Detailed explanation of crystal parameters explained by crystal circuit knowledge

1. The difference between crystal and crystal

1) The crystal oscillator is an abbreviation of active crystal oscillator, also called oscillator. The English name is oscillator. The crystal is the abbreviation of passive crystal oscillator, also called resonator. The English name is crystal. 2) Crystal (passive) is generally a non-polar component that is directly inserted into two pins . It is necessary to use a clock circuit to generate an oscillating signal. Commonly available in 49U, 49S packages. 3) The crystal oscillator (active) is generally a four-pin package with a clock circuit inside, which can generate an oscillating signal only by supplying power. Generally divided into 7050, 5032, 3225, 2520 several package forms.

2. MEMS silicon crystal oscillator is distinguished from quartz crystal oscillator. MEMS silicon crystal oscillator is made of silicon and is made of advanced semiconductor technology. Therefore, in terms of high performance and low cost, there is an obvious advantage of quartz, which is manifested in the following aspects: 1) Fully automated semiconductor process (chip level), no air tightness problem, never stop vibration. 2) The internal temperature compensation circuit is included, no temperature drift, and the temperature is guaranteed at -40-85 °C. 3) The average trouble-free working time is 500 million hours. 4) Seismic performance is 25 times that of a quartz oscillator. 5) Support any frequency point of 1-800MHZ, and accurately output 5 digits after the decimal point. 6) Support 1.8V, 2.5V, 2.8V, 3.3V multiple working voltage matching. 7) Support various precision matching such as 10PPM, 20PPM, 25PPM, 30PPM, 50PPM. 8) Support all standard size packages of 7050, 5032, 3225, 2520. 9) Standard four-pin, six-pin package, without any design changes, directly replace the quartz oscillator. 10) Supports product types such as differential output, single-ended output, voltage control (VCXO), and temperature compensation (TCXO). 11) 300% market growth rate, expected to replace more than 80% of the quartz oscillator market within three years.

3. The equivalent circuit of the 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 dynamic capacitor also called equivalent series capacitor; L1 is dynamic inductor also called equivalent series inductance; R1 is dynamic resistance also called equivalent series resistance; C0 is static capacitor is also called equivalent parallel capacitor.

There are two most useful zero phase frequencies in this equivalent circuit, one of which is the resonant frequency (Fr) and the other is the antiresonant frequency (Fa). When a crystal element is actually applied to an oscillating circuit, it is generally also coupled to a load capacitor to work together to operate the crystal at a frequency between Fr and Fa, which is determined by the phase and effective reactance of the oscillating 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 component specification, which is the ideal operating frequency desired by the user in circuit design and component purchase.

4.2 Adjusting the frequency difference

The maximum allowable deviation of the operating frequency from the nominal frequency at the reference temperature. Commonly expressed as ppm

If the ppm is converted into the percent sign "%": 1 ppm = 0.0001%.

However, in most scientific and technical journals, the ppm is not used, but the thousandth mark "‰" is used, and the ppm is converted into ‰: 1ppm = 0.001‰.

Ppm refers to part per million, and b, t represents billion and trillion, respectively.

That is, 1ppm = 10^-6 orders of magnitude, similarly ppb, ppt, etc., -9 times and -12 times, respectively.

4.3 Temperature deviation The allowable deviation of the operating frequency from the operating frequency over the reference temperature over the entire temperature range. Commonly used in ppm.

4.4 Aging rate refers to the frequency drift caused by time under specified conditions. This indicator is necessary for precision crystals, but it “has no 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 levels. Allowed" (according to the IEC announcement). The best solution to the aging problem can only be through close consultation between the manufacturer and the user.

4.5 Resonant resistance (Rr) refers to the equivalent resistance of a crystal element at the resonant frequency. When the effect of C0 is not considered, it is also approximately equal to the so-called crystal dynamic resistance R1 or equivalent series resistance (ESR). This parameter controls the quality factor of the crystal component and also determines the level of crystal oscillation in the applied circuit, thus affecting the stability of the crystal so that it can ideally oscillate. So it is an important indicator parameter for crystal components. In general, for a given frequency, the smaller the crystal box is selected, the higher the average value of ESR may be. In most cases, the resistance value of a specific crystal component cannot be predicted during the manufacturing process, but only guaranteed. The resistance will be lower than the maximum given in the specification.

4.6 Load Resistor Resistance (RL) is the resistance of a crystal component in series with a specified external capacitor at the load resonant frequency FL. For a given crystal body, the load resonance resistance value depends on the load capacitance value working with the component, and the resonance resistance after the load capacitance on the string is always greater than the resonance resistance of the crystal element itself.

4.7 The load capacitance (CL) together with the crystal element determines the effective external capacitance of the load resonant frequency FL. The CL in the crystal component specification is a test condition and a use condition. This value can be appropriately adjusted according to the situation when the user uses it to fine tune the actual working frequency of the FL (that is, the manufacturing tolerance of the crystal can be adjusted). However, it has a suitable value, otherwise it will bring deterioration to the oscillating circuit. The value is usually 10pF, 15pF, 20pF, 30pF, 50pF, ∝, etc., when CL is marked as ∝, it is applied in the series resonant type circuit. Do not add a load capacitor, and the operating frequency is the crystal (series) resonant frequency Fr. Users should be aware that for certain crystals (including unpackaged oscillator applications), the deviation of the actual capacitance of the circuit of ±0.5pF can produce ±10 under a given load capacitance of a production specification (especially for small load capacitances). ×10-6 frequency error. Therefore, load capacitance is a very important order specification.

4.8 Static Capacitance (C0) Equivalent Circuit Capacitance in the static arm. Its size depends mainly on the electrode area, wafer thickness and wafer processing.

4.9 Dynamic Capacitance (C1) The capacitance in the dynamic arm in the equivalent circuit. Its size depends mainly on the electrode area, and is also related to the parallelism of the wafer and the amount of fine adjustment.

4.10 Inductance in the dynamic arm in the dynamic inductance (L1) equivalent circuit. Dynamic inductance and dynamic capacitance are a pair of correlation quantities.

4.11 Resonant frequency (Fr) is the lower of the two frequencies at which the electrical impedance of the 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, which is approximately equal to the so-called series (branch) resonance frequency (Fs). This frequency is the natural resonant frequency of the crystal. In the design of the high-stability crystal oscillator, it is the design parameter when the crystal oscillator is stably operated at the nominal frequency, the frequency adjustment range is determined, and the frequency fine-tuning device is set.

4.12 Load Resonance Frequency (FL)

It means that under the specified conditions, the crystal element is connected in series or in parallel with a load capacitor, and the combined impedance appears as one of two frequencies when resistive. In series load capacitance, FL is the lower of the two frequencies; FL is the higher of the two when paralleling the load capacitance. For a given load capacitance value (CL), the actual effect is that the two frequencies are the same; and this frequency is the actual frequency that is exhibited in the circuit for most applications of the crystal, and is also the manufacturer. To meet the user's test indicator parameters for the product to meet the nominal frequency requirements.

4.13 Quality factor (Q) The quality factor is also called the mechanical Q value. It is an important parameter reflecting the performance of the resonator. It has the following relationship with L1 and C1: Q=wL1/R1=1/wR1C1 As above, the larger the R1 The lower the Q value, the greater the power dissipation and the instability of the frequency. On the contrary, the higher the Q value, the more stable the frequency.

4.14 Level of drive is a measure of the excitation condition applied to a crystal element expressed in terms of dissipated power. The frequency and resistance of all crystal components vary to some extent with the excitation level. This is called excitation level correlation (DLD), so the excitation level in the order specification must be the excitation in the crystal application circuit. Level. Because of the inherent excitation level correlation of the crystal components, the user must pay attention to and ensure that the excitation level is too low and the oscillation is poor or the excessive excitation frequency is abnormal when the oscillator circuit is designed and used.

4.15 Excitation Level Correlation (DLD) Due to the piezoelectric effect, the excitation level forces the harmonic oscillator 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 the internal and external friction of the quartz resonator. Friction loss is related to the velocity of the vibration mass. When the oscillation is no longer linear, or when the tensile or strain, displacement or acceleration inside or outside the quartz oscillator or the mounting point reaches a critical point, the friction loss will increase. This causes a change in frequency and resistance. The main causes of DLD failure during processing are as follows. The result may be that it cannot be oscillated : 1) There is particulate contamination on the surface of the resonator. The main cause is the unclean environment or illegal contact with the wafer surface; 2) mechanical damage of the resonator. The main cause is the scratches generated during the grinding process. 3) There are particles or silver balls in the electrode. The main cause is that the vacuum chamber is unclean and the coating rate is not suitable. 4) The mounting is poor electrode contact; 5) There is mechanical stress between the bracket, the electrode and the quartz plate.

4.16 DLD2 (unit: ohm)

The difference between the maximum and minimum values ​​of the load resonance resistance at different excitation levels. (eg: from 0.1uw to 200uw, a total of 20 steps).

4.17 RLD2 (unit: ohm)

The average value of the load resonance resistance at different excitation levels is closer to the value of the resonance resistance Rr, but is larger. (eg: from 0.1uw to 200uw, a total of 20 steps).

4.18 Spurious Response All crystal components have other frequency responses in addition to the main response (frequency required). The way to reduce the parasitic response is to change the geometry of the wafer, the electrodes, and the wafer processing process, but at the same time change the dynamic and static parameters of the crystal. Measurement of spurious response 1) SPDB uses DB to represent the difference between the amplitude of Fr and the maximum parasitic amplitude;

2) SPUR resistance at maximum parasitic; 3) SPFR minimum resistance parasitic distance from resonant frequency, expressed in Hz or ppm.

5. Classification of crystal oscillators

5.1 Package Quartz Oscillator (SPXO)

A quartz oscillator that does not apply temperature control and temperature compensation. The frequency temperature characteristics depend on the stability of the quartz crystal itself. 5.2 Temperature-compensated crystal oscillator (TCXO) adds a temperature compensation loop to reduce the crystal oscillator whose frequency changes due to changes in ambient temperature.

5.3 Voltage Controlled Crystal Oscillator (VCXO)

A quartz oscillator that controls the external voltage to change or modulate the output frequency. 5.4 Constant Temperature Groove Crystal Oscillator (OCXO)

The quartz oscillator or quartz crystal oscillator is held at a constant temperature in a constant temperature bath to control the quartz oscillator whose output frequency can maintain a small amount of change at ambient temperature.

In addition to the above four oscillators, with the application of PLL, Digital, Memory technology, the diversified quartz oscillators of other functions have also increased rapidly.