The following is the procedures of Negative resistance measurement method:
1.Open the crystal output side pin in the circuit used , and connect a precision variable resistor in series with the crystal, as shows the following:  | Rs: The value of the crystal's ESR. VR: Precision variable resistor. Oscilloscope: Must use active probe |
2.Adjusting the VR to start oscillation.
3.Increaseing the VR until oscillation stop.
4.Measuring the VR value.
5.Calculating the -R using in sum of the RS and VR, as shows the following equation:
-R=-(Rs+VR)
6.It is preferable to -R, usually, -R>=-(5~10)xRs
So the limit of the crystal's ESR can depend on the equation to design.
*The circuit noise will affect the test result. | 
TECHNICAL TERMS
