Crystal drive level is the amount of power dissipated in a crystal which is usually specified in microwatts or milliwatts. It can be calculated by measuring the excitation current flowing through the crystal. There are multiple ways of measuring drive level. Using a current probe is most reliable which is similar to current flowing through a resistor. Using a FET probe is also common but has disadvantages on an inverter type oscillator, like a Pierce. On an inverter type oscillator, there is phase difference between the signal at the crystal input and output which make it difficult to accurately measure the voltage across the crystal. The equations and examples below will show the requirements for measuring drive level using a current probe. Refer to application note AN-830 for more details. For other questions not addressed by the Knowledge Base, please submit a technical support request.
 
The drive level can be calculated by using the RMS drive current (Id) and the load resonance resistance (RL). Refer to equation 1.
 
Since RL varies with load capacitance, equation 2 can be used to calculate RL.
 
Combining equation 1 and 2 yields equation 3.
 
When RR is at FS (series resonance frequency), RR is equal to R1 (Motional Resistance).
 
If using a series resonance crystal, CL goes to infinity.

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AN-830 Quartz Crystal Drive Level Application Note PDF 143 KB