Measurement using a spectrum analyzer with a loop antenna
The load capacitance is the circuit equivalent capacitance looking the circuit system from the two wire leads of crystal. The operation frequency in the circuit is defined by load capacitance and crystal.
When designing an oscillator circuit, negative resistance (-R) is very important parameter to consider. To maintain stable oscillation at a constant frequency, it must have enough negative resistance (|-R|) to compensate for the resistance of the crystal.
Add resistor “VR” to the crystal unit in series and ensure that the oscillation starts or stops. The approximate negative resistance of the circuit is the value obtained by adding the effective resistance “RL” the maximum resistance “VR” when the oscillation starts or stops after gradually making VR value larger. The best value of Negative Resistance (-R) is 3~5 times of crystal ESR(R1). If the Negative Resistance (-R) is too small, could make Crystal unstable.
Electric-power or current level under the specified conditions of a crystal unit. Drive level is expressed in terms of milliwatt or microwatts.
Excessive drive level will result in the crystal fracture or make the frequency drift in a long time.
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DL = I2x RL
RL = R1(1+C0/CL)2
R1 : Equivalent series resistance
RL : Loaded resistance of resonance
I : Current of pass in the crystal
