In nuclear physics experiments, errors of timing measurements result from the contribution of disturbances, intrinsic at the electronic counting system. The counting system is affected by the statistical variations of the input pulse shape. We have studied for a tunnel diode basic monostable circuit the threshold behaviour and delay jitter vs input rise time. Therefore we give an operative definition of the threshold and calculate its variation for exponential and ramp input shaped pulses. The results are critically tested with experimental circuits. We find finally the counting lossess to be ascribed to the jitter of the input pulses rise time. The results obtained may be straightly applied to all switching circuits in a more general fashion.