Detailed introduction to crystal oscillators and crystal parameters

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Detailed introduction to crystal oscillators and crystal parameters [Copy link]

1. The difference between crystal oscillator and crystal
(1) Crystal oscillator is the abbreviation of active crystal oscillator, also called oscillator. The English name is oscillator. Crystal is the abbreviation of passive crystal oscillator, also called resonator. The English name is crystal.
(2) Passive crystal oscillator (crystal) is generally a non-polar component with two pins directly plugged in, which requires the help of a clock circuit to generate an oscillation signal. Common packages include 49U and 49S.
(3) Active crystal oscillator (crystal oscillator) is generally a surface-mounted package with four pins, with a clock circuit inside, and can generate an oscillation signal only by power supply. It is generally divided into 7050, 5032, 3225, and 2520 packaging types.
2. The difference between MEMS silicon crystal oscillator and quartz crystal oscillator
MEMS silicon crystal oscillator uses silicon as raw material and is manufactured using advanced semiconductor technology. Therefore, in terms of high performance and low cost, it has obvious advantages over quartz, which are specifically reflected in the following aspects:
(1) Fully automated semiconductor process (chip level), no airtightness problems, and never stops oscillating.
(2) Contains temperature compensation circuit, no temperature drift, -40-85℃ full temperature guarantee.
(3) MTBF 500 million hours.
(4) Anti-vibration performance 25 times that of quartz oscillator.
(5) Supports any frequency point from 1-800MHZ, accurate to 5 decimal places.
(6) Supports 1.8V, 2.5V, 2.8V, 3.3V multiple working voltage matching.
(7) Supports 10PPM, 20PPM, 25PPM, 30PPM, 50PPM and other precision matching.
(8) Supports all standard size packages of 7050, 5032, 3225, 2520.
(9) Standard four-pin and six-pin packages, no design changes are required, directly replace quartz oscillators.
(10) Supports differential output, single-ended output, voltage control (VCXO), temperature compensation (TCXO) and other product types.
(11) With a market growth rate of 300%, it is expected to replace more than 80% of the quartz oscillator market within three years.
3. Equivalent circuit of crystal resonator

The figure above is a simplified circuit with the same impedance characteristics as the crystal resonator near the resonant frequency. Among them: C1 is the dynamic capacitance also known as the equivalent series capacitance; L1 is the dynamic inductance also known as the equivalent series inductance; R1 is the dynamic resistance also known as the equivalent series resistance; C0 is the static capacitance also known as the equivalent parallel capacitance.
There are two most useful zero-phase frequencies in this equivalent circuit, one is the resonant frequency (Fr) and the other is the anti-resonant frequency (Fa). When the crystal element is actually used in the oscillation circuit, it is generally connected with a load capacitor, and the crystal works at a frequency between Fr and Fa. This frequency is determined by the phase and effective reactance of the oscillation circuit. By changing the reactance conditions of the circuit, the crystal frequency can be adjusted within a limited range.
4. Key parameters
4.1 Nominal frequency
refers to the frequency specified in the crystal element specification, that is, the ideal operating frequency that the user hopes to achieve when designing the circuit and purchasing the components. 4.2 The maximum allowable deviation of the operating frequency from the nominal frequency when
adjusting the frequency difference reference temperature. It is usually expressed in ppm (1/106). 4.3 Temperature frequency difference The allowable deviation of the operating frequency relative to the reference temperature within the entire temperature range. It is usually expressed in ppm (1/106). 4.4 Aging rate refers to the frequency drift caused by time under specified conditions. This indicator is necessary for precision crystals, but it "does not have clear test conditions, but is continuously monitored by the manufacturer through planned sampling of all products. Some crystal components may be worse than the specified level, which is allowed" (according to the IEC announcement). The best solution to the aging problem can only be achieved through close consultation between manufacturers and users. 4.5 Resonant resistance (Rr) refers to the equivalent resistance of the crystal element at the resonant frequency. When the effect of C0 is not considered, it is also approximately equal to the so-called dynamic resistance R1 of the crystal or the equivalent series resistance (ESR). This parameter controls the quality factor of the crystal element and also determines the crystal oscillation level in the applied circuit, thus affecting the stability of the crystal and whether it can be ideally started. Therefore, it is an important indicator parameter of the crystal element. In general, for a given frequency, the smaller the crystal box selected, the higher the average ESR may be; in most cases, the resistance value of a specific crystal element cannot be predicted during the manufacturing process, but it can only be guaranteed that the resistance will be lower than the maximum value given in the specification. 4.6 Load resonant resistance (RL) refers to the resistance of the crystal element when it is connected in series with the specified external capacitance at the load resonant frequency FL. For a given crystal element, its load resonant resistance value depends on the load capacitance value working with the element. The resonant resistance after the load capacitance is connected in series is always greater than the resonant resistance of the crystal element itself. 4.7 Load capacitance (CL) together with the crystal element determines the effective external capacitance of the load resonant frequency FL. CL in the crystal element specification is a test condition and a use condition. This value can be adjusted appropriately according to the specific situation when the user uses it to fine-tune the actual operating frequency of FL (that is, the manufacturing tolerance of the crystal can be adjusted). But it has a suitable value, otherwise it will cause deterioration to the oscillation circuit. Its value is usually 10pF, 15pF, 20pF, 30pF, 50pF, ∝, etc. When CL is marked as ∝, it means that it is used in a series resonant circuit, no load capacitance should be added, and the operating frequency is the (series) resonant frequency Fr of the crystal. Users should note that for some crystals (including unpackaged oscillator applications), under a certain production specification set load capacitance (especially when the load capacitance is small), a deviation of ±0.5pF in the actual circuit capacitance can produce a frequency error of ±10×10-6. Therefore, load capacitance is a very important order specification indicator. 4.8 Static capacitance (C0) The capacitance in the static arm of the equivalent circuit. Its size mainly depends on the electrode area, the thickness of the wafer and the wafer processing technology. 4.9 Dynamic capacitance (C1) The capacitance in the dynamic arm of the equivalent circuit. Its size mainly depends on the electrode area, and is also related to the parallelism of the wafer and the size of the fine-tuning amount. 4.10 Dynamic inductance (L1) The inductance in the dynamic arm in the equivalent circuit. Dynamic inductance and dynamic capacitance are a pair of related quantities. 4.11 Resonant frequency (Fr) refers to the lower of the two frequencies at which the electrical impedance of a crystal element is resistive under specified conditions. According to the equivalent circuit, when the effect of C0 is not considered, Fr is determined by C1 and L1 and is approximately equal to the so-called series (branch) resonant frequency (Fs). This frequency is the natural resonant frequency of the crystal. In the design of a high-stability crystal oscillator, it is used as a design parameter to ensure that the crystal oscillator works stably at the nominal frequency, determine the frequency adjustment range, and set the frequency fine-tuning device.

4.12 Load resonant frequency (FL)
refers to one of the two frequencies when the crystal element is connected in series or in parallel with a load capacitor under specified conditions, and its combined impedance presents a resistive nature. When the load capacitor is connected in series, FL is the lower of the two frequencies; when the load capacitor is connected in parallel, FL is the higher of the two frequencies. For a given load capacitance value (CL), in practical terms, the two frequencies are the same; and
this frequency is the actual frequency that the crystal will exhibit in the circuit when used in most applications, and is also a test parameter that manufacturers use to meet user requirements for products that meet nominal frequency requirements.

4.13 Quality Factor (Q)
Quality factor, also known as mechanical Q value, is an important parameter that reflects the performance of the resonator. It has the following relationship with L1 and C1:
Q=wL1/R1=1/wR1C1
As shown in the above formula, the larger the R1, the lower the Q value, the greater the power dissipation, and it will also cause frequency instability. Conversely, the higher the Q value, the more stable the frequency.
4.14 Drive level (Level of drive)
is a measure of the excitation conditions applied to the crystal element, expressed in terms of dissipated power. The frequency and resistance of all crystal elements vary to a certain extent with the change of the drive level, which is called the drive level dependency (DLD). Therefore, the drive level in the order specification must be the drive level in the actual application circuit of the crystal. Because of the inherent drive level dependency of the crystal element, when designing the oscillation circuit and using the crystal, the user must pay attention to and ensure that the drive level is too low to cause poor oscillation or excessive drive frequency abnormality.
4.15 Drive level dependence (DLD)
Due to the piezoelectric effect, the drive level forces the resonator to produce mechanical oscillations. In this process, the acceleration work is converted into kinetic energy and elastic energy, and the power consumption is converted into heat. The latter conversion is caused by friction inside and outside the quartz resonator.
Friction losses are related to the speed of the vibrating particles. When the oscillation is no longer linear, or when the tension or strain, displacement or acceleration inside the quartz resonator or on its surface and mounting point reaches a critical value, the friction losses will increase. This will cause changes in frequency and resistance. The main reasons for poor DLD
during processing
are as follows, and the result may be that oscillation cannot be started: 1) There is particulate contamination on the surface of the resonator. The main reasons are unclean production environment or illegal contact with the wafer surface;
2) Mechanical damage to the resonator. The main reason is scratches caused by grinding.
3) There are particles or silver balls in the electrode. The main reasons are unclean vacuum chamber and inappropriate coating rate.
4) The mounting is a poor contact between the electrode;
5) Mechanical stress between the bracket, electrode and quartz wafer.
4.16 DLD2 (unit: ohm)
The difference between the maximum and minimum values of the load resonant resistance at different excitation levels. (For example: from 0.1uw~200uw, a total of 20 steps).
4.17 RLD2 (unit: ohm)
The average value of the load resonant resistance at different excitation levels <is close to the value of the resonant resistance Rr, but is larger>. (For example: from 0.1uw~200uw, a total of 20 steps).
4.18 Parasitic response
All crystal components have other frequency responses in addition to the main response (the required frequency). The way to reduce the parasitic response is to change the geometric size of the chip, the electrode, and the chip processing technology, but at the same time it will change the dynamic and static parameters of the crystal.
Measurement of parasitic response
1) SPDB The difference between the amplitude of Fr and the maximum parasitic amplitude is expressed in DB;
2) SPUR The resistance at the maximum parasitic;
3) SPFR The distance between the minimum resistance parasitic and the resonant frequency, expressed in Hz or ppm.
5. Classification of crystal oscillators
5.1 Package quartz oscillator (SPXO)
A quartz oscillator that is not temperature controlled or temperature compensated. The frequency-temperature characteristics depend on the stability of the quartz oscillator crystal itself.
5.2 Temperature compensated quartz oscillator (TCXO)
A quartz oscillator that has an additional temperature compensation circuit to reduce the frequency variation caused by ambient temperature fluctuations.
5.3 Voltage controlled quartz oscillator (VCXO)
A quartz oscillator that controls an external voltage to change or modulate the output frequency.
5.4 Oven-controlled quartz oscillator (OCXO) A quartz oscillator that
uses a constant temperature bath to keep the quartz oscillator or quartz oscillator crystal at a certain temperature, and controls its output frequency to maintain a very small change at ambient temperature.
In addition to the above four oscillators, with the application of PLL, Digital, and Memory technologies, other diversified quartz oscillators with other functions are also increasing rapidly.