Why is Zener avalanche noise saw tooth shaped?
Consider that you effectively have this:







simulate this circuit &ndash; Schematic created using CircuitLab

where C is the junction capacitance, plus any external capacitance (leads, breadboard, etc). Some of the current from R1 leaks through D1, but the rest charges C. Once the voltage reaches a certain level, avalanche breakdown occurs and current flows from C until the avalanche stops. Then the current begins charging C again.

To calculate C you first need to know the leakage. Decrease V1 until the noise disappears. Then measure the current. Then increase V1 back to 30V. Measure the rising slope of the noise dV/dt. Measure the average value of V. The current through R1 is approximately constant at (30V - V)/100kohm. Subtract the leakage current from this, then use I=C dV/dt to calculate the capacitance.