Electrical Parameters of Passive crystal Oscillator
Electrical parameters of passive crystal oscillator
1. Nominal frequency
It refers to the frequency specified in the specification of crystal elements, that is, the ideal working frequency expected by users in circuit design and component selection.
2. Frequency tolerance/accuracy
The maximum deviation (PPM:1/1000000) of operating frequency from the nominal frequency at reference temperature under specified conditions.
3. Frequency stability
The allowable deviation value of working frequency relative to reference temperature in the whole working temperature range under specified conditions.
4. Aging
Refers to the frequency drift caused by time under specified conditions. This index is necessary for precision crystal, but it "has no clear test conditions, but is continuously supervised by the manufacturer through planned sampling inspection of all products. Some crystal elements may be worse than the specified level, which is allowed" (according to the IEC notice).
5. Resonance resistance (RR)
It refers to the equivalent resistance of the passive crystal oscillator at the resonant frequency. When the effect of C0 is not considered, it is also approximately equal to the dynamic resistance R1 or equivalent series resistance (ESR) of the passive crystal oscillator.
This parameter not only controls the quality factor of the passive crystal oscillator but also determines the oscillation level of the crystal oscillator in the applied circuit, thus affecting the stability of the crystal oscillator and whether it can start up ideally. So it is an important index parameter of the crystal oscillator. Generally, for a given frequency, the smaller the selected crystal oscillator is, the higher the average value of ESR may be; In most cases, the resistance value of a specific crystal element can not be predicted in the manufacturing process, but can only be guaranteed to be lower than the maximum value given in the specification.
6. Load resonant resistance (RL)
It refers to the resistance of the crystal oscillator connected in series with specified external capacitance
at load resonance frequency FL. For a given crystal oscillator, the value of load resonant resistance depends on the value of external capacitance working with the element. The resonant resistance of the capacitor connected in series is always greater than that of the crystal oscillator itself.
7. Load capacitance (CL)
Together with the passive crystal oscillator, the effective external capacitance of the load resonant frequency FL is determined. CL in crystal element specification is a test condition as well as a use condition.
This value can be adjusted according to the user's specific situation to fine-tune the actual working frequency of FL (that is, the manufacturing tolerance of crystal can be adjusted). But it has a suitable value, otherwise, it will bring deterioration to the oscillation circuit.
Its value is usually 10pF, 15pF, 20pF, 30pf, 50pF, ∝ and so on. When CL is marked as ∝, it means that it is applied in the series resonant circuit, no external capacitor is added, and the working frequency is the Series resonant frequency FR of the passive crystal oscillator. We should note that for some crystal oscillators (including chip applications without package), the deviation of the actual capacitance of the circuit of ± 0.5pf can produce a frequency error of ± 10ppm under a given load capacitance (especially when the load capacitance is small). Therefore, load capacitance is a very important order specification index.
8. Static capacitance (C0)
It is the capacitance in the static arm of the equivalent circuit. Its size mainly depends on the electrode area,
wafer thickness and wafer processing technology.
9. Motional Capacitance(C1)
It is the capacitance in the dynamic arm of the equivalent circuit. Its size mainly depends on the electrode area, in addition, it is also related to the parallelism of the quartz wafers and the size of fine adjustment.
10. Motional inductance
It is the inductance in the dynamic arm of the equivalent circuit. Dynamic inductance and dynamic capacitance are a pair of related quantities.
11. Resonance frequency (FR)
It refers to the lower one of the two resistive frequencies 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 series resonant frequency (FS). This frequency is the natural resonant frequency of quartz crystals. In the design of high stability crystal oscillator, it is used as the design parameter to make the crystal oscillator work stably at the nominal frequency, determine the frequency adjustment range, set the frequency fine-tuning device, and other requirements.
12. Load resonant frequency (FL)
It refers to one of the two frequencies when the crystal oscillator is connected in series or in parallel with a load capacitor that their combined impedance is resistive under specified conditions. When the load capacitors are connected in series, FL is the lower of the two frequencies; When the load capacitors are connected in parallel, FL is the higher frequency. For a given load capacitance value (CL), the two frequencies are the same in practical effect; Moreover, this frequency is the actual frequency of crystal oscillators in most applications, and it is also the test index parameter for manufacturers to meet the user's requirements of the nominal frequency.
13. Quality factor (Q)
The quality factor, also known as the mechanical Q value, is an important parameter to reflect the performance of the resonator.
14. Drive Level
It is a measure of the excitation conditions applied to a crystal element, expressed in terms of dissipated power. The frequency and resistance of all passive crystal oscillators change with the change of excitation level to a certain extent, which is called excitation level correlation (DLD), Therefore, the excitation level in the order specification must be the excitation level in the practical application circuit of the crystal oscillator. Because of the inherent drive level dependence of crystal oscillators, users must pay attention to and ensure that there is no phenomenon of poor starting or abnormal over-excitation frequency due to too low a drive level when designing an oscillation circuit and using a passive crystal oscillator.
15.DLD
Due to the piezoelectric effect, the drive level forces the oscillator to produce mechanical oscillation. In this process, the acceleration work is converted into kinetic energy and elastic performance, and the power consumption is converted into heat. The conversion of the latter is caused by the internal and external friction of the quartz wafer. The friction loss is related to the velocity of the vibrating particle. When the vibration is no longer linear, or when the mounting point of tensile or strain, displacement or acceleration of the quartz wafer at its surface or internal reaches the critical value, the friction loss will increase. This results in changes in frequency and resistance.
16.DLD2
The difference between the maximum value and the minimum value of loading resonant resistance under different drive levels.
17.RLD2
The average value of load resonant resistance under different drive levels.
18. Parasitic response
In addition to the main response (required frequency), all crystal oscillators have other frequency responses.
The way to reduce the parasitic response is to change the geometry of the wafer, the electrode and the wafer processing technology, but at the same time, it will change the dynamic and static parameters of the crystal.