G7 of the transistor T7, between the output voltage Vddx of the current source Iddx and the gate G9 of the transistor T9, and between the output voltage Vddx of the current source Iddx and the gate G11 of the transistor T11. Also for the same reason, the same voltage difference VN is present between the gate G8 of the transistor T8 and ground, between the gate G10 of the transistor T10 and ground, and between the gate G12 of the transistor T12 and ground.
As can be seen in the circuit of FIG. 2, the voltage between the output of the current source Iddx and ground is VN+VP.
In this way, all the stages of the circuit of FIG. 3, namely the bias circuit BC, the regulator/LPF circuit R/LPF, the oscillator stage OSC, the current-mode comparator CMC and the output stage OS, are well-balanced independent of the processing corner of the total circuit, the actual temperature in use and the injected current in use.
The DC voltage at the output of the operational amplifier OA1 follows the DC voltage present at the inverting and non-inverting inputs of the operational amplifier OA1 which are controlled to be the same.
The resistor R3 at the output of the operational amplifier OA1 prevents that a rail-to-rail swing of the voltage Vx1 is causing linearity errors in the regulator and LPF circuit R/LPF.
The operational amplifier OA1 is connected in a closed loop wherein the output of the operational amplifier OA1, which is connected to the input of the oscillator stage OSC, is fed back to the inverting input of the operational amplifier OA1 via resistor R2. The non-inverting input of the operational amplifier OA1 is connected through the resistor R4 to the output of the oscillator stage OSC. So, stated differently the input and the output of the oscillator stage OSC are feedback to the inverting and non-inverting inputs of the operational amplifier OA1. In use, these inverting and non-inverting inputs will have the same DC voltage level. In this way, the operational amplifier OA1 of the regulator and LPF circuit R/LPF controls that the DC voltage level at the input of the oscillator stage OSC is the same as the DC voltage level at the output of the oscillator stage OSC. So, the operational amplifier OA1 substitutes resistor R1 in the prior art setup of FIG. 1, the advantage being that such an operational amplifier OA1 dissipates less electrical energy in use.
However, providing only this feedback circuit with the operational amplifier OA1 would cause a relatively slow start-up of the total circuit.
Therefore, the series connection of resistor R7 and switch SW2 has been provided which has the same function as resistor R1 in the circuit according to the prior art (FIG. 1), however, only at the time of starting the circuit. I.e., at the time of starting, the switches SW1 and SW2 are both closed such as to allow current to flow. Oscillator crystal X1 oscillates and provides an oscillating signal at its output terminals x11 and x12. An oscillating voltage is built up across the resistor R7. This oscillating signal across oscillator crystal X1 is output to the inverting and non-inverting input terminals of the operational amplifier OA1 in the R/LPF circuit such that its output also generates an oscillating signal which is, then, provided to the rest of the circuit, as explained above. In the circuit of FIG. 3, after initialisation, i.e., when the operational amplifier OA1 provides a stable oscillating signal at its output, switches SW1 and SW2 are opened such that no current will flow anymore through them. Then, no current will flow anymore through resistor R7 either, thus saving energy. Even though, after initialisation, the resistor R7 is disconnected from the circuit, the operational amplifier OA1 in R/LPF circuit causes the DC voltage difference across the oscillator crystal X1 to be 0 (zero). I.e., both terminals x11 and x12 are controlled to be at the same DC voltage VN. A method to control the opening of switches SW1 and SW2 is to count a predetermined number of generated pulses by the oscillator after start-up. To that end, the output signal of the oscillator can be fed to a counting circuit which counts the number of pulses of the generated signal and which is arranged to control the opening/closing of switches SW1b and SW2. For example SW1 and SW2 are opened by such a counting circuit if 1024 pulses are counted.
On average, in a preferred embodiment, DC voltages Vx1, Vx2, VN, and VP are substantially the same in the circuit of FIG. 3. Here, “substantially” means that these DC voltages are intended to have the same values but they may differ slightly in practice, due to tolerances in design of the different used transistors.
In the oscillator stage OSC of the circuit of FIG. 2 no amplitude control is required as the current source Iddx defines the voltage Vddx. The oscillator stage OSC is a push/pull stage with a double gm if compared with a single transistor “grounded source” configuration at the same current. Apart from a small voltage drop across the respective resistors R5 and R6, the voltage at the interconnected drains D7 and D8 swings between ground and voltage Vddx. The resistors R5 and R6 soften the clamping of Vx2.
Apart from the resistors R5 and R6, the current mode comparator CMC is a copy of the oscillator stage OSC, but the oscillator stage OSC drives a heavy load, i.e. the load capacitors C6 and C8, that requires relative high current, while the current-mode comparator CMC itself drives a tiny load, i.e. output circuit OC, that allows the output of the output circuit OC to jump from “rail to rail”, i.e. between voltage Vddx and ground. The voltage drop at peak currents over the resistors R5 and R6 boosts the current-gain of the current-mode comparator CMC.
The output stage OS is a copy of the current-mode comparator CMC. However, the respective sizes of the transistors T11 and T12 differ from the respective sizes of the transistors T9 and T10, such that the current consumption is very low, the output stage OS form a relatively small load to the current-mode comparator CMC, and can drive a relatively heavy load itself.
To summarize, the basic functionalities of the respective functional blocks of FIGS. 2 and 3 are as follows:

    
    
        Current source Iddx: arranged to provide a constant current to the entire current driven oscillator circuit; eventually Iddx can have a temperature coefficient to fine tune the Temperature Coefficient TC performance of the oscillator.
        Bias circuit BC: arranged to provide a well-defined first DC output voltage VBC,out;
        Regulator and LPF circuit R/LPF: arranged to provide a same second DC output voltage VR/LPC at two different output terminals while allowing an oscillating voltage signal to be present between these two output terminals;
        Oscillator stage OSC: arranged to provide a first oscillating signal VOSC,out;
        Current-mode comparator CMC: arranged to receive the oscillating signal VOSC and to provide an amplified oscillating signal. This amplified oscillating signal clamps between ground and Vddx.
        Output stage OS: arranged to form a small load to the output of the current mode comparator CMC and to allow driving the level shifter L.
        Level shifter L: arranged to convert the logic level of the output signal VCMC,out of the current-mode comparator CMC to a required logic level of the circuits receiving the oscillator signal of the oscillator as shown in FIGS. 2 and 3.
    
    


FIG. 4 shows an alternative bias circuit BC′ to the one shown in FIG. 2 as well as an example of the operational amplifier OA1 of the oscillator circuit in which same reference number refer to the same components as in FIGS. 2 and 3.
The alternative bias circuit BC′ of FIG. 4 includes the regulator OA1 and comprises a plurality of transistors Tj (j=13, 14, . . . 17). Each one of these transistors Ti has a respective gate Gj, a source Sj, and a drain Dj.
The alternative bias circuit BC′ of FIG. 4 includes a thirteenth N-type transistor T13 having its source terminal S13 connected to the drain terminal D5 of transistor T5, its drain terminal D13 connected to the drain terminal D4 of transistor T4, to its gate G13 and to the gate G5 of transistor T5, a fourteenth N-type transistor T14 having a source S14 connected to the drain D6 of the transistor T6 and a gate G14 connected to the first end r21 of the resistor R2 (not shown in FIG. 4), a fifteenth N-type transistor T15 having a source S15 connected to the drain D6 of the transistor T6 and a gate G15 connected to the first end r41 of the resistor R4 (not shown in FIG. 4), a sixteenth P-type transistor T16 having a drain D16 connected to the drain D14 of the transistor T14, a gate G16 connected to the drain D14 of the transistor T14, and a source S16 connected to the current source Iddx, and a seventeenth P-type transistor T17 having a drain D17 connected to the drain D15 of the transistor T15 and to the first end r31 of the resistor R3 (not shown in FIG. 4), a gate G16 connected to the drain D14 of the transistor T14, and a source S17 connected to the current source Iddx. By doing so, gate G15 is the inverting input and gate G14 is the non-inverting input, whereas the node of drain D15/drain D17 is the output of the regulator.
In the set-up of FIG. 4, the additional cascode transistor T13 is characterized by having W13/L13>>β, where β=W/L of transistor T5, resulting in a lower gate-source voltage Vgs13, and the drain-source voltage Vds5 across drain D5 and source S5 meeting the condition: Vds5>Vdsat5, where Vdsat5 is the saturation voltage of transistor T5. So, transistor T5 is in saturation.
In an example, transistor T6 meets the condition 2*W/L and the condition Vds6>Vdsat6. So, transistor T6 is in saturation too.
For both transistors T14 and T15 are characterized by having W14/L14>>β and W15/L15>>β.
Because of these conditions being met, the regulator stage R/LPF (here comprising transistors T6, T14, T15, T16, T17 is having all transistors in saturation if:

Vin+≈Vin−≈VN≈Vddx−VP,

The current driven crystal oscillator circuit illustrated in FIG. 2 may work in a transceiver mode wherein stabilized frequencies for radio transmitters and/or receivers can be generated. In this mode, rising/falling edges of the generated oscillating signal as eventually produced should be as accurate as possible such that they define moments in time at which certain actions may start/end are as accurate as possible. This is achieved by injecting relatively more current in the oscillator circuit by current source Iddx such that lower phase noise is obtained.
Circuit dimensions are linear proportional to a large extend to the crystal frequency. The following main parameters may apply to the circuit of FIG. 2:
fXTAL frequency of the crystal X1 (e.g. 16 MHz).
Iddx supply current injected into the oscillator.
IS start-up current.
fLPF turn-over frequency of the low-pass filters C4, R2 and C5, R4.
CDDX VDDX smoothing capacitor (e.g. 50 pF; C7 in FIG. 2).
RSPN equivalence series resistance of resistors R5 and R6 in the oscillator stage OS.
RSTART resistance of the feedback resistor in the start-up mode (R7 in FIG. 2).
RS equivalence series resistance of resistor (R3 in FIG. 2) and the regulator OA1 in the OA/LPF.
βBIAS channel width divided by channel length of the channel of the NMOS transistor of the bias circuit BS (T5 in FIG. 2).
βOSC channel width divided by channel length of the channel of the NMOS transistor in the oscillator stage OS (T8).
βCP1 channel width divided by channel length of the channel of the NMOS transistor of the current-mode comparator CMC (T10 in FIG. 2).
βCP2 channel width divided by channel length of the channel of the NMOS transistor of the output circuit OC (T12 in FIG. 2).

    
    
        αPN channel width divided by channel length of the channel of the PMOS transistor divided by channel width divided by channel length of the channel of the NMOS transistor in the bias circuit BC, oscillator stage OScurrent-mode comparator CMC and output circuit OC.
    
    


CL Load capacitors (specified by crystal manufacturer; C6, C8 in FIG. 2).
In one embodiment of the current driven crystal oscillator circuit of FIG. 2 working in a transceiver mode, the plurality of circuit dimensions of FIG. 2 may have the following values:
Iddx≈fXTAL*2 pA/Hz (≈32 μA at fXTAL=16 MHz; ≈64 nA at fXTAL=32 kHz)
fLPF≈fXFAL/40 (≈400 kHz at fXTAL=16 MHz; e.g. C=2 pF and R=200 kΩ)
RSPN≈60 mV/Iddx (≈1800Ω at fXTAL=16 MHz)
βBIAS≈250 μA/Iddx (≈8≈0.2*2.4 μm/60 nm at fXTAL=16 MHz;

    
    
        so if L=60 nm then W=0.2*2.4μ=0.48 μm)
    
    


βOSC≈62.5 mA/Iddx (≈2000≈50*2.4 μm/60 nm at fXTAL=16 MHz)
βCP1≈12.5 mA/Iddx (≈400≈10*2.4 μm/60 nm at fXTAL=16 MHz)
βCP2≈1.25 mA/Iddx (≈40≈1*2.4 μm/60 nm at fXTAL=16 MHz)
αPN 2.5 and the channel width divided by channel length of the channel of T6 is 2*βBIAS.
These parameters may have a value in a range from 50% to 150% of the above nominal values. Preferably, these parameters may have a value in a range from 75% to 125% of the above nominal values and even more preferably, these parameters may have a value in a range from 90% to 110% of the above nominal values.
In this way, the current provided by the current source Iddx of the current driven crystal oscillator circuit of FIG. 2 working in a transceiver mode will be distributed between the different blocks in the following way: current Ibias of the bias circuit will be ≈2.5% of current Iddx, current Iosc of the oscillator stage OS will be ≈78% of current Iddx, current Icp1 of the current-mode comparator CMC will be ≈14% of current Iddx, and current Icp2 of the output circuit OC will be ≈0.5% of current Iddx. The remaining 5% of Iddx current is consumed by the regulator (OA1).
FIG. 5, FIG. 6, FIG. 7, FIG. 8 and FIG. 9 illustrate simulation results of the current driven crystal oscillator circuit of FIG. 2 working in a transceiver mode. In FIG. 5, FIG. 6, FIG. 7 and FIG. 8 the horizontal axes represent in microamperes the current Iddx of the current driven crystal oscillator circuit of FIG. 2 working in a transceiver mode. The vertical axes represent: in FIG. 5, Vx1, Vx2, and avg_vddx represented in volts, in FIG. 6, the ratio of swing voltage represented in volts to Vddx represented in volts, in FIG. 7, the efficiency factor defined by the ratio of swing voltage represented in volts to current Iddx represented in amperes, and in FIG. 8, the duty cycle of the output signal as provided by the level shifter LIn FIG. 9, the horizontal axis also represents in microamperes the current Iddx, which represents the total current of the whole current driven crystal oscillator circuit of FIG. 2; working in a transceiver mode and along the vertical axis five curves are illustrated which represent respectively the percentage % of current Iddx that goes to ICP2, the percentage % of current Iddx that goes to current Ibias, the percentage % of current Iddx that goes to current Ireg, the percentage % of current Iddx that goes to current ICP1 and the percentage % of current Iddx that goes to current IOSC.
As shown in the simulations illustrated in FIG. 5, FIG. 6, FIG. 7, FIG. 8 and FIG. 9 the current Iddx is swept from 10 microampere to 300 microamperes, the capacitors C6 and C8 have a capacitance of 18 picofarads, and the circuit works at a temperature of 25 degrees Celsius.
To accelerate the start-up of the crystal oscillator a relative high current (IS in FIG. 2) is injected in the transceiver mode configuration while resistors R5 and R6 are shorted by a switches and switch SW2 (in FIG. 2) is closed. More than 90% of the injected current (sum of Iddx and IS) flows in the oscillator stage (T7 and T8) maximizing its transconductance. Start-up resistor RSTART (R7 in FIG. 2) takes care that the input and output voltage of the oscillator stage are approximately equal at start-up independent of the regulator (OA1 in FIG. 2). A 12-bit clock counter (212=4096 clock pulses) which is reset by a POR (Power-On Reset)-signal can be used to switch from start-up mode to transceiver mode after starting the oscillator. Resistor RSTART (R7 in FIG. 2) can be 100 kΩ and IS can be 1 mA for a 16 MHz crystal. For a lower crystal frequency a higher value for RSTART can be used in combination with a lower start-up current (IS).
The current driven crystal oscillator circuit illustrated in FIG. 2 can also work in a timer mode in order to provide a stable clock signal. In this embodiment, requirements as to phase noise are less strict because the timer will count an average number of oscillation cycles by counting the number of rising/falling edges only. So, here, the current as injected into the oscillator circuit by current source Iddx can be much less than in the transceiver mode.
In one embodiment of the current driven crystal oscillator circuit of FIG. 2 working in a timer mode, the plurality of circuit dimensions of FIG. 2 may have the following values:

    
    
        Iddx≈fXTAL*125 fA/Hz(≈2 μA at fXTAL=16 MHz; ≈4 nA at fXTAL=32 kHz)
        fLPF≈fXTAL,/40 (≈400 kHz at fXTAL=16 MHz)
        RSPN≈0 mV/Iddx (≈0Ω at fXTAL=16 MHz→closed switch over R5 and R6 in FIG. 2)
        βBIAS≈16 μA/Iddx (≈8≈0.2*2.4 μm/60 nm at fXTAL=16 MHz;
        
            so if L=60 nm then W=0.2*2.4μ=0.48 m)
        
        
        βOSC≈1.25 mA/Iddx (≈640≈16*2.4 μm/60 nm at fXTAL=16 MHz)
        βCP1≈16 μA/Iddx (≈8≈0.2*2.4 μm/60 nm at fXTAL=16 MHz)
        βCP2≈8 μA/Iddx (≈4≈0.1*2.4 μm/60 nm at fXTAL=16 MHz)
        αPN≈2.5 and the channel width divided by channel length of the channel of T6 is 2*βBIAS
    
    


These parameters may have a value in a range from 50% to 150% of the above nominal values. Preferably, these parameters may have a value in a range from 75% to 125% of the above nominal values and even more preferably, these parameters may have a value in a range from 90% to 110% of the above nominal values.
In this way, the current provided by the current source I1 of the current driven crystal oscillator circuit of FIG. 2 working in a timer mode will be distributed between the different blocks in the following way: current Ibias of the bias circuit will be ≈0.5% of current I1, current Iosc of the oscillator stage OS will be ≈97% of current I1, current Icp1 of the current-mode comparator CMC will be ≈1% of current I1, and current Icp2 of the output circuit OC will be ≈0.1% of current I1. The remaining 1% of I1 current is consumed by the regulator (OA1).
FIG. 10, FIG. 11, FIG. 12 and FIG. 13 and FIG. 14 illustrate simulation results of the current driven crystal oscillator circuit of FIG. 2 working in a timer mode. In FIG. 10, FIG. 11, FIG. 12 and FIG. 13 and FIG. 14, the same results as in FIG. 5, FIG. 6, FIG. 7, FIG. 8 and FIG. 9, but for the timer mode, are represented, respectively. In the simulations for obtaining the results represented in FIG. 10, FIG. 11, FIG. 12 and FIG. 13 and FIG. 14, the current Iddx varies between 0.7 microampere and 3 microamperes, the capacitors C6 and C8 have a capacitance of 8 picofarads, and the circuit temperature works at 25 degrees Celsius.
FIG. 15, FIG. 16, FIG. 17 and FIG. 18 also illustrate simulation results of the current driven crystal oscillator circuit of FIG. 2 working in a timer mode. In FIG. 15, FIG. 16, FIG. 17 and FIG. 18, the horizontal axes represent temperature in degrees Celsius. The vertical axes represent: in FIG. 15, Vx1 and Vx2 represented in volts, in FIG. 16, the ratio of swing voltage represented in volts to Vddx represented in volts, in FIG. 17, the efficiency factor defined by the ratio of swing voltage represented in volts to current Iddx represented in amperes, and in FIG. 18, the duty cycle of the output signal as provided by the level shifter L, wherein for each of the four figures, three curves are shown for each parameter representing simulation results for nominal process corner and for the extremes of the processing corners: the so-called ‘fast’ corner and the ‘slow’ corner. In the slow corner all process parameters are set to the processing limits which results to the slowest possible circuits.
FIG. 19 illustrates simulation results of the current driven crystal oscillator circuit of FIG. 2 working in a timer mode. In FIG. 19, the horizontal axis represents temperature in degrees Celsius and the vertical axis represents the percentage of current Iddx that goes to current IOSC. Also here three curves are shown.
During the simulations for obtaining the results represented in FIG. 10-19, the current Iddx was set at 1.5 microamperes, the capacitors C6 and C8 had a capacitance of 8 picofarads, and the circuit temperature sweep over processing corners.
To summarize, the design of the presented oscillator circuit can be optimized for, e.g., three embodiments:

    
    
        Timer mode: examples of figures are: iosc=2 μA; C6=C8≈8 pF; Vddx≈450 mV;
        
            Vx_pp [=peak-to-peak voltage across the crystal X1]≈0.7*Vddx; phase_noise (10 kHz)≈−116 dBc/Hz;
            (iosc may be as low as 1 μA in case C6=C8≈3 pF (parasitics only);
        
        
        Transceiver mode: examples of figures are: iosc=32 μA;
        
            5 pF<C6=C8<18 pF; Vddx≈750 mV;
            Vx_pp≈1.1*Vddx;
            phase_noise (10 kHz)<−146 dBc/Hz
        
        
        Start-up mode: iosc=1 mA; C6=C8=18 pF; Vddx<1.4V;
        
            Vx_pp→accelerated start-up
        
        
    
    


The three embodiments described above wherein the oscillator circuit illustrated in FIG. 2 can work in a transceiver mode, a timer mode, or a accelerated start-up mode, may be combined in a single circuit wherein the circuit may comprise separate, distinct (transistor) elements in the oscillator circuit for each mode and suitably arranged switches arranged to connect or disconnect them to the rest of the oscillator circuit depending on the mode the circuit should work in. For instance, the circuit according to this embodiment may comprise two versions of transistor T10, one of them with an area value of 10x and another one with an area value of 0.15x, wherein 1x=2.4 μm/60 nm. If the circuit is working in a transceiver mode the transistor from the two versions of T10 with an area value of 10x will be the one connected to the circuit by means of these switches, while if the circuit is working in a timer mode, the transistor with an area of 0.15x will be the one connected to the circuit by means of these switches.
It will be clear to a person skilled in the art that the scope of the invention is not limited to the examples discussed in the foregoing, but that several amendments and modifications thereof are possible without deviating from the scope of the invention as defined in the attached claims. While the invention has been illustrated and described in detail in the figures and the description, such illustration and description are to be considered illustrative or exemplary only, and not restrictive. The present invention is not limited to the disclosed embodiments but comprises any combination of the disclosed embodiments that can come to an advantage. The invention is limited by the attached claims and their technical equivalents only.
Variations to the disclosed embodiments can be understood and effected by a person skilled in the art in practicing the claimed invention, from a study of the figures, the description and the attached claims. In the description and claims, the word “comprising” does not exclude other elements, and the indefinite article “a” or “an” does not exclude a plurality. In fact it is to be construed as meaning “at least one”. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the invention.

## Claims

1. An oscillator circuit comprising an oscillator stage and a first current source arranged to drive said oscillator stage, the oscillator stage comprising an oscillator stage input terminal, an oscillator stage output terminal, an oscillator arranged to provide an oscillating signal between said oscillator stage input terminal and said oscillator stage output terminal, said oscillator circuit comprising an operational amplifier with an inverting input, a non-inverting input and an operational amplifier output, said oscillator stage input terminal and said oscillator stage output terminal being coupled to said inverting input and non-inverting input, and said operational amplifier output being coupled to the oscillator stage input terminal such that said oscillator stage input terminal and said oscillator stage output terminal are controlled to have a same DC voltage level.

2. The oscillator circuit according to claim 1, further comprising a first capacitor connected to the oscillator stage input terminal and a second capacitor connected to the oscillator stage output terminal.

3. The oscillator circuit according to claim 2, further comprising a third capacitor connected to the inverting input of the operational amplifier and a fourth capacitor connected to the non-inverting input of the operational amplifer.

4. The oscillator circuit according to claim 1, wherein said operational amplifier output is connected to said the oscillator stage input terminal via a first resistor.

5. The oscillator circuit according to claim 4, wherein said oscillator stage comprises a series connection of a first transistor and a second transistor connected at a first junction, said first transistor having a first gate and said second transistor having a second gate, said first gate and said second gate being connected to one another at a second junction, said second junction being connected to a first oscillator terminal of the oscilator and said first junction being connected to a second oscillator terminal of the oscillator.

6. The oscillator circuit according to claim 5, wherein the series connection of said first transistor and said second transistor are arranged such as to receive an oscillator stage current from said first current source.

7. The oscillator circuit according to claim 6, wherein one side of said series connection of said first transistor and said second transistor is connected to said first current source via a second resistor, and an other side of said series connection of said first transistor and said second transistor is connected to ground via a third resistor.

8. The oscillator circuit according to claim 5, wherein said operational amplifier is arranged to receive electrical power from a third transistor which is arranged in a current mirror arrangement with a fourth transistor arranged in a bias circuit, wherein such fourth transistor is arranged to conduct a predetermined DC bias current.

9. The oscillator circuit according to claim 8, wherein said bias circuit comprises a fifth transistor which is arranged in a diode arrangement in series with said fourth transistor and is arranged to receive current from said first current source.

10. The oscillator circuit according to claim 9, wherein said oscillator stage input terminal is, either directly or indirectly, connected to a current mode comparator, said current mode comparator comprising a series connection of a sixth transistor and a seventh transistor connected at a third junction, said sixth transistor (T9) having a sixth transistor gate and said seventh transistor having a seventh transistor gate, said sixth transistor gate and said seventh transistor being connected to one another at a fourth junction, said fourth junction being arranged to receive an oscillating signal from said oscillator stage input terminal, and said series connection of said sixth transistor and said seventh transistor being arranged to receive current from said first current source.

11. The oscillator circuit according to claim 10, wherein said current mode comparator has a current mode comparator output connected to an output circuit, said output circuit comprising a series connection of an eighth transistor and a ninth transistor connected at a fifth junction, said eighth transistor having an eighth transistor gate and said ninth transistor having a ninth transisitor gate, said eighth transistor gate and said ninth transistor being connected to one another at a sixth junction, said sixth junction being arranged to receive a signal from said current mode comparator output, and said series connection of said eighth transistor and said ninth transistor being arranged to receive current from said first current source.

12. The oscillator circuit according to claim 11, wherein said output circuit has an output circuit output connected to an input of a level shifter which is arranged to shift a logic level of an output circuit output signal.

13. The oscillator circuit according to claim 4, wherein the oscillator circuit comprises a second resistor which is arranged in a series connection with a first switch, which series connection of said second resistor and said first switch is connected between said first oscillator terminal and said second oscillator terminal, the oscillator circuit being arranged to switch on said first switch at start-up of the oscillator circuit and to switch off said first switch after a start-up phase.

14. The oscillator circuit according to claim 13, wherein the oscillator circuit comprises a series connection of a second current source and a current source switch, said series connection of a second current source and a current source switch being arranged in parallel to said first current source.

15. The oscillator circuit according to claim 11, wherein a crystal X1 has an oscillating frequency fx1, the fourth transistor comprises a channel having a channel width divided by a channel length equal to βBIAS, the second transistor comprises a channel having a channel width divided by a channel length equal to βosc, the seventh transistor comprises a channel having a channel width divided by a channel length equal to βCP1, the ninth transistor comprises a channel having a channel width divided by a channel length equal to βCP2, the fifth transistor comprises a channel having a channel width divided by a channel length equal to βBIAS′, the first transistor comprises a channel having a channel width divided by a channel length equal to βosc′, the sixth transistor comprises a channel having a channel width divided by a channel length equal to βCP1′, the eighth transistor comprises a channel having a channel width divided by a channel length equal to βCP2′, the third transistor comprises a channel having a channel width divided by a channel length equal to β6, and wherein the first current source Iddx has a range from (1.5*fx1*2) picoampere/Hertz to (0.5*fx1*2) picoampere/Hertz, βBIAS has a range from (375/Iddx) microampere to (125/Iddx) microampere, βosc has a range from (93.75/Iddx) miliampere to (31.25/Iddx) miliampere, βCP1 has a range from (18.75/Iddx) miliampere to (6.25/Iddx) miliampere, βCP2 has a range from (1.875/Iddx) miliampere to (0.625/Iddx) miliampere, βBIAS′ has a range from ((2.5*375)/Iddx) microampere to ((2.5*125)/Iddx) microampere, βosc′ has a range from ((2.5*93.75)/Iddx) miliampere to ((2.5*31.25)/Iddx) miliampere, βCP1′ has a range from ((2.5*18.75)/Iddx) miliampere to ((2.5*6.25)/Iddx) miliampere, βCP2′ has a range from ((2.5*1.875)/Iddx) miliampere to ((2.5*0.625)/Iddx) miliampere and β6 has a range from (750/Iddx) microampere to (250/Iddx) microampere.

16. The oscillator circuit according to claim 15, wherein the first current source Iddx has a range from preferably (1.25*fx1*2) picoampere/Hertz to (0.75*fx1*2) picoampere/Hertz and more preferably is (fx1*2) picoampere/Hertz, βBIAS preferably has a range from (312.5/Iddx) microampere to (187.5/Iddx) microampere and more preferably is 250/Iddx microampere, βosc preferably has a range from (93.75/Iddx) miliampere to (31.25/Iddx) miliampere and more preferably is 62.5/Iddx miliampere, βCP1 preferably has a range from (15.625/Iddx) miliampere to (9.375/Iddx) miliampere and more preferably is 12.5/Iddx miliampere, βCP2 preferably has a range from (1.5625/Iddx) miliampere to (0,9375/Iddx) miliampere and more preferably is 1.25/Iddx miliampere, βBIAS′ preferably has a range from ((2.5*312.5)/Iddx) microampere to ((2.5*187.5)/Iddx) microampere and more preferably is (2.5*250)/Iddx microampere, βosc′ preferably has a range from ((2.5*93.75)/Iddx) miliampere to ((2.5*31.25)/Iddx) miliampere and more preferably is (2.5*62.5)/Iddx miliampere, βCP1′ preferably has a range from ((2.5*15.625)/Iddx) miliampere to ((2.5*9.375)/Iddx) miliampere and more preferably is (2.5*12.5)/Iddx miliampere βCP2′ preferably has a range from ((2.5*1.5625)/Iddx) miliampere to ((2.5*0,9375)/Iddx) miliampere, more preferably is (2.5*1.25)/Iddx miliampere and β6 preferably has a range from (625/Iddx) microampere to (375/Iddx) microampere and more preferably is 500/Iddx microampere.

17. The oscillator circuit according to claim 11, wherein a crystal X1 has an oscillating frequency fx1, the fourth transistor (T5) comprises a channel having a channel width divided by a channel length equal to βBIAS, the second transistor (T8) comprises a channel having a channel width divided by a channel length equal to βosc, the seventh transistor (T10) comprises a channel having a channel width divided by a channel length equal to βCP1, the ninth transistor (T12) comprises a channel having a channel width divided by a channel length equal to βCP2, the fifth transistor (T4) comprises a channel having a channel width divided by a channel length equal to βBIAS′, the first transistor (T7) comprises a channel having a channel width divided by a channel length equal to βosc′, the sixth transistor (T9) comprises a channel having a channel width divided by a channel length equal to βcp1′, the eighth transistor (T11) comprises a channel having a channel width divided by a channel length equal to βCP2′, the third transistor (T6) comprises a channel having a channel width divided by a channel length equal to β6 and wherein the first current source Iddx has a range from (fx1*187.5) femtoampere/Hertz to (fx1*62.5) femtoampere/Hertz, βBIAS has a range from (24/Iddx) microampere to (8/Iddx) microampere, βosc has a range from (1.875/Iddx) miliampere to (0.625/Iddx) miliampere, βCP1 has a range from (24/Iddx) microampere to (8/Iddx) microampere, βCP2 has a range from (1.875/Iddx) microampere to (0.625/Iddx) microampere, βBIAS′ has a range from ((2.5*24)/Iddx) microampere to ((2.5*8)/Iddx) microampere, βosc′ has a range from ((2.5*1.875)/Iddx) miliampere to ((2.5*0.625)/Iddx) miliampere, βCP1′ has a range from ((2.5*24)/Iddx) microampere to ((2.5*8)/Iddx) microampere, βCP2′ has a range from ((2.5*1.875)/Iddx) microampere to ((2.5*0.625)/Iddx) microampere and β6 has a range from (48/Iddx) microampere to (16/Iddx) microampere.

18. The oscillator circuit according to claim 17, wherein the first current source Iddx preferably has a range from (fx1*156.25) femtoampere/Hertz to (fx1*93.75) femtoampere/Hertz and more preferably is (fx1*125) femtoampere/Hertz, βBIAS preferably has a range from (20/Iddx) microampere to (12/Iddx) microampere and more preferably is (16/Iddx) microampere, βosc has a range from (1.5625/Iddx) miliampere to (0,9375/Iddx) miliampere and more preferably is 1.25/Iddx miliampere, βCP1 preferably has a range from (20/Iddx) microampere to (12/Iddx) microampere and more preferably is (16/Iddx) microampere, βCP2 preferably has a range from (10/Iddx) microampere to (6/Iddx) miliampere and more preferably is 8/Iddx miliampere, βBIAS′ preferably has a range from ((2.5*20)/Iddx) microampere to ((2.5*12)/Iddx) microampere and more preferably is ((2.5*16)/Iddx) microampere, βosc′ has a range from ((2.5*1.5625)/Iddx) miliampere to ((2.5*0,9375)/Iddx) miliampere and more preferably is (2.5*1.25)/Iddx miliampere, βCP1′ preferably has a range from ((2.5*20)/Iddx) microampere to ((2.5*12)/Iddx) microampere and more preferably is ((2.5*16)/Iddx) microampere, βCP2′ has a range from ((2.5*10)/Iddx) miliampere to ((2.5*6)/Iddx) miliampere and more preferably is (2.5*8)/Iddx miliampere and βBIAS preferably has a range from (40/Iddx) microampere to (24/Iddx) microampere and more preferably is (32/Iddx) microampere.