The simplest method of measuring frequency with an oscilloscope, is to determine [using the gratitude] the distance between two identical points in a waveform and multiply this by the timebase setting to obtain the period and hence the frequency. This method whilst quick, is subject to at least at 5 percent error. More accurate methods of frequency measurement using an oscilloscope, utilize it as display detector in comparison techniques, employing a stable known frequency from an external source and the scope’s x-y display facility.
Consider first the problem of adjusting an oscillator’s output frequency to be equal to that of a standard frequency signal.
Applying one of this signals to all x plates of the oscilloscope and the other to the plate will result in an eclipse type display that will vary in shape at a rate dependent on the difference in frequency between two signals. As the oscillators frequency is adjusted to be equal to the standard frequency, the frequency of the eclipse will become slower and when the two frequencies are equal, will cease altogether. Thus, it becomes apparent that differences between two signals which have approximately equal frequency may be determined by counting the rotations of the eclipse in a known time. In this manner, extremely accurate frequency comparisons may be made using an oscilloscope. For example, if an oscillators output frequency changes with respect to a 1KHz standard by one cycle [one rotation of the display] is 0s, then the difference in frequency between the two is a tenth of Hz or 0.01 percent.
Another method of determining frequency ratios using an oscilloscope is to supply a common frequency to the X and Y inputs, these voltages having a 900 phase shift [to give a circular display], and applying a higher frequency signal to the Z modulator input. This higher frequency signal varies the intensity of the circular display, there being one bright and one dark section, for each multiple of X-Y input frequency. This method, whilst giving a display which is easy to interpret is limited in application to extract multiples of input frequencies, with the additional requirement that the Z nput frequency must be larger than the X-Y input.
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