1. Field of Invention
This invention relates to circuits for electronic timepieces and in particular to circuits for electronic timepieces which are adjustable.
2. Description of the Prior Art
Although the high accuracy and excellent stability of electronic timepieces, especially those with quartz crystal oscillators, it is generally recognized, their popularity has not been up to expectations. The underlying reason for this has been their high cost as timepieces and problems encountered in the mass production of the quartz crystal oscillator and other segments of the electronic timepiece.
Although high precision of individual parts is a necessary condition for mass production of quartz crystal timepieces, discrepancies must inherently be allowed to exist to a certain extent. For instance, it is necessary to provide some means of adjusting for a combination of a quartz crystal oscillator having a certain natural frequency and its oscillator circuit. Frequently a trimmer condenser or the like is presently used for this type of adjustment. However, during the manufacturing process, crystals that have a natural frequency which is unadjustable by means of a trimer condenser or like means must be discarded. Furthermore, according to the prior art the frequency of the oscillator circuit has been continuously adjusted by altering the capacitances of trimer condensers and thus changing the value of the perameter of the feedback element of the oscillator circuit that contains the quartz crystal oscillator.
However, this method of frequency adjustment by means of a trimer capacitor has the following shortcomings:
(1) The adjustable range of the frequency is limited as a result of the constraints of the natural frequency and the quality factor of the quartz crystal oscillator;
(2) Since the trimer condenser is bulky and expensive, it is not a suitable component for a wrist watch; and
(3) The use of an adjustable component which has poor long term stability results in adverse effects upon the precision of the timepiece as a result of temperature changes and aging of the component.
In veiw of these defects, so-called digital frequency adjustment has appeared in practice. Digital frequency adjustment usually takes the form of a conventional frequency adjustment method via a trimer condenser or independent adjustment in which the division ratio of the divider circuit is changed by a combination of integrated circuits. However, the digital frequency adjustments currently practiced are of the so-called unit directional adjustment type in which the divider is equiped with either an adder or a subtractor circuit, and unlike the trimer capacitor frequency adjustment method, the frequency of the quartz crystal oscillator circuit is changed from its natural value either to a lower value or to a higher value but not both. This system has shortcomings such as a limitation in flexibility when a quartz crystal oscillator is processed for frequency adjustments and a difficulty in coping with the frequency change due to aging of the quartz crystal oscillator.