Compensating temperature null characteristics of self-compensated oscillators

Abstract

Techniques are described that enables controlling the TNULL characteristic of a self-compensated oscillator by controlling the magnitude and direction of the frequency deviation versus temperature, and thus, compensating the frequency deviation.

Claims

1. An oscillator circuit comprising: an oscillator comprising: one or more frequency determining tank circuits; and one or more amplifiers coupled to the one or more tank circuits; circuitry for causing a phase shift between voltage and current of the one or more tank circuits, for causing the oscillator to operate at a temperature null operating point of reduced frequency variation over a temperature null temperature range, in accordance with a temperature null characteristic; one or more output buffers coupled to the oscillator; and a temperature compensation circuit comprising a temperature sensor and a control circuit coupled to the temperature sensor for generating one or more compensation signals, the one or more compensation signals being generated so as to counteract temperature-dependent frequency variations of the temperature null characteristic and being applied to the oscillator for reducing temperature variations over frequency at least within the temperature null temperature range; wherein the oscillator circuit is configured to generate the one or more compensation signals during normal operation, distinct from calibration, and to vary the one or more compensation signals in response to temperature sensed using the temperature sensor during normal operation; and wherein the oscillator circuit, in addition to operating as a self-compensated oscillator, operates also as a temperature-compensated oscillator.

2. The oscillator circuit of claim 1, wherein the one or more compensation signals are applied to the circuitry for causing a phase shift; thus controlling the phase shift of the one or more tank circuits to follow a desired function across temperature.

3. The oscillator circuit of claim 1, wherein the one or more compensation signals are applied to the one or more tank circuits; thus controlling the impedance of the one or more tank circuits to follow a desired function across temperature.

4. The oscillator circuit of claim 1, wherein the one or more compensation signals are applied to the one or more output buffers; thus controlling the input impedance of the one or more output buffers to follow a desired function across temperature.

5. The oscillator circuit of claim 1, wherein the one or more compensation signals are applied to the one or more tank circuits and the one or more output buffers.

6. The oscillator circuit of claim 1, wherein the control circuit comprises one or more profile generators for generating one or more temperature-dependent signals.

7. The oscillator circuit of claim 1, wherein the one or more compensation signals are analog.

8. The oscillator circuit of claim 1, wherein the one or more compensation signals are digital.

9. The oscillator circuit of claim 1, wherein the oscillator is an I/Q oscillator comprising an I oscillator core and a Q oscillator core, and two coupling transconductance cells coupling the I oscillator core and the Q oscillator core, transconductances of the coupling transconductance cells being chosen to cause the oscillator to operate at the temperature null operating point.

10. The oscillator circuit of claim 9, wherein the one or more compensation signals are used to vary the coupling transconductances as a function of temperature.

11. A method of producing a temperature-compensated oscillator signal, comprising: operating an oscillator at a temperature null operating point of reduced frequency variation over a temperature null temperature range, in accordance with a temperature null characteristic; sensing temperature using a temperature sensor; generating one or more temperature-dependent compensation signals so as to counteract temperature-dependent frequency variations of the temperature null characteristic; and applying the one or more compensation signals to the oscillator for reducing temperature variations over frequency at least within the temperature null temperature range; wherein the oscillator circuit is configured to generate the one or more compensation signals during normal operation, distinct from calibration, and to vary the one or more compensation signals in response to temperature sensed using the temperature sensor during normal operation; and wherein the oscillator, in addition to operating as a self-compensated oscillator, operates also as a temperature-compensated oscillator.

12. The method of claim 11, comprising: using one or more tank circuits to determine an oscillator frequency; and using the one or more compensation signals to influence a phase shift between voltage and current of the one or more tank circuits.

13. The method of claim 11, comprising applying the one or more compensation signals to the one or more tank circuits.

14. The method of claim 11, comprising using one or more output buffers to produce one or more output signals, and applying the one or more compensation signals to the one or more output buffers.

15. The method of claim 14, comprising applying the one or more compensation signals to the one or more tank circuits and the one or more output buffers.

16. The method of claim 11, comprising using one or more profile generators for generating one or more temperature-dependent compensation signals.

17. The method of claim 14, wherein the one or more compensation signals are analog.

18. The method of claim 14, wherein the one or more compensation signals are digital.

19. The method of claim 14, wherein the compensation signals are a mix of analog and digital signals.

20. The oscillator circuit of claim 1, wherein the compensation signals are a mix of analog and digital signals.

21. A method of controlling an oscillator comprising a phase shift network and a temperature compensation circuit comprising a temperature sensor, the method comprising: using the phase shift network to cause the oscillator to operate at a temperature null at which variations of frequency as a result of variations in temperature are reduced, the oscillator operating as a self-compensated oscillator; and at least within a region of operation about the temperature null, using the temperature compensation circuit to counteract expected frequency variations in accordance with temperature readings from the temperature sensor; wherein the oscillator generates one or more compensation signals during normal operation, distinct from calibration, and varies the one or more compensation signals in response to temperature sensed using the temperature sensor during normal operation; wherein the oscillator, in addition to operating as a self-compensated oscillator, operates also as a temperature-compensated oscillator.

22. An oscillator circuit comprising: a phase shift network; and a temperature compensation circuit comprising a temperature sensor; wherein the oscillator circuit is configured to: use the phase shift network to cause the oscillator circuit to operate at a temperature null at which variations of frequency as a result of variations in temperature are reduced, the oscillator circuit operating as a self-compensated oscillator; and at least within a region of operation about the temperature null, the temperature compensation circuit being configured to counteract expected frequency variations in accordance with temperature readings from the temperature sensor; wherein the oscillator circuit is configured to generate one or more compensation signals during normal operation, distinct from calibration, and to vary the one or more compensation signals in response to temperature sensed using the temperature sensor during normal operation; and wherein the oscillator circuit is configured to, in addition to operating as a self-compensated oscillator, operate also as a temperature-compensated oscillator.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The present invention may be further understood from the following detailed description in conjunction with the appended drawing figures. In the drawing:

(2) FIG. 1 is a block diagram of a self-Compensated Oscillator (SCO).

(3) FIG. 2 is a graph illustrating a theoretical TNULL characteristic based on the first order temperature model.

(4) FIG. 3 is a graph illustrating three possible practical TNULL characteristics versus the theoretical TNULL characteristic derived from the first order model.

(5) FIG. 4 is a block diagram of a SCO with the TNULL characteristic compensation applied through varying the LC tank phase versus temperature (Phase Compensation).

(6) FIG. 5 is a graph illustrating SCO frequency deviation before and after applying the compensation of the TNULL characteristic.

(7) FIG. 6 is a graph illustrating compensating the frequency deviation of the SCO inside and outside the TNULL range.

(8) FIG. 7A is a block diagram of a SCO with TNULL characteristic compensation applied through varying the LC tank impedance (Z.sub.tank) versus temperature (Impedance Compensation).

(9) FIG. 7B is a block diagram of a SCO with TNULL characteristic compensation applied through varying the input impedance of the following buffer (Load Compensation).

(10) FIG. 8 is a block diagram of a SCO with TNULL characteristic compensation using phase compensation, impedance compensation and load compensation simultaneously.

(11) FIG. 9A is a block diagram of an example of a compensation block that can be utilized to compensate the SCO TNULL characteristic.

(12) FIG. 9B is a block diagram of an example of a compensation block that can be utilized to compensate the SCO TNULL characteristic.

(13) FIG. 9C is a block diagram of an example of a compensation block that can be utilized to compensate the SCO TNULL characteristic.

(14) FIG. 9D is a block diagram of an example of a compensation block that can be utilized to compensate the SCO TNULL characteristic.

(15) FIG. 10 is a block diagram of a an IQ SCO with phase compensation applied on the IQ SCO.

(16) FIG. 11 is a phasor diagram of the IQ SCO of FIG. 10.

(17) FIG. 12 is a block diagram of a an IQ SCO with impedance compensation applied on the IQ SCO.

(18) FIG. 13 is a diagram of digitally controlled capacitor units for compensating Z.sub.tank.

(19) FIG. 14 is a diagram of an analog varactor for compensating Z.sub.tank.

DETAILED DESCRIPTION

(20) Referring now to FIG. 4, there is shown the SCO with the proposed Phase Compensation applied. In FIG. 4, blocks 401, 403 and 405 correspond to blocks 101, 103 and 105 of FIG. 1. As explained in Hanafi, the (Φ) control is adjusted such that the oscillator operates at Φ.sub.GNULL. Afterwards, the compensation block 407 generates a temperature-dependent control signal S(T). This control signal is then used to control the (Φ) block in order to generate a phase (Φ) between the voltage and current which follows a specific profile across temperature. The SCO output frequency depends on the value of Φ according to a specific sensitivity function; thus, the SCO exhibits a temperature-dependent frequency shift corresponding to the control signal S(T). The control signal profile across temperature is adjusted such that the generated frequency shift substantially cancels the TNULL characteristic (the inherent behavior of the oscillator deviation at TNULL) as shown in FIG. 5.

(21) The control signal profile can also be adjusted to compensate the oscillator inherent frequency deviation outside the TNULL operation range as well, as shown in FIG. 6. As illustrated in FIG. 6, the SCO is operating at the TNULL of the temperature range of T.sub.o−ΔT to T.sub.o+ΔT and the oscillator deviates significantly outside this range. The control signal in this case is used to compensate the frequency deviation outside the TNULL range as well.

(22) FIG. 7A presents a variation of the proposed compensation mechanism. In FIG. 7A, blocks 701a, 703a and 705a correspond to blocks 101, 103 and 105 of FIG. 1. In this case, a compensation block 707a is used to induce the compensating frequency shift by varying the value of Z.sub.tank i.e. S(T) controls the tank impedance of the SCO. As proposed earlier, the induced frequency shift can compensate the SCO inherent frequency deviation inside and outside the TNULL range.

(23) Moreover, FIG. 7B shows a further compensation mechanism. In FIG. 7B, blocks 701b, 703b and 705b correspond to blocks 101, 103 and 105 of FIG. 1. This time a compensation block 707b provides a compensation signal S(T) that controls the input impedance of the active circuit which interfaces the SCO. Normally, an oscillator is followed by an active buffering circuit such as output buffer 709b which delivers the oscillator signal from the oscillator to the required recipients while protecting the oscillator from any possible undesired loading. The input impedance (Z.sub.in) of such a buffer is considered as a part of the SCO tank impedance; thus, controlling the buffer input impedance (Z.sub.in) across temperature induces a controllable frequency shift across temperature. This compensation mechanism is denoted as “Load Compensation”. Load compensation can compensate the SCO inherent frequency deviation inside and outside the TNULL range.

(24) Finally, the SCO can be compensated using a mix of all these techniques phase compensation, impedance compensation and load compensation as shown in FIG. 8. In FIG. 8, blocks 801, 803 and 805 correspond to blocks 101, 103 and 105 of FIG. 1. Compensations blocks 807a, 807b and 807c provide phase, impedance and load compensation, respectively.

(25) Generally, the control signal (S(T)) generated by the compensation block can take several forms depending on the SCO architecture. For example and not for limitation, the control signal can be an analog signal, digital signal or a mix of both analog and digital signals. Furthermore, the control signal can be a voltage signal, a current signal or a mix of both current and voltage signals.

(26) FIG. 9A to FIG. 9D shows different examples for generating the control signal (S(T)). In FIG. 9A, the temperature sensor 901 detects the die temperature and generates an analog signal (A(T)) that is substantially proportional to temperature. Afterwards, a control circuit, the profile generator block 903a, utilizes A(T) to generate S(T) with the specific temperature-dependent profile required to compensate the SCO TNULL characteristic. In FIG. 9A, the whole compensation process is done in the analog domain.

(27) FIG. 9B illustrates a different concept. In FIG. 9B, blocks 901 and 903a correspond to blocks 901 and 903a of FIG. 9A. Herein, the SCO frequency is controlled using a digital signal; it is a Digitally-Controlled SCO (DCSCO). Thus, the control signal (S(T)) is transferred into the digital domain using an Analog-to-Digital Converter (ADC) 905 and then used to compensate the DCSCO.

(28) In FIG. 9C, the output of the temperature sensor 901 is transferred into the digital domain by an ADC 907 and then the compensation profile is generated digitally (block 903d). The digital control signal is then used to compensate the DCSCO. FIG. 9D illustrates a concept similar to FIG. 9C except that the output of the digital compensation block is transferred back to the analog domain using a Digital-to-Analog Converter (DAC) 911 and then used to compensate the SCO.

(29) Furthermore, the topologies explained in FIG. 9A to FIG. 9D are for the sake of example and not for limitation. For instance, the SCO can be compensated using a combination of these topologies depending on the SCO architecture.

Compensation Examples

(30) The following description presents some techniques for the proposed TNULL characteristic compensation. The presented techniques are demonstrated just for example and not for limitation.

Example I

(31) FIG. 10 shows the LC oscillator in a quadrature configuration. The quadrature configuration consists of two identical oscillator cores, the I-core and the Q-core, coupled together with the transconductance cells “g.sub.mc” (1011a and 1011b). The I-core includes a tank 1001I and an amplifier 1003I. The Q-core includes a tank 1001Q and an amplifier 1003Q. As explained in Hanafi, the IQ oscillator can be configured to work as an SCO. The phase between the voltage and the current in the tank circuits is given by:

(32) $\Phi ={\tan }^{-1}\left(\frac{g_{mc}}{g_m}\right)$

(33) Where g.sub.mc is the coupling transconductance and g.sub.m is the oscillator core transconductance. The initial phase is adjusted to force the oscillator to operate at the TNULL Phase (−Φ.sub.GNULL). The compensation block 1007 then generates a temperature-dependent profile that modulates the g.sub.mc/g.sub.m values and thus modulating the V-I phase. The control signal can modulate either g.sub.m or g.sub.mc and can be of analog nature, digital nature or a mix between analog and digital.

(34) FIG. 11 shows the phasor diagram for the IQ oscillator. The oscillator is initially adjusted to operate as an SCO by adjusting V-I angle to Φ.sub.GNULL. Afterwards, the compensation block modulates the Φ.sub.GNULL by ΔΦ(T) using the control signal S(T). The modulated ΔΦ(T) should induce a frequency shift that cancels the inherent frequency deviation of the SCO.

Example II

(35) FIG. 12 illustrates the proposed compensation for a quadrature oscillator core through the LC tank impedance. In FIG. 12, block 1201I, 1201Q, 1203I, 1203Q, 1211a and 1211b correspond to blocks 1001I, 1001Q, 1003I, 1003Q, 1011a and 1011b of FIG. 10. As explained in Example I, the g.sub.mc/g.sub.m ratio is chosen to adjust the V-I phase to operate at the Null Phase TNULL. Afterwards, the compensation block modulates the tank impedance Z.sub.tank to compensate the oscillator inherent frequency deviation. The control signal can be in analog and/or digital form, and can modulate any part of the tank impedance as explained above.

(36) FIG. 13 shows an example of compensating Z.sub.tank 1300, including a tank circuit 1310. In this example, the capacitive part of Z.sub.tank is modified using capacitor units 1301-1, 1301-2, . . . , 1301-n that are digitally switched on or off to compensate the frequency deviation of the SCO. Each capacitor unit includes a capacitor and a switch (e.g., C.sub.1 and S.sub.1 in the case of capacitor unit 1301-1).

(37) FIG. 14 shows another example for compensating the capacitive part of Z.sub.tank. In FIG. 14, elements 1400 and 1410 correspond to elements 1300 and 1310 in FIG. 13. In this case, an analog varactor 1403 is connected in parallel to the tank circuit and its control voltage S(T) is supplied by the compensation block. A hybrid solution can utilize both the digitally-controlled capacitor units and the analog-controlled varactor as well.

(38) It will be appreciated by those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential character thereof. The foregoing description is therefore intended in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims, not the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.