GB2248353A

GB2248353A - Frequency reference circuit

Info

Publication number: GB2248353A
Application number: GB9021259A
Authority: GB
Inventors: Roger James Williamson; Andrew Llewellyn Miles
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1990-09-29
Filing date: 1990-09-29
Publication date: 1992-04-01
Anticipated expiration: 2010-09-29
Also published as: GB9021259D0; GB2248353B

Abstract

A quartz crystal controlled frequency reference circuit has phase lock loop means for comparison of a pair of harmonic frequencies whereby to provide a temperature characteristic signal. Each harmonic is abstracted via a respective phase locked loop. This overcomes the stability problems associated with conventional circuits requiring filters. <IMAGE>

Description

FREQUENCY REFERENCE CIRCUIT This invention relates to frequency reference circuits, and in particular to circuits in which the frequency maintaining element is a piezoelectric resonator.

High quality close tolerance piezoelectric, e.g. quartz crystal, resonators are used in a variety of applications requiring an accurate reference frequency signal. A particular problem with piezoelectric resonators is that of change of frequency with changes in temperature. Although this frequency change is relatively small it is significant, e.g. in frequency synthesis applications, where the highest accuracy is required. In an attempt to overcome this problem various techniques have been devised for determining the resonator temperature by frequency comparison of two or more vibrational modes. For example, US Patent No.

4,872,765 describes a technique in which a pair of vibrational harmonics or overtones are compared to give a measure of the resonator temperature. Such a technique requires some means of characterising the two harmonic frequencies from the resonating crystal.

Conventionally this is effected by use of LC filters each tuned to a corresponding harmonic frequency. The two harmonic signals are then amplified and mixed to give a beat frequency signal indicative of the crystal temperature. It has been found however that the frequency selective nature of the filters required to abstract the harmonic signal has a detrimental effect on the stability of the resonator. Specifically, uncertainties in the filter circuit component values combine with the resonator equivalent circuit resulting in a resonant frequency that is no longer determined entirely by the crystal parameters. This limits the accuracy that may be achieved An object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a frequency reference oscillator, including a piezoelectric crystal resonator, means for driving the resonator in a resonant mode including harmonics of that mode, first and second phase locked loops associated with the resonator amd each adapted to oscillate at a respective harmonic frequency and means for comparing the frequencies of the phase locked loops whereby to derive a measure of the oscillator temperature.

According to the invention there is further provided a frequency reference oscillator having means for temperature compensation of its output frequency, the oscillator including a quartz crystal resonator, means for driving the resonator in a resonant mode including harmonics of that mode, first and .seco-nd phase locked loops each comprising a voltage controlled oscillator and a phase comparator and each tuned to a respective harmonic frequency whereby in use to lock to said harmonic frequency, means for comparing the frequencies of the phase locked loops whereby to derive a measure of the oscillator temperature, and means responsive to said temperature measure for comparison of said output frequency.

The technique allows the filtering required to separate the harmonics to be accomplished at effectively zero Q factor and thus with substantially no contribution to frequency error. The frequency selectively is effected by control of the voltage controlled oscillator of each loop. As the system approaches equilibrium, the oscillator control voltage assumed a constant value. Thus any phase or frequency variability tends to zero.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a part-schematic diagram of a dual mode frequency reference circuit; and Fig 2 shows one form of phase discriminator for use in the oscillator circuit of Fig. 1.

Referring to Fig. 1, the circuit includes a piezoelectric resonator Q1, e.g. a quartz crystal resonator, coupled in the emitter circuit of a bipolar pnp transistor TR1. Feedback for maintaining oscillation of the resonator is provided by a pair of phase locked loops coupled between the collector and base of the transistor. Each loop comprises a voltage controlled oscillator VCO1, VC02 and a phase sensitive detector or discriminator PSD1, PSD2. In the circuit of Fig. 1 each phase-locked loop oscillates at the respective harmonic frequency, the resonator being employed as a frequency/phase discriminator element.

Oscillation of the resonator Q1 is maintained via a Q-boost circuit GB1 coupled in the feedback path between the collector and base of the transistor TR1, In a typical arrangement one phase-backed loop runs at about three times the frequency of the other loop. It will be appreciated that, as the crystal Q1 is a high Q device, unless the voltage controlled oscillator frequency of each loop is within a few kilohertz of the respective crystal harmonic frequency, that loop is unlikely to lock and will thus 'freewheel' at some intermediate frequency. This problem is overcome by restricting the range of each voltage controlled oscillator.In the arrangement of Fig. 1, each voltage controlled oscillator is provided with a respective frequency controlling crystal Qll, Q21 whereby the frequency range of that oscillator is restricted about the appropriate harmonic frequency of the crystal Q1. Other techniques of restricting the oscillator frequency ranges will be apparent to those skilled in the art. For example, when the circuit arrangement is employed in a microprocessor controlled frequency synthesiser, the microprocessor may be programmed to apply a range voltage to each voltage controlled oscillator to sweep the frequency of that oscillator upwards until the harmonic frequency is detected In use, each phase locked loop locks on to the corresponding harmonic.Determination of the resonator temperature may be effected by comparison of the two loop frequencies to derive a difference signal the frequency of which is temperature dependent. When the frequency reference circuit is employed in the construction of a frequency synthesiser, the t-emperature dependent signal may be employed to control or adjust the frequency division algorithm such that the output synthesised frequency remains at a constant, temperature independent, value.

Referring now to Fig. 2, this illustrates an oscillator and an associated phase discriminator for use in the circuit of Fig. 1. The figure shows the oscillator coupled to the phase discriminator circuit of one of the two phase cked loops. For clarity the voltage controlled oscillator is omitted from Fig. 2.

The current through the drive transistor TR21 is mirrored via transistors TR31 and TR32, the latter being coupled to the tail circuit of a transistor long-tailed prior structure comprising transistors TR33 and TR 34.

One transistor (TR33) of the pair is supplied with a constant reference voltage via transistors TR35 and TR36, while the other transistor (TR34) of the pair is fed with the output signal from the voltage controlled oscillator (VCO). The phase relationship between the VCO signal and the resonator signal determines the currents flowing through the two transistors of the long-tailed pair. This in turn provides an output control voltage for the VCO whereby the oscillator is phase locked to the resonator harmonic frequency.

The phase discriminator and the crystal Q10 are driven with the same frequency, i.e. the crystal frequency, but in phase quadrature. These drive signals are referred to as I-excitation and Q-excitation in Fig.

2, the crystal Q1 being driven via the I-excitation signal. Transistor TR30, resistor R3Oand capacitor C30 may be incorporated in the oscillator circuit to provide ESR connection.

The phase difference between the crystal current and the I-excitation varies rapidly with frequency. This current appears in the collector of transistor TR21 and is mirrored in transistor TR32-. The current through transistor TR32 is directed alternately to the collectors of transistors TR33 and TR34 by the Q-excitation signal. This process alternately changes and discharges the loop filter capacitor C30 by small amounts. When the crystal current signal is exactly in phase with the I-signal, and hence exactly in quadrature with the Q-signal, the charge and discharge currents of the capacitor C30 become identical thus resulting in a steady VCO output control voltage. If the phase difference alters from quadrature with the Q-signal, charge starts to accumulate on the capacitor C30 thus charging the VCO control voltage until the frequency of the phase locked loop has been corrected sufficiently to restore the precise quadrature phase relationship.

It will be appreciated that whilst the above circuits have been described with particular reference to use in a frequency synthesiser, they are by no means limited to that particular application.

Claims

1. A frequency reference oscillator, including a piezoelectric crystal resonator, means for driving the resonator in a resonant mode including harmonics of that mode,-first and second phase locked loops associated with the resonator amd each adapted to oscillate at a respective harmonic frequency and means for comparing the frequencies of the phase locked loops whereby to derive a measure of the oscillator temperature.

2. A frequency reference oscillator having means for temperature compensation of its output frequency, the oscillator including a quartz crystal resonator, means for driving the resonator in a resonant mode including harmonics of that mode, first and second phase locked loops each comprising a voltage controlled oscillator and a phase comparator and each tuned to a respective harmonic frequency whereby in use to lock to said harmonic frequency, means for comparing the frequencies of the phase locked loops whereby to derive a measure of the oscillator temperature, and means responsive to said temperature measure for comparison of said output frequency.

3. A frequency reference oscillator as claimed in claim 2 wherein the frequency range of each said voltage controlled oscillator is restricted substantially to the frequency of the corresponding harmonic mode.

4. A frequency reference oscillator as claimed in claim 3, wherein the frequency range of each said voltage controlled oscillator is determined by a crystal resonator tuned to the respective harmonic frequency.

5. A frequency reference oscillator substantially as described herein with reference to and as shown in the accompanying drawings.

6 A frequency synthesiser incorporating a frequency reference oscillator as claimed in any one of claims 1 to 5.