Mode coupled tuning fork type quartz crystal vibrator and method of tuning

A mode coupled quartz crystal vibrator coupling two different modes of vibration, comprising:

• a quartz crystal plate cut in the shape of a tuning fork having a base region and two coplanar tuning fork arms extending from the base for vibrating in a first flexural mode of vibration (F.sub.1) and a secondary torsional mode of vibration (T.sub.0), said vibrator utilizing the first overtone of the flexural mode and the fundamental vibration of the torsional mode and having excitation electrodes on both principal surfaces of said plate;
• first adjust electrode means for adjusting f.sub.F, the resonant frequency of the main vibration;
• and
• one of second adjust electrode means for adjuting .DELTA.f, the difference between the resonant frequency of the main vibration f.sub.F and the resonant frequency of the secondary vibration f.sub.T designated .DELTA.f=f.sub.F -f.sub.T and third adjust electrode means for adjusting R=f.sub.T /f.sub.F, wherein f.sub.F and f.sub.T are the resonant frequencies at 20.degree. C., for adjusting the frequency-temperature characteristics of the vibrator,
• said first electrode means and one of said second and third adjust electrode means are formed in the region near the top of the tuning fork arms.

A method for forming a mode coupled tuning fork type quartz crystal vibrator having two opposed principal planar surfaces, said vibrator coupling two different modes of vibration, the main vibration being the flexural mode (F.sub.1) with the resonant frequency of the first overtone of the flexural mode being designated f.sub.F, and the sub-vibrational mode being the torsional mode (T.sub.0), the resonant frequency of the fundamental vibration of the torsional mode being designated f.sub.T, comprising:

• forming first adjust electrode means for adjusting f.sub.F by disposing electrodes having a substantially triangular shape for finely adjusting f.sub.F and substantially rectangular electrodes roughly adjusting f.sub.F at the region of the top of the tuning fork arms on both principal surfaces of each arm;
• forming second adjust electrode means for adjusting one of .DELTA.f=f.sub.F -f.sub.T and R=f.sub.F /f.sub.T, (f.sub.F and f.sub.T being the resonant frequencies at 20.degree. C.), by disposing substantially rectangular electrodes at the region of the top of the tuning fork arms on both principal surfaces of each arm,
• roughly adjusting f.sub.F by one of the steps of removing a strip of electrode material from and adding a strip of metal to each rectangular electrode for roughly adjusting f.sub.F to a value of about 100 ppm lower than a preselected frequency f.sub.F0 ;
• adjusting one of .DELTA.f and R by one of the steps of removing a strip of electrode material from and adding a strip of metal to each rectangular electrode for adjusting .DELTA.f so that .DELTA.f is adjusted to a preselected value .DELTA.f.sub.0 ;
• and
• finely adjusting f.sub.F to the preselected value f.sub.F0 and at the same time maintaining .DELTA.f at the preselected value .DELTA.f.sub.0 by one of the steps of removing a strip of electrode material from and adding a strip of electrode material to each triangular electrode for finely adjusting f.sub.F.