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    Summary of crystal oscillator and matching capacitance




    1. Matchingcapacitance ---- load capacitance refers to the capacitance required for normaloscillation of crystal oscillator. Generally, the external capacitance is tomake the equivalent capacitance at both ends of crystal oscillator equal to orclose to the load capacitance. In the case of high demand, the capacitance toground of IC input should also be considered. Generally, the capacitance atboth ends of crystal oscillator is twice of the required load capacitance. Inthis way, the parallel connection is close to the load capacitance.

    2. Loadcapacitance refers to the total external effective capacitance across both endsof the crystal in the circuit. It is a test condition and a use condition. Whenapplied, the accurate frequency can be obtained by adjusting the value of loadcapacitance. The magnitude of the capacitor mainly affects the resonantfrequency of the load and the equivalent resonant resistance of the load.

    3. Generally,increasing the load capacitance will decrease the oscillation frequency, whiledecreasing the load capacitance will increase the oscillation frequency.

    4. Loadcapacitance refers to the sum of all effective capacitances inside and outsidethe IC block connected by two leads of the crystal oscillator, which can beregarded as the series connection capacitance of the crystal oscillator in thecircuit. Different load frequency determines the oscillation frequency of theoscillator. For crystal oscillator with the same nominal frequency, the loadcapacitance is not necessarily the same. Because the quartz crystal oscillatorhas two resonance frequencies, one is the low load capacitance crystaloscillator of the series wiper crystal oscillator, the other is the high loadcapacitance crystal oscillator of the parallel wiper crystal oscillator.Therefore, when the crystal oscillator with the same nominal frequency isinterchanged, the load capacitance must be one to one, and it is not allowed tobe interchanged rashly, otherwise, the electrical appliance will not worknormally.

    Resistance besidecrystal oscillator (parallel and Series)

    A 22K resistor isconnected in series at the output end of a circuit, and a 10m resistor isconnected between the output end and the input end of a circuit. This isbecause a linear operational amplifier is connected inside the chip end of thecrystal oscillator, and the input is output 180 degrees in reverse direction.The network composed of load capacitance resistor at the crystal oscillatorprovides another 180 degrees of phase shift, and the phase shift of the wholeloop is 360 degrees, which meets the requirements of oscillation. The phasecondition also requires that the closed-loop gain is greater than or equal to 1before the crystal can work normally.

    The resistancefunction of the crystal oscillator input-output connection is to generatenegative feedback, to ensure that the amplifier works in the linear area withhigh gain, generally at the M ohm level, the resistance at the output end andthe load capacitance form a network, providing 180 degree phase shift, and atthe same time, it plays the role of current limiting, so as to prevent theinverter output from over driving crystal oscillator and damaging crystaloscillator.

    The resistance inseries with the crystal oscillator is often used to prevent the crystaloscillator from being driven excessively. The result of crystal vibration overdriving is to reduce the loss of crystal vibration gradually, which will leadto the rise of frequency and the early failure of crystal vibration. It can alsobe used for drive level adjustment. Used to adjust drive level and vibrationmargin.

    Generally, theinside of Xin and xout is a Schmitt Inverter, and the inverter can not drivecrystal oscillation. Therefore, a resistor is connected in parallel at bothends of the inverter, and the output signal is fed back 180 degrees to theinput end to form a negative feedback amplifier circuit. In terms of theresistance of the crystal, the equivalent resistance of the resistor and thecrystal is in parallel. Think about whether the resistance is large or not.What is the effect of small resistance on the impedance of crystal?

    The function ofresistance is to add a feedback loop to the inverter inside the circuit to forman amplifier. When the crystal is in it, the AC equivalent of the feedback loopwill resonate according to the crystal frequency. Because the Q value of thecrystal is very high, the resistance will not affect the output frequency in alarge range. In the past, when the stability of this circuit was tested, it wastested that it can be normally started from 100k to 20m, but it will affect thepulse width ratio.

    The Q value of thecrystal is very high. What does the Q value mean? The series equivalentimpedance of the crystal is Ze = re + jxe, Re < < | jxe|. Generally, thecrystal is equivalent to a very high Q inductance, which is equivalent to avery small conductor resistance of the inductance. Q generally reaches theorder of 10-4.

    Avoid damaging thecrystal when the signal is too strong. The resistance is generally large,usually several hundred K.

    The resistance inseries is used to limit the amplitude of oscillation, and the two capacitors inseries are usually about 20 ~ 30p according to the crystal vibration of LZ,mainly used to fine tune the frequency and waveform, and affect the amplitude.The resistance in series depends on the IC spec, some are used for feedback,and some are countermeasures for EMI.

    However, thesmaller re is, the larger RP will be after converting it into parallel equivalentimpedance, which has a ready-made formula. The equivalent RP of the crystal isvery large. The external shunt resistance is connected to this RP, so the RPvalue is reduced ----- > the re value is increased ----- > the Q value isreduced.




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