Piezoelectric oscillator for ultrasonic waves

Piezoelectric effect

Certain crystals (quartz, Rochelle salt, tourmaline) develop opposite electric charges on opposite faces when they are compressed or stretched along a certain direction (direct effect). Conversely, when an alternating electric field is applied across them, the crystal alternately expands and contracts (inverse effect). If the frequency of the field equals the natural frequency of the crystal, it vibrates with large amplitude (resonance) and produces ultrasonic waves.

Natural frequency of a quartz plate of thickness t: f = (n/2t) √(Y/ρ), n = 1, 2, 3 …, where Y is Young's modulus and ρ the density. A thin plate gives a high frequency.

Piezoelectric oscillator (construction)

A quartz plate is cut perpendicular to the electric (X) axis and held between two metal plates, forming a capacitor C1. This is connected in parallel with the coil L1 in the collector (tank) circuit of a tuned-collector transistor (or valve) oscillator. The coil L2 in the base circuit is coupled to L1 for positive feedback.

  +Vcc
    |
    +--- L1 ---+--- C (variable) ---+
    |          |                    |
    |      collector           [quartz between
 transistor                     two metal plates, C1]
    |          |                    |
  base <-- L2 (coupled to L1)       |
    |__________________________ ____|

Tank circuit = L1 in parallel with the variable capacitor C and the quartz capacitor C1.

Working

1. When the supply is switched on, the L1-C tank circuit oscillates at f = 1/(2π√(L1C)). L2, coupled to L1 by mutual induction, feeds back energy to the base and keeps the oscillation going. Since the crystal plates are connected across L1, an alternating voltage of the same frequency is applied to the crystal.
2. The crystal contracts and expands at this frequency. C is varied until the tank frequency equals the natural frequency of the crystal; resonance then occurs and the crystal vibrates with maximum amplitude.
3. The vibrating faces send ultrasonic waves of the same frequency into the surrounding medium (air or oil).
4. Feedback keeps the oscillation steady. Frequencies range from about 20 kHz up to several MHz.

Advantages: stable frequency, easy to adjust, high-frequency output. Used for sonar, flaw detection and ultrasonic cleaning.