Working principle and characteristics of crystal oscillator, crystal oscillator and clock generator


1. Crystal (crystal), some RD call it a crystal oscillator: a quartz crystal is passive with two legs and has no direction. It needs an input from an IC or other external crystal oscillator. only when a frequency is generated does it have no direction. The quartz crystal also needs an inverter, and a load capacitor (load capacitor) can form a generator. The quartz crystal component consists of a quartz crystal plate and a shell. , Electrical components commonly known as crystals and crystal oscillators were called crystal resonators at the beginning of my country.

You can see that the normal components of a quartz crystal (two pins) are not directional, but when one pin (pin) is connected to the shell, it can be directional. The crystal is a kind of electromechanical device, which is made of low power loss quartz crystal, which is precisely cut and ground, coated with electrodes and welded with leads. This type of crystal has a very important characteristic: if it is electrified, it will produce mechanical vibrations, and vice versa, if a mechanical force is applied to it, it will generate electricity. This characteristic is called the electromechanical effect.

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

Because the chemical properties of quartz crystal are very stable, the thermal expansion coefficient is very small, and its oscillation frequency is also very stable. Because the geometric size can be controlled very precisely, its resonant frequency is also very accurate. - What is the purpose of the inverter and load capacitor? ? why? - Form a positive feedback so that it can start to vibrate. The difference between a resonator and a clock generator is that the resonator is the simplest oscillator without any compensation, and the clock generator, as we usually say, consists of a resonator plus ic A circuit implements its function.

Taking vcxo as an example: A voltage controlled crystal oscillator (VCXO) is a crystal oscillator whose generation efficiency can be varied or modulated by adding a control voltage via infrared.


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

VCXO is mainly used for negative feedback phase locking and frequency modulation technology. And it should be clear how to decide how to choose?

2. An oscillator (oscillator), some RDs are also called crystal oscillators, usually have four legs, are directional, have a power supply, ground, and clock output pins, and have crystals and oscillator circuits inside. The frequency can be generated directly without input source input. The frequency is calibrated at the factory. Features: convenient application, stable frequency, less electromagnetic radiation. But the price is more expensive than crystal. Crystal oscillators are called crystal oscillators for short. Generally, they are active functional components composed of quartz crystal components, ICs, resistors and capacitors, and shells. They can output stable frequency signals when the power is turned on.

A crystal oscillator typically has 4 pins (outputs), all of which are directional and marked on the pattern or manual. Resonator: The equivalent function in the circuit is the frequency selection network, which is the main component of the oscillation circuit and determines the frequency stability of the oscillator. Types: quartz crystal, ceramics, LC, dielectric materials such as resonators. If the quartz crystal interacts with the amplifier circuit to form positive feedback, and the loop gain is greater than one, a self-excited oscillation signal will be generated. This is the basic principle behind a quartz crystal. Choice-------According to the specific requirements of the IC you use: 1) Can only use an external clock, then choose a clock generator, or use a crystal oscillator + inverter + capacitor to form an oscillator, choose according to price and convenience, 2 ) If you can use an external clock, you can also use a crystal oscillator, then use a crystal oscillator, 3) If you can only use a crystal oscillator, select a crystal oscillator. The difference between passive crystal oscillator and active crystal oscillator, scope and use:

1. Passive Crystal - The passive crystal must use a DSP on-chip oscillator and the datasheet provides recommended connection methods. The passive crystal has no voltage problem, and the signal level is variable, that is, it is determined according to the waveform. The same crystal can be applied to different voltages and can be used in a DSP with a different clock signal. voltage requirements, and the price is usually lower, so it is recommended to use crystals for general applications if conditions allow, which is especially suitable for manufacturers with a rich product range and large batches.

Compared to a crystal oscillator, the disadvantage of passive quartz is the poor signal quality. Usually, an exact match of the peripheral circuit (capacitance, inductance, resistance, etc.) is required fori signal conditioning) and configuration of peripheral devices. circuit must be done when replacing a crystal with a different frequency Adjust accordingly. It is recommended to use high-precision quartz crystals and try not to use low-precision ceramics.

2. Active Crystal Oscillator - An active crystal oscillator does not need an internal DSP oscillator, the signal quality is good, relatively stable, and the connection method is relatively simple (mainly to filter the power supply, usually using a capacitor and an inductor). PI type filter network, the output signal can be filtered with a low resistance resistor), and no complex configuration circuit is required.

Typical uses for active crystal oscillators are: one pin is suspended, two pins are grounded, three pins are connected to the output, and four pins are connected to voltage. Compared to passive crystal oscillators, the disadvantage of active crystal oscillators is that their signal levels are fixed and an appropriate output level must be selected, which is less flexible and expensive.

For applications with sensitive timing requirements, I personally think an active crystal oscillator is better because you can choose a more accurate crystal oscillator or even a high performance temperature compensated crystal oscillator. Some DSPs do not have an internal oscillator circuit and can only use active crystal oscillators such as the 6000 TI series. Active crystal oscillators are usually larger than passive ones, but now many active crystal oscillators are surface mounted and their volume is comparable to the volume of crystals, and some are even smaller than many crystals.

A few points to pay attention to: 1. The DSP that needs frequency multiplication needs to set up the PLL peripherals configuration scheme, mainly isolation and filtering;

2.Crystal oscillators below 20MHz are mainly fundamental frequency devices with good stability. Most crystal oscillators above 20 MHz are harmonics (such as 3rd harmonic, 5th harmonic, etc.) and their stability is poor. Therefore, it is highly recommended to use low frequency devices. After all, the peripheral configuration required by the PLL circuit for frequency multiplication is mainly capacitors, resistors, and inductors, and its stability and price are much better than crystal oscillator devices;

3. The length of the clock signal track is as short as possible, the line width is as large as possible, and the distance between it and other printed lines is as large as possible, close to the device layout and wiring, if necessary, can reach the inner layer and surround it with ground wires;


4. There are special design requirements when a clock signal is applied externally viabackplane, and you need to familiarize yourself with the relevant materials in detail. In addition, some explanations need to be made: Generally speaking, the stability of a crystal oscillator is better than that of a crystal, especially in the field of precision measurements. Most high-end crystal oscillators are used, so that various compensation technologies can be integrated. Reducing the complexity of the design. Just imagine, if you use quartz and then develop your own signal conditioning, anti-jamming and temperature compensation, what would be the complexity of the design? Here we design RF circuits and other high clock applications, we use high precision temperature compensated crystal oscillators, and industrial oscillators cost several hundred yuan each.

If you cannot find a suitable crystal oscillator for your application, that is, the complexity of the design exceeds the level of the finished crystal oscillator on the market, you must design it yourself. In this case, you must use a crystal, but these crystals should not be ordinary crystals on the market, but special high-grade crystals such as ruby ​​crystals and more. The situation in the higher requirements area is even more special. The clocks we use here in high-precision testing are provided even by atomic clocks, rubidium clocks and other equipment. They are connected through special RF connectors. They are large equipment and quite heavy. Crystal oscillator: A so-called crystal oscillator and a quartz crystal clock combined. However, since more resonators are used in consumer electronics products, crystal oscillators are generally understood to be equivalent to resonators in general concept. The latter is usually called a bell. This article presents some technical indicators that are sufficient to show the performance of a crystal oscillator. Understanding the meaning of these indicators will help communication design engineers successfully complete a design project and at the same time can significantly reduce the cost of purchasing a complete set. machine manufacturer.

----total frequency difference: during the specified time, the maximum frequency difference between the frequency of the crystal oscillator and the specified nominal frequency due to all combinations of the specified operating and non-operating parameters.

----Note: The total frequency difference includes the maximum frequency difference caused by frequency temperature stability, frequency temperature accuracy, frequency aging rate, frequency supply voltage stability, and frequency load stability. It is generally used only when dealing with short-term frequency stability, but other indicators of frequency stability are not strictly required. For example: precision guidance radar.

----Temperaturefrequency stability: at nominal power supply and load, the maximum frequency deviation allowed without an implicit reference temperature or with an implicit reference temperature within the specified temperature range.

----fT=±(fmax-fmin)/(fmax+fmin)----fTref=±MAX[|(fmax-fref)/fref|,|(fmin-fref)/fref|] fT: Frequency stability with respect to temperature (without implied reference temperature) ---- fTref: Frequency stability with respect to temperature (with implied reference temperature) ---- fmax: Highest frequency measured in the specified temperature range ---- fmin: Highest low frequency measured in the specified temperature range----fref: frequency measured at the specified reference temperature

----Explanation: An fTref crystal oscillator is more difficult to manufacture than an fT crystal oscillator, so the price of an fTref crystal oscillator is higher.

----Typical values ​​of frequency and temperature stability of crystal oscillators used in some electronic systems are shown in the table below:

----Some of the frequency temperature stability indicators in the table should be frequency temperature stability indicators with an implicit reference temperature, but they are not shown. (1 ppm=1×10-6; 1 ppm=1×10-9).

----Warm-up time for frequency stabilization: Based on the stable output frequency of the crystal oscillator, the time required from power on until the output frequency is less than the specified frequency tolerance.

----Note: In most applications, the crystal oscillator is turned on for a long time, but in some applications, the crystal oscillator needs to be turned on and off frequently. At this time, the frequency stabilization heats up. must consider the uptime index (especially for military communication stations used in harsh environments, when the temperature stability of the frequency ≤ ± 0.3 ppm (-45 ℃ ~ 85 ℃) is required), use the OCXO as the local oscillator, and the frequency stabilization warm-up time will be at least 5 minutes, and using the DTCXO will only take more than ten seconds).

----Frequency aging rate: The ratio between the oscillator frequency and the time that the oscillator frequency is measured under constant ambient conditions. This long-term frequency drift is caused by slow changes in the crystal resonator components and oscillator circuit components, and the maximum rate of change after a given period of time (e.g. ±10ppb/day, 72 hours after power-up.After a few hours) or the maximum total change can be used. frequency over a specified period of time (for example, ±1 ppm/(first year) and ±5 ppm/(ten years)).

----Explanation: TCXO aging rate: ±0.2ppm~±2ppm (first year) and ±1ppm~±5ppm (ten years) (Except in special circumstances, TCXO rarely uses the daily frequency aging rate indicator, so how even in laboratory conditions the change in frequency caused by the changetemperature will be much higher than the daily aging of the frequency of a thermally compensated quartz oscillator, so this indicator loses its practical value). OCXO aging rate: ±0.5ppb~±10ppb/day (72 hours after power on), ±30ppb~±2ppm (first year), ±0.3ppb~±3ppm (ten years).

----Voltage frequency control range: adjust the frequency control voltage from the reference voltage to the specified end voltage, the minimum change in the peak value of the crystal oscillator frequency.

----Explanation: The reference voltage is +2.5V, and the specified terminal voltage is +0.5V and +4.5V. 0.5V, the frequency change amount of the frequency control voltage is +130 parts per million, then VCXO frequency control voltage control voltage range is expressed as: ≥ ± 100ppm (2.5V ± 2V). Voltage Controlled Frequency Response Range: As the modulation frequency changes, the ratio between the peak frequency deviation and the modulation frequency. This is usually indicated by a specified modulation frequency that is a few dB below the specified modulation reference frequency.

----Note: The frequency response of VCXO frequency voltage regulation range is 0~10KHz.

----Linearity of frequency voltage regulation: a measure of the output voltage transfer characteristics of an input frequency compared to an ideal (straight-line) function, which expresses the allowable non-linearity of the entire range of frequency deviation in percent.

----Note: Typical VCXO frequency voltage regulation linearity: ≤±10%, ≤±20%. A simple method for calculating the linear frequency voltage control VCXO (when the polarity of the frequency voltage control is positive):

----frequency voltage regulation linearity=±((fmax-fmin)/f0)×100%----fmax: VCXO output frequency at maximum control voltage voltage----fmin: VCXO at minimum output frequency when voltage is controlled by voltage ----f0: voltage-controlled center frequency of voltage----single sideband phase noise £(f): ratio of phase modulated sideband power density to carrier power at position away from carrier f .

Overview of crystal oscillators

1. Type of crystal oscillator:

1.Conventional crystal oscillator The packaged crystal oscillator (PXO) is the simplest and most applicable oscillator whose main control element is a crystal element. Since it does not use temperature control and temperature compensation methods, its frequency-temperature characteristics are mainly determined by the crystal components used.

2. Voltage Controlled Crystal OscillatorA voltage controlled crystal oscillator (VCXO) is a crystal oscillator that uses an external control voltage to offset or modulate the output frequency. The temperature-frequency characteristics of VCXO are similar to those of PXO, we mainly determinecrystal components used.

3. TemperatureCompensatedCrystalOscillator (TCXO) temperature-compensated crystal oscillator includes a digitally compensated crystal oscillator (DCXODigitallyCompensatedCrystalOscillator) and a microcomputer-compensated crystal oscillator ( MCXOMicrocomputerCompensatedCrystalOscillator). The device uses an analog compensation circuit or a digital compensation method to compensate for the frequency-temperature characteristics of the crystal element by changing the reactance of the crystal load with temperature to obtain a crystal oscillator that reduces its frequency-temperature offset.

4. Temperature Controlled Crystal OscillatorA temperature controlled crystal oscillator (OCXO) is at least a quartz oscillator in which a quartz element is placed in a heat shield (such as a constant temperature bath) to control its temperature so that the crystal temperature remains practically unchanged.

5. Temperature compensated voltage controlled crystal oscillator (VCTCXO) A combination of a temperature compensated crystal oscillator and a voltage controlled crystal oscillator.

6.Voltage control - constant temperature crystal oscillator (VCOCXO) Combination of constant temperature crystal oscillator and voltage controlled crystal oscillator.

2.The main parameters of the crystal oscillator Frequency accuracy: When the rated supply voltage, rated load resistance, reference temperature (252°C) and other conditions remain unchanged, the frequency of the crystal oscillator will correspond to its set rated value. value Maximum allowable deviation, i.e. (fmax-fmin)/f0;

★Temperature stability: other conditions remain unchanged, the maximum change in the output frequency of the crystal oscillator in the specified temperature range relative to the allowable frequency deviation of the sum of output frequency extremes within the temperature range, that is, (fmax-fmin) / (fmax+fmin);< /p>

★ Frequency adjustment range: Change the output frequency range by adjusting the variable element of the crystal oscillator. Frequency adjustment function: ① adjust the output frequency to the set value within the frequency range;

②Due to aging or other reasons, the output frequency of the crystal oscillator deviates, adjust the output frequency to the specified value.

★FM characteristics (voltage control): including FM frequency offset, FM sensitivity and FM linearity. ①Frequency modulation deviation: output frequency difference when the control voltage of the voltage controlled crystal oscillator changes from the rated maximum value to the minimum value. ②Frequency modulation sensitivity: output frequency change caused by the change blockI have a voltage controlled crystal oscillator plus a control voltage. ③ FM Linearity: It is a measure of the transmission characteristics of a modulation system compared to an ideal straight line (least squares). Usually expressed as a percentage deviation from an ideal straight line within a given range

★Load characteristic: the rest of the conditions remain unchanged, the maximum allowable frequency deviation of the output frequency of the crystal oscillator relative to the output frequency at rated load within the specified load range.

★Voltage characteristic: Other conditions remain unchanged, the maximum allowable frequency deviation of the output frequency of the crystal oscillator relative to the output frequency at the rated supply voltage within the specified supply voltage range.

★Interference: The ratio of the power of the discrete frequency component of the spectrum in the output signal, which is not related to the fundamental frequency (except for subharmonics) and the fundamental frequency, expressed in dBc.

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

★Frequency aging: Under certain environmental conditions, the process of drifting the output frequency of a system over time due to the aging of components (mainly quartz resonators). It is usually measured by the frequency difference over a certain time interval. For highly stable crystal oscillators, since the output frequency drifts approximately linearly in one direction over long operating times, the aging rate (the relative change in frequency per unit time) is often used to measure it. For example: 10-8/day or 10-6/year etc.

★Daily fluctuation: After the oscillator has passed the specified warm-up time, it is measured every hour for 24 hours, and the test data is calculated by the formula S=(fmax-fmin)/f0 to obtain the daily fluctuation.

★ Starting characteristics: during the given warm-up time, the maximum change in the generator frequency value is represented as V=(fmax-fmin)/f0.

★Phase noise: A measure of short-term stability in the frequency domain. Expressed as the ratio £(f) of SSB noise to carrier noise, £(f) is directly related to noise fluctuation spectral density Sφ(f) and frequency fluctuation spectral density Sy(f) expressed in terms of the following formula: f2S( f)=f02Sy( f)=2f2£(f) f - Fourier frequency or deviation from the carrier frequency f0 - how to choose the carrier frequency of the crystal oscillator Choosing a crystal oscillator

Given certain parameters, the design engineer can select an oscillator suitable for the application

---- Countless electronic circuits and applications today require precise timing or reference clocks. Crystal clocks are well suited for many of these applications.

----Clock generators are supplied in various configurations, which are characterized by a widespectrum of electrical characteristics. There are several different types: voltage controlled crystal oscillators (VCXO), temperature compensated crystal oscillators (TCXO), oven controlled crystal oscillators (OCXO), and digitally compensated crystal oscillators (DCXO). Each type has its own unique characteristic performance.

----Frequency stability considerations---- One of the main characteristics of crystal oscillators is stability within 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.

----Design engineers must carefully determine the actual needs of a particular application, and then specify the stability of the generator. Too high a rate means you will spend more money.

----For applications requiring frequency stability of ±20ppm or more, conventional uncompensated crystal oscillators can be used. For ±1 to ±20 ppm stability, TCXO should be considered. For stability below ±1 ppm, an OCXO or DCXO should be considered.

----Output----Other parameters to consider are output type, phase noise, jitter, voltage regulation, load stability, power consumption, package shape, shock and vibration, and electromagnetic interference (EMI). ). The crystal oscillator can be compatible with HCMOS/TTL, ACMOS, ECL and sine wave output. Each type of output signal has its own unique waveform characteristics and applications. Attention should be paid to the requirement for three states or additional outputs. For some applications, symmetry, rise and fall times, and logic levels are also specified. Many DSP chipsets and communications often require strong symmetry (45% to 55%) and fast rise and fall times (less than 5 ns).

----Phase Noise and Jitter----Phase noise measured in the frequency domain is an accurate measure of short-term stability. It is measured to within 1 Hz of the center frequency and is typically measured down to 1 MHz.

----The oscillator phase noise improves at frequencies further from the center frequency. The TCXO and OCXO, as well as other crystal oscillators using fundamental or harmonic modes, have the best phase noise performance. Oscillators that use a PLL synthesizer to generate the output frequency typically exhibit worse phase noise performance than oscillators that do not use PLL technology.

---- Jitter is related to phase noise but is measured in the time domain. Jitter, expressed in picoseconds, is available in RMS or peak value.

is the peak measurement. Many applications, such as communications networks, wireless communications, ATMs, and SONET, require stringent mixing requirements. It is necessary to pay close attention to the characteristicsm jitter and phase noise generators used in these systems.

----Influence of power supply and load----The frequency stability of the generator is also affected by fluctuations in the voltage of the generator's power supply and fluctuations in the load of the generator. The right choice of oscillators can minimize these effects. Designers should verify generator performance under recommended supply voltage tolerances and loads. An oscillator rated for only 15 pF cannot be expected to perform well at 50 pF. Generators operating at supply voltages higher than recommended will also exhibit poor waveform and stability.

---- For devices that require battery power, power consumption must be considered. Products that use 3.3V need to design oscillators that work at 3.3V. ---- Lower voltage allows the product to run with low power consumption. Most of today's commercially available surface mount generators operate at 3.3V. Many perforated generators using legacy 5V devices are being redesigned to operate at 3.3V.

----Packaging is similar to other electronic components, and clock generators are also using smaller and smaller sizes. For example, the M-tron M3L/M5L series surface mount oscillators are now available in 3.2 x 5.0 x 1.0mm packages. Typically, smaller devices are more expensive than larger surface mount or through-hole mount devices. Small packets are often a compromise between performance, output selection, and frequency selection.

----Working Environment----The actual application environment of the oscillator should be carefully considered. For example, strong vibration or shock can cause problems for oscillators.

----Besides the possibility of physical damage, vibration or shock can cause erratic operation at certain frequencies. This external noise can cause frequency spikes, increased noise levels, and intermittent generator failure. ---- For applications requiring special EMI compatibility, EMI is another priority. In addition to using proper motherboard layout techniques, it is important to select a clock generator that provides the least amount of radiation. In general, oscillators with slower rise/fall times exhibit better EMI performance.

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