Pulse width modulation (PWM) is widely used in different applications. PWM transform the information in the amplitude of a bounded input signal into the pulse width output signal, without suffering from quantization noise. The frequency of the output signal is usually constant. In this paper the new PWM system with frequency changing (PWMF) is described. In PWMF the pulse width and also frequency is changed, therefore 2 independents information are simultaneously transmitted, and PWM and frequency modulation (FM) are simultaneously used. But such system needs fast demodulator separately for PWM and FM. The circuit for fast demodulation of PWMF signal is also described and measuring results are presented. All described circuits were constructed and measured.