常见问题
A Piezoelectric Quartz Crystal Unit is an electronic component used in frequency control, filtering, and clocking applications.
It consists of a quartz resonator equipped with electrodes, housed in a hermetically sealed package that provides some means of connection to a circuit.
Piezoelectric Quartz Crystal Units are usually called "crystals."
The differences are in size, shape and some performance characteristics. AT-cut crystal blanks are circular, and AT-strip cut crystal blanks are rectangular. Both types are AT-cuts, but several differences in performance characteristics exist because of the different geometry of the blanks.
In general, given the same frequency and overtone, the AT strip will have a lower value of Shunt Capacitance (Co) and Motional Capacitance (C1). It will also have a higher value of Effective Series Resistance (ESR) and Motional Inductance (L1). Because pullability is a function of Co and C1, AT strip cut crystals have less pullability than ocular AT cut crystals.
All quartz crystal resonators have a series resonant frequency (fs, frequency of lowest impedance). At this frequency, the crystal appears resistive in the circuit.
Crystals can be "pulled" from this series frequency by adding reactance (capacitance) in series with the crystal. When operated in combination with an external load capacitance (CL) the crystal oscillates in a frequency range slightly above its series resonant frequency. This is the parallel (load resonant) frequency.
When ordering a parallel crystal, always specify the nominal parallel resonant frequency and the amount of load capacitance in picoFarads (pF).
Alternatively, a standard value of CL (such as 20pF) can be ordered; the capacitor values are then calculated to match the crystal CL.
Note that a series type will oscillate in a parallel oscillator, and vice versa. The observable difference is a frequency offset of less than + 1000ppm (0.1 %) from the nominal frequency.
Yes, this industrial temperature range (-40 ºC to + 85 ºC ) is optional on most crystals. Depending on the crystal package, temperature ranges of up to -55 ºC to +105 ºC may be available.
Contact ARGO's engineering department to discuss your specific requirements.
使用晶体振荡前必须考虑之事项…
在开始设计使用晶体振荡器前，以下问题请详加考虑，
因为这些问题将是晶体决定规格之关键。
1. 线路上需要什么频率?
2. 常温下(25℃)频率可容许多少误差?
3. 操作温度范围是多少?
4. 在操作温度范围内可允许多少偏移误差量?
5. 晶体振荡在线路上是并联或串联振荡?
6. 如是并联振荡，负载电容是多少?
7. 频率推动变化量(pull ability)是否很重要?
8. 您所需之晶体振荡器大小尺寸及包装为何?
在开始设计使用晶体振荡器前，以下问题请详加考虑，
因为这些问题将是晶体决定规格之关键。
1. 线路上需要什么频率?
2. 常温下(25℃)频率可容许多少误差?
3. 操作温度范围是多少?
4. 在操作温度范围内可允许多少偏移误差量?
5. 晶体振荡在线路上是并联或串联振荡?
6. 如是并联振荡，负载电容是多少?
7. 频率推动变化量(pull ability)是否很重要?
8. 您所需之晶体振荡器大小尺寸及包装为何?
The HC49U crystal utilizes a circular AT cut crystal blank. Due to their small size, most surface mount crystals use a rectangular AT strip cut crystal blank. ("Strip" refers to the rectangular shape of the blank.)
While both are AT cut blanks, several differences exist due to their different geometry. In general, given the same frequency and overtone, an AT strip cut will have a lower Shunt Capacitance (CO) and Motional Capacitance (C1) than the AT cut. Because pullability is a function of the capacitance ratio CO/C1, AT strip crystals have less pullability.
Quartz crystals are not linear devices and they do not behave in a linear way. But, it is theoretically possible to achieve symmetrical pulling.
Pullability is the amount by which the frequency of a crystal will change when the circuit condition is switched from series to parallel resonance.
Pullability is also used to describe the frequency change that occurs when the load capacitance is switched from one value to another.
"Activity dips" are sudden increases in the resistance of a crystal that may perturb the frequency.
I would not make the change without a thorough investigation. Some of the SMD (surface mount device) crystal units are comparable to conventional crystal units, others are not.
In general, SMD's have higher resistance, differing values of shunt and motional capacitance, and are more sensitive to drive level. The pullability characteristics of these devices may differ significantly from a conventional crystal.
I would recommend a fairly exhaustive qualification sequence before making the switch.
Drive Level - HC49U crystals are typically rated 1.0mW max, while the HC49S and most SMD crystals are rated at only 0.5mW or 0.1mW maximum. See question 11 for more information on exceeding the maximum drive level of a crystal.
Pullability -some applications require tuning the frequency of the oscillator, either by mechanically or electrically changing the value of load capacitance.
The HC49S and surface mount crystals have less pullability than the HC49U. It is important to verify whether the HC49S or the surface mount crystal will have sufficient pull range for the application. Generally, an HC49S has one half the pullability as an HC49U.
ESR - The Equivalent Series Resistance (ESR) is generally higher with the HC49S and surface mount crystals, and can cause a problem if the oscillator circuit does not have sufficient loop gain.
Exceeding the maximum drive level (power dissipation) of a crystal can lead to an increased rate of aging, Drive Level Dependency (DLD) problems, increased number and intensity of activity dips that can stop oscillation and, at very high drive levels, breakage of the crystal blank.