Crystal oscillator, crystal oscillator, clock oscillator working principle and characteristics

1, crystal, some RD called crystal oscillator: quartz crystal, is passive two feet, no direction, requires IC or other external crystal oscillator input, only to generate frequency, is non-directional. The crystal also needs an inverter, and the loading capacitor can form an oscillator. The quartz crystal component consists of a quartz crystal piece and a casing to form a passive piezoelectric component, commonly known as a crystal, a crystal oscillator, which was called a crystal resonator in China.

It can be seen that the normal quartz crystal component (two legs) is non-directional, but when a terminal (pin) is connected to the casing, there is a possibility of directivity. Crystal is an electromechanical device made by precision cutting and grinding a quartz crystal with low electrical loss and plating the electrode lead. This crystal has a very important characteristic. If it is energized, he will produce mechanical oscillation. On the contrary, if he is given mechanical force, he will generate electricity. This characteristic is called electromechanical effect.

They have a very important feature, and their oscillation frequency is closely related to their shape, material, cutting direction and so on.

Since the chemical properties of quartz crystals are very stable, the coefficient of thermal expansion is very small, and the oscillation frequency is also very stable. Since the control geometry can be very precise, the resonance frequency is also very accurate. - What is the inverter, what is the loading capacitor used for? ? why would? - Form positive feedback, so that it can oscillate. Resonators and clocks are different. The resonator is the simplest oscillator without any compensation. The vibration that we usually say is composed of a resonator plus ic. A loop to achieve its own function.

Taking vcxo as an example: a voltage controlled crystal oscillator (VCXO) is a quartz crystal oscillator that has a variable oscillation efficiency or can be modulated by an infrared plus control voltage.

VCXO is mainly composed of a quartz resonator, a varactor diode and an oscillating circuit. Its working principle is to change the capacitance of the varactor by controlling the voltage, thereby "pulling" the frequency of the quartz resonator to achieve the purpose of frequency modulation.

VCXO is mostly used for phase-locked technology and frequency negative feedback modulation. And deciding how to choose should be very clear, right?

2. Oscillator, some RD is also called crystal oscillator, generally has four feet, is directional, has power, ground and clock output pins, has crystal and oscillation circuit inside, does not need input and input signal source, The frequency can be generated directly. The frequency is calibrated at the factory. Features: easy to apply, stable frequency, less electromagnetic radiation. But the price is more expensive than the crystal. Quartz crystal oscillator is called crystal oscillator. It is generally composed of quartz crystal component, IC and RC and external components. It can output stable frequency signal when power is applied.
The crystal oscillator is generally 4 feet (lead end), and has directionality, which is marked in the sample or the specification. Resonator: The equivalent function in the circuit is a network with frequency selection. It is the core component of the oscillator circuit. It determines the frequency stability of the oscillator. Quartz crystal, ceramic, LC, medium. Resonators of materials. The quartz crystal is combined with the amplifying circuit to generate a self-oscillation signal if the line is positive feedback and the loop amplification factor is greater than one. This is the basic principle of quartz crystals. Select ------- according to the specific requirements of the IC you are using, 1) Only use an external clock, then select the clock, or use the crystal + inverter + capacitor to form the oscillator, according to the price and convenience 2) If an external clock is available, or a crystal oscillator can be used, then a crystal oscillator is used. 3) If only a crystal oscillator can be used, the difference between the crystal oscillator passive crystal and the active crystal oscillator, the application range and usage are selected:

1, passive crystals - passive crystals need to use the DSP on-chip oscillator, there is a recommended connection method on the datasheet. The passive crystal has no voltage problem, the signal level is variable, that is, it is determined according to the oscillation circuit. The same crystal can be applied to a variety of voltages, and can be used for a variety of different clock signal voltage requirements of the DSP. And the price is usually lower, so for general applications, if the conditions permit the use of crystals, this is especially suitable for producers with large product lines.

The defect of passive crystals relative to crystal oscillators is that the signal quality is poor. It is usually necessary to accurately match the peripheral circuits (capacitors, inductors, resistors, etc. used for signal matching). When replacing crystals of different frequencies, the peripheral configuration circuit needs to be adjusted accordingly. . It is recommended to use a quartz crystal with higher precision, and try not to use ceramic vigilance with low precision.

2, active crystal oscillator - active crystal oscillator does not require DSP internal oscillator, signal quality is good, relatively stable, and the connection method is relatively simple (mainly to do power supply filtering, usually using a capacitor and inductor composed of PI type filter network The output uses a small resistance resistor to filter the signal. It does not require complicated configuration circuitry.

The usual usage of active crystal oscillator: one foot is left floating, two feet are grounded, three feet are connected to output, and four feet are connected to voltage. Compared to passive crystals, the active crystal oscillator has a defect in that its signal level is fixed, and it is necessary to select a suitable output level, which is less flexible and expensive.

For applications that are sensitive to timing requirements, the individual believes that the active crystal oscillator is good, because a more sophisticated crystal oscillator can be used, even a high-grade temperature-compensated crystal oscillator. Some DSPs do not have a start-up circuit inside, and only active crystals, such as TI's 6000 series, can be used. Active crystals are typically larger than passive crystals, but many active crystals are now surface-mount, comparable in size to crystals, and even smaller than many crystals.

Points to note: 1. DSPs that require multiplier need to be configured with PLL peripheral configuration circuits, mainly for isolation and filtering;

2. The crystal oscillators below 20MHz are basically fundamental frequency devices with good stability. Most of them above 20MHz are harmonics (such as 3rd harmonic, 5th harmonic, etc.), and the stability is poor, so it is strongly recommended. The use of low-frequency devices, after all, the peripheral configuration required for multiplier PLL circuits is mainly capacitance, resistance, inductance, its stability and price are far better than the crystal crystal device;

3. The length of the clock signal trace should be as short as possible, the line width should be as large as possible, and the distance from other printed lines should be as large as possible, close to the layout and wiring of the device, and if necessary, the inner layer can be removed and surrounded by the ground line;

4. There are special design requirements when introducing the clock signal from the outside through the backplane. It is necessary to refer to the relevant materials in detail. In addition, there are some explanations: in general, the stability of the crystal oscillator is better than that of the crystal, especially in the field of precision measurement. Most of them use high-grade crystal oscillators, so that various compensation technologies can be integrated. Reduce the complexity of the design. Imagine if you use a crystal and then design your own waveform shaping, anti-jamming, and temperature compensation, what would the design complexity look like? Here we design high-frequency circuits such as RF circuits, which are high-precision temperature-compensated crystal oscillators, and industrial grades are several hundred yuan.

If the application of special fields cannot find a suitable crystal oscillator, that is to say, the complexity of the design exceeds the level of the finished crystal oscillator on the market, it must be designed by itself. In this case, the crystal should be selected, but these crystals are definitely not on the market. Ordinary crystals, but special high-end crystals, such as ruby ​​crystals. The higher requirements of the field are more special. The clocks we use here for high-precision testing are even provided by atomic clocks, cuckoo clocks, etc., connected by dedicated RF connectors, which is a large device and quite bulky. Crystal oscillator: a so-called quartz crystal resonator and a quartz crystal clock oscillator. However, since resonators are used more in consumer electronics, the general concept of crystal oscillators is equivalent to resonators. The latter is usually referred to as the clock vibration. This article describes some technical indicators that can demonstrate the performance of a crystal oscillator. Understanding the meaning of these indicators will help communication design engineers to successfully complete the design project, and can also greatly reduce the procurement cost of the whole machine manufacturer.

Total frequency difference: The maximum frequency difference between the crystal oscillator frequency and the given nominal frequency caused by the combination of the specified working and non-working parameters within the specified time.

---- Description: The total frequency difference includes the maximum frequency difference caused by frequency temperature stability, frequency temperature accuracy, frequency aging rate, frequency power supply voltage stability and frequency load stability. It is generally only used in the case of short-term frequency stability, but not in other frequency stability indicators. For example: precision guided radar.

----Frequency temperature stability: Under the nominal power supply and load, the operating system is within the specified temperature range without the implicit reference temperature or the maximum allowable frequency offset with the implicit reference temperature.

----fT=±(fmax-fmin)/(fmax+fmin)----fTref=±MAX[|(fmax-fref)/fref|,|(fmin-fref)/fref|]fT:frequency Temperature stability (without implied reference temperature)----fTref: Frequency temperature stability (with implied reference temperature)----fmax: The highest frequency measured within the specified temperature range----fmin: Specification The lowest frequency measured in the temperature range----fref: the frequency measured by the specified reference temperature

---- Description: The crystal oscillator with fTref index is more difficult to produce than the crystal oscillator with fT index. Therefore, the price of crystal oscillator with fTref index is higher.

The typical frequency and temperature stability indicators of crystal oscillators used in several electronic systems are shown in the following table:

---- There is a part of the frequency temperature stability index in the table should be the frequency temperature stability index with the implicit reference temperature, but it is not shown. (1 ppm = 1 x 10-6; 1 ppb = 1 x 10-9).

----Frequency stable warm-up time: The time required from the power-on to the output frequency is less than the specified frequency tolerance based on the stable output frequency of the crystal oscillator.

----Note: In most applications, the crystal oscillator is powered for a long time. However, in some applications, the crystal oscillator needs to be turned on and off frequently. At this time, the frequency stable warm-up time indicator needs to be taken into consideration (especially For military communication stations used in harsh environments, when frequency temperature stability is required to be ≤±0.3ppm (-45°C～85°C), OCXO is used as the local oscillator, and the frequency stable warm-up time will be no less than 5 minutes. It takes only a dozen seconds to use DTCXO.

Frequency aging rate: The relationship between oscillator frequency and time when measuring oscillator frequency under constant environmental conditions. This long-term frequency drift is caused by slow changes in the crystal components and oscillator circuit components. The maximum rate of change after a specified time limit (eg ±10 ppb/day, after 72 hours of power-up), or the maximum total time within the specified time limit. Frequency changes (eg, ±1ppm/(first year) and ±5ppm/(ten years)) are expressed.

---- Description: The frequency aging rate of TCXO is ±0.2ppm～±2ppm (first year) and ±1ppm～±5ppm (ten years). (Unless special circumstances, TCXO rarely uses the daily frequency aging rate index. Because even in laboratory conditions, the frequency change caused by temperature changes will greatly exceed the frequency aging of the temperature-compensated crystal oscillator every day, this indicator loses its practical significance). The frequency aging rate of OCXO is ±0.5 ppb to ±10 ppb/day (after 72 hours of power-on), ±30 ppb to ±2 ppm (first year), and ±0.3 ppm to ±3 ppm (ten years).

----Frequency voltage control range: The minimum peak value of the crystal oscillator frequency is adjusted from the reference voltage to the specified end voltage and the crystal oscillator frequency.

----Description: The reference voltage is +2.5V, the specified end voltage is +0.5V and +4.5V, and the frequency change of the voltage controlled crystal oscillator is -110ppm at +0.5V frequency control voltage, at +4.5V frequency When the voltage is changed by +130ppm when the voltage is controlled, the VCXO voltage control frequency voltage control range is expressed as: ≥ ± 100ppm (2.5V ± 2V). Voltage Control Frequency Response Range: The relationship between the peak frequency offset and the modulation frequency when the modulation frequency changes. Usually, the specified modulation frequency is expressed by a few dB lower than the specified modulation reference frequency.

----Description: The frequency response of the VCXO frequency voltage control range is 0~10kHz.

Frequency-Frequency Control Linearity: A measure of the output frequency-input control voltage transfer characteristic compared to the ideal (straight line) function, which is a percentage of the allowable nonlinearity of the entire range of frequency offsets.

---- Description: The typical VCXO frequency voltage control linearity is: ≤±10%, ≤±20%. The simple VCXO frequency voltage control linear calculation method is (when the frequency voltage control polarity is positive):

----Frequency voltage control linear=±((fmax-fmin)/f0)×100%----fmax: VCXO output frequency at maximum voltage-controlled voltage----fmin: VCXO at minimum voltage-controlled voltage Output frequency----f0: voltage-controlled center voltage frequency----single-side phase noise £(f): the ratio of the power density of a phase-modulated sideband to the carrier power from the carrier f.

Crystal Oscillator Overview

First, the crystal oscillator type:

1. Ordinary crystal oscillator PackagedCrystalOscillator (PXO) The simplest and most suitable oscillator whose basic control element is a crystal element. Since temperature control and temperature compensation are not used, its frequency-temperature characteristics are mainly determined by the crystal components used.

2. Voltage Controlled Crystal Oscillator VoltageControlledCrystalOscillator (VCXO) uses a crystal oscillator with an external control voltage to bias or modulate its frequency output. The frequency-temperature characteristics of VCXO are similar to PXO and are primarily determined by the crystal components employed.

3. The Temperature Compensated Crystal Oscillator (TCXO) includes a DCXODigitallyCompensated Crystal Oscillator and a MCXO Microcomputer Compensated Crystal Oscillator. The device internally uses an analog compensation network or digital compensation to compensate for the frequency-temperature characteristics of the crystal components with changes in crystal load reactance with temperature to achieve a crystal oscillator that reduces its frequency-temperature offset.

4. The Oven Controlled Crystal Oscillator (OCXO) is a crystal oscillator that controls the temperature of the crystal element in a heat shield (such as a constant temperature bath) to keep the crystal temperature substantially constant.

5, voltage control - temperature compensation crystal oscillator (VCTCXO) temperature compensation crystal oscillator and voltage control crystal oscillator combination.

6. Voltage Control - A combination of a constant temperature crystal oscillator (VCOCXO) oven controlled crystal oscillator and a voltage controlled crystal oscillator.

Second, the crystal oscillator main parameters ★ frequency accuracy: the nominal supply voltage, nominal load impedance, reference temperature (252 ° C) and other conditions remain unchanged, the crystal oscillator frequency relative to its specified nominal maximum allowable Deviation, ie (fmax-fmin)/f0;

★Temperature stability: Other conditions remain the same, the allowable frequency offset value of the maximum variation of the output frequency of the crystal oscillator in the specified temperature range relative to the sum of the output frequency extremes in the temperature range, ie (fmax-fmin)/( Fmax+fmin);

★ Frequency adjustment range: Change the range of output frequency by adjusting a variable component of the crystal. The effect of frequency adjustment is: 1 to adjust the output frequency to a predetermined value within the frequency range;

2 Due to aging or other reasons, the output frequency of the crystal oscillator is shifted, and the output frequency is adjusted to a specified value.

★ FM (voltage control) features: including FM frequency offset, FM sensitivity, FM linearity. 1 FM frequency offset: The output frequency difference when the control voltage of the voltage controlled crystal oscillator changes from the nominal maximum value to the minimum value. 2 FM sensitivity: The amount of change in the output frequency caused by the voltage-controlled crystal oscillator change unit plus the control voltage. 3 FM linearity: is a measure of the transmission characteristics of a modulation system compared to an ideal straight line (least squares). Usually expressed as a percentage of the ideal line within the specified range

★Load characteristics: Other conditions remain unchanged, and the maximum allowable frequency offset of the crystal oscillator output frequency relative to the output frequency under the nominal load within the specified variation range.

★Voltage characteristics: Other conditions remain unchanged, and the maximum allowable frequency offset of the crystal oscillator output frequency relative to the output frequency at the nominal supply voltage within the specified variation range.

★ Clutter: The power ratio of the discrete spectral components in the output signal to the main frequency without harmonics (except for the subharmonics) and the main frequency, expressed in dBc.

★ Harmonic: The ratio of the harmonic component power Pi to the carrier power P0, expressed in dBc.

★Frequency aging: The system drift of the output frequency over time due to aging of components (mainly quartz resonators) under specified environmental conditions. It is usually measured by the frequency difference within a certain time interval. For highly stable crystals, the output frequency is measured in a nearly linear unidirectional drift over a long period of time, often measured by the aging rate (relative frequency change per unit time). Such as: 10-8/day or 10-6/year.

★Day fluctuation: It means that the oscillator is measured every hour after the specified warm-up time, continuously measuring for 24 hours, and the test data is calculated by S=(fmax-fmin)/f0, and the daily fluctuation is obtained.

★ Power-on characteristics: The maximum change in the oscillator frequency value during the specified warm-up time is expressed by V = (fmax - fmin) / f0.

★ Phase noise: Frequency domain metric for short-term stability. Using the ratio of single sideband noise to carrier noise £(f), £(f) is directly related to the spectral density Sφ(f) of the noise fluctuation and the spectral density Sy(f) of the frequency fluctuation, expressed by the following equation: f2S( f)=f02Sy(f)=2f2£(f)f—Fourier frequency or deviation from carrier frequency; f0—Selection of carrier frequency for crystal oscillator

Pay attention to some parameters, the design engineer can choose the oscillator suitable for the application.

Today, countless electronic circuits and applications require precise timing or clock reference signals. Crystal clock oscillators are ideal for many applications in this area.

The clock oscillator is available in a variety of packages and is characterized by a wide range of electrical performance specifications. There are several different types: voltage controlled crystal oscillator (VCXO), temperature compensated crystal oscillator (TCXO), oven crystal oscillator (OCXO), and digitally compensated crystal oscillator (DCXO). Each type has its own unique properties.

---- Frequency stability considerations - One of the main characteristics of the crystal oscillator is the stability in the operating temperature, which is an important factor in determining the price of the oscillator. The higher the stability or the wider the temperature range, the higher the price of the device.

The design engineer must carefully determine the actual needs for a particular application and then specify the stability of the oscillator. Too high a target means more money.

For applications with a frequency stability requirement of ±20ppm or higher, a normal uncompensated crystal oscillator can be used. For stability from ±1 to ±20 ppm, TCXO should be considered. For stability below ±1 ppm, OCXO or DCXO should be considered.

----Output---Other parameters that must be considered are output type, phase noise, jitter, voltage stability, load stability, power consumption, package form, shock and vibration, and electromagnetic interference (EMI). The crystal oscillator is HCMOS/TTL compatible, ACMOS compatible, ECL and sine wave output. Each output type has its unique waveform characteristics and uses. Attention should be paid to the requirements of tri-state or complementary output. Symmetry, rise and fall times, and logic levels are also specified for some applications. Many DSP and communication chipsets often require strict symmetry (45% to 55%) and fast rise and fall times (less than 5 ns).

Phase noise and jitter—The phase noise obtained in the frequency domain is a true measure of short-term stability. It measures up to 1 Hz of the center frequency and typically measures 1 MHz.

The phase noise of the oscillator is improved at frequencies away from the center frequency. TCXO and OCXO oscillators, as well as other crystal oscillators using fundamental or harmonic methods, have the best phase noise performance. An oscillator that uses a phase-locked loop synthesizer to produce an output frequency generally exhibits poor phase noise performance than an oscillator that uses a non-phase-locked loop technique.

The jitter is related to phase noise, but it is measured in the time domain. The jitter expressed in picoseconds can be a valid value or a peak.

- Peak measurement. Many applications, such as communication networks, wireless data transmission, ATM and SONET requirements, must meet stringent saturation specifications. It is important to pay close attention to the jitter and phase noise characteristics of the oscillators used in these systems.

The impact of the power supply and load - the frequency stability of the oscillator is also affected by oscillator supply voltage fluctuations and oscillator load changes. Proper selection of the oscillator minimizes these effects. The designer should verify the performance of the oscillator under the recommended supply voltage tolerances and load. It is not expected that an oscillator that can only be rated for 15pF will perform well when driving 50pF. Oscillators operating above the recommended supply voltage will also exhibit poor waveform and stability.

For devices that require battery power, power must be considered. Introducing a 3.3V product is bound to develop an oscillator that operates at 3.3V. The lower voltage allows the product to operate at low power. Most commercially available surface mount oscillators today operate at 3.3V. Many perforated oscillators using conventional 5V devices are being redesigned to operate at 3.3V.

The package is similar to other electronic components, and the clock oscillator is also available in smaller and smaller packages. For example, M-tron's M3L/M5L series of surface mount oscillators are now available in a 3.2 x 5.0 x 1.0 mm package. In general, smaller devices are more expensive than larger surface mount or perforated package devices. Small packages often have a trade-off between performance, output selection, and frequency selection.

----Working environment----The actual application environment of the oscillator needs careful consideration. For example, high vibration or shock levels can cause problems for the oscillator.

In addition to possible physical damage, vibration or shock can cause erroneous actions at certain frequencies. These externally induced disturbances can cause frequency jitter, increased noise footprint, and intermittent oscillator failure. EMI is another priority for applications that require special EMI compatibility. In addition to using the appropriate PCB motherboard layout techniques, it is important to choose a clock oscillator that provides the least amount of radiation. In general, oscillators with slower rise/fall times exhibit better EMI characteristics.

For frequencies below 70MHz, an HCMOS oscillator is recommended. For higher frequencies, an ECL type oscillator can be used. ECL type oscillators usually have the best total noise rejection, and even at lower frequencies of 10 to 100 MHz, the ECL type is slightly better than other types of oscillators.