A TEMPERATURE STABILIZED DEVICE WITH ACCELERATED RESPONSE TO POWER SUPPLY

VARIATIONS

FIELD OF THE INVENTION

The present invention relates to temperature stabilized electronic devices, wherein a device's transient response to power supply variations is accelerated in order to shorten the time period required to bring the device's temperature back to the control setpoint and, as a result, to reduce the undesirable transient changes in the device's key performance parameters.

BACKGROUND OF THE INVENTION

Key performance parameters of many electronic devices are often dependent on the ambient temperature. For example, the frequency of a signal generated by an electronic oscillator is dependent on a number of external parameters, such as ambient temperature, power supply voltage, mechanical acceleration, with frequency instability due to ambient temperature changes being particularly pronounced. A number of techniques have been developed to reduce the sensitivity of key performance parameters in electronic devices to variations in ambient temperature. One such technique is a device's temperature stabilization, wherein the temperature-sensitive parts of an electronic device are placed in a temperature stable environment, thus reducing the effects of ambient temperature variations.

One example of a temperature stabilized electronic device is an oven-controlled electronic oscillator. In an Oven-Controlled Oscillator (OCO), the device's temperature is controlled and maintained at a predetermined level, usually set a few degrees higher than the maximum operating temperature of the device; the result is that the oscillator's temperature variations are substantially reduced compared to the variations in the ambient temperature, resulting in improved output frequency stability. Examples of Oven-Controlled Oscillator devices are: Oven-Controlled Crystal Oscillator (OCXO) comprising an oscillator circuit with a crystal resonator, Oven-Controlled MEMS Oscillator (OCMO) comprising an oscillator circuit with a microelectromechanical systems (MEMS) resonator, Oven-Controlled SAW Oscillator (OCSO) comprising an oscillator circuit with a surface acoustic wave (SAW) resonator, and others. ln order to maintain a stable device assembly temperature in a temperature stabilized device, a closed loop temperature control technique is often deployed. The diagram in Fig. 1 (prior art) provides an example of such a control loop. As shown therein, the temperature of the device assembly 1 is sensed by means of a temperature sensor 2, the sensed temperature value ("device assembly temperature") is compared to the required temperature value ("setpoint"), the value of temperature error is worked out as the difference between the setpoint value and the actual device assembly temperature value, and the temperature error is then processed by the error processing block 3, the output of which controls one or several heaters 4 in order to minimize the temperature error value, thus bringing the device's temperature closer to the setpoint. Any of a number of known control algorithms can be used in the error processing block. For example, a proportional control algorithm can be used; in this control method, the output of the error processing block (i.e., the heater control signal) is arranged to be proportional to the temperature error:

where S _H c is the value of the heater control signal, K _P is the proportional control coefficient (also known as proportional gain), and ΔΤ is the temperature error value (i.e., the difference between the required temperature "setpoint" value and the value of the actual temperature of the device assembly). Depending on specific circuit implementation, the heater control signal can be either a voltage signal VHC or a current signal l _H c-

It follows from expression (1) that the proportional control algorithm will result in a permanently present non-zero temperature error, often called "an offset". Indeed, as soon as the temperature error reaches a zero value (i.e., when the device assembly temperature is equal to the desired "setpoint" value), the value of the heater control signal SHC becomes zero too, which in turn will result in the device assembly temperature gradually deviating from the desired "setpoint" temperature. Providing the proportional gain value K _P is set correctly and the control loop is stable, a steady state will be achieved where the SHC and ΔΤ values are stable and non-zero, i.e. a permanent temperature "offset" is present. In order to eliminate or at least minimize the undesirable "offset", the proportional control method can be enhanced by adding a constant term to the control expression :

where C _H c is the added constant heater control term. It follows from expression (2) that the constant control term C _H c, if set correctly, will indeed eliminate the offset error, but its value will be optimal only for a certain single value of the ambient temperature.

In order for the additional term to suit the whole operating range of ambient temperatures, the additional term can be generated as a time integral of the temperature error ΔΤ; such a control algorithm is known as Proportional-Integral (PI) and functions in accordance with the following expression:

SHC = Κ _Ρ *ΔΤ + Ki*JAT*at (3) where K _P is the proportional gain,

ΔΤ is the temperature error value,

Ki is the integral gain,

0t indicates integration over time.

The integral action Ki*fAT*0t in expression (3) replaces the constant term C _H c in expression (2) and represents a term that is automatically adjusted depending on the temperature error value ΔΤ. Due to its integrating nature, the Integral action in a PI controller is lagging in time, i.e. it takes some time for the integral action to take full effect.

Besides Proportional and Proportional-Integral, other known control algorithms can be used to control the assembly temperature in temperature stabilized devices. An important point is that the heater control signal S _H c in any of such algorithms is generated as a function of only one variable - the temperature error:

One of the implications of expression (4) is that the control algorithm does not directly take into account changes in the power supply level. This means that if the temperature stabilized device's power supply voltage changes, this change will cause a change in the amount of power dissipated in the heater(s) and result in a change of the device assembly temperature and of the temperature error value. It will take considerable time for the deviation in the temperature error to be processed by the temperature control loop and for the resulting action to take its effect. In a PI control's case, the ΔΤ change will be corrected eventually, but only after a certain period of time, the latter comprised of (a) the time required to integrate the changing ΔΤ value in the integrator Κι*/ΔΤ*όΐ and (b) for the device assembly to acquire the required temperature; the time period (b) will depend on the amount of thermal mass presented by the temperature stabilized device assembly.

Another implication of a change in power supply voltage level is that power dissipated in circuits and components of the temperature stabilized device other than the heaters will change too. Since this dissipated power also contributes to the device assembly heating, the change in dissipated power will result in an additional temperature error. Again, the temperature control loop in a prior art temperature stabilized device will eventually "take care" of this change, but only after a period of time due to the nature of the integral control action and the device's thermal mass.

The purpose of the present invention is to offer techniques that allow to reduce the time period required to bring the temperature stabilized device's temperature back to the setpoint following a change in power supply voltage, and to reduce the undesirable effects of the temperature disturbance on key performance parameters of the temperature stabilized device, or at least offer alternative if not improved techniques for mitigating the transient effects caused by power supply voltage changes in temperature stabilized devices. For example, in oven-controlled oscillator (OCO) devices, the use of the techniques of the invention may shorten, and reduce the magnitude of, the transient temperature disturbances experienced by the device as a result of power supply variations, which results in shortened and reduced variations in the output frequency generated by the OCO devices.

SUMMARY OF THE INVENTION The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

In its first aspect, this invention presents a temperature stabilized device that can be powered by a power supply voltage source, the said device comprising a heater circuit for heating the device assembly, and a temperature control circuit arranged to generate a heater control signal to control the amount of power dissipated in the heater circuit, and wherein the heater control signal is arranged to be dependent on, and electronically adjusted with changes in, the power supply voltage level.

In at least some embodiments the temperature control circuit is arranged to control the amount of power dissipated in the heater circuit to maintain the device assembly temperature within at most ±5%, more preferably at most ±1%, more preferably at most ±0.01% of the device's intended operating ambient temperature range.

In at least some embodiments the heater control signal is arranged to be electronically adjusted with changes in the power supply voltage level that do not exceed ±10%, more preferably ±5%, more preferably ±1%.

In a second aspect of the invention, in the abovementioned temperature stabilized device an electronic circuit is arranged to reduce the heater control signal before the said signal is applied to the heater circuit or to a heater driver circuit.

Thus, there are two main aspects to the present invention. I. In the first aspect of the invention, additional circuitry is arranged in the temperature stabilized device to perform the following functions: (a) sensing the power supply voltage level, and (b) generating a heater control signal which depends not only on the temperature error value, but also on the sensed power supply voltage value. In other words, for a temperature stabilized device of the invention the expression (4) above is replaced by the following expression:

where S _H c is the value of the heater control signal,

ΔΤ is the value of the temperature error,

Vs is the supply voltage value.

The advantage of generating the heater control signal as a function of both the temperature error and the power supply voltage is that if the power supply voltage changes, the heater power is adjusted accordingly by the control algorithm in order to maintain the amount of heater power as required to maintain a stable device assembly temperature; moreover, since the heater control signal adjustment is done electronically, it takes place immediately following the supply voltage change and considerably faster than over the time period that would have been required for the temperature control loop to bring the device temperature to the requisite setpoint. The heater control signal generated as per the expression (5) can be said to be "power supply compensated".

II. In the second aspect of the invention, yet another additional circuit is introduced, in order to further accelerate the temperature stabilized device's transient response to power supply changes. In a temperature stabilized electronic device, power is dissipated not only in the heater(s), but also in every other electronic circuit and electronic component of the device. For example, in an OCO device, power dissipation also takes place in voltage regulator circuits, oscillator circuits, output buffer circuits, etc. This internally dissipated power causes additional heating of the device, i.e. heating the device in addition to heating provided by the heater(s). At high ambient temperatures the temperature dependent heater power is reduced, and the internally dissipated power becomes comparable to that or even exceeds it.

When power supply voltage level changes, the heater power can be adjusted without any considerable delay using the technique described as the first aspect of the invention. However, the internally dissipated power change will also take place and the temperature error caused by such a change will not be corrected equally fast, as it will require to be corrected by the temperature control loop, with delays associated with the thermal mass of the device and, sometimes, with the nature of the control algorithm deployed.

In order to accelerate the temperature stabilized device's response to changes in internally dissipated power resulting from supply voltage changes, in embodiments an additional circuit is introduced to subtract a small amount of power supply compensated heater control signal, with the subtracted amount corresponding to the amount of internally dissipated power. This is further explained with reference to Figures 2 and 3. In steady state, the temperature stabilized device assembly temperature is stable; its stability is achieved by heating the device assembly to compensate for the heat being lost to the ambient environment. As described above and shown in Fig. 2, the temperature stabilized device heating is done by heaters that dissipate power P _H i and by internally dissipated power P _D .

P∑ = PHI + PD (6) where P∑ is the total power heating the temperature stabilized device,

PHI is the power dissipated in the heater(s), P _D is the internally dissipated power.

In a conventional temperature stabilized device, both P _H i and P _D values are power supply voltage dependent and in case of a power supply voltage change, both P _H i and P _D will change, causing a temperature error that will be processed by the temperature control loop and minimized by it after a certain period of time due to the thermal mass of the device. ln a temperature stabilized device implemented according with the first aspect of the present invention, in case of a power supply voltage change the heater power P _H i will be maintained by the additional circuit as described in part I herein : i.e., the heater control signal SHC is power supply compensated and will be instantly adjusted to account for the supply voltage change, and the heater power will be maintained at the level required to maintain the required device temperature. However, the other component of the total power heating the temperature stabilized device - the internally dissipated power P _D - will also change following the supply voltage change, causing a temperature error that will take time to be corrected by the temperature control loop. In order to further improve the response time to supply voltage change, the heating arrangement is modified according to the second aspect of this invention and illustrated in Fig. 3.

In this arrangement, a small amount of the power supply compensated heater control signal is subtracted before the said signal is applied to the heater(s) or to the heater driver circuit, with the subtracted amount corresponding to the amount of internally dissipated power PD. This presents to the heater control loop as though it is controlling the internally dissipated power too, which allows the technique described in part I of the present invention to instantly compensate the internally dissipated power for power supply changes.

Assuming - in order to simplify the explanation of the concept - that the subtracted amount of heater control signal corresponds to the amount of internally dissipated power P _D , the total heating power in a device shown in Fig. 3 is represented by the following expression: P _∑ = (P _H2 - PD) + PD (7)

Since the total amount of power required to maintain a stable OCO's temperature is the same in arrangements shown in Figures 2 and 3, the following expression is true:

PHI + PD = (PHZ - PD) + PD (8) which means that P _H2 = PHI + PD (9)

Thus, in arrangement of Fig. 3 and according to the second aspect of this invention, subtracting a small amount of power supply compensated heater control signal before it is applied to the heater(s) or to the heater driver circuit results in an increase in the heater power requested by the control loop, and since the control loop request is generated using the technique disclosed as the first aspect of this invention, the total heating power in the temperature stabilized device is electronically compensated for power supply changes, and will react to such changes considerably faster than in a conventional temperature stabilized device.

A more detailed description of the second aspect of the present invention is presented below, as applied to an Oven-Controlled Oscillator (OCO) device. Many contemporary OCO devices comprise voltage regulator circuits in order to generate supply voltages characterized by higher stability that the unregulated externally supplied power supply voltage.

The total power P _∑ dissipated in the OCO assembly is a combination of power dissipated in the OCO's heater(s) P _H i and power P _D dissipated in all other circuits internal to the OCO: P _∑ = P _H1 + P _D

The internally dissipated power PD can be broken down into power dissipated by circuits powered from internally regulated supplies and drawing regulated supply current l _r , and power dissipated by circuits powered from the unregulated supply that draw unregulated supply current l _ur . Current l _r is not dependent on the external supply voltage V as it is drawn by circuits running off internal regulated supplies, whereas the (usually considerably smaller) current l _ur does vary with supply voltage V as it is drawn by circuitry directly connected to the external unregulated supply - hence current l _ur is shown to be a function of V in the following expression :

P _D = I _R * V + I _UR (V) * V

Similarly, the heater power can be expressed in terms of the heater current l _H c and voltage P _H1 = I _HC * V

where l _H c is generated as per the first aspect of the present invention, for example as inversely proportional to supply voltage V, and can be described by the following expression:

In the expression above is the temperature error signal, T _G is the temperature sensor gain of the error processing stage, is the power supply compensated heater control signal generated according to part I hereof, and C _G is the current gain of the heater driver circuit.

According to the second aspect of the invention, a small amount of the heater control signal is subtracted before it is applied to heater driver circuit: IHC2 — (AT * T _c * ^H /y— Ic * C _G

This extra current l _c has to be accounted for in the total power budget, so the regulated current l _r increases by l _c to become l _r i:

I _rl = I _r + I _c

The new heater power P _H 2 is represented by:

?Η2 ⁼ IHC2 * V = (ΔΓ * T _C * ) * C _c * V = AT * T _C * I _H * C _c — I _c * C _c * V and the new value of non-heater power dissipated in the device P _D i is:

P _D1 = I _rl * V + l _ur (V) * V = Or + Ic) * V + I _ur (V) * V

The new total power P _∑ is then the sum of P _H 2 and P _D i P _∑ = P _H2 + P _D1 = AT * T _C * I _H * C _C - I _C * C _C * V + (I _r + l _c ) * V + I _ur (V) * V

P∑ = PH + PDI = T * T _G * I _H * C _G + {-I _c * C _G + (I _r + l _c ) + I _ur (V)} * V P∑ = PH2 + PDI = AT * T _c * I _H * C _C + {-I _c * (Q - 1) + I _r + I _ur (V)} * V

It follows from the latter expression that, if l _c is made equal to a certain value as shown below, then the term in brackets {} tends to zero, and the total power P _∑ is largely independent of supply voltage and to a greater degree than applying the technique of part I alone. Therefore the assembly responds to supply voltage variations quicker than when allowing the thermal control loop to correct for the said variations. The optimal value of l _c required for this technique is:

₌ Or + IurQO

(C _c - 1)

For this technique to work best, current l _ur should be much less than l _r , which is usually the case in a well-designed OCO device. Also, in order to keep the total power consumption as low as possible, the current gain of the heater drive circuit C _G should be large, for example greater than 30.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described with reference to the accompanying figures in which, - FIG. 1 shows a temperature control loop in a conventional (prior art) temperature stabilized device, FIG. 2 shows heating power sources in a temperature stabilized device, FIG. 3 shows heating power sources in a temperature stabilized device incorporating the second aspect of the present invention,

FIG. 4 shows an example of an embodiment of a temperature stabilized device incorporating the first aspect of the present invention, FIG. 5 shows an example of an implementation of a power supply sensing circuit,

FIG. 6 shows a graph of temperature control gradient plotted as a function of adjustable circuit resistor value,

FIG. 7 shows an example of a circuit implementation to generate a power supply dependent heater control signal, FIG. 8 shows an embodiment of a temperature stabilized device that incorporates both the first and the second aspects of the invention,

FIG. 9 shows a way to implement the current subtraction function, FIG. 10 shows an example of a heater driver and heater circuit,

FIG. 11 shows an example of a heater driver and heater circuit with an added heater control signal subtraction circuit,

FIG. 12 shows an embodiment of a current sink module,

FIG. 13, 14, 15, and 16 illustrate performance benefits obtainable by utilizing the techniques of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 4 illustrates an embodiment of a temperature stabilized device that incorporates the first aspect of the present invention. The device assembly temperature is sensed using a temperature sensor 1 and compared with the required device assembly temperature ("setpoint") to work out the temperature error value that is the input to the error processing block 2. This block generates a heater control signal according to a chosen control algorithm as described earlier herein. In a conventional temperature stabilized device, the heater control signal would be connected to a heater(s) driver circuit, whereas in the device of the invention an additional circuit - power supply compensation circuit 3 - is introduced, in order to produce a heater control signal that is adjusted for changes in, and therefore is dependent on, the level of power supply voltage. The latter - VSUPPLY - is therefore a necessary second input to the power supply compensation block 3 as shown in Fig. 4. Thus, the resulting power supply compensated heater control signal at point 4 is a function of both the present value of temperature error and the present value of power supply voltage.

The power supply compensated heater control signal at point 4 is connected as an input to the heater(s) driver circuit 5, which drives the heater(s) 6 to control and maintain a stable temperature of the device assembly 7.

Since the heater control signal 4 is continuously adjusted for any changes in power supply level (which includes regular adjustments at very short intervals, e.g. every 30ms), such changes will be responded to by the temperature stabilized device of the invention considerably faster than in a conventional temperature stabilized device.

One way to implement the power supply compensation circuit is by implementing it as a multiplier circuit that produces a product of the signal at the output of the error processing block and a signal that linearly decreases with power supply voltage. In other words, in one implementation the output of the supply sensing circuit is multiplied by the heating demand signal, and the resulting signal is used to drive the heating devices.

An implementation of a power supply sensing circuit is shown in Fig. 5. The circuit is comprised of bipolar transistors Qref, Q0 and Ql, NMOS transistors M0 and Ml with a size ratio of M, current source Iref, and emitter resistors 0 and Rl, with currents l ₀ and li running through these resistors. V _REG , V _U RE _G and VREF are the regulated, unregulated, and reference voltages used by the circuit, whereas the IOUT is the power supply sensing signal generated by the circuit. Qref, Q0 and Ql are configured so that the reference voltage is replicated on the emitters of Q0 and Ql, essentially fixing the voltage at these points. As the unregulated supply voltage changes, the current 10 also changes. An increase in the unregulated supply voltage increases the current 10, and a decrease in unregulated supply voltage decreases the current 10. The current mirror (comprising M0 and Ml) with a current gain of M, subtracts M times the 10 current from current II. The difference is the output current signal (IOUT) representing a power supply voltage dependent signal.

The heating power can be expressed as:

Power = V _vureg x I _out In this circuit,

¼-eg— ¼-ef X

= l ₁ - M X I ₀

(¼?ureg ^— ^ref)

Io =

Ro

(Vyreg ^_ V _ref )

Ii =

R^

The heating power can now be expressed as:

The minimum sensitivity of the heating power to supply voltage variations will be achieved when dPower

= 0

dV, vureg

Since

dPower V _ref 2 X V _vure g V _re f

- M X

d yureg RQ Ri

The optimal value of M for given values of RO and RI is:

Vref Ri

M =— X ¹

Rn 2 X V _vure g _re f

Thus, by altering the value of the resistor R ₀ , it is possible to minimize the heater power supply sensitivity for any given supply voltage, as illustrated in Fig. 6 in which the temperature control gradient (in units of °C/V) is plotted as a function of the resistor value, with optimal resistor value being in the vicinity of the plot crossing the horizontal axis (i.e., where the sensitivity to power supply variations is close to zero). Alternatively, the value of M can be altered to obtain optimal power supply sensitivity for given values of R0 and RI. In embodiments of the invention values of M and R0/R1 ratios can be co- selected.

An embodiment of a full circuit implementation to generate power supply dependent heater control signal is shown in Fig. 7. This circuit is comprised of the power supply sensing circuit shown in Fig. 5, and additional circuitry to generate a product of the power supply sensing signal IOUT and the HEATER DEMAND signal (used in conjunction with the HEATER REFERENCE signal) generated by the temperature error processing circuit as described earlier. The additional circuits are comprised of NPN transistors Q2, Q3, Q10 - Q17, resistors R6 and R7, and current sources 13 and 14. In this circuit, the HEATER REFERENCE and HEATER DEMAND are fed into a differential pair Q2, Q3 with the currents set by sources 13, 14, and operating conditions set with R7, Q17 and Q18. Difference signals from Q2 and Q3 are then fed into Qll and Q10 that split the current lOUT from the supply sensing circuit depending on the difference signals. Q12 and Q13 act as a current mirror such that a difference of the current in Q10 and Qll feeds into a subsequent current mirror Q14 and Q15 generating the final output current, thus generating a power supply compensated HEATER SIGNAL to be used as the heater control signal to control the power dissipated in the device's heaters.

Fig. 8 illustrates an embodiment of a temperature stabilized device that incorporates both the first and the second aspects of the invention. Compared, and in addition, to the device structure shown in Fig. 4, an additional circuit 8 "heater control signal subtraction" is introduced to subtract a small part of the power supply compensated heater control signal. As explained earlier herein, this presents to the heater control loop as though it is controlling the internally dissipated power too, which allows the power supply compensation technique of the first aspect of the present invention to instantly or almost instantly compensate the internally dissipated power for power supply changes.

One way to implement the current subtraction function is by adding a simple programmable source that sinks a settable amount of current from the power supply compensated heater control signal, as shown in Fig. 9.

An example of a heater driver and heater circuit implementation is shown in Fig. 10; the heater control current Iheat is multiplied by a factor of 68 in the current mirror formed by the two NMOS transistors with a size ratio of 68 to 1, and the larger transistor is used as a heater for the device assembly.

An example of adding to the circuit shown in Fig. 10 a heater control signal (current) subtraction circuit is shown Fig. 11; the subtraction circuit is labelled as "Current Sink Module", in which terminal Iref accepts a reference current used by the subtraction circuit, terminal Iselect is a digital bus carrying binary signals to select the correct amount of heater control signal (current) to be subtracted, and the subtracted current flows through the la terminal into the circuit ground reference.

An example of an embodiment of the Current Sink Module is shown in Fig. 12. In this circuit the reference current Iref is mirrored from transistor Nl to transistors N2, N3, and N4 in the ratios of 1, 2, and 4 times. The switches SI, S2, and S3 are controlled by the signals of the Iselect bus to sink a binary weighted value of the reference current from 0 to 7 times through terminal la.

Figures 13 to 16 illustrate performance benefits obtainable by utilizing the techniques of the invention. Figures 13 and 14 show measured transient responses of a conventional OCXO to a power supply voltage change. Fig. 13 shows a transient output frequency change that follows a 5% power supply voltage rise; as shown therein, the frequency is destabilized as a result of the supply voltage change, with the transient disturbance taking more than 30 seconds to settle and resulting in a 12 ppb p-p frequency deviation during the transient response. A power supply voltage drop by 5% also results in a transient output frequency change of a similar magnitude and duration, as shown in Fig. 14.

An OCXO device comprising an embodiment of the invention exhibits a significantly accelerated response to step changes in power supply voltage. As shown in Fig. 15, the transient output frequency response to a rise in power supply voltage in an OCXO of the invention takes place over less than 1 second and is not accompanied by any significant frequency deviation compared to the deviations in a conventional OCXO. An OCXO device of the invention exhibits a similarly short transient response, without any significant frequency deviations, to a 5% power supply voltage drop as shown in Fig. 16.