Abstract: An input signal is converted into a dc voltage proportional to the frequency of the input signal in a frequency-voltage converter, while saw tooth waves are generated in a saw tooth wave generator triggered with the input signal. The dc voltage is applied to a variable resistance circuit where the resistance is changed corresponding to the dc voltage. The period of such saw tooth waves is controlled with the variable resistance circuit so that the saw tooth waves may follow the input signal frequency for automatically indicating at all times a desired number of waveforms on the screen of a cathode-ray tube. The variable resistance circuit may be controlled with means for free running operation of the saw tooth wave generator during the absence of the input signal.