quartz crystal
Quartz (SiO2) is composed of two elements, silicon and oxygen.
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Cutting angle and vibration mode
According to different purposes, quartz crystals can be made by cutting quartz crystal rods into crystal pieces according to specific crystal orientations. The vibration profile, frequency variation, and characteristics are shown in the figure:
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Frequency
This frequency specifically refers to the performance indicators of crystal components, expressed in MHz or KHz.
Frequency deviation
The allowable deviation of the nominal frequency at a certain temperature (usually 25 ℃) is expressed as a percentage (%) or parts per million (ppm).
Frequency Stability
Stability refers to the allowable deviation of the nominal frequency within a certain temperature range, which is specified as 0 at 25 ℃, expressed as a percentage (%) or parts per million (ppm) of the nominal frequency. As mentioned earlier, this parameter is closely related to the chamfer of quartz chips.
Frequency Temperature Characteristics
The frequency temperature characteristic curve of AT thickness cut quartz crystal as the cutting angle changes. Due to the temperature characteristic of AT cutting frequency being equivalent to a cubic equation, it has good frequency stability over a wide temperature range.
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Working temperature range
The temperature range within which quartz crystal components operate within the specified error.
Storage temperature range
The temperature range within which a crystal maintains its standard characteristics in a non working state.
equivalent circuit
A quartz crystal that generates the main resonant frequency can be expressed as an equivalent circuit, which generally includes a series circuit composed of inductance, capacitance, and resistance, and a capacitor connected in parallel with this series circuit, as shown in the figure.
Here, C0 is the static capacitance, including the static capacitance between electrodes and the stray capacitance between terminals.
When considering quartz crystal components as an electronic and mechanical vibration system, L1 and C1 are its equivalent constants. Due to the fact that these two constants depend on factors such as cutting type, cutting angle, chip size, and electrode structure, and can be repeatedly adjusted, the accuracy of quartz crystal components can be achieved very high.
R1 represents oscillation loss, which is controlled by cutting method, assembly method, chip shape, and chip size.
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Load capacitance (CL)
Once any external capacitor is connected in series with a quartz crystal component, it becomes a determining factor in its resonant frequency. When the load capacitance changes, the frequency will also change accordingly. Therefore, when used in circuits, the standard load capacitance is often used to fine tune the frequency to the desired value.
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Static capacitance (C0)
Static capacitance between electrodes and stray capacitance in the installation system.
Equivalent series resistance (ESR, Rr, R1)
The resistance value of a crystal at its resonant frequency, ESR represents the impedance of the crystal, measured in ohms.
Drive level
A function of the excitation current flowing through a crystal. The excitation level is the numerical value of power loss in the crystal. The maximum power is the power consumed by most power devices to maintain operation while ensuring normal electrical parameters, measured in mW or uW. The excitation level should be maintained at the minimum value required to ensure normal and stable oscillation of the quartz crystal, in order to avoid poor aging characteristics and crystal damage
overtone crystal
Crystals usually operate at the fundamental frequency, but with slight adjustments to the circuit, they can operate at the third, fifth, seventh, and ninth harmonics. In order to ensure that the overtone crystal vibrates at specific harmonics, its profile angle, parallelism, and surface smoothness have been specially modified.
insulation resistance
The resistance between leads or between leads and the casing.
Quality factors
The "Q" value is the quality factor of dynamic arm resonance in the equivalent circuit of a crystal. The maximum stability that an oscillating circuit can achieve is directly related to the Q value of the crystal in the circuit. The higher the Q value, the smaller the crystal bandwidth ("F"), the steeper the change in reactance value (fs fa), and the smaller the impact of external reactance on the crystal.
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CL=(C1 × C2)/(C1 C2) Stray capacitance Stray capacitance can vary between 2pF-6pF
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pay attention to
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When applied to CMOS oscillation circuits, Rd in the circuit diagram is essential to maintain the excitation level within a specific numerical range and achieve stable oscillation.
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C1 and C2 must be within the range of 10-31pF. If C1 is less than 10pF or C2 is greater than 30pF, oscillation is easily affected by different circuit conditions, which can increase the excitation level or decrease the negative resistance, resulting in unstable oscillation frequency.
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When wiring crystal oscillation circuits, they should be as short as possible.
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The stray capacitance between the circuit and the grounding part should be reduced.
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The crystal oscillation circuit should avoid bridging with other circuit components.
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Ultrasonic cleaning can cause degradation of crystal properties
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parasitism
All quartz resonators have parasitic (unexpected) oscillation responses outside the main frequency. They are represented in the equivalent circuit diagram as additional response loops formed by R1, L1, and C1. The ratio of the impedance RNW of parasitic response to the impedance Rr of the main resonant wave is usually expressed in terms of attenuation constant dB and defined as parasitic attenuation
aNW=-20 · lg
For oscillating crystals, 3 to 6dB is completely sufficient For filtering crystals, the usual requirement is to exceed 40dB This specification requirement can only be achieved through special design processes and the use of very small dynamic capacitors The achievable attenuation decreases with the increase of frequency and the number of overtones The parasitic attenuation of typical planar quartz chip resonators is better than that of planar convex or double-sided convex chip resonators When determining parasitic response parameters, an acceptable level of parasitic attenuation and the relative relationship between parasitic frequency and main frequency should be determined simultaneously
In AT cutting, for planar chips, the "discordant response" only exists between 40 and 150 kHz of the main response, while for flat convex or double-sided convex chips, the parasitic response is between 200 and 400 kHz In the above measurement methods, parasitic response attenuation can be measured when it reaches 20 to 30dB, and for higher attenuation Compensation for C0 is necessary
DLD
The amplitude of the mechanical vibration of a quartz oscillator increases proportionally with the amplitude of the current The relationship between power and response impedance is Pc=12qR1. High excitation power can cause resonance damage or electrode evaporation. The maximum allowable power should not exceed 10mV
Due to the power oscillation of L1 and C1 resistance, there exists Qc=Q x Pc. If Pc=1mV, Q=100000, Qc is equivalent to 100W Due to low Pc power, the oscillation amplitude will exceed, ultimately causing the frequency of the crystal to shift upward
As the frequency of crystal overtones increases, the dependence on excitation power becomes more significant The above figure shows typical results, but the precise expected results are still influenced by factors such as crystal design and processing, mechanical chip parameters, electrode size, and dispensing conditions
It can be seen that the excitation power must be carefully determined to maintain a good relationship between the crystal in production and use
Nowadays, the excitation power of a semiconductor oscillation circuit is generally 0.1mV, so it is also generally carried out at 0.1mV during crystal production
A high-quality crystal can easily vibrate, and its frequency remains stable as it gradually increases from 1nW Nowadays, semiconductor circuits with very low power at both ends of the crystal can also work well at very low power
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The above figure shows a comparison of the operating curves of a crystal with or without dependence on excitation power
Crystals with poor vapor deposition electrodes and insufficient surface cleanliness can exhibit high impedance at low power, as shown in the figure. This effect is called excitation power dependence (DLD). Typically, DLD is tested at 1-10mV and then 1mV during production, and the impedance changes can be used as a standard for testing Obviously, increasing the testing content will significantly increase the cost of crystal production
The DLD limit value can be quickly determined using appropriate testing instruments, but only qualified/unqualified tests can be conducted IEC Draft 248 covers the measurement methods for the dependence of excitation power developed according to (DIV) IEC444-6
Providing an optimized oscillation loop with sufficient feedback and good pulses can greatly eliminate internal problems of oscillation
aging
The change in working frequency within a specific time range is generally expressed as the maximum value, and the unit is parts per million (ppm/year) of the annual frequency change. There are many reasons why frequency varies over time, such as sealing characteristics and integrity, manufacturing process, material type, operating temperature, and frequency.
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Common calculation formulas:
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