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# A summary of the crystal vibration and the matched capacitance

2018-06-30 14:22:37 次浏览分类：Industry Dynamics

1. matching capacitance - load capacitance refers to the capacitance needed for normal vibration of the crystal oscillator. The general external capacitance is to make the equivalent capacitance at the two ends of the crystal oscillator equal to or close to the load capacitance. For high occasions, the earth capacitance of the IC input should also be considered. The capacitance at both ends of the general crystal oscillator is two times the required load capacitance. This parallel is close to the load capacitance.

2. load capacitance refers to the total external effective capacitance across the ends of the crystal in the circuit. He is a test condition and a condition of use. When applied, the accurate frequency can be obtained by adjusting the load capacitance value. The size of the capacitor mainly affects the load resonance frequency and the equivalent load resonance resistance.

3. generally speaking, increasing the load capacitance will cause the oscillation frequency to decrease, while reducing the load capacitance will increase the oscillation frequency.

4. load capacitance means that the two lead wires of crystal oscillator connect the sum of all the effective capacitance inside and outside the IC block, which can be regarded as a series of capacitors in the circuit. Different load frequencies determine the oscillation frequency of the oscillator. For a crystal oscillator with the same nominal frequency, the load capacitance is not necessarily the same. Because the quartz crystal oscillator has two resonant frequencies, one is a low load capacitive oscillator with a series wiping oscillator, and the other is a high load capacitive oscillator that wiped the crystal oscillator in parallel. Therefore, when the crystal oscillator with the same nominal frequency is interchanged, it is also necessary to ask the load capacitor to be one or more, so that it can not be swerve, otherwise it will cause electrical work to be abnormal.

Resistance near the crystal oscillator (parallel and Series)

A circuit connects the resistance of a 22K in its output end. A resistance is connected between the output and the input ends of the circuit. This is due to a linear operational amplifier inside the chip end that connects the crystal oscillator. The input is output in reverse 180 degrees, and the network of load capacitance electrical resistance at the crystal oscillator provides a other 180 degree phase. The phase shift of the entire loop is 360 degrees, which satisfies the phase condition of the oscillation. At the same time, the closed-loop gain is greater than or equal to 1, and the crystal works normally.

The resistance effect of the input and output connection of the crystal oscillator is to produce negative feedback, which ensures that the amplifier operates in the linear region of high gain, generally at the M European level, the resistance of the output end and the load capacitance form a network, which provides 180 degree phase shift, at the same time, it plays the role of current limiting and prevents the reverse device output from driving the crystal vibration and damaging the crystal.

Resistance in series with crystal oscillator is often used to prevent the crystal oscillator from being overdriven. The result of the excessively driving of the crystal oscillator is to reduce the contact electroplating of the crystal oscillator by gradual loss, which will cause the increase of frequency and lead to the early failure of the crystal oscillator, and can also be used to adjust the drive level. It is used to adjust drive level and hair margin margin.

The interior of Xin and Xout is generally a Schmidt inverter. The inverter can not drive the crystal shock. Therefore, a resistor is paralleled in the opposite ends of the inverter. The resistance completes the output signal back 180 degrees back to the input to form negative feedback, which forms a negative feedback amplification circuit. The crystal is on resistance, resistance and the equivalent resistance of the crystal. Resistance is a parallel connection. If you want to think about whether the resistance is large or the resistance is small, it has little effect on the impedance of the crystal.

The function of the resistor is to add a feedback loop inside the circuit to form an amplifier. When the crystal is in it, the exchange of the feedback loop is equivalent to the crystal frequency, because the Q value of the crystal is very high, so the change of the resistance in a large range will not affect the output frequency. In the past, when testing the stability of this circuit, it was normal to start from 100K to 20M, but it would affect the pulse width ratio.

The Q value of the crystal is very high. What does the Q value mean? The series equivalent impedance of the crystal is Ze = Re + jXe, Re<< |jXe|, and the crystal is generally equivalent to a high and very high inductance of a Q, and the resistance of the conductor is very small and small. Q generally reaches the order of 10^-4.

Avoid signals that are too strong for bad crystals. Generally, the resistance is large, and it is usually hundreds of K.

The series of resistance is used to limit the amplitude of the oscillation, and the two capacitors in according to LZ's crystal vibration of dozens of MHZ is generally around 20~30P, mainly used with the micro tuning frequency and waveform, and influence the amplitude, and the resistance to go in is to see IC spec, some are used for feedback, and there is a EMI strategy.

However, when the equivalent impedance is converted to parallel connection, the smaller the Re, the larger the Rp is. The equivalent Rp of the crystal is very large. The resistance on the outside is connected to the Rp, so the Rp value is reduced - - increasing Re - - reducing Q.

On crystal oscillator

Quartz crystal oscillator is a high precision and high stability oscillator. It is widely used in all kinds of oscillating circuits, such as color TV, computer and remote control. It is used in frequency generator, clock signal for data processing equipment and reference signal for specific system.

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