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Ashining technical articles - crystal clock circuit design

pubdate:2018/06/27 13:46:00      posted by:      visit times:636

        The crystal oscillator acts like a human heart in a digital circuit system, and the crystal oscillator circuit provides a basic clock signal for the digital circuit system.

        Ashining Technology is rigorous and demanding in the design of crystal oscillator circuit, adheres to strict technical verification, and provides users with highly reliable wireless communication products.
        In the current digital wireless communication, the quality of the crystal signal greatly affects the communication quality of the entire system.
        The main parameters of the crystal oscillator are nominal frequency, adjustment frequency difference, aging frequency difference, equivalent resistance, static capacitance, load capacitance and so on. The specific choices need to be assessed separately according to actual needs and considered comprehensively. It is not recommended to pursue a single parameter indicator.
        Poor crystal scheme design can seriously affect the stability and reliability of the whole system. Many engineers choose crystal oscillators by pursuing lower tolerances, while ignoring the importance of clock signal quality. Generally, the inexpensive active crystal oscillator has severe jitter, waveform distortion, large phase noise, large harmonic components and wide distribution, which are only suitable for general system. Ashining doesn’t recommend use that, because its severe harmonic components tend to crosstalk to other parts of the system, resulting in greatly reduced system reliability and stability, such as transmission loss, module deadlock, and even crosstalk to its own power supply. The system affects the normal operation of other components, and may also directly radiate to the free space via the antenna, seriously interfering with the normal operation of the surrounding equipment.




picture 1 - Clock signal eye diagram of a module using an inexpensive active crystal oscillator



[picture 2 - Spectrum of the clock signal of a module using an inexpensive active crystal oscillator]



        The choice of crystal oscillator should be based on specific needs. Taking the AS32 series wireless module as an example, the AS32 series wireless module is a cost-effective LORA wireless serial port module developed by Ashining Technology based on Semtech SX1278 wireless transceiver chip.



【图三-Semtech LoRa modulation transceivers Crystal Specification 】


(来源http://www.semtech.com/images/datasheet/an120014-xo-guidance-lora-modulation.pdf )

        Ashining Technology strictly selects the excellent crystal oscillator scheme based on the design specifications provided by Semtech through a large number of experimental tests. Its design needs to be comprehensively considered and strictly verified. In addition to ensuring accurate and stable oscillation frequency, it is also necessary to consider signal crosstalk, load matching, environmental adaptation and many other issues.



             

[picture 4 - AS32-TTL-100 Wireless Module Clock Signal Eye Diagram]


        The crystal scheme used by the AS32-TTL-100 is accurate and stable, and the phase noise is clean. Meets the Semtech design requirements shown in Figure 3. Accurate, clean and stable clock signal is an important support for long-term stable operation of the system, and it is a solid guarantee for reliable system communication.


[picture 5 –as32-ttl-100 Wireless module crystal oscillator clock signal spectrum ]


        The harmonic of as32-ttl-100 clock signal is cleaner, which reduces the problem of signal crosstalk, and greatly improves the stability and reliability of the system. Ashining technology company adhering to the  design concept of theory and experimental verification combination, seriously provide users with stable and reliable wireless communications products.







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