Essentially all commercially available cathode ray oscilloscopes include time base sweep circuits designed such that an electron beam is swept linearly across the screen of a cathode ray tube at a preselected rate. Preselection is performed via front panel switches. The selectable sweep rates typically range from seconds/centimeter to tenths of a microsecond/centimeter, usually in a 1-2-5-10 sequence.
Observation of a periodic or quasi-periodic waveform on a standard cathode ray oscilloscope is usually accomplished by manual selection of an appropriate sweep rate, adjustment of the sweep trigger controls and vertical amplifier attenuator setting. The waveform is observed directly on the cathode ray tube while the period or frequency of the waveform may be determined by employing a screen reticle on the face of the cathode ray tube and a calibrated deflection rate switch setting.
While satisfactory for many applications, this observation and measurement technique has several disadvantages. For example, measurement of frequency of a displayed signal involves some mental calculation. One cycle of the signal is first observed on the screen of the cathode ray tube. Using the screen reticle, the period is determined from the deflection rate switch setting. Then the period is arithmetically inverted to provide frequency. This technique typically results in a 5-10% error.
As another disadvantage, at any given deflection rate setting, the number of cycles displayed is a function of the frequency of the input signal. As the frequency of the input signal changes, the number of cycles displayed on the cathode ray tube of the oscilloscope changes proportionally. When the frequency of the input signal is substantially higher than the deflection rate of the oscilloscope, a large number of cycles is displayed on the screen, making visual analysis impracticable. To display a small number of cycles of the signal across the entire screen, the deflection rate of the oscilloscope must be manually increased by means of the deflection rate control.
It is often desirable to provide a display of a predetermined number of cycles of an input signal independent of the frequency thereof. Systems have been provided in the prior art for this purpose. The systems of which I am aware, although generally somewhat satisfactory, are quite complex and not practical for general applications requiring low cost. This is because several stages of signal processing are utilized for automatically synchronizing the deflection rate of the deflection signal to the frequency of the input signal. In addition, the systems include no means for displaying the frequency or period of the input signal.