The meaning of the three capacitors of the crystal oscillator

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The meaning of the three capacitors of the crystal oscillator

Posted Date: 2024-01-25

Quartz crystal resonator (Xtal) is a widely used frequency component. Manufacturers list basic parameters such as nominal frequency, frequency stability, operating temperature, load capacitance, etc. in their specifications. Some also Provides parasitic capacitance (Shunt capacitance) and dynamic capacitance (Motional capacitance) indicators.

For example, the recommended values ​​of the load capacitance CL of the crystal oscillator in Figure 1 are 9pF and 12.5pF, the dynamic capacitance C1 is 6.0fF, and the parasitic capacitance Cs is 1.2pF. So, what do these three capacitors represent? What is the magnitude of the dynamic capacitance unit fF?

Figure 1. 32.768kHz crystal oscillator specification provided by the manufacturer

Load capacitance and parasitic capacitance

Load capacitance is the sum of all effective capacitances of the entire oscillation circuit with the crystal oscillator as the core. The size of this CL value determines the operating frequency of the oscillator. By adjusting the load capacitance, the operating frequency of the oscillator can be adjusted to the nominal value.

The load capacitance CL is given in the manufacturer's specifications, and the standard values ​​are 12.5 pF, 16 pF, 20 pF, and 30pF. The relationship between CL capacitance and resonant frequency is not linear. It can be seen from Figure 2 that when CL becomes smaller, the frequency deviation becomes larger; when CL increases, the frequency deviation decreases.

Figure 2. Crystal oscillator circuit and load capacitance definition

Load capacitance CL consists of CG, CD, and CS. In addition to the two external capacitors in the oscillation circuit, it also includes the parasitic capacitance (shunt capacitance) between the two pins of the crystal, the parasitic capacitance of the two pins of the IC chip, and the stray capacitance from the PCB. In Figure 2, CG is the total capacitance from the input pin of the crystal oscillator circuit to gnd (including the external CG capacitor and the tiny crystal oscillator pin); CD is the total capacitance from the output pin of the crystal oscillator circuit to gnd (including The external CD capacitor and the tiny crystal oscillator pin are two parts); CS is the total parasitic capacitance (shunt capacitance). Here, parasitic capacitance (CS) is sometimes also called parallel capacitance. The specific value can be found in the specification book. The size generally ranges from 0.2 to 8pF. For example, the parasitic capacitance (shunt capacitance) in the specification book shown in Figure 1 is 1.2pF. . As for Ci and Co from IC chip pins, they can generally be found in the chip manual. Compared with CS, the Ci and Co values ​​of the chip are much smaller and can be completely ignored. Next, we calculate the values ​​of the external capacitors CG and CD. In order to maintain the load balance of the crystal, CG=CD is generally required in practical applications, so the formula in Figure 3 becomes: CL = CG/2 + CS= CD/2 + CS. Introducing the CL and CS data in Figure 1, we can get: CG/2 = CD/2 = CL - CS = 12.5pF - 1.2pF = 11.3pFCG = CD = 22.6pF

It is worth noting that many IC chips have internal compensation capacitors (internal capacitance). Users only need to select the appropriate quartz crystal according to the load capacitance value recommended by the IC chip datasheet, and there is no need to add additional capacitance. However, due to the uncertainty of parasitic circuits, it is best to reserve space for CG/CD in actual design.

Dynamic capacitance and unit fF

In addition to the load capacitance and parasitic capacitance, the characteristic parameters of the crystal oscillator also include the capacitance index - dynamic capacitance. Some crystal oscillator manufacturers will provide this index, and the size is usually several fF. Here, we need to note that fF is a small capacitance unit: 1f = 10-151F = 103mF = 106μF = 109nF = 1012pF = 1015fF 1fF = 10-3pF = 10-6nF = 10-9μF = 10-15F It can be seen that the dynamics of the crystal oscillator Capacitance (Motional capacitance) is a very small value and has minimal impact on load capacitance. No wonder most crystal oscillator manufacturers do not provide this parameter!

Review Editor: Huang Fei

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