Domestic substitution is considered from the perspective of supply chain security. In recent years, the business income of NDK and KDS in Japan has shown a downward trend. Especially since the second half of 2018, the trade friction between China, the United States and Japan and South Korea has intensified. In order to ensure the safety of the industrial chain, well-known domestic manufacturers of communications, complete machines, and home appliances have actively The component industry seeks domestic substitution, which accelerates the import substitution of mid-to-high-end products such as miniaturized SMD resonators, TCXO oscillators, and TSX thermal crystals. When replacing the foreign crystal oscillator brand in the BOM table, there are some technical issues that need to be mentioned:

    1. Load capacitance CL

     The nominal frequency in the specification of the quartz crystal resonator is measured under the condition of a certain load capacitance CL. When using it, two load matching capacitors of equal value must be connected and adjusted to the optimum value in order to obtain the nominal oscillation frequency.

     Here, the ceramic capacitors C1 and C2 for matching purposes are of equal value, and the sum of their capacitance plus any stray capacitance from the crystal package, oscillator input pins, and board traces constitutes the total load capacitance (Cload). It's not easy to accurately calculate all the stray and parasitic capacitances in a circuit, so you can make an estimate (usually around 4 to 6pF) and measure the output frequency to see if you need to adjust the value of the capacitor. If the total CLoad is greater than the specified CL, the oscillation frequency will be reduced. If CL is too low, the frequency will be higher. If CL is too high and low, the oscillator may not start at all. If the crystal oscillator is replaced, C1 and C2 will of course have to be recalculated.

    2. The coordination of circuit negative resistance and resonator resonant resistance

    Usually, the oscillator circuit will have a negative resistance value in the design, and this negative resistance value is usually related to the feedback coefficient of the circuit and the amplification factor of the active element. Quartz resonator, as a frequency-selective element, has an equivalent resonant resistance. This resonance resistance is related to the package size and frequency of the resonator. Generally, the smaller the size of the resonator, the larger the resonance resistance; the lower the frequency, the greater the resonance resistance. Oscillation becomes unstable when the negative resistance of the circuit is less than twice the resistance of the quartz resonator. In particular, the working voltage of the oscillator circuit fluctuates, and the dispersion of the active components and the component values of the feedback link will cause the negative resistance to deviate from the design value. If the resonator is replaced, the resonant resistance of the quartz resonator has changed, and the normal working conditions required by the oscillation circuit are destroyed, and the feedback circuit needs to be adjusted. It is recommended to adjust the negative resistance to about 3-10 times the resistance of the quartz resonator. .

    3. Parasitic problems of crystal components

    If the replacement of the quartz resonator results in a frequency not at the required value and the circuit does not work properly, this may be caused by multiple spurious responses. This phenomenon rarely occurs. If it occurs, most of them occur on high-frequency crystal oscillators, which need to be negotiated with the crystal oscillator manufacturer.