Crystal Oscillators

Crystal Oscillators

Due to the variations in temperature, humidity, transistor, and circuit constants, etc. The frequency stability is usually poor in conventional radio frequency oscillators (using LC circuits). Stable oscillations are required in most applications. In such conditions,  a crystal oscillator works great. It has a high level of stability and quality. In a crystal oscillator, a resonant tuned (tank) circuit is made of a piezoelectric crystal.

Piezoelectric effect

The piezoelectric effect is a highly essential feature of quartz crystals. When an AC voltage is applied, it vibrates at the applied voltage's frequency. On the other hand, if mechanically force to vibrate, it produces an ac voltage. The piezoelectric effect may also be found in Rochelle salt and tourmaline, in addition to quartz.

The most resistant are tourmaline, even though it has the least piezoelectric activity. It is also the most costly. It is used at very high frequencies on occasion. The most piezoelectric activity is found in Rochelle salt, but it is mechanically the weakest. They are readily broken. Microphones, headsets, and loudspeakers are all made with Rochelle salts.

Quartz is a compromise between the Rochelle salt's piezoelectric activity and the tourmaline's strength. It is both affordable and abundant in nature. It is mostly utilized in RE oscillators as a crystal.

Characteristics of crystal oscillator

The crystal is appropriately cut and then placed between two metal plates for use in electronics oscillators, as illustrated in the figure. Let's observe what happens when an ac source is linked across the crystal. Because two metal plates separated by a dielectric operate as a capacitor, even when the attached crystal is not vibrating, it is comparable to a capacitance Cm.

Fig: Quartz Crystal Circuit

Mechanical vibrations are built up when an ac voltage is supplied to the crystal. The natural resonance frequency of these Equivalent circuit vibrations is determined by a variety of variables. Dimensions of the crystal, how the surface is orientated about the axis, and how the crystal is placed are some of these considerations. Although the crystal possesses an electric-mechanical resonance, an analogous electrical resonant circuit may be used to simulate it in motion, as illustrated in the figure.

L = 137H, C = 0.0235, and R = 15 K are typical values for 90 kHz. This is equal to a Q of 5500. The mounting capacitance Cm is substantially higher than the capacitance (C = 3.5pF).

When compared to a discrete LC circuit, the crystal's extraordinarily high Q-value is a standout characteristic. The use of crystals may produce Q values of almost 106, whereas a discrete LC circuit seldom exceeds 100. The extraordinarily high Q of a crystal ensures a relatively steady oscillation frequency.

There are two resonant frequencies in the crystal. First, a series resonant frequency fs, inductance L resonates with capacitance C. The series branch exhibits a parallel resonance with capacitance Cm above the frequency fs. Parallel resonant frequency (FP) is the name given to this frequency. The crystal oscillator has a capacitive reactance above this frequency. Only between the frequencies Fs and FP do the crystal act as an inductor. If the crystal is utilized as an inductor in the circuit, the oscillation frequency must be between Fs and FP. The crystal's two frequencies are temperature-dependent. It is feasible to have a frequency drift of less than 1 part in 1010 by maintaining the crystal in temperature-controlled ovens.

Transistor crystal oscillator

The circuit of a crystal oscillator is shown in the diagram below. In the collector, a tank circuit L1-C1 is installed, and the crystal is installed in the base circuit. Coil L2, which is inductively connected to coil L1, provides feedback. The feedback winding is linked in series with the crystal. The natural frequency of the LC circuit is roughly equivalent to the crystal's natural frequency.

Fig: Crystal Oscillator Circuit 

Capacitor C1 will charge when the power is switched on. Oscillations are created as the capacitor discharges. The crystal oscillator produces oscillations as a result of the positive feedback. The frequency of oscillation in the circuit is controlled by the crystal. Because the crystal is connected to the base circuit, it has a far greater impact on the frequency of the circuit than the LC circuit. As a result, the entire circuit oscillates at the crystal's inherent frequency, causing the circuit to create a resonant frequency.


1. Because the crystal's frequency is independent of temperature, these oscillators have a high degree of frequency stability.

2. It can create high-frequency oscillations.


1. They are delicate and should only be used in low-power circuits.

2. Oscillation frequency cannot be adjusted significantly.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

Post a Comment

Previous Post Next Post