Don't underestimate the negative resistance and load capacitance, otherwise the crystal oscillator will go on strike

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Don't underestimate the negative resistance and load capacitance, otherwise the crystal oscillator will go on strike [Copy link]

In the hardware system, the oscillation circuit plays the role of "system pulse", and requires efforts in driving capability, compensation circuit, load capacitance, etc. If not handled properly, the crystal oscillator will not achieve the expected effect, and may even fail to oscillate.

1. Negative electrode resistance

Driving a crystal oscillator requires a very small amount of electrical power, the specific amount depends on the oscillator crystal unit. This power is very small, and if too much driving force is applied, it will cause the product characteristics to be damaged or destroyed.

Figure 1. Negative resistor (r) in an oscillator circuit

The compensation circuit is used to adjust the start-up time. Unless sufficient negative resistance is provided in the oscillation circuit, the oscillation start-up time will increase or no oscillation will occur. To avoid this, it is necessary to provide sufficient negative resistance when designing the circuit. The specific method is to connect the resistor (r) and the crystal unit in series to the circuit, and adjust (r) so that the oscillation occurs (or stops) to obtain the best value. It is recommended that (r) > CI × (5 to 10)).

2. Load capacitance

Load capacitance is essential. If the load capacitance in the oscillation circuit is different, the oscillation frequency may deviate from the designed frequency.

Figure 2. Oscillation circuit parameter setting reference

The approximate expression for the load capacitance in the circuit is CL = CG × CD / (CG + CD) + CS, where CS represents the stray capacitance of the circuit.

The load capacitance is generally 5-15pF, and most crystal oscillator manufacturers give recommended values in their specifications. For example, the Taitien crystal oscillator specification recommends 15pF for OZ-I type, and no more than 3pF for XX and XZ types. The Tangshan Jingyuan TCXO-7050 crystal oscillator specification recommends 15pF.