Crystal oscillator with variable-gain and variable-output-impedance inverter system

A crystal oscillator with variable gain and variable output impedance inverter system includes an inverter, a variable impedance feedback circuit, connected between the output and input of the inverter, a crystal oscillator system, having a crystal with first and second electrodes connected across the input and output of the inverter; a serial variable impedance circuit connected between the inverter output and an electrode of the crystal and a control circuit for temporarily, during start up mode, increasing the impedance of the feedback circuit and decreasing the impedance of the serial circuit relative to the stationary mode impedances and then returning the feedback impedance to the lower impedance level and the serial circuit to the higher impedance level that promotes high frequency stability of the oscillator in the normal, stationary mode, of operation.

FIELD OF THE INVENTION

This invention relates to an improved oscillator with a variable gain and variable output impedance inverter system which provides both very reliable start-up and high frequency stability during stationary operations.

BACKGROUND OF THE INVENTION

Contemporary microcontrollers and processors typically contain an inverter-amplifier that is suitable for use as a part of feedback oscillator when it is connected to an external crystal and some other components. A typical crystal oscillator system includes an inverter amplifier with two extra resistances Rf and Rs. Feedback resistance Rf is connected between the input and the output of the inverter/amplifier and provides proper DC biasing and negative feedback. Serial limiting resistance Rs is connected from the output of inverter-amplifier to one of the crystal pins and ensures proper output impedance and power for the crystal oscillator system. The other components of the crystal oscillator system are capacitors CL1and CL2connecting the electrodes of the crystal to ground (GND). The values of CL1and CL2are determined by the manufacturer's specifications according to the intended use of the crystal. There is a limited choice of component selection to provide specified crystal oscillator characteristics. It is basically limited to the choice of values of Rf and Rs. A desired characteristic of crystal oscillator systems is frequency stability in the specified range of operating voltages, temperatures and variations of parameters of crystal and inverter/amplifier. Another important characteristic of crystal oscillator systems is the ability for fast oscillation start up on power up of supply voltage. Yet another important characteristic of a crystal oscillator system is a drive level compatible with the specified power dissipation in the crystal. Usually crystal manufacturers give the operating voltage drive levels of the crystal in microwatts for specified crystal long and short term stability. Drive levels of the crystal directly effect the stability of the crystal oscillator system: frequency stable crystal oscillator system should have a drive voltage level no more than the specified drive voltage level. Unfortunately the values of Rf and Rs are very difficult to chose to satisfy all main crystal oscillator characteristics: for better frequency stability the value of Rf needs to be small and the value of Rs needs to be high; for reliable oscillation start-up the value of Rf needs to be high and the value of Rs needs to be small. That forces a compromise between the values of Rf and Rs to partly satisfy both frequency stability and reliable start-up of the crystal oscillator and results in a non-optimal operation of the crystal oscillator. The problem is exacerbated for so called “low power” crystal oscillator systems which have very small size and relatively low price compared with so called “high power” crystal oscillator systems. The majority of existing processors have been designed for “high power” crystals which make it more difficult to use them with “low power” crystals.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an improved crystal oscillator which provides both very reliable start up and high frequency stability during stationary operation.

It is a further object of this invention to provide such an improved crystal oscillator with a variable gain and variable output impedance inverter.

It is a further object of this invention to provide such an improved crystal oscillator with a variable gain and variable output impedance inverter system which optimizes operation in both the start up and stationary modes.

It is a further object of this invention to provide such an improved crystal oscillator with a variable gain and variable output impedance inverter system which provides low feedback impedance and high series impedance for stationary mode operation and high feedback impedance and low series impedance, temporarily, during start up mode.

The invention results from the realization that an improved crystal oscillator which provides both very reliable start up as well as high frequency stability during stationary operation can be achieved using a control circuit to temporarily, during start up mode, increase the impedance of the feedback circuit and decrease the impedance of the serial circuit to provide fast start up, then return the feedback impedance to the lower impedance level and the serial circuit to the higher impedance level that promotes high frequency stability of the oscillator in the normal, stationary mode of operation.

This invention features a crystal oscillator with a variable gain and variable output impedance inverter system. There is an inverter and variable impedance feedback circuit connected between the output and input of the inverter. The crystal oscillator system includes a crystal having first and second electrodes connected across the input and output of the inverter. A serial variable impedance circuit is connected between the inverter output and an electrode of the crystal. A control circuit temporarily, during start up mode, increases the impedance of the feedback circuit and decreases the impedance of the serial circuit relative to the stationary mode impedances.

In a preferred embodiment the impedances may be resistances. The feedback impedance circuit may include first and second resistances in series and a first switch shunting one of the resistances. The serial impedance circuit may include at least a third resistance and a second switch shunting the third resistance. There may be a fourth resistance in series with the third resistance. The first and second resistances may be equal and the third and fourth resistances may be equal. The crystal system may include a load capacitor connected from each of the crystal electrodes to ground. The control circuit may include a comparator responsive to the crystal oscillator voltage for temporarily increasing the impedance of the feedback circuit and decreasing the impedance of the serial circuit during start up mode until the crystal oscillator voltage reaches a predetermined reference level. The reference level may be approximately one half or more of the stationary mode crystal oscillator voltage. The control circuit may include a timer for temporarily increasing the impedance of the feedback circuit and decreasing the impedance of the serial circuit during start up mode for a period of approximately 500 cycles of the crystal oscillator frequency. The control circuit may include a comparator responsive to the crystal oscillator voltage for temporarily operating the switches during start up mode until the crystal oscillator voltage reaches a predetermined reference level. The control circuit may include a timer for temporarily operating the switches during start up mode for approximately 500 cycles of the crystal oscillator frequency.

DETAILED DESCRIPTION OF THE INVENTION

There is shown inFIG. 1a prior art crystal oscillator10including an inverter amplifier12, with a feedback resistor14(Rf) connected between the output16and input18of inverter12. A crystal system20including crystal22has its electrode24connected to input18of inverter12and has its electrode26connected through serial resistance28(Rs) to output16. Crystal system20also includes load capacitors30and32which contribute to the tank circuit of crystal22. The values of capacitors30and32are specified by the crystal manufacturer according to the intended use of crystal22. Resistor14provides DC biasing of inverter12and also provides negative local feedback for inverter12: this determines amplifier gain and tolerance of the inverter to changes in supply voltage and other environmental conditions. Serial resistance28stabilizes the output voltage of inverter amplifier12and is used to control the crystal drive level which is determined as the amount of AC energy supplied by the amplifier to the crystal system20. Good frequency stability in the stationary mode of operation is provided by the minimum required level of drive voltage and minimal coupling of the crystal system20to inverter amplifier12. This in turn requires a relatively small value of feedback resistance14and a relatively large value of the serial resistance28. These conditions provide long term reliability and lower power consumption which is especially important when the so called “low power” miniaturized quartz crystals are used. In contrast reliable start up of oscillator10requires relatively large values of feedback resistance14and relatively small values of resistance28. Thus the optimal values for resistances14and28are opposite for the different modes of the crystal oscillator10operation. That is, the stationary mode and the start up mode. This forces the design to compromise the feedback resistance Rf14and the serial resistance Rs28values to at least partly satisfy the requirements for stability and reliable start up while optimizing neither.

In accordance with this invention crystal oscillator10a,FIG. 2, includes a feedback circuit14awhich has variable impedance and a serial circuit28awhich includes variable impedance. In addition, there is a control circuit40which may include either a timer42or a comparator44. Control circuit40operates feedback circuit14aand serial circuit28a, so that it maintains feedback impedance circuit14aat a low impedance and serial impedance circuit28aat a high impedance in the normal, stationary, mode of operation, but temporarily, during start up mode, increases the impedance of the feedback circuit and decreases the impedance of the serial circuit, relative to the stationary mode impedances, to ensure a fast, reliable start up and then returns the feedback impedance to the lower impedance level and the serial circuit to the higher impedance level that promotes the high frequency stability of the oscillator in the normal stationary mode of operation. Control circuit40may use either timer42or comparator44to accomplish this. Comparator44compares, for example, the voltage on crystal system22a, Vosc, to a reference voltage which is generally approximately one half of the voltage across the oscillator in stationary mode. When Vosc reaches the reference level comparator44temporarily shifts the impedances in feedback circuit14aand serial circuit28a. Alternatively, timer42may be used for the same purpose. Timer42may temporarily adjust the impedances in the same manner for a period of approximately 500 cycles of the crystal oscillator fundamental frequency. It is understood that at approximately 1,000 cycles of operation at the fundamental frequency the start up is virtually complete, thus at 500 cycles the start up mode is well underway and there is no further need to continue the start up mode boost as the start up will continue properly from that point.

In one embodiment, a crystal oscillator10b,FIG. 3, according to this invention may have a variable impedance feedback circuit14b, which includes two resistances14bb(Rf1) and14bbb(Rf2) with a switch50shunting resistance14bbband the variable serial impedance28b(Rs1) may include a single resistance28bbaccompanied by a shunt switch52so that when shunt switch52is closed there is no resistance in that line. Or there may be a second resistance (Rs2)28bbbso that even when shunt switch52is closed there remains the resistance28bbb. Typically, but in no way limiting, the resistance14bbmay be 1 meg Ω. Resistance14bbbmay be, again without limiting the invention, a different value or the same 1 M Ω value. Similarly, with the serial variable impedance circuit28bresistance28bbmay be 1 k Ω for example and if there is a second resistance28bbbit may be a different value or it may be 1 k Ω. Typically, capacitors30band32bwill be between 10 and 30 ρf depending upon the frequency and other parameters. The supply voltage Vcc can be 3.3 volts. Typically the output16bwill be approximately 90% of that or 2.97 volts in stationary mode and so the oscillator voltage will be about 70% of that or 2.3 volts. In that case the reference voltage at comparator44bwill be roughly half of that at or 2.3 volt/2=1.15 volts.

In operation in the stationary mode when switch50is closed switch52is open. Resistance14bbcorresponds to low amplifier gain and high band width with low nonlinear distortion and high immunity to supply voltage variation and temperature changes, all of which serve to provide high frequency stability. Serial impedance circuit28bhas an impedance equal to the resistance of resistance28bplus28bb; this corresponds to low a low drive level for the crystal and further helps to provide high frequency stability. In contrast, at the start up mode, switch50is open and switch52is closed. Now the feedback impedance circuit has an impedance of resistance14bbplus14bbbwhich corresponds to high amplifier gain and helps to provide a reliable start up of the oscillator despite a number of different crystal parameters. The impedance of the serial impedance circuit now is only that of resistance28bbb, if in fact there is one, otherwise the impedance is zero or very nearly zero. The value of zero or the value of resistance28bbb, for example, 1 k Ω corresponds to high drive level of crystal and helps to provide better start up conditions.