Introduction to the working principles of four types of crystal oscillators

Crystal oscillator refers to cutting quartz crystals (referred to as chips), quartz crystal resonators, quartz crystals or crystals from a certain direction angle, and crystal oscillators; But adding IC to the packaging to form a resonant circuit is called a crystal oscillator. Its products are generally packaged in plastic shells, as well as glass shells, porcelain, or plastic packaging. This article recommends the following four types of crystal oscillators: temperature controlled crystal oscillators (OCXO), temperature compensated crystal oscillators (TCXO), general crystal oscillators (SPXO), and suppressed crystal oscillators (VCXO).

(1) Temperature controlled crystal oscillator (hereinafter referred to as OCXO)

This type of crystal vibration uses constant temperature bath technology to solve the problem of temperature stability. The crystal is placed in the constant temperature bath, and the temperature control working point is set to maintain the constant temperature of the bath body, which is not affected by external temperature within a certain range, achieving the effect of stable output frequency. The crystal vibration is mainly used in various communication devices, including switches, SDH transmission equipment, mobile communication direct amplifiers, GPS receivers, broadcasting stations, digital televisions, and equipment. According to the necessity of the user, this type of crystal oscillator can be equipped with voltage control pins.

The main characteristic of OCXO is that due to the use of constant temperature bath technology, the frequency and temperature characteristics are the same in all types of crystal vibrations. Due to the precise circuit principle, its short stability and phase noise are good. The main drawbacks are high power consumption and large volume, which require heating for 5 minutes for normal operation. The common indicators of crystal vibration produced by our company are as follows:

(2) Temperature compensation of crystal oscillator (TCXO)

The TCXO temperature compensated crystal oscillator is a quartz crystal oscillator that comes with a temperature compensation circuit to reduce the oscillation frequency changes caused by surrounding temperature changes. The principle of temperature compensation is to compensate for the frequency drift of the resonator caused by changes in operating temperature by changing the load capacitance in the oscillation circuit.

The function of crystal vibration is to provide the basic clock signal for the system. Generally speaking, a system shares a crystal vibration to facilitate synchronization of all components. Some communication systems use different crystal vibrations for the fundamental frequency and radio frequency, and maintain synchronization by electronically adjusting the frequency.

Crystal vibration is typically used in conjunction with phase-locked loop circuits to provide the required clock frequency for the system. If different subsystems require clock signals of different frequencies, they can be provided through different phase-locked loops connected to the same crystal vibration.

The solution for temperature stability adopts some temperature compensation methods. The key principle is to control the output frequency of crystal vibration and achieve a stable output frequency by sensing the working temperature. Traditional TCXO uses analog devices for compensation. With the development of compensation technology, many large-scale intelligent compensation TCXOs have begun to appear. This intelligent compensation TCXO is also known as DTCXO. When we use a microcontroller for compensation, we call it MCXO. Due to the use of digital technology, this type of crystal oscillator has high accuracy in temperature characteristics and can adapt to a wide operating temperature range, mainly used in fields and difficult application environments. With the joint efforts of most R&D personnel, our company has independently developed high-precision MCXO, which has a globally designed structure and a highly automated production and testing system. The monthly output can reach 5000 units, and its structural design is shown in the following figure.

(3) General crystal oscillator (SPXO)

This is a simple crystal oscillator, commonly known as a clock oscillator, which works by clearing the "pressure control", "temperature compensation", and "temperature compensation" AGC in Figure 3, which are completely completed by the free oscillation of the crystal. This type of crystal oscillator is mainly used in areas with low stability requirements.

(4) Suppressed crystal oscillator (VCXO)

This is classified based on whether crystal vibration has pressure control function. The crystal vibration with pressure control input pins is called VCXO. The above three types of crystal vibrations can all be equipped with pressure control ports.

Total frequency difference of crystal oscillator indicator: The error between the crystal oscillator frequency and the standard frequency is caused by all combinations of required operating and non operating parameters.

Explanation: The total frequency difference includes frequency temperature stability, frequency aging rate error, frequency voltage characteristics, and frequency load characteristics. Generally speaking, we only pay attention to its short-term frequency stability, and choose places that do not strictly require other frequency stability indicators. For example, precision guided radar.

Frequency stability: Any crystal vibration, frequency instability, and different levels. The curve of the output frequency of crystal vibration over time is shown in Figure 2. The figure shows three frequency instability factors: aging, drift, and short stability.

Curve 1 uses 0.1 seconds to detect the state, displaying the crystal oscillator being short and stable; Curve 3 detects the state for 100 seconds, displaying crystal drift; Curve 4 takes one day to detect the situation. Display crystal oscillator aging.

Frequency temperature stability: When operating within the required temperature range under standard power supply and load, there is no allowable frequency deviation for implied standard temperature or implied standard humidity.

ft=±(fmax-fmin)/(fmax fmin)

ftref ＝±MAX[｜(fmax-fref)/fref｜，｜(fmin-fref)/fref｜]

Ft: Frequency temperature stability (without implicit standard temperature)

Ftref: Frequency temperature stability (with implicit standard temperature)

Fmax: Frequency required for measurement within the ambient temperature range

Fmin: Frequency of measurement within the required ambient temperature

Fref: Frequency required for standard temperature measurement

Note: The production difficulty of Ftref indicator crystal oscillators is higher than that of Ftref indicator crystal oscillators, therefore the price of Ftref indicator crystal oscillators is higher.

Startup characteristic (stable frequency preheating time): refers to the rate of change from a period of time (such as 5 minutes) after restart to another period of time (such as 1 hour) after restart. It represents the stable rate of crystal vibration. This indicator is very useful for commonly used switching instruments such as frequency meters.

Indications: In most applications, crystal oscillators are powered on for a long time, but in some applications, crystal oscillators must be frequently started and turned off, and frequency stability preheating time indicators need to be considered (especially for military communication radio stations commonly used in harsh environments, when frequency and temperature stability are specified, ≤ ± 0.3ppm (-45 ℃~85 ℃), with OCXO as the local oscillator, frequency stability, and preheating time not less than 5 minutes, Using MCXO only takes more than ten seconds.

Frequency aging rate: The relationship between oscillator frequency and time when measuring oscillator frequency under constant natural conditions. This long-term frequency drift is caused by slow changes in crystal components and oscillator circuit components. Therefore, the rate of frequency shift is called the aging rate, which can be used to determine the rate of change after the required period (such as ± 10ppb/day, after 72 hours of power on), or to determine the total frequency of change within the specified period (such as ± 1ppm/(year) and ± 5ppm/(ten years).

Crystal aging is caused by problems such as stress, pollutants, residual gases, and structural process defects during the production of crystals. The stress takes some time to stabilize. A crystal cutting method called "stress compensation" (SC cutting method) enables the crystal to have good characteristics.

Pollutants and residual gas molecules can attach to crystal chips or oxidize crystal electrical levels. The higher the oscillation frequency, the thinner the crystal chip used, and the greater the impact. This effect takes a long time to gradually stabilize, and this stability will repeat with changes in temperature or operating state - causing pollutants to concentrate or disperse on the crystal surface again. Therefore, low-frequency crystal oscillators have better aging rates than high-frequency crystal oscillators, longer working time crystal oscillators than shorter working time crystal oscillators, and continuous operating crystal oscillators than intermittent operating crystal oscillators.

Display: The aging rates of TCXO are: ± 0.2ppm~± 2ppm (annually) and ± 1ppm~± 5ppm (10 years) (TCXO rarely selects daily frequency aging rate indicators except for special circumstances, because even under laboratory conditions, frequency changes caused by temperature changes can greatly exceed the daily frequency aging of temperature compensated crystal oscillators, so this indicator loses its specific significance). The aging rate of OCXO is ± 0.5ppb to ± 10ppb/day (after 72 hours of power on), ± 30ppb to ± 2ppm (annually), and ± 0.3ppm to ± 3ppm (ten years).

Short term stability: Short term stability, observed for 1ms, 10ms, 100ms, 1s, 10s.

The output frequency of crystal vibration is affected by the internal power supply (Q value of the crystal, noise of components, stability of the power supply, operating status, etc.), and the bandwidth is unstable. After measuring a series of frequency values, calculate using the Allen equation. Phase noise can also reflect short stability (requiring special instrument measurement).

Recurrence: Definition: After long-term stability of crystal vibration, turn off t1 (such as 24 hours), start t2 (such as 4 hours), measure frequency f1, turn off t1, start t2, and measure frequency f2. Reproduce=(f2-f1)/f2.

Frequency voltage control range: Adjust the frequency control voltage from the reference voltage to the specified endpoint voltage, and observe the peak change in crystal oscillator frequency.

Indications: The reference voltage is+2.5V, and the required endpoint voltages are+0.5V and+4.5V. The voltage controlled crystal oscillator has a frequency change of -2ppm when the frequency control voltage is+0.5V, and a frequency change of+2.1ppm when the frequency control voltage is+4.5V. The VCXO voltage control frequency voltage control range is ≥ ± 2ppm (2.5V ± 2V), with a positive slope and a positive linear shape of 2.4%.

Voltage controlled frequency response range: The relationship between peak frequency deviation and modulation frequency when the modulation frequency changes. Generally speaking, multiple DBs indicate that the specified modulation frequency is lower than the specified modulation reference frequency.

The frequency response of VCXO frequency voltage control category is 0-10kHz.

Frequency voltage controlled linear: A measurement of the transmission characteristics of the output frequency input control voltage. Compared to the ideal (linear) function, the frequency deviation representing the entire category can allow for nonlinearity.

Display: Typical VCXO frequency voltage control lines are: ≤ ± 10%, ≤ ± 20%. The simple calculation method for VCXO frequency voltage controlled linear is (when the frequency voltage controlled polarity is positive):

Frequency voltage controlled linear shape=± (fmax fmin)/f0) × 100%

Fmax: VCXO output frequency at voltage controlled voltage

Fmin: VCXO output frequency at voltage controlled voltage

F0: Pressure control center voltage frequency

Single sideband phase noise £ (f): The ratio of the power of the phase modulated sideband to the carrier power at the offset carrier f.

Output waveform: The output waveform can be divided into waveform and sine wave.

Waveforms are mainly used for digital communication system clocks, while square waves mainly require indicators such as output level, duty cycle, rise/fall time, and driving ability.

With the rapid development of science and technology, high-quality signal sources must be used as increasingly complex baseband data carriers for communication, radar, high-speed data transmission and similar systems. These frequency band components that cannot exist under ideal conditions (parasitic modulation in the carrier), when the carrier signal containing parasitic amplitude modulation and phase modulation (unclean signal) is modulated by the baseband signal of the passenger data, will significantly deteriorate the signal quality and data error rate of the transmission. Therefore, the cleanliness (bandwidth purity) of carrier signals as the carrier of transmission signals directly affects the quality of communication. For sine waves, it is usually necessary to provide data on harmonics, noise, and power.

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