WO2003007469A1

WO2003007469A1 - Balanced oscillator circuit

Info

Publication number: WO2003007469A1
Application number: PCT/NL2001/000542
Authority: WO
Inventors: Paulus Thomas Maria Van Zeijl; Peter-Paul Vervoort
Original assignee: Telefonaktiebolaget L.M. Ericsson
Priority date: 2001-07-13
Filing date: 2001-07-13
Publication date: 2003-01-23

Abstract

An oscillator circuit (2) at least including: at least one resonator body (Crys) having a first resonator contact (crys-p) connected to at least one first circuit ocntact (outp) and a second resonator contact (crys-n) connected to at least one second circuit contact (outn); an amplifier section connected to said first resonator contact and said second resonator contact. The oscillator circuit is a balanced circuit, wherein the first resonator contact is connected to a first amplifier section (Mn1; Mc1; Qn1Qn1) and the second resonator contact is connected to a second amplifier second (Mn2; Mc2; Qn2, Qn2) similar to the first amplifier section in a substantially equivalent way.

Description

Title: Balanced oscillator circuit

The invention relates to an oscillator circuit at least including: a least one resonator body having a first resonator contact connected to at least one first circuit contact and a second resonator contact connected to at least one second circuit contact; an amplifier section connected to said first resonator contact and said second resonator contact.

In the art, oscillator circuits are generally known, for example from the the United States Patent publication US 6 249 155 and European Patent publication EP1 096 663. The known circuits are single-ended circuits using one transistor as an amplifier, or are connected as a single ended circuit. However, the known crystal oscillators are disadvantageous because they inject the fundamental crystal-frequency into the substrate and/or ground if the circuit is an integrated circuit. The fundamental frequency can then be picked up by other circuits.

The invention seeks to overcome or at least reduce this disadvantage. Therefore, according to the invention an oscillator circuit as described above is characterised in that the oscillator circuit is a balanced circuit, wherein the first resonator contact is connected to a first amplifier section and the second resonator contact is connected to a second amplifier section similar to the first amplifier section in a substantially equivalent way. In a circuit according to the invention the signal of fundamental frequency is not injected into the substrate and transferred to other circuits because the circuit is a balanced circuit, i.e. the signals from both crystal contacts are processed in a similar way since each signal is available in the circuit in a zero phase and an anti-phase signal. When the signal is injected into the substrate, a compensating antiphase signal is injected into the substrate as well. Since a signal and the same signal but 180 degrees shifted in phase cancel each other, the resulting injected signal is zero. Furthermore, a balanced circuit has a high power supply rejection, whereby variations in the supply voltage do not affect the performance of the oscillator circuit significantly or at least affect the performance less than for a circuit with a low power supply rejection. An oscillator circuit in accordance with the present invention is particularly suitable for implementation as an integrated circuit.

The invention further provides a method for generating a signal of a fundamental frequency. Such a method may be performed with an oscillator circuit according to the invention. Specific embodiments of the invention are set forth in the dependent claims.

Further details, aspects and embodiments of the invention will be described with reference to the figures in the attached drawing.

Fig. 1 shows an equivalent circuit of a piezo-electric crystal; Fig. 2 shows a circuit diagram of a first example of an embodiment of an oscillator circuit according to the invention;

Fig. 3 shows a circuit diagram of a second example of an embodiment of an oscillator circuit according to the invention;

Fig. 4 shows a circuit diagram of a third example of an embodiment of an oscillator circuit according to the invention;

Fig. 5 shows a circuit diagram of a complementary version of the example shown in fig. 3; and

Fig. 6 shows a circuit diagram of a bipolar version of the example shown in fig. 4. Fig. 1 shows an equivalent circuit of a piezo-electric crystal. A piezoelectric crystal develops a voltage over its surface if a mechanical pressure is exerted on the crystal. Likewise a pressure will develop over the crystal if a voltage is applied to the crystal. Due to said pressure the crystal will mechanically deform. This property is well known and may be used for the generation of oscillating signals with the fundamental or Eigen frequency of the crystal. Electronically, a piezo-eletric crystal may be represented by an equivalent circuit 1 as is shown in fig. 1. The equivalent circuit 1 includes a series circuit LCR with a parallel capacitance C2. The series circuit LCR consists of an inductor LI connected in series to a series capacitor Cl and a resistor Rl. The circuit 1 is an oscillator circuit with two modes of oscillating, a series mode and a parallel mode. Both the series mode and the parallel mode have a resonance frequency. If the crystal is used as an oscillator source, the crystal will generate a signal of one of these resonance frequencies. These frequencies are called the fundamental crystal frequencies. The series resonance frequency of the circuit is determined by the capacitance Cl and inductance LI. The series resonance frequency may mathematically be described by:

^{S ~} 2 H- C1 In equation (1) fs is the series resonance frequency and LI and Cl are the reactances of the inductor LI and the series capacitor Cl respectively. The parallel resonance frequency is determined by the impedances of the inductor LI and capacitors Cl and C2 and may mathematically be described by

In equation (2), fp is the parallel resonance frequency. As can be seen in fig. 1, the piezo-electric crystal includes a resistor Rl.

Therefore, the crystal dissipates power and the amplitude of the oscillations will decay in time. In order to compensate for the loss of energy, the crystal has to be connected to a power supply of some kind. The power supply may also be used to amplify the signal of fundamental frequency. A circuit providing compensation for the losses in the crystal and amplification of the signal is called the oscillator circuit.

Fig. 2 shows an example of an oscillator circuit 2 according to the invention. The shown oscillator circuit 2 includes a piezo-electric crystal Crys. The crystal has a first crystal contact Crys_p and a second crystal contact Crys_n. The first crystal contact Crys_p and the second crystal contact Crys_n are connected to a positive output outp and a negative output outn of the oscillator circuit 2.

The circuit 2 is a balanced circuit, i.e. the signals from both crystal contacts are processed in a similar way. The signal of fundamental frequency is not injected into the substrate because of this balanced circuit, since each signal is available in the circuit as an zero phase and an anti-phase signal. When a signal is injected into the substrate, an antiphase signal is injected into the substrate as well. Since a signal and the same signal but 180 degrees phase shifted cancel each other, the resulting injected signal has a value of zero. The balanced circuit further has a high power supply rejection, whereby variations in the supply voltage do not affect the performance of the oscillator circuit significantly or at least affect the performance less than for a circuit with a low power supply rejection. When the crystal is oscillating, a signal of the fundamental crystal frequency is present at each of the crystal contacts Crys_ j, Crys_n. The signal at the first crystal contact Crys_p has a phase difference of substantially 180 degrees with respect to the signal at the second crystal contact Crys_n, i.e. they are substantially in antiphase. In fig. 2, each of the crystal contacts is connected to ground via a capacitor C11,C12 respectively. These capacitors form the load capacitance that is required for most crystals. However, the circuit still functions if the capacitors CU,C12 are not present.

Connected to the first crystal contact Crys_p is an first amplifier section Mnl. In the example of fig. 1, the first amplifier section Mnl is a voltage-to- current amplifier, that is a device which outputs a certain output current in dependence of the voltage of an input signal, also known in the art as a gm- stage. It is likewise possible to use a current-to- voltage amplifier, that is a device which outputs a certain output voltage in dependence of the current of an input signal, as an amplifier section. In general, the amplifier section may be any type of negative resistor. The drain dl of the first gm-stage Mnl is connected to the first crystal contact, the source si is connected to the ground gnd and the gate gl of the first gm-stage is connected to the second crystal output Crys_n. The first gm-stage Mnl is provided with a bias current by a first bias current source Ibias 1, connected to a voltage supply Vdd and the drain dl of the n-type MOSFET Mnl.

The second crystal contact Crys_n is connected to the drain d2 of a second amplifier section Mn2 in a similar way. The gate g2 of the second amplifier section Mn2 is connected to the drain dl of the first amplifier section Mnl and the first crystal contact Crys_p. The source s2 of the second amplifier section is connected to the ground and the second amplifier section is provided with a bias current by a second bias current source Ibias2 connected to the drain d2.

The positive input (the gate) of each of the gm-stages is connected to the positive output (i.e. the drain) of the other gm-stage because the drain dl,d2 of each of the n-type MOSFETs Mnl,Mn2 is cross-connected to the gate gl,g2 of the other n-type MOSFET. Thereby a positive feedback loop is obtained, which will cause the oscillator circuit to oscillate.

In the embodiment of fig. 2, the first gm stage is implemented as a n- type Metal Oxide Semiconductor Field Effect Transistor (n-type MOSFET). However, the gm-stage may be implemented as a different device, such a p- type MOSFET, a complementary MOSFET, a bipolar transistor or any other type of amplifier.

Fig. 3 shows a circuit diagram of a second example of an oscillator circuit according to the invention . The circuit of fig. 3 is substantially equivalent to the circuit shown in fig.2. The circuit in fig. 3 differs from the circuit in fig.2 in that the output of each of the gm-stages Mnl,Mn2 is indirectly connected to the input of the other gm-stage via a capacitor Ccl,Cc2 instead of by a direct connection. Furthermore, the input gl,g2 of each gm- stage is connected to the output dl,d2 of same of the gm-stages Mnl,Mn2 via a resistor R1,R2. Therefore, each gm-stage is provided with a positive feedback loop.

The feedback and cross-connection via capacitors Ccl and Cc2 prevent lock-up of the oscillator circuit in a situation with zero loop gain in which the circuit will not function as an oscillator. This situation may occur in the circuit shown in fig. 2 if one of the gm-stages is in a non-conducting state (when the gate-source voltage is zero) forcing the other gm-stage in a conducting stage (i.e. the gate source voltage is equal to the supply voltage). Due to the positive feedback in the loop this is a stable situation from which the circuit cannot recover.

Resistors Rl and R2 ensure that both transistors are switched as diodes for low frequencies, thus providing a DC-operating point in which the loopgain will not be equal to zero. For high frequencies the capacitors Ccl and Cc2 provide a positive feedback path enabling the circuit to oscillate at the fundamental frequency of the crystal. The values of Rl, R2, Ccl and Cc2 must be chosen in such a way that the circuit will only oscillate at the fundamental frequency of the crystal, i.e. the resonance frequency of this circuit must be substantially equal to the fundamental frequency of the crystal. The resonance frequency fres may mathematically be described by:

In equation (3), Cl and C2 are the capacitance of the capacitors Cl and C2 of fig 1 and Cll and C12 are the capacitances of the capacitors Cll and C12 in fig. 3.

The circuit of fig. 3 has a positive feedback loop that does not involve the crystal. This may cause an undesired relaxation-type like oscillation behaviour. In fig. 4 a third example of an oscillator circuit according to the invention is shown in which the circuit is prevented from behaving in this way. In the third example shown in fig. 4 , the positive feedback loop is again implemented via capacitors Ccl and Cc2. The positive outputs dl,d2 of the gm- stages Mnl,Mn2 are connected to each other via resistors Rl, R2. Hereby a common-mode feedback is present by means of resistors Rl, R2, R3 and R4 and the gm-stages form a differential amplifier. The connection between resistors Rl and R2 is a common mode point CM. At the common mode point CM the circuit develops a common-mode voltage which is used as the DC- biasing voltage for the transistors Mnl and Mn2. The common-mode voltage is decoupled with a capacitor Cdecouple. For AC or RF signals the capacitor Cdecouple acts a ground. Thereby the two balanced part of the circuit are separated for AC or RF signals. The capacitor Cdecouple decouples the common-mode point for small circuit imbalance. Properly dimensioning the components in the circuit will lower the loopgain of the relaxation-type like oscillation below 1, while maintaining a loopgain larger than one for the desired crystal oscillation. This dimensioning may be obtained by using a circuit simulator or calculating the gain formulas, which is generally known in the art.

A complementary version of the circuit of fig. 3 is shown in fig. 5. In fig. 5 instead of n-type MOSFETs, MOS inverters with local feedback are used as gm-stages. Each of the MOS invertors Mcl,Mc2 includes a n-type MOSFET Mnl,Mn2 and a p-type MOSFET Mpl,Mp2. The input of the MOS invertor is connected to the output via a resistor Rl resp. R2. Each of the MOS invertor outputs is connected to the input of the other MOS invertor via a capacitor Ccl resp. Cc2. The energy consumption of the circuit shown in fig. 5 is lower because both an n-type MOSFET and a p-type MOSFET share the same supply current, whereby the same loopgain at a lower supply current is obtained . The circuit of fig. 5 also has the advantage that it requires no external biasing current. In a similar fashion a complementary version of the circuit of Fig. 4 may be designed by replacing the n-type MOSFETs with MOS invertors. An oscillator circuit in accordance with the invention may also be realised in bipolar technology. In that case any n-type MOSFETs are substited by npn bipolar junction transistors and any p-type MOSFETs are substituted by pnp BJTs. In such a bipolar circuit, the supply voltage is limited to 2*VBE, wherein VBE is the base-emitter voltage.

In fig. 6 a bipolar version of the circuit of fig. 5 is shown in which the inputs, i.e. the base of the BJTs are directly connected to the output, i.e. the collectors, of the other transistors and emitter degeneration of the transistors is used to prevent lock-up of the circuit. Each of the emitters of the BJTs Qnl,Qn2,Qpl,Qp2 is connected to a degeneration resistor Rel-Re4 and a decoupling capacitor Cel-Ce4 in parallel. For high frequencies the capacitors Cel-Ce4 acts as a bypass of the resistor Rel-Re4 in parallel therewith. The degeneration resistors Rel-Re4 prevent the circuit to lock-up while the capacitive decoupling of the degeneration resistors Rel-Re4 increases the loopgain at the oscillation frequency.7

In the shown examples of oscillator circuits the oscillation frequency may be tuned by varying the supply voltage. Varying the supply voltage will change the reverse-voltage across the drain-bulk or collector-bulk diodes of the transistors thus changing the value of the capacitance of these diodes. This change in capacitance changes the load capacitance of the crystal and therefore its resonance frequency. Furthermore, in the shown examples the resistors may (partially) be replaced with inductors.

Instead of a piezo-electric crystal, other types of resonator bodies may be connected to the oscillator circuit, such as a tuning fork, a cavity resonator or rhumbatron or an acoustic oscillator, as are generally known in the art.

An oscillator circuit according to the invention is suited for use in an integrated circuit. In general, integrated circuits are smaller, have a lower power consumption and improved performance characteristics compared to circuits comprising discrete components. Thereby an oscillator circuit according to the invention is especially suited for implementation in wireless device with limited power supply and/or which size shoud be as small as possible, such as mobile telephones or devices communicating via the bluetooth protocol. It should be noted that the oscillator circuit may also be implemented with at least one discrete component. Furthermore, an oscillator circuit according to the invention may be implemented in a receiver device or a transceiver (transmitter and receiver) device. A receiver device is an electronic device for the reception and processing of electro-magnetic signals such as radio signals. A transceiver device is a receiver device which is able to transmit electro-magnetic signals as well.

A receiver device or a transceiver device comprising at least one oscillator circuit according to the invention may be used in any wireless electronic device. A wireless electronic device is any device used for the reception of electro-magnetic signals. A wireless electronic device may comprise a transmitter functionality for the transmission of electro-magnetic signals. The wireless electronic device may for example be a device with a limited power supply (such as a battery), such as mobile telephone, a portable radio or a lap-top computer connected to a other device via a Bluetooth connection. The wireless electronic device may likewise be a device with a virtually unlimited power supply, like a mobile telephone base station or a desktop computer communicating to other device via a Bluetooth connection.

Claims

1. An oscillator circuit (2) at least including: a least one resonator body (Crys) having a first resonator contact (crys_p) connected to at least one first circuit contact (outp)and a second resonator contact (crys_n) connected to at least one second circuit contact (outn); an amplifier section connected to said first resonator contact and said second resonator contact; characterised in that the oscillator circuit is a balanced circuit, wherein the first resonator contact is connected to a first amplifier section (Mnl;Mcl;QnlQpl) and the second resonator contact is connected to a second amplifier section

(Mn2;Mc2;Qn2,Qp2) similar to the first amplifier section in a substantially equivalent way.

2. An oscillator circuit (2) as claimed in claim 1, wherein the first amplifier section and the second amplifier section are voltage-to- current amplifier sections.

3. An oscillator circuit (2) as claimed in claim 1, wherein the first amplifier section and the second amplifier section are current -to -voltage amplifier sections.

4. An oscillator circuit (2) as claimed in any one of claims 1-3, wherein each of the amplifier sections (Mnl,Mn2;Mcl,Mc2;Qnl,Qn2,Qpl,Qp2) has an amplifier input (gl,g2) and an amplifier output (dl,d2) and wherein the amplifier output of each amplifier section is connected through a cross- connection to the amplifier input of the other amplifier section.

5. An oscillator circuit (2) as claimed in claim 4, wherein the cross- connection includes at least one capacitor (Ccl,Cc2).

6. An oscillator circuit (2) as claimed in any one of the preceding claims, wherein the amplifier output (dl,d2) of each amplifier section

(Mnl,Mn2;Mcl,Mc2;Qnl,Qn2,Qpl,Qp2) is connected through a feedback connection to the amplifier input of the same amplifier section.

7. An oscillator circuit (2) as claimed in claim 6, wherein the feedback connection includes at least one passive component (Rl,R2,R3,R4).

8 An oscillator circuit (2) as claimed in claim 7, wherein at least one of said at least one passive components is a resistor (R1,R2,R3,R4).

9 An oscillator circuit (2) as claimed in claim 8, wherein the passive component is an inductor.

10. An oscillator circuit (2) as claimed in any one of claims 7-9, wherein the feedback connection is connected to the output (dl,d2) of the other amplifier section via a second passive component; wherein the second passive components of the first amplifier section and the second amplifier section are connected to each other; and wherein the second passive components are connected to ground via a decoupling component (Cdecouple) which electrically disconnects the first amplifier section (Mnl) and the second amplifier section (Mn2) for signals of radio frequencies.

11 An oscillator circuit (2) as claimed in claim 10, wherein the decoupling component is a capacitor connecting the second passive components to ground.

12. An oscillator circuit (2) as claimed in any one of the preceding claims, wherein the amplifier sections each include at least one transistor.

13. An oscillator circuit (2) as claimed in claim 14, wherein at least one of the transistors is a Field Effect Transistor (FET)

14. An oscillator circuit (2) as claimed in claim 13, wherein at least one of the FETs is an n-type Metal Oxide Semiconductor FET.

15. An oscillator circuit (2) as claimed in claims 13 or 14, wherein at least one of the FETs is an p-type Metal Oxide Semiconductor FET

16. An oscillator circuit (2) as claimed in claim 13, wherein at least one of the FETs is a MOS inverter comprising at least one p-type MOSFET and at least one n-type MOSFET.

17. An oscillator circuit (2) as claimed in claim 12, wherein the amplifier sections include at least one bipolar junction transistor (Qnl,Qn2,Qpl,Qp2).

18. An oscillator circuit (2) as claimed in any one of the preceding claims, wherein each of the resonator contacts (crys-n,crys_p) is connected to ground via a capacitor (CL1,CL20.

19. An oscillator circuit (2) as claimed in any one of the preceding claims, wherein said resonator body is a piezo-electric crystal

20. An oscillator circuit (2) as claimed in any one of the preceding claims, wherein said resonator body is a cavity oscillator.

21. An oscillator circuit (2) as claimed in any one of the preceding claims, wherein said resonator body is a tuning fork oscillator

22. An integrated circuit including an oscillator circuit (2) as claimed in any one of claims 1-21.

23. A method for generating a signal of a fundamental frequency including: exciting a resonator body whereby a first resonator signal of a fundamental resonator frequency is obtained at a first resonator contact and an second resonator signal of fundamental frequency being in anti-phase with respect to the first resonator signal of the fundamental resonator frequency is obtained at a second resonator contact; amplifying the first resonator signal whereby a first amplified signal is obtained; amplifying the second resonator signal in a way similar to the amplifying of the first resonator signal, whereby a second amplified signal being in antiphase with respect to the first amplified signal is obtained; feeding back the amplified signals to the resonator signals.

24. A transceiver device at least including an oscillator circuit (2) as claimed in any one of claims 1-21.

25. A receiver device at least including an oscillator circuit (2) as claimed in any one of claims 1-21.

26. An wireless electronics device at least including an oscillator circuit (2) as claimed in any one of claims 1-21.