LOGO

There are some electronic devices that require a highly stable frequency of AC signals, and LC oscillators have poor stability and are prone to frequency drift
(that is, the generated AC signal frequency is easy to change). 

The use of a special component in the oscillator - quartz crystal, can produce a highly stable signal, the use of quartz crystal oscillator called crystal oscillator.

The crystal oscillator has the piezoelectric effect, that is, the crystal will be deformed after applying voltage to the two poles of the wafer, and in turn,
if the external force causes the wafer to be deformed, the metal sheet on the two poles will produce voltage. If the appropriate alternating voltage is
added to the chip, the chip will produce resonance (the resonance frequency is related to the inclination Angle of the quartz slope, and the frequency
is certain). Crystal oscillator uses a kind of crystal that can convert electrical and mechanical energy into each other, and works in a resonant state to
provide stable and accurate single-frequency oscillation. 

Under normal operating conditions, the absolute accuracy of the ordinary crystal frequency can reach 50 parts per million. Using this characteristic,
the crystal oscillator can provide a more stable pulse, which is widely used in the clock circuit of microchips. The chip is mostly made of quartz
semiconductor material, and the shell is packaged with metal.

The crystal oscillator is often used in connection with the main board, south bridge, sound card and other circuits. Crystal oscillator can be likened
to the "heartbeat" generator of each board, if the "heartbeat" of the main card has a problem, it will make the other circuits fail.

1. Parallel crystal oscillator

The parallel crystal oscillator is shown in Figure 5. The triode VT and R1, R2, R3, R4 constitute the amplifier circuit; C3 is an AC bypass capacitor, which
is equivalent to a short circuit for AC signals. X1 is a quartz crystal that acts as an inductor in a circuit. It can be seen from the AC equivalent diagram that
the circuit is a capacitor three-point oscillator, C1, C2, X1 constitute a frequency selection circuit, the selection frequency is mainly determined by X1,
the frequency is close to fp.

Circuit oscillation process: After the power supply is turned on, the triode VT is switched on, and the changing Ic current flows through the VT, which contains
weak signals of various frequencies from 0 to ∞. These signals are added to the frequency selection circuit consisting of C1, C2 and X1, and the frequency
selection circuit selects f0 signal from it. There is f0 signal voltage at both ends of X1, C1 and C2, and the f0 signal voltage at both ends of C2 is fed back
to the base-emitter of VT for amplification. 

After amplification, the output signal is added to the frequency selection circuit, and the signal voltage at both ends of C1 and C2 increases. The voltage at
both ends of C2 is sent to the VT base-emitter, so repeated, the VT output signal is getting larger and larger, and the amplification of the VT amplifier circuit
gradually decreases, when the amplification of the amplifier circuit is equal to the attenuation factor of the feedback circuit, the output signal amplitude
remains stable, will not increase, and the signal is sent to other circuits.

2. Series crystal oscillator

The series crystal oscillator is shown in Figure 6. The oscillator adopts a two-stage amplifier circuit, quartz crystal X1 in addition to forming a feedback circuit,
but also has a frequency selection function, the selection frequency f0=fs, potentiometer RP1 used to adjust the amplitude of the feedback signal.

(1) Determine the type of feedback circuit

Since the signal is fed back to the VT1 emitter, it is assumed that the instantaneous polarity of the VT1 emitter voltage is "+", the collector voltage polarity
is "+" (the emitter and collector are in phase relationship, and the collector voltage rises when the emitter voltage rises), the base voltage polarity of
VT2 is "+", and the emitter voltage polarity is also "+". 

The voltage of this polarity is fed back to the emitter of VT1 through X1, and the feedback voltage polarity is the same as the assumed voltage polarity, so
the feedback is positive feedback.

(2) The oscillation process of the circuit

After the power supply is turned on, the transistor VT1 and VT2 are switched on, the Ie current with the change of the output of the VT2 emitter contains
signals of various frequencies, and the impedance of the f0 signal of the quartz crystal X1 is very small. The f0 signal is fed back to the emitter of VT1 by
X1 and RP1, and the signal is output from the collector after being amplified by VT1, and then added to the emitter after being amplified by VT2.

 And then through X1 feedback to VT1 amplification, so repeated, VT2 output f0 signal amplitude is getting larger and larger, VT1, VT2 amplifier circuit
amplification factor is getting smaller and smaller, when the amplification factor is equal to the feedback attenuation factor, the output f0 signal
amplitude no longer changes, the circuit output stable f0 signal.

3. Quartz crystal oscillator is divided into non-temperature compensated crystal oscillator, temperature compensated crystal oscillator (TCXO),
voltage controlled crystal oscillator (VCXO), constant temperature controlled crystal oscillator (OCXO) and digital /μp compensated crystal oscillator
(DCXO/MCXO) and other types. Among them, the non-temperature-compensated crystal oscillator is the simplest one, which is called the
standard packaged crystal oscillator (SPXO) in the Japanese Industrial Standard (JIS).

① thermostatically controlled crystal oscillator. Thermostatically controlled crystal oscillator (OCXO) is a crystal oscillator that uses a constant temperature
tank to keep the temperature of the crystal oscillator or quartz crystal oscillator constant, and reduces the oscillator output frequency change caused
by changes in ambient temperature to a minimum, as shown in Figure 15-3. 

In the OCXO, some only put the quartz crystal oscillator in a constant temperature tank, some put the quartz crystal oscillator and related important
components in a constant temperature tank, and some put the quartz crystal oscillator in the internal constant temperature tank, and the oscillation
circuit is placed in the external constant temperature tank for temperature compensation, the implementation of double constant temperature tank control method. 

The use of proportional control of the constant temperature tank can increase the temperature stability of the crystal to more than 5000 times,
so that the oscillator frequency stability is maintained at least 1×10-9. OCXO is mainly used in mobile communication base stations,
defense, navigation, frequency counters, spectrum and network analyzers and other equipment and instruments. The OCXO is composed of a
thermostat control circuit and an oscillator circuit. 

Usually people use thermistor "bridge" composed of differential series amplifier to achieve temperature control. The (Clapp) oscillator circuit
with automatic gain control (AGC) is an ideal technical solution to obtain high stability of oscillation frequency in recent years. In recent years,
the technical level of OCXO has improved greatly.

② Temperature compensated crystal oscillator. Temperature compensated crystal oscillator (TCXO) is a quartz crystal oscillator that reduces
the amount of oscillation frequency change caused by changes in ambient temperature through an additional temperature compensation
circuit, as shown in Figure 7. In TCXO, there are two main compensation methods for quartz crystal oscillator frequency and temperature
drift: direct compensation and indirect compensation.

a. Direct compensation type. The directly compensated TCXO is a temperature compensated circuit composed of a thermistor and a
resistance-capacitance element, which is connected in series with a quartz crystal oscillator in an oscillator. When the temperature changes, the
resistance value of the thermistor and the equivalent series capacitance value of the crystal change accordingly, thus cancelling or reducing the
temperature drift of the oscillation frequency.

 The compensation circuit is simple, low cost, saves the size and space of printed circuit board (PCB), and is suitable for small and low
voltage and small current occasions. However, when the crystal oscillator accuracy is required to be less than ±1×10-6, the direct
compensation method is not suitable.

b. Indirect compensation type. Indirect compensation is divided into two types: analog and digital. The analog indirect temperature compensation
is a temperature-voltage conversion circuit composed of temperature sensing elements such as thermistor, and the voltage is applied to a varactor-diode
connected to the crystal oscillator, and the nonlinear frequency drift of the crystal oscillator is compensated by the change of the crystal oscillator's series capacitance. 

This compensation method can achieve a high accuracy of ±0.5×10-6, but is limited in low voltage conditions below 3V. Digital indirect
temperature compensation is in the analog indirect temperature compensation circuit in the temperature-voltage conversion circuit
and then add A level of analog/digital (A/D) converter, the analog quantity into digital quantity.

 This method can realize automatic temperature compensation, so that the crystal oscillator frequency stability is very high, but the specific
compensation circuit is more complex, the cost is also high, only applicable to the base station and broadcasting station and other
requirements of high precision.

③ Ordinary crystal oscillator. A common crystal oscillator (SPXO) is a simple crystal oscillator commonly referred to as a clock oscillator. It is a
crystal oscillator that works entirely by crystal free oscillation. This kind of crystal oscillator is mainly used in occasions where the stability
requirement is not high. Figure 8 shows a common crystal oscillator.

④ Voltage controlled crystal oscillator. Voltage controlled crystal oscillator (VCXO) is a quartz crystal oscillator whose oscillation frequency
can be changed or modulated by applying an external control voltage. In a typical VCXO, the frequency of the quartz crystal oscillator is
usually "pulled" by changing the capacitance of the varactor diode by tuning the voltage.

 The VCXO allows a relatively wide frequency control range, with the actual traction degree range of about ±200×10-6 or greater.
If the output frequency of the VCXO is required to be higher than that achieved by the quartz crystal oscillator, the frequency doubling
scheme can be used. Another way to extend the tuning range is to mix the output signal of the crystal oscillator with the output signal
of the VCXO. Compared with a single oscillator, this heterodyne type of two oscillator signal tuning range is significantly extended.