For MHz oscillating crystals, the so-called "fundamental oscillation" (fundamental oscillation) or a harmonic (usually the third or fifth harmonic) is used. The decisive factor here is the mechanical mode of vibration of the quartz crystal, which is based on the piezoelectric effect of quartz. In the MHz range, the fundamental mode (available from PETERMANN-TECHNIK GmbH up to 285 MHz) is typically used in thethickness-shear mode.
Oscillation mode in the MHz range: Thickness shear oscillator (planar shear mode)
What happens in the process?
The quartz crystal is ground and cut (usually AT cut) in such a way that it oscillates in a certain thrust mode when an alternating voltage in the MHz range is applied. The quartz does not move in the longitudinal direction or transversely to the surface, but the oscillation takes place in the plane of the crystal, i.e. like a lateral "shearing".
The resulting movement is reminiscent of two plates lying on top of each other and being displaced against each other - this is the "shearing" movement.
How it works in detail
- AT cut:
- The quartz crystal is cut at a specific angle to the crystal axis (approximately 35°15' to the Z-axis).
- This special cut produces the desired oscillation shape (shear mode) and ensures temperature stability in the MHz range.
- Application of an alternating voltage:
- Electrodes on both sides of the quartz generate an electric field through the crystal.
- Due to the piezoelectric effect, the crystal deforms mechanically (it begins to vibrate) as soon as a voltage is applied.
- The mechanical oscillation in turn generates an electrical voltage - a self-reinforcing effect at the resonance frequency.
- Resonance in the MHz range:
- The thickness of the quartz crystal determines the resonance frequency (at 10 MHz, for example, the thickness is approx. 0.33 mm).
- The fundamental frequency is the lowest possible natural frequency at which the quartz resonates in shear mode.
Why this particular form of vibration?
Reason | Explanation |
Good frequency stability | AT section has a flat temperature frequency behaviour at room temperature up to approx. 70 °C. |
Efficient energy transfer | The shear mode couples well with the electric field and loses little energy. |
Low attenuation in the MHz range | The oscillation is mechanically stable and has a high quality factor (Q factor). |
Small size possible | The thickness-frequency relationship allows high frequencies to be realised with thin quartz. |
Some crystals in the higher MHz range (e.g. 30 MHz, 50 MHz) use the 3rd or 5th harmonic of the same oscillation mode. They therefore oscillate at a multiple of the fundamental frequency.
Why?
- The manufacture of very thin quartz crystals for high fundamental frequencies is mechanically difficult.
- Instead, thicker crystals are used on a higher harmonic - easier to manufacture, but requiring special circuits.
Conclusion
In the MHz range , the fundamental oscillation is used in the thickness shear oscillation mode because it:
high frequency stability,
good quality (low losses),
insensitivity to temperature,
and efficient electrical coupling.
These properties make it ideal for precise clock generators, which are used today in all applications in the electronics sector.
A note on our own behalf: Due to our fundamental resonator designs, we can supply fundamental frequencies of up to 285 MHz - see here: https://www.petermann-technik.de/produkte/quarz-konfigurator.html.
