Oscillators – Solid state active element oscillator – Transistors
Patent
1993-08-16
1994-09-13
Mis, David
Oscillators
Solid state active element oscillator
Transistors
331158, 331177R, 331183, H03B 536
Patent
active
053472373
DESCRIPTION:
BRIEF SUMMARY
This invention relates to voltage controlled oscillators and in particular to oscillators in which a piezo-electric, e.g. quartz crystal provides a frequency determining function.
BACKGROUND OF THE INVENTION
Voltage controlled oscillators are widely used in frequency reference applications such as the construction of phase locked loop devices and tuneable filters. For high quality applications where accurate frequency control is required, a piezo-electric, e.g. quartz crystal element is employed. Such crystals have very precise and stable frequencies. The disadvantage of such an arrangement is that of providing some means of frequency tuning. Conventional circuits that provide this function are somewhat complex and costly.
The object of the present invention is to minimise or to overcome this disadvantage.
SUMMARY OF THE INVENTION
According to the invention there is provided a frequency tuneable crystal oscillator arrangement, including a piezo-electric crystal, means for providing a drive current to the crystal, a reactance into which a current corresponding to the crystal element is fed whereby to develop a corresponding voltage across that reactance, and an adjustable gain amplifier for amplifying said voltage and for feeding said amplified voltage back to the crystal, wherein said reactance is arranged in series with the crystal, and wherein the reactance comprises a plurality of different impedances, there being current steering means for dividing the current corresponding to the crystal current between the impedances whereby to determine the effective value of the reactance in series with the crystal whereby to control the crystal frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described with reference to the accompanying drawings in which:
FIG. 1 shows schematically a Butler oscillator which is introduced for explanatory purposes;
FIGS. 2 and 3 illustrate the manner in which the Butler circuit of FIG. 1 is modified;
FIG. 4 illustrates one embodiment of the present invention;
FIGS. 5 and 6 show further embodiments of the invention;
FIG. 7 shows in schematic form a voltage controlled oscillator arrangement employing the crystal oscillator circuit of FIG. 4; and
FIGS. 8 to 13 illustrate the constitution of the oscillator arrangement of FIG. 7.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIG. 1, there is depicted a Butler oscillator circuit incorporating a crystal X1 connected to a pair of cross coupled field effect transistors Q11 and Q12, each having a load resistor R11, R12 coupled to its drain and a current source CS11, CS12 coupled to its source. It is a feature of this circuit that the impedance Z.sub.o presented at the crystal, when the oscillator is operating in its linear region, is substantially the difference between the source resistance of the transistors and the load impedance. ##EQU1## Referring now to FIG. 2, we have found that the balanced arrangement of the Butler configuration allows the use of current steering to vary, in a controlled manner, the impedance Z.sub.1 presented to the crystal X1. This is achieved by the introduction of a further pair of transistors Q21, Q22 each having means V21, V22 for controlling the difference between its gate voltage and that of its associated transistor Q11 or Q12 respectively. Usually V21=V22. The impedance `seen` by the crystal X.sub.1 is then given the expression ##EQU2##
It will be appreciated that gmn are each voltage dependent and to a first order approximation
This illustrates the principle whereby we employ current steering to control the reactance presented to the crystal.
Referring to FIG. 3 we have found that a major problem inherent in the Butler arrangement can be overcome. The Butler circuit can operate as a source coupled multi-vibrator with the static capacitance Co of the crystal X1 completing the feedback loop. We overcome this problem by providing a secondary feedback loop, whose gain is inverted, via a compensating capacitor C31 whose value is substantially equal to Co. If w
REFERENCES:
patent: 5034706 (1991-07-01), Betti et al.
patent: 5187450 (1993-02-01), Wagner et al.
Mis David
Northern Telecom Limited
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