How to measure negative resistance?
A.
Using an oscilloscope with a current probe in series with a variable resistor (must be zeroed) as follows (Figure 1), (Figure 2):
After turning on the power, observe the current waveform and increase the value of the variable resistor until oscillation stops (or use a spectrum analyzer to monitor the signal strength; as the variable resistor value increases, the signal strength will gradually weaken until it becomes undetectable). At this moment, the sum of the resistance of the variable resistor(VR) and the equivalent resistance of the crystal represents the external negative resistance(-R). The negative resistance margin is given by ∣−R∣/ESR max ≥5.
Ways to improve insufficient negative resistance:
After turning on the power, observe the current waveform and increase the value of the variable resistor until oscillation stops (or use a spectrum analyzer to monitor the signal strength; as the variable resistor value increases, the signal strength will gradually weaken until it becomes undetectable). At this moment, the sum of the resistance of the variable resistor(VR) and the equivalent resistance of the crystal represents the external negative resistance(-R). The negative resistance margin is given by ∣−R∣/ESR max ≥5.
Ways to improve insufficient negative resistance:
- Reduce the value of the external capacitor and use XTAL with lower CL to make the oscillation circuit reach a matching state.
- Adjust the current limiting resistor value.
- Choose the crystal with lower ESR.
