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)×Rs 　So the limit of the crystal's ESR can depend on the equation to design. ※The circuit noise will affect the test result.