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.
Advantages
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.
Disadvantages
1. They
are delicate and should only be used in low-power circuits.
2.
Oscillation frequency cannot be adjusted significantly.