In situ analyzer of electron emission properties that displays the Fowler-Nordheim (F-N) plot and its location on the Seppen-Katamuki chart directly from the current-voltage values of the field emission device, was developed. Field electron emission characteristics were acquired by applying the voltage that was modulated by an ac signal. An analog circuit, which consists of inexpensive operational amplifiers, was designed so that the circuit converts the current-voltage characteristics to the reciprocal voltage and the logarithm of the current over square voltage. The slope and intercept were also calculated by the analog circuit. The operation of the circuit was confirmed with a light-emitting diode. The electron emission property of an etched tungsten needle emitter was converted to F-N plot, and the property projected to the screen was a straight line. (C) 2003 American Vacuum Society.