Aim\ A fourparameter sine wave curvefitting methd is introduced. As an absolute convergent course

it is developed on the base of threeparameter sine wave curvefitting method. The absolute convergent interval

the absolute convergence criterion and the realization course are presented. The demodulation of AM(Amplitude Modulation) signal of the function generator are discussed and achieved. Methods\ By using one period moving sine wave model

the precise demodulation of AM signal wave is achieved. Results\ In case that the modulating signal is a sine wave

the simulated result of demodulation amplitude is presented. The distortion of demodulation is given which describes each of the other parmeter jitters in each case. At the same time

one group of experimental results are given

which shows that the demodulation results are clear and definite. Conclusions\ The method is excellent in demodulating AM signal of the function generators. High accuracy

high resolution

can be obtained. The method can be applied to measure and calibrate both AM signal generator and its demodulation devices.