Oscillators – Electromechanical resonator – Crystal
Patent
1989-02-10
1990-08-14
Pascal, Robert J.
Oscillators
Electromechanical resonator
Crystal
331176, H03B 532
Patent
active
049490559
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The invention relates to a crystal oscillator compensation circuit comprising an oscillator crystal, briefly called crystal hereinafter, which is excited into oscillation by a crystal oscillator circuit, and an adjustable capacitance for adjusting the frequency of the oscillator.
2. Prior Art
Once a crystal oscillates at a particular frequency due to predetermined dimensions, a predetermined direction of cut and a predetermined control voltage across the adjustable capacitance, this frequency will only be held for a certain temperature and only for a certain time.
To compensate for effects which change the frequency, various compensation circuits have become known.
From GB-A-No. 2 095 005, a crystal oscillator compensation circuit is known, comprising an oscillator crystal which is excited into oscillation by a crystal oscillator circuit, and with an adjustable capacitance for adjusting the frequency of the crystal. This is a crystal oscillating at a relatively low frequency such as is used in clocks. To compensate for the effect of aging of the crystal on the frequency, a reference oscillator circuit comprising a quartz crystal oscillating at a high frequency (for example AT cut) is also provided. The lower frequency is continuously compared with a partvalue of the high frequency, if deviations are found, the frequency of the oscillator crystal is changed by the adjustable capacitance in such a manner that the measured difference becomes zero.
The said circuit only functions if the reference quartz crystal does not age significantly. To achieve this, such a reference crystal must be artificially aged at relatively high temperatures. To keep aging effects of crystals oscillating at a high frequency as low as possible, it is known to age crystals for a relatively long time, for example for some months, at 80 to 90 degrees Celsius. The crystal properties change only little with the course of time after such a treatment.
A circuit for compensating temperature effects instead of aging effects is known from GB-A-No. 2 064 248. The circuit also exhibits an oscillator crystal and an adjustable capacitance for adjusting the frequency of the crystal. In addition, there is a store in which, for a large number of different temperatures, compensation values are recorded which have been determined for each individual crystal in a calibration process in such a manner that the frequency remained unchanged when they were applied. Such a calibration process is extremely time consuming since it is necessary to wait for thermal equilibrium for each of the many calibration steps.
A circuit for compensating acceleration effects is known from U.S. Pat. No. 4 318 063. Apart from the oscillator crystal and the adjustable capacitance for adjusting the frequency of the crystal, this exhibits an acceleration sensor and a logic circuit which, in accordance with a known law relating to the relationship between acceleration and frequency change, calculates a compensation value which is intended to cancel the frequency change occurring during the measured acceleration. The same equation is used for all crystals, that is to say no calibration for each individual crystal is performed as in the circuit for temperature compensation described above.
The above shows clearly that a reliable compensation circuit is lacking, particularly with respect to aging effects. As described, the known compensation circuit is only capable of compensating for aging effects of a slowly oscillating crystal within the extent of those aging effects to which a rapidly oscillating crystal is subject. It is accordingly not possible to compensate for aging effects of the rapidly oscillating crystal.
The invention is based on the object of specifying a crystal oscillator compensation circuit which is capable of reliably compensating for aging effects.
REPRESENTATION OF THE INVENTION
The circuit according to the invention exhibits, like known circuits, an oscillator crystal which is excited into oscillation by a crystal oscillator c
REFERENCES:
patent: 4318063 (1982-03-01), Przyjemski
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