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How to choose a crystal oscillator
The release date:2013/6/9 9:25:11   Reading:3282 times
Customer Self specified temperature crystal oscillator specifications required attention:
     First, crystal indicators is not possible to mention the higher the better, there are different requirements for different occasions, if unreasonably increase the requirements of individual indicators, will certainly result in substantial improvements in the cost of all aspects of the work and to bring unexpected difficult.
   Nominal frequency
  Is a crystal to meet supply voltage, load, voltage-controlled voltage output frequency required conditions, only in accordance with user requirements, the supplier can not be changed.
   Frequency Accuracy
Crystal frequency accuracy of proposed only as a means of validation. This indicator is unimportant, because the targets set too high, after a very short period of time will be exceeded. Such as daily aging rate 1E-8 crystal, frequency accuracy is set at 1E-7, then about 10 days over. It is only for manufacturers to provide a factory-adjusted measures. As one pair of crystal quality can be tested to develop a secondary index, you can mention relaxed and not as a test of the death index.
    VCO range
"Minimum accuracy" and "pulling range" yielded conflicting, they can compromise on pulling range. That is, the voltage control range can leave some degree of affluence. Let wealthy degree of 10% is reasonable.
     Do not think, wealthy degrees (voltage-controlled crystal oscillator range) to obtain bigger and more insurance because PLL but also to meet the "initial maximum frequency deviation" This network access parameters, in addition to the voltage-controlled characteristic linear problem.
     Throughout the VCO range, minimum number of frequency points into? From the above description shows that: "Initial maximum frequency deviation" is the two frequency points maximum frequency interval, so the voltage controlled crystal oscillator Range / initial maximum frequency deviation = minimum frequency point. On the 2,3-level clock is calculated as follows:
    2 minutes:2×4E-7/5E-10=1600;
    3 minutes:2×4.6E-6/1E-8=920。
    The linear voltage controlled characteristic problem also affects the number of frequency points, according to the characteristics of the flat portion (the minimum unit voltage offset) to consider, so the frequency point will increase linearly worse, the more the increase. Taking into account the general properties of the linear voltage controlled requirement is 10%, so the frequency is increased by about 10%. That is, the voltage control range increases by about 20%. 2 minutes to take 2 × 5E-7 frequency points is 2000; 3 minutes take 2 × 6E-7 of frequency points to 1200.
     Thus, a voltage controlled voltage points need to have the same number, which requires 11 bits (2048 to generate points) number of touch converter. Taking into account the following reasons: to take a small frequency interval that is conducive to the improvement of indicators; no 11 of the DAC; their high number of low linear DAC is not particularly reliable. Optional 12-bit DAC suitable suitable.
    Analog PLL clock can be free from this constraint, you can use a wide range of voltage control. As mentioned above, the crystal can not do a very wide range of voltage control, and poor linearity. Fortunately for linear analog PLL ask, you can relax linearity requirements, but still want to VCO range as narrow as possible.
     Temperature Frequency Stability
  Temperature Frequency Stability refers to the crystal oscillator output frequency with the ambient temperature variation, is a very important technical indicators, the temperature of OCXO frequency stability is generally a few ± ppb to hundreds of ± ppb. The temperature range is generally less than -40 ℃ ~ +85 ℃, the user according to actual needs, as selected with a wider range of temperatures, the temperature frequency stability is higher, the higher the cost.
     Output waveform, Dimensions
You can choose according to actual demand, but smaller dimensions, make it difficult for the greater cost is also higher.
     Phase noise and short-term stability
Phase noise and short-term stability, the lower, the higher the cost, the specific reference to the following technical specifications, the user according to actual needs.
   OCXO generally require years of aging and the aging date, targets lower TCXO then look years aging, OCXO day aging generally ± .3 ppb ~ ± 5ppb, the industry generally believe that aging is the day of the aging 100 times. This is also an important indicator of the crystal. Recommended that the user according to actual needs.
      Voltage characteristics and load characteristics
   Voltage characteristics and load characteristics in general to a few tenths of a ± ppb ± ppb. Is determined by the ability of the circuit and the device is determined, the user according to actual needs.
      Power (power current and steady current)
   For the index higher OCXO, the power consumption is smaller, the smaller the current temperature, temperature frequency stability unfavorable, this and the crystal dimensions similar to the power supply voltage and can be quite small, will help save costs.
      Supply Voltage
   Crystal power supply voltage generally: 3.3V, 5.0V, 9.0V, 12V, 15V types, the user should be based on the need to use the design.
   And the user's circuit to match, only according to user requirements, the supplier can not be changed.
  Laser marking, appearance, if necessary, according to user needs marking.
  Using resistance welding package to replace the traditional tin seal, can well avoid the crystal in the envelope of the process, subjected to high temperature, affecting the stability of the crystal or bring other problems.
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