The two capacitor
s called crystal vibration load capacitance, respectively in the crystals of two feet and capacitance to the ground, in dozens of skin hair.
It will affect the resonance frequency of the crystals and the output amplitude, the general order of crystal vibration when the supplier will ask you how much is the load capacitance.
Crystal vibration load capacitance = [
Cd * Cg)
Cd + Cg)
In Cd + Cic + delta C type, Cg is respectively in the crystals of two feet and capacitance to the ground, Cic (
Integrated circuit internal capacitance)
Capacitance on PCB)
Experience value for 3 to 5 pf.
All kinds of logic chip crystals pin can be equivalent to a capacitor three-point oscillator.
Crystals the interior of the pin is usually a inverter, or is an odd number of inverter series.
XO and vibration in crystals output pin with a resistor connected between the input pin XI, for CMOS chip is usually several dozens M M to Europe.
A lot of chip pins internal already contains this resistance, pin external need not pick up.
The resistance is in order to make the inverter with the linear state in oscillation when the initial place, inverter has a lot to gain amplifier, like a in order to start up.
Quartz crystal is connected between the input and output of crystals pin, equivalent to a parallel resonant circuit, oscillation frequency should be parallel resonant frequency of the quartz crystal.
Next to the crystal two capacitance to ground is actually the three-point capacitance circuit the partial pressure of capacitance, earth point is the point of partial pressure.
To meet that partial pressure point as the reference point, the input and output of oscillation pin is inverse, but from the perspective on both ends of the parallel resonant circuit of the quartz crystal, forming a positive feedback to ensure sustained oscillation circuit.
In chip design, these two capacitors have been formed, usually in two equal capacity, the size depends on the technology and the landscape is different, but it is relatively small, not suitable for wide range of frequencies.
When external is about several to dozens of PF, PF depends on frequency and the characteristics of quartz crystal.
It is important to note: the two values of capacitance in series is in resonance circuit in parallel, will influence the oscillation frequency.
When two equal capacitance, feedback coefficient is 0.
5, is generally can meet the oscillation condition, but if it is not easy to start up or oscillation instability can reduce the input of capacitance, and increase the output value to increase the amount of feedback.
compressor start capacitor are required in the manufacture of almost every product and dual start capacitor ac motor capacitor
is one of the most common machines.
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